Hard Light Productions Forums
Off-Topic Discussion => General Discussion => Topic started by: jr2 on June 13, 2014, 03:32:39 pm
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Basically, to get people interested in space travel / science, NASA hired someone who has done stuff for the Star Trek universe to make a plausible model of a ship powered by an Alcubierre drive.
http://www.cnet.com/pictures/the-ixs-enterprise-warp-drive-ship-pictures/
I've gotta say, it looks great. Hummina hummina, where do I get one? :D
http://www.cnet.com/news/mark-rademaker-designing-a-warp-drive-space-ship-for-nasa/
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Typical marketing lipstick on a pig. Alcubierre drives do not work, period.
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Typical marketing lipstick on a pig. Alcubierre drives do not work, period.
[citation needed]
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Typical marketing lipstick on a pig. Alcubierre drives do not work, period.
[citation needed]
He got one off amazon. It sucked, one star, would not attempt FTL travel again.
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Typical marketing lipstick on a pig. Alcubierre drives do not work, period.
[citation needed]
He got one off amazon. It sucked, one star, would not attempt FTL travel again.
Well yeah, you obviously need at least 14 stars to power the FTL engine. He should've read the manual.
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Changes to the curvature of space propagate at the speed of light. At the very best maybe you could make an Alcubierre speedway between places you've already been (but I doubt that's possible either).
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too bad nasa blew its whole budget on graphics artists.
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Typical marketing lipstick on a pig. Alcubierre drives do not work, period.
****'s sake luis we get it, you read an article saying alcubierre drives wouldn't work and you feel the need to beat others over the head with it. please stop it for christ's sake
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Changes to the curvature of space propagate at the speed of light. At the very best maybe you could make an Alcubierre speedway between places you've already been (but I doubt that's possible either).
There's that, and there's also Krasnikov tubes, which could be laid in the wake of travel near the speed of light, and would essentially permit a return trip to just after you left. Multiple Krasnikov tubes could create a closed timelike curve.
Anyway, to those who don't already know, there are a few different problems with Alcubierre drives, not just the mass/energy issue. The research connected to this concept art may be able to get around the mass/energy requirements if a specific interpretation of brane cosmology is correct (which is what they're testing, and it's still pretty cool if it works, even if there are other issues which make Alcubierre drives impossible). If their theory holds, the amount of energy required for a realistic spaceship could be less than 700 kg. This is less than half the energy released in the 2004 Indian Ocean earthquake (also a little more than half the total energy consumed by the US in 2001, but about 4x the total electricity consumed by the US that year, according to Wolfram|Alpha), though negative energy and placement of all this energy would still be a problem. Additionally, being causally disconnected from the walls of the bubble during transit would prevent anyone inside the bubble from steering or decelerating, and particles that build up at the front of the bubble during transit would have enough energy to destroy anything in front of the ship when the bubble comes to rest.
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if those are the only problems then it sounds like they are the best option at the moment.
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Typical marketing lipstick on a pig. Alcubierre drives do not work, period.
****'s sake luis we get it, you read an article saying alcubierre drives wouldn't work and you feel the need to beat others over the head with it. please stop it for christ's sake
No, I didn't "read an article". I consulted with physicists, read quite a lot of material regarding this, and engaged in critical thinking.
I also am aware I'm not the best person to discuss the limitations, but even considering Koprachis' number of 700Kg of energy, it's overtly ridiculous. Not only you have to generate and efficiently manage this energy, you'll even have to turn it into negative energy, which is something we either believe is impossible to exist, or just mindblowingly difficult if we ever come across with some "fluke" like the Casimir Effect and so on.
This idea of creating "lanes" is good for scifi, but please. Anything that is going faster than light from any other perspective is either creating paradoxes or hidden from us by event horizons, and anything that is beyond event horizons can never "go back" to us.
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heh. Kg of energy.
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It's a perfectly valid unit if you assume c=1. :) Natural units are weird like that. And it's a usable unit even if you're not using them. E=mc2, "700kg of energy" is what you get from fusing 350kg of matter with 350kg of antimatter.
No, I didn't "read an article". I consulted with physicists, read quite a lot of material regarding this, and engaged in critical thinking.
Dunno what physicists were those, but, let me clear something up. Alcubierre drives do work. Moreover, we can make one that produces a measurable effect right now. This is not science fiction, not speculation, but actual, hard science. It was discussed on KSP forums in somewhat more depth, but the general idea is, an Alcubierre starship is an engineering problem, not a physics one.
The only question is, can an Alcubierre drive go FTL? The answer is no. But assuming negative energy is possible, it could, in a mind-boggingly weird way, mess with the structure of spacetime itself so it "moves faster than light", not only without ever moving, but without violating relativity and causality. I'm currently trying to learn the very basics of algebra that can describe this, and I've got enough already at this point, so don't ask me how exactly it works, but it does. :) Rest assured, it is incredibly nonintuitive how this stuff works, but it seems that mathematics do work out. The biggest problem is this negative energy, which only exists as a part of a theory at this point. Remember, though, antimatter (essentially "negative matter" from a certain point of view, though with positive mass) and neutrinos started out that way, too, as did Higgs bosons and many other heavy particles, so I'm optimistic.
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Aaaand everyone misses the gorgeous model. Go figure.
re: alcubi drives not working: read up on all the science disproving yhe Wright brothers' hair-brained heavier than air flying machine concept.
Does this prove it will work? No. It simply means, that just because science says something doesnt work, doesn't necessarily mean it won't. You might just be making the attempt in the wrong way. Then again, there might be no feasible way! Haha! But it's always fun to try, you learn more regardless of the results being successful or not. :nod:
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Actually, in this case, science says it does work. :) In fact, Flyer 1 was also based on sound calculations and engineering. Same with Alcubierre, IIRC, the effect has already been observed. As I just detailed above, the only problem is getting the drive to "go FTL".
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Aaaand everyone misses the gorgeous model. Go figure.
re: alcubi drives not working: read up on all the science disproving yhe Wright brothers' hair-brained heavier than air flying machine concept.
Does this prove it will work? No. It simply means, that just because science says something doesnt work, doesn't necessarily mean it won't. You might just be making the attempt in the wrong way. Then again, there might be no feasible way! Haha! But it's always fun to try, you learn more regardless of the results being successful or not. :nod:
The case against the Alcubierre drive is much more solid than that, and you should really know better than to equivocate the two. The requirement to have a bunch of exotic matter, which has never been observed to exist and for which no method of manufacturing exists, is a massive dealbreaker.
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That only applies to the FTL version, though. Now, I haven't done any calculations on just how efficient the STL one would be, but if we could travel at, say, something like 90% c for less than a ton of matter/antimatter fuel, I'd say this ship could still get built. Mars in 4 minutes, anyone? I don't know if it can be that efficient without going into negative energy, though.
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I was strictly speaking about the FTL aspect of it. Regarding a more modest objective I do not possess enough knowledge to know one way or the other, but the mechanism sounds energetically costly (euphemism) and overall crazy. I'm not expecting Elon Musk presenting the Alcubierre Drive in the next 50 years for the Falcon 23.
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Basically, to get people interested in space travel / science, NASA hired someone who has done stuff for the Star Trek universe to make a plausible model of a ship powered by an Alcubierre drive.
http://www.cnet.com/pictures/the-ixs-enterprise-warp-drive-ship-pictures/
I've gotta say, it looks great. Hummina hummina, where do I get one? :D
http://www.cnet.com/news/mark-rademaker-designing-a-warp-drive-space-ship-for-nasa/
Because when I think scientific accuracy, I immediately think Star Trek.
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they got ipads and sliding doors right on, why should they be wrong on this one?
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I was strictly speaking about the FTL aspect of it. Regarding a more modest objective I do not possess enough knowledge to know one way or the other, but the mechanism sounds energetically costly (euphemism) and overall crazy. I'm not expecting Elon Musk presenting the Alcubierre Drive in the next 50 years for the Falcon 23.
Just a note, "Falcon 23" would imply a 23-engine monstrosity along the lines of the old N1 rocket. :) Elon probably knows better than to try that. Also, Alcubierre drive isn't much good as a liftoff engine, so if developed, it'd probably go on a Dragon. And the effect is being studied by NASA (look up dr. Harold White). The energy cost might not be as big as it seems. Bending spacetime to do our bidding does sound crazy, but noone who ever achieved anything said "this is crazy, it'll never work" (well, maybe except for a brief exclamation when working out the kinks of the new idea). :) Turns out it's already been successfully done in small scale, and it apparently didn't black out the continent, so maybe the energy requirements aren't that bad.
Also, while FTL version is the one people are the most excited about, STL one is based on actually proven physics (well, it works, or at least seems to) and either way, an important stepping stone towards FTL one, should the latter be possible. Even if it doesn't, there's a lot of potential in this form of propulsion, especially since it isn't restricted by Newton's 3rd law. Alcubierre drive is essentially reactionless, so neither the Tsiolkovsky Rocket Equation nor Newton's equations limit it's efficiency. It's not like there are no other limiting factors, but it's something to consider.
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oh man i can't resist
"700kg of energy" is what you get from fusing 450kg of matter with 450kg of antimatter.
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oh nb this isn't even true on any level, depending on the type of matter you use an annihilation reaction will produce neutrinos; so you're right back where you started
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oh man i can't resist
I'd strongly advise you to do so, because you're wrong.
What I said is true. 700kg of matter can literally be equated to 700kg of energy. That's what E=mc2 means, unless you're one of the ones who don't accept Relativity. 350kg of antimatter+350kg of matter, when anihilating, will release that energy, generally in some other form than matter an antimatter. Now, for my simplistic example I assumed that you end up with a handful of "pure energy" (photons, they have no rest mass, only kinetic and potential energy). Now, about the only true thing you said is that you can also get neutrinos out of that; they do have rest mass, which means you get (kinetic and potential) energy + mass of neutrinos. I didn't get into this, since it's not useful for explaining why mass is a form of energy, and therefore one can be expressed using units for other.
In general, there are 3 kinds of energy in the universe. There's kinetic energy, potential energy and mass. They can and often do change into one another, and can be measured with the same units. For extra fun, try also expressing time in meters, since you can also do that thanks to relativity. :) The laws of the universe become surprisingly beautiful in this form, and you don't need to fully delve into natural units (which will make your head hurt until you get used to them).
Now, the problems of getting useful energy out of an A-M reaction is another thing entirely. I won't delve into that. You evidently mixed up with what I wanted to say, since neutrinos are indeed pretty useless as far as power generation goes. So are very high energy photons (gamma rays). They just pass through the collector. But exactly what you get depends on what you put in, and I'm pretty sure there's a reaction that could work for power generation with good efficiency.
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450kg + 450kg = 900kg.
Also, where the hell are you getting (and storing) 450kgs of AM?
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Bah, that's what I get for doing everything in Mathematica these days. Fixed. :) I've got that problem a lot, I properly split and calculate a triple integral, then I screw up addition or mix up something equally trivial. :) In my defense, I'm not the only one with this problem (you usually let Mathematica put in numbers once you know how the equations look like, so it's not a big handicap). But you're right, I should've probably double-checked the post.
As for how to get 350kg of AM, that's just theoretical divagations, while it's nothing that can't be done physics-wise, it's obviously quite hard to do if you actually tried to launch such a ship. So's a propulsion-scale STL Alcubierre drive, but it's not like it ever stopped anyone from discussing it. :)
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Excuse me for thinking that discussing the practical issues with actually building and powering an Alcubierre drive are a necessary part of any discussion involving Alcubierre drives.
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TBH, I didn't intend that particular idea to serve as a power source - just explaining what "700kg of energy" is. About 63000 petajoules, or 4*1038 electronovolts, or 6.3*1026 ergs, or even 6*1016 BTU. Whatever. A tremendous amount of energy, but it doesn't seem impossible. Now, we go find some good fusion fuel. According to AtomicRockets, D+He3 gives us 353.23 TJ/Kg. Now it's a rather trivial division to find we'll need about 180T of fusion fuel, assuming we can reclaim 100% energy produced (or that AtomicRocket counted in the loss in it's figure). That's a lot, but there's much more water on Earth than that. He3 could be a problem, if you want to propose a different fuel mix, just give me the numbers for it (gotta love Mathematica, once you come up with a routine, you can run it for any good data). :) Now the question boils down to two factors: making fusion power work and lifting 180T or something of fuel into space. Soviets could've had the capacity to do the latter by the 90s (proposed Energia-Vulkan booster could probably make that much to LEO) if it wasn't for politics, so could US with the Nova rocket. The former is being researched and we're making good progress. So we could probably power the thing by fusion just fine.
Now, could we afford this? Nope, not today. But someday it might change. It's definitely not physically impossible, even with 10 times less efficient fusion reaction (D-T, for example), it boils down to an engineering problem, and one that can be solved "brute force" by building more superheavy LVs. Also, 700kg is for an FTL version, I imagine that the STL one would be much cheaper in terms of energy.
I think that discussing engineering details of a car before the idea of a wheel has been fully proven is somewhat far-fetched, but if White's research is enough of a proof for you, we can talk that, sure. :) But IMO, it's a whole different discussion.
One nice thing is, I noticed one problem with the concept thanks to this discussion. Assuming it's an actual proposal, there's no way it could run on fusion, there's not enough space for fuel tanks. 180T of light elements, even if you liquefy them, takes a lot of space. Now, Star Trek ships are A-M powered, but as you highlighted, I'd rather take my chances with 72T of deuterium and 108T of He3, and building an efficient fusion reactor. :)
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The STL probe version can probably run on fusion, but you'd definitely need an AM powerplant and engine (along with solar infrastucture to generate the AM fuel) to go faster.
Honestly, it seems that the FTL version really needs a space elevator due to sheer tonnage. Since not only do you need the powerplant and exotic matter ring, you then have the hab module, and reaction mass. (300-400t ship?)
Now wait, before you go "crazy scifi things needs another one!" With current materials science we have the tensile strength to build an elevator in a lower gravity well like the moon. One on Earth may be "10 years away, forever" though.
So, lunar or martian elevator and assembly for a starship makes the most sense.
AM v.s. fusion, if something goes wrong you're dead anyway so I don't see a reason to prefer the less efficient form of energy storage.
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Dunno about reaction mass. While it's true Alcubierre drive doesn't accelerate you by itself in physical sense, reaction mass required depends on what you want to do. With a plasma-based heatshield (because if you've got fusion, you should be able to do that, too), you could try for an aerocapture at your destination, therefore you might only need a bit of xenon for ion-based RCS system (hey, you're running of fusion, you're not short on electrical power!). The ship's gonna be heavy, but I don't think that a space elevator is a strict necessity. It'd slash the launch costs immensely, but if you're not scared of nukes, gas core-engined LV could be enough to handle the logistics.
And as I said, if the "700kg of energy" requirement holds, running the FTL version of fusion doesn't seem so impossible anymore. I'm not sure what's the relation between this energy and mass of spacecraft (there being no relation would likely be too good to be true...), but launching 200-400T to LEO can be done with todays' technology, albeit at quite a cost (which Elon Musk is soon gonna drive down, BTW. Methane is cheap).
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and the 700Kg energy requirement is recently down from the mass of Jupiter, which was down from 'more energy than the visible universe' so we've been seeing a progressing trend in the direction this thing is going in.
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So, lunar or martian elevator and assembly for a starship makes the most sense.
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Utopia Planitia Shipyards, here I come! :D
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Also, while FTL version is the one people are the most excited about, STL one is based on actually proven physics (well, it works, or at least seems to) and either way, an important stepping stone towards FTL one, should the latter be possible.
According to wiki the results are inconclusive..
https://en.wikipedia.org/wiki/Talk%3AWhite%E2%80%93Juday_warp-field_interferometer
Remember, though, antimatter (essentially "negative matter" from a certain point of view, though with positive mass) and neutrinos started out that way, too, as did Higgs bosons and many other heavy particles, so I'm optimistic.
I am just an armchair physicist but I am not aware of any evidence that negative mass is plausible, either experimental or theoretical. While Higgs bosons or antimatter was predicted as likely existing, so thats different. There are lots of particles that could exist in some farfetched theory but dont exist in reality.
I also found this article by a string theorist highly critical of FTL Alcubierre drive concept:
http://motls.blogspot.sk/2013/07/relativity-bans-faster-than-light-warp.html
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I also found this article by a string theorist highly critical of FTL Alcubierre drive concept:
http://motls.blogspot.sk/2013/07/relativity-bans-faster-than-light-warp.html
He's also a climate change denialist. Although it shouldn't affect the way I read his physics (and his statements certainly seem to make sense in that article, even though I wish they didn't), it kinda makes me wonder about his scientific nonce.
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Motl suffers from the tragically common affliction in physicists and engineers and architects of thinking he can understand absolutely anything based on his aptitude at physics.
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He's also, as has been said, a string theorist, not a relativist. White, on the other hand, actually specializes in what he's doing. I think that he already considered points raised in the article. While it does make sense, the restrictions mentioned all seem awfully basic. It's not something you just miss, while layman's understanding of a warp drive does indeed boil down to what was "debunked", I'm sure White's approach already took those restrictions into account. He simply wouldn't be working on something so trivially impossible.
As for the interferometer experiment being inconclusive, I wouldn't despair. They did notice a tendency, even if it isn't strong enough to actually bring a solid conclusion, it's probably just a matter of building a bigger/more precise interferometer. I'd say, give them time, given the scale of the experiment, it's to be expected that the effect would not be very strong.
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Also, while FTL version is the one people are the most excited about, STL one is based on actually proven physics (well, it works, or at least seems to) and either way, an important stepping stone towards FTL one, should the latter be possible.
According to wiki the results are inconclusive..
https://en.wikipedia.org/wiki/Talk%3AWhite%E2%80%93Juday_warp-field_interferometer
Remember, though, antimatter (essentially "negative matter" from a certain point of view, though with positive mass) and neutrinos started out that way, too, as did Higgs bosons and many other heavy particles, so I'm optimistic.
I am just an armchair physicist but I am not aware of any evidence that negative mass is plausible, either experimental or theoretical. While Higgs bosons or antimatter was predicted as likely existing, so thats different. There are lots of particles that could exist in some farfetched theory but dont exist in reality.
I also found this article by a string theorist highly critical of FTL Alcubierre drive concept:
http://motls.blogspot.sk/2013/07/relativity-bans-faster-than-light-warp.html
Well, we have literally no idea what dark energy is. But if it is negative energy, that means negative mass can potentially exist.
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Dark matter and dark energy both... Kinda feel phlogiston-ish to me, although I freely admit I don't have the physics background to back that up. Has there been any conclusive evidence yet to rule out a MOND-like modification of long range gravitational effects?
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I believe this (http://en.wikipedia.org/wiki/Bullet_cluster) is generally considered the piece of evidence that finally killed MOND.
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So that ship model looks pretty cool
Makes me kinda want to start modeling stuff again and putting it in Freespace
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Hey, deathfun...
Kudos for getting the point of the thread, have a cookie.
:lol:
Not referring to anyone who was actually discussing the theory etc, just referring to the 'gaaaaaaa alcubi doesn't work, burn, heretic!' tone that was going on for a bit. I mean, it was mainly about the art, guys. :p
Sorry I couldn't post screenies, the site wouldn't allow the pictures to be menu-clicked. That would have helped set the tone I imagine. Bah, mobile browsing sucks sometimes... :doubt:
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Aaaand everyone misses the gorgeous model. Go figure.
re: alcubi drives not working: read up on all the science disproving yhe Wright brothers' hair-brained heavier than air flying machine concept.
Does this prove it will work? No. It simply means, that just because science says something doesnt work, doesn't necessarily mean it won't. You might just be making the attempt in the wrong way. Then again, there might be no feasible way! Haha! But it's always fun to try, you learn more regardless of the results being successful or not. :nod:
The case against the Alcubierre drive is much more solid than that, and you should really know better than to equivocate the two. The requirement to have a bunch of exotic matter, which has never been observed to exist and for which no method of manufacturing exists, is a massive dealbreaker.
Are you sure this is not because we are looking back at the Wrights with so much more acrued knowledge now, while Alcubierre is still a big question mark?
Can you really be sure that with another 100 or 200 years of more knowledge we won't be looking at both of them in the same way?
I mean.... not so long ago "breaking the sound barrier" or hah traveling to the moon.... was completely in the realm of science fiction and people speculated whether it will be possible at all ... yes, now we know it's easy... the point is that before we knew that we had all kinds of educated people arguing that it's impossible and utter nonsense :P
I only know that I can't say with certainity whether it will or will not work at some point in the future - with the limited information we do have at this point, wouldn't you agree?
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Are you sure this is not because we are looking back at the Wrights with so much more acrued knowledge now, while Alcubierre is still a big question mark?
Can you really be sure that with another 100 or 200 years of more knowledge we won't be looking at both of them in the same way?
I mean.... not so long ago "breaking the sound barrier" or hah traveling to the moon.... was completely in the realm of science fiction and people speculated whether it will be possible at all ... yes, now we know it's easy... the point is that before we knew that we had all kinds of educated people arguing that it's impossible and utter nonsense :P
I only know that I can't say with certainity whether it will or will not work at some point in the future - with the limited information we do have at this point, wouldn't you agree?
I guess that the distinction is that, for the Wrights, they could look at birds, bats, insects - lots of examples that what they wanted to do was physically possible. It was purely an engineering challenge that was always going to be solved sooner or later, because nature had already solved it. Similarly, for things like breaking the sound barrier, or landing on the moon - there were challenges, absolutely. But each one was simply a matter of refining and improving existing techniques and technologies. The physics was nothing spectacularly new - even for things like turbulent flow at mach 1, the physics was never exotic - just vastly complex.
An Alcubierre drive is in an entirely different category. We have zero evidence that anything, under any circumstances, under any reference frame can move at a velocity of greater than C, and there are some very, very fundamental physical laws that say that we almost certainly will never find any. The methods people propose to get around it are ingenious, and maybe one day they will work - I really, really hope that they will - but surely you can see the difference between an engineering challenge exploiting known physics and a physics challenge trying to challenge our fundamental understanding of reality?
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I only know that I can't say with certainity whether it will or will not work at some point in the future - with the limited information we do have at this point, wouldn't you agree?
No. I do agree that we can't know what the future will hold, but I am not holding my breath for the small possibility that the future holds exactly the kind of breakthroughs that would enable us to turn the Alcubierre drive into reality.
I mean, if you're taking the analogy to the early days of powered flight further, remember all those insane machines people came up with? The ones that, to a modern mind, are clearly not going to work, ever? It's far more likely that this drive system is going to end up in the same pile than it is for it to work.
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Note, the "sound barrier" analogy is somewhat better one. It was, at one point, presumed that aerodynamic forces will tend to infinity as one approaches speed of sounds. Turns out Prandtl-Glauert model simply failed at Mach 1, as the air behaves very differently after that. Granted, they did have an example, too (bullets went supersonic long before planes), but it demonstrates a situation where a normally working theory stops to apply.
But before breaking the sound barrier, maybe we should learn to fly? If White's "mini-Alcubierre" works, then it's a good thing for FTL proponents, since it'd demonstrate the concept is sound, even if the speed is limited. However, if it doesn't work, well, that's good too, because it means that a theory we're using isn't good (since it postulates STL Alcubierre drive is definitely possible), and that we don't know a lot more than we think. So either way, it opens up new perspectives in the field. So I'd say, let's wait for White to get some better results.
Also note, you don't need FTL to travel to the stars. If Alcubierre drive is limited by the speed of light, that would probably also mean it's subject to time dilation and Lorenz contraction, much like everything else. Assuming the numbers quoted before hold for an STL system, you could accelerate with the thing to some ridiculous speed like 9.999c and let relativity work for you. You won't travel faster then light, but at this point, distance between you and the target shrinks, so you'll still be at your destination much faster. Of course, the people planetside will think otherwise, but that's their problem, isn't it? :) And with some imagination, you could write stories just as interesting with this kind of drive (in fact, some did. "Forever War", for instance, deals with using time dilation to cover long distances).
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Just thinking... bigger warp engines means more exotic mass and more speed.
So really an optimal design would be deconstructing a small moon, have an equatorial warp ring and a warp field extending just above it's surface.
Probably contain water and fuel storage within the outer sphere. Then put rotational habitat rings inside of it. (probably layer a few at different angles)
So it'd basically be a combination of Rama and the Deathstar.
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Just thinking... bigger warp engines means more exotic mass and more speed.
So really an optimal design would be deconstructing a small moon, have an equatorial warp ring and a warp field extending just above it's surface.
Probably contain water and fuel storage within the outer sphere. Then put rotational habitat rings inside of it. (probably layer a few at different angles)
So it'd basically be a combination of Rama and the Deathstar.
And lucky for us we got one of those small moons in orbit already ....... uh ;) :P
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No, I didn't "read an article". I consulted with physicists, read quite a lot of material regarding this, and engaged in critical thinking.
Were these physicists you consulted specialists in general relativistic field equations, and did they explain to you why the Alcubierre solution is invalid? If so, then I urge you to urge them to submit their critique to a relevant journal, perhaps ApJ, so that others can examine it. If not, then you may want to reconsider your views.
Not only you have to generate and efficiently manage this energy, you'll even have to turn it into negative energy, which is something we either believe is impossible to exist, or just mindblowingly difficult if we ever come across with some "fluke" like the Casimir Effect and so on.
But cosmological observations indicate that a negative value for vacuum energy does exist, and it works perfectly fine in the framework of general relativity as well. The problem is not 'are repulsive effects on space-time possible?' but rather 'how do we achieve them on a local scale?'. The latter might not be possible or practical, but this is quite a difference from saying the mechanism itself doesn't exist.
This idea of creating "lanes" is good for scifi, but please. Anything that is going faster than light from any other perspective is either creating paradoxes or hidden from us by event horizons, and anything that is beyond event horizons can never "go back" to us.
Admittedly I am not very familiar with the 'lanes' idea, but I don't think this is an attempt of explaining away paradoxes except by people who aren't up to snuff on general relativity and/or don't fully understand how the drive works. Your argument here sounds very persuasive, but it suffers similar flaws.
The first part of your argument is an assertion that objects moving with v>c (i.e. tachyons, or having a space-like trajectory through space-time) either invokes paradoxes or is hidden by an event horizon. The first consequence is correct, the second is not. Tachyons do not produce event horizons (just follow the null geodesics coming off of it), but they do probably invoke paradoxes, because they enable situations where effects can precede their own causes in certain reference frames.
The second part deals with objects that are behind an event horizon, with an assertion that they cannot 'return to us'. This is correct in a general sense, e.g. particles that have fallen into black holes or which lie beyond the cosmological event horizon. They cannot return to us because in order for them to do so, they must follow a space-like trajectory which invokes the problem above (among others, like the presumed acceleration past c).
But Alcubierre drive is neither of these cases. (http://iopscience.iop.org/0264-9381/11/5/001/pdf/0264-9381_11_5_001.pdf) The trajectory of the ship is not space-like, but time-like, and feeling zero accelerative forces. The motion is instead due to changes in the space-time metric around the ship. Event horizons may be produced, but these are of very different form than those of black holes or the large-scale universe. They are non-static and non-permanent.
To truly examine the validity of the Alcubierre solution in context of producing paradoxes or not, you have to examine space-time intervals within the metric. People have done so and to the best of my knowledge have thus far not found any glaring problems.
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@Alcubierre Fanboys:
Proposition:
Changes to the curvature of space-time cannot propagate faster than the speed of light.
Discuss.
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@Alcubierre Fanboys:
Proposition:
Changes to the curvature of space-time cannot propagate faster than the speed of light.
Discuss.
@Aardwolf
That is not how discussion works. Please cite papers that support your position.
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I assumed at least someone would be familiar with this.
Speed of Gravity (http://en.wikipedia.org/wiki/Speed_of_gravity) on Wikipedia.
The speed of gravitational waves in the general theory of relativity (http://en.wikipedia.org/wiki/General_theory_of_relativity) is equal to the speed of light (http://en.wikipedia.org/wiki/Speed_of_light) in vacuum, c. Within the theory of special relativity (http://en.wikipedia.org/wiki/Special_relativity), the constant c is not exclusively about light; instead it is the highest possible speed for any interaction in nature. Formally, c is a conversion factor for changing the unit of time to the unit of space. This makes it the only speed which does not depend either on the motion of an observer or a source of light and/or gravity. Thus, the speed of "light" is also the speed of gravitational waves and any other massless particle (http://en.wikipedia.org/wiki/Massless_particle). Such particles include the gluon=http://en.wikipedia.org/wiki/Gluon (http://gluon=http://en.wikipedia.org/wiki/Gluon) (carrier of the strong force (http://en.wikipedia.org/wiki/Strong_force)), the photons (http://en.wikipedia.org/wiki/Photon) that make up light, and the theoretical gravitons (http://en.wikipedia.org/wiki/Graviton) which make up the associated field particles of gravity (a theory of the graviton requires a theory of quantum gravity (http://en.wikipedia.org/wiki/Quantum_gravity), however).
Emphasis mine.
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@Alcubierre Fanboys:
Proposition:
Changes to the curvature of space-time cannot propagate faster than the speed of light.
Discuss.
And they don't in the Alcubierre solution. Read the paper linked to in the post above yours.
The most important lesson in general relativity, the lesson they drill into you in the first chapter/lecture in any good course/textbook (http://www.amazon.com/By-Charles-W-Misner-Gravitation/dp/B008UBJXKO/ref=sr_1_3?ie=UTF8&qid=1405109744&sr=8-3&keywords=gravitation+misner) of the topic, is that space-time physics must be looked at locally. Locally, nothing moves faster than light in the Alcubierre solution, and space-time perturbations propagate at c. You're suffering the same misconception (http://www.publish.csiro.au/?paper=AS03040) people have when they claim that superluminal motions at great cosmological distances in the expanding universe defies relativity.
Ed: I find it really mind-boggling that people think they can so easily refute a solution of general relativistic field equations with only the most basic notions of general relativity.
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Let's try a graphical approach to describe how far the effects of the Alcubierre drive have reached at any time. If "a picture is worth a thousand words", I'm making a sevenfold profit here.
Let's work in one dimension (so we have x and t), and use c=1, so that in undistorted space a particle moving at light-speed is a line parallel to x=t.
Let's use a simplified model of the distortion, since we're only concerned with how far the farthest extent of it has reached. We'll say that space is either distorted or not, and that in distorted space, a particle moving at light-speed is a line parallel to x=2t.
At t=0, we emit a "distortion front". Its trajectory is x=t.
At t=1, we emit another. Its trajectory is initially x=2(t-1). This intersects x=t at (2,2). It has now reached undistorted space, and its trajectory changes to match the original.
At t=2, we emit another. Its trajectory is initially x=2(t-2). This intersects x=t at (3,3). The same thing happens.
.
.
.
You're probably going to tell me this isn't how it works. Well since I'm going to the trouble of describing graphs of how I think the effect would propagate, you can go to the trouble of correcting this model.
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You're right, I'm going to tell you that that is not how it works. :)
You're yet again trying to examine motions by invoking a global inertial reference frame. This doesn't work because your reference frame contains non-static curvature which cannot be removed by a simple coordinate transformation, and is therefore not an inertial frame of reference at all. Therefore your conceptions about what must happen do not hold valid.
This is the whole reason general relativity was derived in the first place -- to deal with motions when global frames are not available. Space-time physics must be examined locally.
Try reading the supplementary material I have been providing you with before continuing to make arguments out of ignorance of the subject.
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Can't read Greek, and learning Greek wouldn't be time-economical.
Ok, tell me this then: the instant the Alcubierre drive is turned on, what is the farthest distance its distortion effect could have reached? How can there not be a "front" beyond which the effect has yet to be observed?
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Can't read Greek, and learning Greek wouldn't be time-economical.
If you do not understand geometrodynamics, then why are you participating in a discussion of it as if you have any expectation of educating those who do? If you want to understand why your conceptions are wrong, then you need to educate yourself. The means of doing so have been kindly provided. Sorry if you don't have the time, but I have no sympathy for one's choice in continuing to deliberately argue out of ignorance.
Your latest question suffers the same conceptual problems that I explained in the prior post. I won't repeat it again, you know how to read.
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Is there or is there not a "front" beyond which the effects of the Alcubierre drive are not felt?
The 5 seconds it takes to respond "there is" or "there isn't" are less valuable than the 10 hours it would take for me to learn even the basics of geometrodynamics.
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Can a quantum system which was prepared to have σz=+1 have the state σx=+1 and σz=+1? It takes 5 seconds to respond "yes" or "no" and this is more valuable than however long it takes to explain why it is an improper question by explaining the quantum uncertainty principle.
[/equivalent question for purpose of demonstrating why it is ill-posed]
The avenue that you are trying to pursue is that the propagation speed of Alcubierre's distortion field must propagate faster than c in order for it to achieve its claimed effect of superluminal travel, yet it cannot do this because fields cannot propagate faster than c. Therefore there is a logical inconsistency and therefore the drive is impossible.
Congratulations, you, a person who doesn't understand nor want to put in the effort of understanding general relativity apparently just showed that an entire community of people who are experts in it that they are wrong. Shall we believe this? No. In much the same way as I don't believe a creationist disproved evolutionary theory by any criticisms they've yet made. They simply have misconceptions just as you currently do.
The logical inconsistency is remedied by investigating motions with an appropriate reference frame in an appropriate coordinate system. If you do so, you will find that locally, the ship is at rest while distortions to the field propagate at c, and globally, these appear faster than c. An analysis of the metrics shows that this does not produce paradoxes nor does it violate general relativity. Quite the contrary, it is a solution to it.
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Just for the record, this thread is awesome. It's always great to listen to people who know their **** arguing physics.
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As an aside, as I don't wish for anyone to misunderstand me: I am not some sort of 'Alcubierre fanboy'. I'm not convinced that it is feasible. I'm not convinced that it is even possible. I actually suspect that there may be a mistake in the formulation of the Alcubierre solution, or that it can't be used to produce effectively superluminal speeds.
I'd love to examine it myself, but I lack the skills. I study astrophysics and cosmology, I have taken courses on general relativity, own the Gravitation textbook (a landmark text in the field) I linked to in an earlier post, and am quite comfortable with the language and mathematics. Nevertheless, thoroughly examining the details of Alcubierre's solution is still way beyond my ability. I don't know that it is right. I only know that people a hell of a lot more capable than myself think it is, and to my knowledge nobody has shown otherwise yet. I am completely serious when I ask people who think it's wrong to try to submit a critique to a journal.
To semi-quote Herra from a prior thread, I have yet to see someone examine the equations and say "hey, this minus sign needs to be a plus", or "this space-time interval doesn't properly equate with that one." The only things I have seen, including in this thread, are people trying to argue that it can't work through the use of simplistic arguments of general relativity that have never failed to be based upon an inadequate understanding of how general relativity actually works.
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I don't get what the problem is with a "global reference frame". Why not pick a patch of space with no big gravity gradients, scatter a bunch of observers throughout it with zero relative velocity between them, and tell them to keep transmitting "ok" until they detect their velocity relative to the rest of the observer network is (significantly) nonzero. Alcubierre ship starts docked with an observer, gets clear of it before doing its space-distorting business, deliberately runs over some observers with its Alcubierre bubble, then comes to a stop and docks with another observer at the far end of the test area.
Oh. That relies on the bubble having a definite size outside of which its effect is negligible. But it probably falls off at 1 / r2, so there can't be an "outside".
But that's ignoring curvature change propagation stuffs. I still think there's something weird with how an Alcubierre drive would "start up".
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Such a network of observers, occupying a region which is flat, experiences proper time equal to coordinate time. Nothing mysterious there, we're just back to special relativity. This is a perfectly fine reference frame in which to do physics, until they become interested in the motion of the Alcubierre ship.
If they think special-relativistically, the network will think the path of the ship is space-like. They will think this blatantly violates physics if they don't recognize the curvatures involved. If they are smart, they will suppose curvature must be involved because as far as anyone knows, relativity prevents space-like trajectories. If they are really smart, they'll test this by examining the gravitational lensing of stars around the ship to map the curvature.
When they do so, they'll find that the ship is not following a space-like trajectory, it is following a time-like one. In fact, it is following a geodesic. It is exactly equivalent to a ship in free fall. Locally, meaning examining sufficiently small reference frames such that the curvature appears flat, there is nothing going faster than light. The ship is at rest, and the distortions to the space-time around it propagate at c.
Again, this is the whole reason general relativity was derived in the first place. Many situations in space-time physics involve accelerated observers or deviations from flatness. When this happens, our intuitions from special relativity go right out the window. For a fine example, just consider that there are galaxies at very large cosmological distances which are receding much faster than c, and we can see them. It is only if we think special relativistically that this should freak us out.
Added:
That relies on the bubble having a definite size outside of which its effect is negligible. But it probably falls off at 1 / r2, so there can't be an "outside".
No, the field is extremely localized and does not follow 1/r2. (Graphics of the field are pretty widespread, including in the original paper itself if you had actually bothered to try looking at it.) This also produces extreme tidal forces as a consequence, but only significantly so outside of the ship if field parameters are carefully chosen.
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Mmm. Yes, the pictures I've seen didn't look like 1 / r2, but virtually nothing in physics has a sharp cutoff outside of which the field is exactly 0.
I don't get how you resolve things when point A says it can propagate changes to point B in 1 nanosecond and point B doesn't think changes from point A can propagate to it that soon. Does the problem just magically go away because you're taking the limit as dt approaches zero, and so both A and B's perception of dx also approaches zero?
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it's because the space in front of it is getting smaller and the space behind it is getting bigger. the ship is sitting motionless, the space is 'moving' for lack of a better word, the field propagates at C, it moves across space at 299792458m/s meters in front are getting smaller, meters behind are getting bigger.
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Sort of what Bobboau said. The measure of distances, (and not just distances in space, but distances in space-time) change in front of and behind the ship in anti-symmetrical ways (one expands, the other contracts). Any and every time you try examining such a system with a global, flat coordinate system, you're doomed to fail miserably at understanding what's happening. This is why you keep having trouble, Aardwolf. It would be a lot easier if you had some familiarity with what a reference frame means and how they work within curved space-time.
Trust me, people who haven't taken special relativity usually struggle for a long time before finally grasping it. I was no exception. General relativity is even harder. If you have not studied general and are trying to understand Alcubierre drives, you are going to have a very bad time.
Does the problem just magically go away because you're taking the limit as dt approaches zero, and so both A and B's perception of dx also approaches zero?
Nope, this has nothing to do with time differentials. Or magic, for that matter. ;)
virtually nothing in physics has a sharp cutoff outside of which the field is exactly 0.
"Sharp cutoff" = discontinuity = non-differentiable = failure of standard space-time physics. There is no discontinuity in the Alcubierre field.
The geometry of the field is described by the curvature tensor, which depends on the metric (which is defined explicitly in the paper) and the stress-energy tensor (also defined explicitly).
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I feel strongly compelled to add that your latest method of argument is truly astounding.
"I think this field seems wrong because I've not seen many with that kind of geometry before. Nevermind that it's a solution of GR and I haven't done any sort of critical analysis, let alone have familiarity with GR." Brilliant! You should submit that one immediately. :p
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Assuming one could be built that functioned as advertised, could an Alcubi drive work on - planet for near instantaneous travel to other parts of the globe (and possibly cheap transit to orbit)?
Or would something interfere?
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Possibly, but I would certainly not recommend trying it. The tidal effects generated at the edge of the field would pretty much tear apart anything it encounters.
Come to think of it, I imagine this is what the 'safe lanes' thing that Luis was talking about (should be) all about, rather than paradox avoidance.
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I thought the latest calculations indicated no tidal forces would be generated? Maybe I'm mistaken.
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/me looks at his own post
You know why that seems like an odd method of argument? Because it's not an argument. Arguments don't start with "I don't get how you resolve". The next time you feel "strongly compelled" to say something snarky like that, don't.
I don't get how you resolve things when point A says it can propagate changes to point B in 1 nanosecond and point B doesn't think changes from point A can propagate to it that soon.
If something is in the path of the ship, what does it see? Question mark.
It seems to me that something in the path of the ship could see that all of the space within 1 light-year of it is "flat" (by observing markers placed throughout it), and then less than 1 year later, it's roadkill. It can be roadkill instantly, because the observations of things exactly one light year away are already one year old by the time they are observed. I think I figured it out. You were the opposite of helpful.
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I thought the latest calculations indicated no tidal forces would be generated? Maybe I'm mistaken.
I'm not familiar with latest calculations (please feel free to link to them though!), but any kind of Alcubierre solution is necessarily going to produce tidal forces somewhere. Tidal forces are consequences of space-time curvature, and you don't have Alcubierre drive without curvature somewhere.
Tidal forces can be described as this: Two objects separated by some distance, but following initially parallel trajectories, will deviate from parallel if the space-time they occupy is curved. In language of general relativity, each is following a perfectly straight line (geodesic) through space-time and feels no forces at all, but because the space-time is curved, these straight lines don't remain parallel. So it seems that the objects are being squeezed together or torn apart by some force, which we call tidal force.
Alcubierre drives produce powerful tidal forces outside of the ship, near edge of the field, because that is where curvature is most extreme.
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/me looks at his own post
You know why that seems like an odd method of argument? Because it's not an argument. Arguments don't start with "I don't get how you resolve". The next time you feel "strongly compelled" to say something snarky like that, don't.
>Enters thread with an argument (http://www.hard-light.net/forums/index.php?topic=87809.msg1752676#msg1752676) about A.D.
>Calls upon same argument (http://www.hard-light.net/forums/index.php?topic=87809.msg1756169#msg1756169) immediately after someone explained why someone else's notions about A.D. are wrong and why, with ****ty attempt at a conversational opener: "Alcubierre Fanboys: …discuss:"
>Is told that the argument is wrong and why in great detail, complete with links to supplementary material and a recommendation of reviewing it before making further arguments.
>Expresses disinterest/inability of self-education and proceeds to make further arguments
>Becomes angry when he is made a fool of.
I've been following a form of tit-for-tat response method, so consider your own behavior if the replies make you angry. I am happy to explain this stuff to anyone who expresses that they want to learn. I am less-than-impressed by those who act that they are going to be able to grasp something after being told what they need to do to achieve proper understanding (and why), and refusing to do it.
You'll also note that I've been answering all of your questions directly.
If something is in the path of the ship, what does it see? Question mark.
It first sees changes in curvature of the space time it occupies, then possibly is destroyed by tidal forces, then impacts the Alcubierre ship itself. The proper time it takes to do so may be extremely short, and this may very well confuse you if you continue to think in perspective of global reference frames.
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No. You responded to this post
Mmm. Yes, the pictures I've seen didn't look like 1 / r2, but virtually nothing in physics has a sharp cutoff outside of which the field is exactly 0.
I don't get how you resolve things when point A says it can propagate changes to point B in 1 nanosecond and point B doesn't think changes from point A can propagate to it that soon. Does the problem just magically go away because you're taking the limit as dt approaches zero, and so both A and B's perception of dx also approaches zero?
with this
I feel strongly compelled to add that your latest method of argument is truly astounding.
WHAT ARGUMENT? There is no argument being advanced in that post. Do you want me to go through it sentence by sentence and demonstrate that for you? You probably don't. But that's even more reason why you need to be shown how you're wrong.
Mmm.
Pensive.
Yes, the pictures I've seen didn't look like 1 / r2,
Concession that the hypothesis from the prior post is, in retrospect, stupid.
but virtually nothing in physics has a sharp cutoff outside of which the field is exactly 0.
Tangent. Illustrations appear to approach zero as distance from the bubble increases. Nonetheless it is possible for a function to be continuously differentiable, be nonzero for part of its domain, and zero for some other continuous subset of its domain; e.g. f(x) = 1 - sqrt(1 - x2) for x < 0, 0 for x >= 0 ... But since there's not much like that in physics, it probably isn't that.
I don't get how you resolve things when
Expression of bewilderment under a certain circumstance, to be specified forthwith.
point A says it can propagate changes to point B in 1 nanosecond and point B doesn't think changes from point A can propagate to it that soon.
Specification of the aforementioned circumstance, which is seemingly paradoxical.
Does the problem just magically go away because you're taking the limit as dt approaches zero, and so both A and B's perception of dx also approaches zero?
Hypothesis regarding how to resolve the aforementioned paradox.
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Well, here's one bit but it's not the one I saw : I'll keep looking for that.
http://www.space.com/17628-warp-drive-possible-interstellar-spaceflight.html
Link to thread discussing this, and in the top to a video by Dr White, maybe they misunderstood him?
http://forum.kerbalspaceprogram.com/threads/82877-Hypothetical-effects-of-the-hypothetical-Alcubierre-drive/page2
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While Aardwolf vents I thought it might be fun to pose a question for anyone who is interested in the extremely non-intuive results that can occur in strongly curved space-time:
Suppose sub-space drives as seen in Freespace are real (this isn't important to the question, but just for fun and to allow the following situation). Suppose that you are in a ship that has just exited from a sub-space jump gone horribly wrong. The drive suffered irreparable damage and forced you to exit subspace right next to a supermassive black hole. Before you can react, you fall inside the event horizon. You are now unable to escape -- doomed to hit the singularity in some finite amount of time in the future. But your engines are still operable, and they allow arbitrarily large acceleration in any direction. You decide that although escape is impossible (and it is), you can at least try to maximize the amount of time you have left before you meet the singularity. What action would you pursue? (You may treat yourself and your ship as being impervious to the accelerative forces of your engines, as well as to the tidal forces within the black hole).
@Jr2: Thanks for the links. As far as I can tell, the first link isn't discussing tidal forces, but rather the amount of energies required to produce the field, with more recent calculations suggesting these would be not as large as previously supposed. As for tidal forces, I don't see anyone saying that it produces no tidal forces, but rather that they can be made to be reasonably small within the ship. Outside the ship, however, they may be enormous. Basically, whenever you look at a figure showing the geometry of the Alcubierre field, the tidal force at a given point is going to be proportional to how "steep" the field is there.
If there are no tidal forces anywhere, then there is no Alcubierre field, and the drive is not achieving anything. :)
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Suppose sub-space drives as seen in Freespace are real (this isn't important to the question, but just for fun and to allow the following situation). Suppose that you are in a ship that has just exited from a sub-space jump gone horribly wrong. The drive suffered irreparable damage and forced you to exit subspace right next to a supermassive black hole. Before you can react, you fall inside the event horizon. You are now unable to escape -- doomed to hit the singularity in some finite amount of time in the future. But your engines are still operable, and they allow arbitrarily large acceleration in any direction. You decide that although escape is impossible (and it is), you can at least try to maximize the amount of time you have left before you meet the singularity. What action would you pursue? (You may treat yourself and your ship as being impervious to the accelerative forces of your engines, as well as to the tidal forces within the black hole).
Logic and Kerbal Space program would suggest that the best method would be to accelerate into an orbit around the black hole and hope that you don't hit any objects.
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Given the non-intuitive disclaimer, I'm going to say that (for whatever reason) accelerating towards the black hole would actually do the trick. Possibly due to a time dilation effect, but I only have a middling grasp on special relativity, and essentially nothing on the general side.
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Logic and Kerbal Space program would suggest that the best method would be to accelerate into an orbit around the black hole and hope that you don't hit any objects.
This makes me wish there was a fully relativistic version of KSP with ability to fly around/into black holes, but holy crap how hard would that be to do... :shaking:
Anyway, a great thought, but unfortunately, there is no such thing as a stable orbit inside the event horizon of a black hole unless the particle or ship can move faster than the speed of light. Within the horizon, any allowed trajectory is destined to hit the singularity in finite time, so all that can be done is maximize the amount of time it takes to do so.
There's a very weird way of describing this, which is that inside the event horizon, the roles of space and time interchange. Outside the horizon, you have complete freedom to move in any spatial direction, but you are compelled to move forward in time and the only thing you can do is slow it down (by moving really fast). Inside the horizon, you have complete freedom to move in time, but you are compelled to move in the direction of decreasing distance from the singularity. In other words, avoiding the singularity inside a black hole is no more possible than avoiding the progression of time in normal space.
Given the non-intuitive disclaimer, I'm going to say that (for whatever reason) accelerating towards the black hole would actually do the trick. Possibly due to a time dilation effect, but I only have a middling grasp on special relativity, and essentially nothing on the general side.
Time dilation effects are definitely important here. It will turn out though that accelerating towards the singularity is a bad choice -- you'll end up hitting it sooner.
It turns out that accelerating away from the singularity is also a bad choice! You'll end up hitting it sooner.
It can be shown that the longest proper time (proper meaning it's how much time you experience and count out if you look at your clock) occurs if your path is a geodesic -- freefall. If you're in the event horizon, the best you can do is do nothing at all. Any other action you try to take will simply make the journey to the singularity shorter.
This problem can be seen worked out (warning: math) here. It's number 12-14.
http://dafix.uark.edu/~danielk/Relativity/HW8Soln.pdf (http://dafix.uark.edu/~danielk/Relativity/HW8Soln.pdf)
"The more you struggle, the shorter you live." Black holes are like cosmic quicksand.
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Were these physicists you consulted specialists in general relativistic field equations, and did they explain to you why the Alcubierre solution is invalid? If so, then I urge you to urge them to submit their critique to a relevant journal, perhaps ApJ, so that others can examine it. If not, then you may want to reconsider your views.
You are assuming these papers are being sufficiently taken seriously by the physics community for them to enact the necessary work...
Look, for me this is very simple and you do not require a degree of physics to state the obvious: you cannot contradict General Relativity and move any piece of information faster than light in any given reference frame. Now these people think they are smart and have gathered a "smart-ass" way to deal with this "prohibition", but when you even everything out, it turns out that you didn't get what you wanted in the first place. Irrespectively of how technically and "locally" you can think you can outsmart Nature, in everyone else's point of reference frame, impossible things are happening right at front of them: an object is at place X=0 at time t=0 and then at place X=1 light minute at time t=1.1 minutes (it took 0.1 minutes to travel 1 light minute). Impossible events (just like the Picard Maneuver) and paradoxes abound here.
I'll grant you all the local wizardry and shenanigans, but what makes the whole point moot is just by looking at the larger vicinity of events. The space around the phenomena is just the typical minkowksy spacetime and in such a thing you cannot have any phenomena going back in time, period. This is strickly forbidden by general relativity. If you allow yourself to say "Well but perhaps it is indeed possible to travel backwards in time!" then the onus of writing a paper saying such a thing is on these revolutionaires, not me! These are the ones going for the Nobel prize, not me!
To truly examine the validity of the Alcubierre solution in context of producing paradoxes or not, you have to examine space-time intervals within the metric. People have done so and to the best of my knowledge have thus far not found any glaring problems.
They have narrowed their investigations to the very local things that were happening inside the "warp bubble" or immediately outside of it, etc. Sometimes, it's enough to just look at the outside of the box and see how amazingly simple it actually is to disprove these notions. Kinda reminds me of stuff like the Zeno paradox and so on. You just needed to look at the problem from a different angle, you don't need any amazing analytical gifts here.
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an object is at place X=0 at time t=0 and then at place X=1 light minute at time t=1.1 minutes
I don't think there's anything unusual about an object moving 1 light minute in 1.1 minutes.
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My bad, I phrased it incorrectly. The point is the object is being observed by a third party near X = 0, and this third party observes the object at X = 1 light minute away at t = 1.1 minutes, indicating the object traveled 1 light minute at 0.1 minutes. This is still on the "possible" side of the equations, for instance if the observer is travelling towards X "final spot" at 0.9 c while the object is travelling at c.
However, if you are near the X "final spot", weirder things start to happen, like seeing the object near you almost a minute before it disappears from its initial location (let's discard the big explosion at the front of the "wave" as predicted).
Now the impossible thing is when the frames are placed in such a way that information is passed through to the past light cone of any one of these objects, creating causal paradoxes. The object could be moving in such a way that for certain frames of reference it could "kill its own past self" (grandfather's paradox), which would be obviously impossible.
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>astrophysicist who has studied GR says the alcubierre drive isn't completely infeasible
>architect who has 'read some papers' and 'consulted some physicists' says only an idiot would think that
yeah sure
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>astrophysicist who has studied GR says the alcubierre drive isn't completely infeasible
>architect who has 'read some papers' and 'consulted some physicists' says only an idiot would think that
yeah sure
And Phantom Hoover ad hominems. I am an architect indeed, but not a stupid one. I did end up 6th place in the national physics Olympiads back in the day, only because I didn't know much of thermodynamics... But yeah I understand your skepticism. Here, take Lubos Mötl criticism:
http://motls.blogspot.pt/2013/07/relativity-bans-faster-than-light-warp.html
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I would like to point out that if we manage to solve the practical questions (which are not exactly trivial matters) of modifying space-time geometry at will with reasonable energy cost, it would be insanely cool whether Alcubierre Drive as an FTL transportation device works or not.
Things like antigravity, artificial gravity, and reaction massless propulsion systems come to mind*. It would practically be open season for colonizing anything in the Solar System, and it would make it possible to craft large scale, self-sufficient space colonies possibly capable of traveling to nearby star systems. It'd just take a while.
Of course there are other problems associated with subluminal space travel at speeds approacing any appreciable fraction of c. Like hitting all the photons in the way that aren't traveling away from you. And those photons being ridiculously blue-shifted to very high energies. And that's before we look at all the matter particles in the way.
You'd need a pretty large ablative shield for long distance travel.
*Since magic (http://tvtropes.org/pmwiki/pmwiki.php/Main/MagicFromTechnology) was mentioned earlier on the thread, how do you guys feel about a Bag of Holding? ;7
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>astrophysicist who has studied GR says the alcubierre drive isn't completely infeasible
>architect who has 'read some papers' and 'consulted some physicists' says only an idiot would think that
yeah sure
And Phantom Hoover ad hominems. I am an architect indeed, but not a stupid one. I did end up 6th place in the national physics Olympiads back in the day, only because I didn't know much of thermodynamics... But yeah I understand your skepticism. Here, take Lubos Mötl criticism:
http://motls.blogspot.pt/2013/07/relativity-bans-faster-than-light-warp.html
Please do not let this thread degenerate into dueling authorities. Luis, get used to the fact that we have absolutely no idea of your credentials in the area, whereas watsisname's are well known. You're skeptical about the Alcubierre drive, and you have sources to back that skepticism up, and that's fine. But posting something that (at least to me) read a lot like a more eloquent version of Aardwolf's uninformed opinions, without any of those sources, doesn't create a favourable impression of your arguments.
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And Phantom Hoover ad hominems. I am an architect indeed, but not a stupid one. I did end up 6th place in the national physics Olympiads back in the day, only because I didn't know much of thermodynamics...
that doesn't mean ****, unless you're trying to convince me that being good at high school physics gives you an intimate understanding of tensor algebra and differential geometry
Here, take Lubos Mötl criticism:
http://motls.blogspot.pt/2013/07/relativity-bans-faster-than-light-warp.html
motl is so notorious for completely forgetting that his expertise has limits that setting his opinion against any kind of consensus of any experts at all might as well be worthless
oh hey, he's a climate change denialist as well. that where you get your 'expert advice' on that one as well?
e:
Your humble correspondent realizes that many readers are left-wing, anti-string-theory fighters. So they probably smoke marijuana and this is my modest attempt to help them.
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motl is so notorious for completely forgetting that his expertise has limits that setting his opinion against any kind of consensus of any experts at all might as well be worthless
Except that this topic is where his actual expertise lies. While he certainly has strange opinions in other areas and likes to stir the pot with controversial claims, I have yet to see any evidence that he is a crank or against consencus of the experts when it comes to physics.
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Lubos is a very questionable source regarding social, political and global warming issues (although he's not a global warming denier at all, that's just sleazy scumbbaggery that has been thrown at him... well because he was a jackass about it). Lubos' blog is also the very best blog source for theoretical physics in the entire internet.
Your humble correspondent realizes that many readers are left-wing, anti-string-theory fighters. So they probably smoke marijuana and this is my modest attempt to help them.
His humour is also .... not for everyone! (Hey, I'm left wing and I can take all his shenanigans because hey they are irrelevant)
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yeeeaaaaahhh......................... :/
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Lubos is a very questionable source regarding social, political and global warming issues (although he's not a global warming denier at all, that's just sleazy scumbbaggery that has been thrown at him... well because he was a jackass about it). Lubos' blog is also the very best blog source for theoretical physics in the entire internet.
Your humble correspondent realizes that many readers are left-wing, anti-string-theory fighters. So they probably smoke marijuana and this is my modest attempt to help them.
His humour is also .... not for everyone! (Hey, I'm left wing and I can take all his shenanigans because hey they are irrelevant)
But for someone who hasn't put in the time to familiarize himself with this person, all of these things do call hid judgment into question. I'm sorry, but I for one can't really take him seriously in anything.
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Lubos is a very questionable source regarding social, political and global warming issues (although he's not a global warming denier at all, that's just sleazy scumbbaggery that has been thrown at him... well because he was a jackass about it). Lubos' blog is also the very best blog source for theoretical physics in the entire internet.
But what makes him lose all credibility in my eyes is when he asserts that proponents of both the Alcubierre drive and loop quantum gravity are fools who just haven't noticed that their theories ignore basic principles of physics. I know he's a very good theoretical physicist. I also know he's not the only good theoretical physicist.
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Lubos is a very questionable source regarding social, political and global warming issues (although he's not a global warming denier at all, that's just sleazy scumbbaggery that has been thrown at him... well because he was a jackass about it). Lubos' blog is also the very best blog source for theoretical physics in the entire internet.
But what makes him lose all credibility in my eyes is when he asserts that proponents of both the Alcubierre drive and loop quantum gravity are fools who just haven't noticed that their theories ignore basic principles of physics. I know he's a very good theoretical physicist. I also know he's not the only good theoretical physicist.
Sometimes studying theories that are most likely wrong makes sense to see if you dont stumble upon something interesting anyway, so I would not call them fools (tough some of them may be, not everyone with a degree has to be an expert, and this is a difficult subject). However it is true that those two theories are on the fringe of theoretical physics and not many physicists take them seriously as corresponding to some physical reality. There is probably a reason for that, so I dont see why his aversion towards LQG and aclubierre drive is in any way a negative.
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Lubos is a very questionable source regarding social, political and global warming issues (although he's not a global warming denier at all, that's just sleazy scumbbaggery that has been thrown at him... well because he was a jackass about it). Lubos' blog is also the very best blog source for theoretical physics in the entire internet.
But what makes him lose all credibility in my eyes is when he asserts that proponents of both the Alcubierre drive and loop quantum gravity are fools who just haven't noticed that their theories ignore basic principles of physics. I know he's a very good theoretical physicist. I also know he's not the only good theoretical physicist.
I think that's actually a pretty straightforward fair accusation against QLT. Very few people besides Smolin himself takes his pet theory seriously. Most physicists however are just nice chaps and wouldn't say its all crap, because... they are nice people*. Lubos ain't nice.
* but when pressed they will give you strong hints of their displeasure at the theory. It sort of breaks a lot of **** of what we know is true.
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But for someone who hasn't put in the time to familiarize himself with this person, all of these things do call hid judgment into question. I'm sorry, but I for one can't really take him seriously in anything.
You realise this is merely a very personal prerrogative right. You're basically saying "I don't know this chap, I can't afford to know and he sounds slimy so I'm not going to, and therefore I won't entertain him at all". It's your prerrogative of course, but it sounds pretty subjective for me. Lubos is indeed an expert on the field and his insights and calls have been right throughout all these years regarding the evolution of physics, string theory, black holes and so on.
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But for someone who hasn't put in the time to familiarize himself with this person, all of these things do call hid judgment into question. I'm sorry, but I for one can't really take him seriously in anything.
You realise this is merely a very personal prerrogative right. You're basically saying "I don't know this chap, I can't afford to know and he sounds slimy so I'm not going to, and therefore I won't entertain him at all". It's your prerrogative of course, but it sounds pretty subjective for me. Lubos is indeed an expert on the field and his insights and calls have been right throughout all these years regarding the evolution of physics, string theory, black holes and so on.
Of course it's highly subjective. That's why I said that I can't take him seriously :P
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My bad! I would love for him to be wrong on this one.
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But what makes him lose all credibility in my eyes is when he asserts that proponents of both the Alcubierre drive and loop quantum gravity are fools who just haven't noticed that their theories ignore basic principles of physics. I know he's a very good theoretical physicist. I also know he's not the only good theoretical physicist.
Hey, PH is right once! We should probably celebrate that day (at least for those of you who are not celebrating the Moon landing yet). :)
To be fair, though many theoretical physicists see themselves as the "only good theoretical physicists", especially if their theories clash with the others. This is because a theory needs to be consistent with itself and the experiments - rejecting everything else is a fair game. As such, some scientists might work with a theory that allows Alcubierre drives and other might work with one that doesn't. Both will have solid reasoning behind them, and probably explain the universe equally well as far as the experiments are concerned. Until an actual test rig tells who's right, they'll stay with their theory and also probably consider the other foolish for not taking for granted what they do.
The reason I'm personally on White's side of this debate is that he is actually experimenting with the effect. His results were "inconclusive" last time I checked, but this is the best hope we have - theorizing is nice, but once you get down to actually doing it, you're really answering questions. Theories are just theories, while obviously useful for directing the experimental effort, they need experiments to actually mean anything but a bunch of pretty equations. That's why I went with experimental physics afterall. The skeptics might theorize all day, but if White's machinery says "it works!", and someone else's rig replicates this afterwards (it's important not too get too excited about revolutionary results. A loose cable will make you FTL neutrinos, for instance :) ), then it's them who'd have to sit down and remake their theories.
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yeah, White is putting up.
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he's not a global warming denier at all
Yes, he prefers the term "climate optimist". Which is to say, he is absolutely a global warming denier.
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You're playing with semantics. He always accepted the greenhouse effect. This word "denialist" is just political slimeballing. What he does is a bit different than the usual denialist, he just doesn't believe there's good evidence for the water vapor positive feedback that the more "alarmist" models predict that the atmosphere has. And then he loses his **** whenever some leftist loses his **** in some newspaper or site or whatever, and it's good fun to watch if you have popcorn by your side.
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You're playing with semantics. He always accepted the greenhouse effect. This word "denialist" is just political slimeballing. What he does is a bit different than the usual denialist, he just doesn't believe there's good evidence for the water vapor positive feedback that the more "alarmist" models predict that the atmosphere has. And then he loses his **** whenever some leftist loses his **** in some newspaper or site or whatever, and it's good fun to watch if you have popcorn by your side.
I think that trash like yourself cannot be debated. It must be destroyed. What is written on the "climate denial" page is just crime, the people who are responsible for it are criminals, and as soon as I get the opportunity to collaborate with someone on their liquidation, I will do it.
(source (http://scienceblogs.com/stoat/2007/08/29/fun-from-a-czech-physicist-and/#comment-4871))
I think Motl does more than enough "political slimeballing" of his own name with his own words.
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I would like to point out that if we manage to solve the practical questions (which are not exactly trivial matters) of modifying space-time geometry at will with reasonable energy cost, it would be insanely cool whether Alcubierre Drive as an FTL transportation device works or not.
Things like antigravity, artificial gravity, and reaction massless propulsion systems come to mind*. It would practically be open season for colonizing anything in the Solar System, and it would make it possible to craft large scale, self-sufficient space colonies possibly capable of traveling to nearby star systems. It'd just take a while.
QFT :cool:
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I think Motl does more than enough "political slimeballing" of his own name with his own words.
Oh I didn't say it was "undeserved" or anything. I said it was false.
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reaction massless propulsion systems
this was White's main area of interest that led him to Alcubirre type drives IIRC, his other big thing before this was quantum thrusters, ion drives that use virtual particles as their propellant.
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Look, for me this is very simple and you do not require a degree of physics to state the obvious: you cannot contradict General Relativity and move any piece of information faster than light in any given reference frame provided the reference frame lacks curvature or other such departures from special relativistic mechanics.
Fixed that for you. It doesn't take a degree in physics to state the "obvious", but it might take one to state what is correct. :) Read the supplementary material provided here (http://www.hard-light.net/forums/index.php?topic=87809.msg1756173#msg1756173) for more information.
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I'm not debating that this kind of "superluminous travel" using space itself doesn't exist, of course I accept that given the premise that space is stretching and so on then all of this is possible. What I am suggesting is that the process must be viewed in its entirety and what seems like a smart cheat against FTL prohibitions is really not when you see the whole process in its entirety. Inflation was a very special event that was, apparently, very homogeneous in nature and precise, in the sense that all space knew at the start the kind of stretching it would undertake and all that was (apparently) required for it to slow down at some point was a combination of gravity and this weird astro constant. But all of this was already in place. You didn't need to superluminally "tell" space to stretch itself.
Not here. IOW, if you are a light hour away from me, no matter how powerful your Alcubierre Drive may be, "space" has to know by the means of some piece of information wave how precisely it would stretch. This information is necessarily driving in c speeds inside the "previous / current" curvature of space.
After having done this, then you can proclaim all you want in how your ship was (in a common sense view) travelling faster than c and I have no issues with that part of the process claim. Of course that having bent space-time, all bets are off.
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this is all very 'obvious' luis but until you can convince me that it actually follows from the einstein field equations i am going to continue dismissing you
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ok, let's put this into slightly different terms. lets say you are near the event horizon of a very large black hole, lets say it is a light hour wide, lets also say this black hole is rotating extremely fast. now for the sake of argument you have really really powerful thrusts that can output arbitrarily powerful thrust of finite amount for as long as needed. you are not inside the event horizon, only near it, and you are thrusting directly away as exactly enough thrust to not get pulled in. the black hole is rotating extremely fast and as a result there is an extremely pronounced frame dragging effect on the space you are occupying. So much so that from a third party several light years (and year years obviously) further/later away that it seems as if you orbit the black hole every hour even though from your perspective you are thrusting directly away from the black hole remaining motionless relative to it. Note the part about how you seem to trace out a path of >1(light hour) * PI every hour.
relativity is very 'obvious' it's why it was so easy for us to figure it out. :)
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That's still a very standard spacetime distortion story. The third person is not even observing the real distortion of spacetime for the light ray paths have also been distorted while trying to get out of that gravity well. In that sense, the spaceship is "travelling" even faster than you imply. But here's the thing: that's not a FTL story. There's no way that spaceship can "use" the black hole's fast rotation in order to travel faster than light, by for instance get inside that particular "orbit" and get out of it in the "other side" as a FTL quick spin to the other side of the BH. This is because when you calculate everything, you've used so much energy and "time" (from the third party point of view!) getting into that position and out of it that if you just closed you eyes in the beggining of that operation and opened in the end of it, the spaceship wouldn't have traveled from A to B in any FTL fashion.
The only way the maths would have made it FTL would require the ship to go through the event horizon but then it can't go back...
Now I know I'm probably oversimplifying here. Lots of **** going on in fast rotating BHs. Still. All that "FTL" shenanigan was just an useless illusion. Alcubierre proposed something remarkably different from this, that this is an illusion that actually works, is actually useful.
But now consider BHs again. They are a useful comparison in this sense: they don't warp space around it in FTL fashion. They always use graviton waves to do the job. I'll ask it again: how does the space itself in the path ahead of the warp bubble "knows" how to bend? It cannot know this unless something told it this information. This information cannot be carried superluminally, or alternatively we are talking about an infinite regression: we need a warp drive to build a warp drive.
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But now consider BHs again. They are a useful comparison in this sense: they don't warp space around it in FTL fashion. They always use graviton waves to do the job.
What? You have proof that gravitons exist? Share with the class, please.
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Fine. If you want to stick with "observed" entities, substitute "Graviton waves" with "Gravitational waves".
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What I am suggesting is that the process must be viewed in its entirety and what seems like a smart cheat against FTL prohibitions is really not when you see the whole process in its entirety.
It is, otherwise we would have no idea what the proper travel time would be for those on the ship vs. distant observers.
You have some confusion in that you think we are looking at what happens locally because it is difficult or impossible to examine it globally, therefore we are missing something important about what's actually going on. That is not the case. If it was, then we would have no way of comparing proper time intervals for those on the ship vs. distant observers. When we say we are looking at what happens locally, we do that because that is how you examine motions within curved space-time.
This is an important point that cannot be overstated -- general relativity upholds what we already knew from special relativity. Light still always propagates at c, and allowed time-like trajectories are still less than c. The difference is that in the flat space-time with unaccelerated observers of special relativity, it doesn't matter how far apart the observers are -- provided an understanding of Lorentz transformations they'll all agree on whether an object has a space-like, time-like, or null trajectory. But in the curved space-time of general relativity, distance between observers does matter, and if you don't account for it then observers can easily be confused and think a trajectory is space-like when it is not.
Examples: Objects within black hole event horizons and ergospheres, galaxies at large cosmological distances, wormholes, and Alcubierre drives.
One way of dealing with curvature is to examine it on sufficiently small scales. Just as the (roughly) spherical surface of the Earth seems flat when you're standing on it, space-time curvature appears flat if examined on sufficiently small scales. Those who are familiar with calculus might be reminded of tangent lines, and the similarity is not superficial. By examining motions locally, you see what they actually are through the space-time itself. The global view in turn will tell you what the curvature is (provided you understand its source) and how motions are affected by it.
So in a nutshell, the central problem that Luis and Aard have here is that they're trying to assert what is or is not allowed according to general relativity on a global perspective, yet general relativity doesn't actually assert such a thing. If it did, then the observed motions of galaxies at large distances would violate general relativity. On the contrary, a proper understanding of GR's principles would reveal that the motions of these galaxies is actually a brilliant proof that it works. The Alcubierre drive is a fine solution to GR as well -- it works as long as it is possible to transform space-time geometry in the stated way, which comes down to the three energy conditions that it does indeed violate. (Note, though, that the energy conditions are not "laws", like the 2nd law of thermodynamics. There are known phenomena that violate one or more of them.)
I strongly suggest that Luis et. al refine their argument from "This solution violates basic principles of GR" to "is it possible for the required transformation to occur?"
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I'll ask it again: how does the space itself in the path ahead of the warp bubble "knows" how to bend? It cannot know this unless something told it this information. This information cannot be carried superluminally
Apparently, light cones are not all they're cracked up to be... (http://arstechnica.com/science/2014/07/quasiparticles-carry-entanglement-to-near-infinite-speeds/)
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I strongly suggest that Luis et. al refine their argument from "This solution violates basic principles of GR" to "is it possible for the required transformation to occur?"
Very fine comment, all of it, but it didn't address any of my concerns, since it dealt with basic questions on space-time curvature that I had already agreed with from the start. My concern isn't about the Bubble itself being "anti GR", it's about "how do you tell spacetime itself to do this without informing it so superluminally".
For instance, I do get it that if spacetime bends at the front of the bubble in order to become extremelly small, then you can travel it really "fast" without being fast at all. What I don't get is how you can teach spacetime in very precise coordenates to do this without signalling it. And if spacetime at the front isn't "bent", then all the information that gets to it must do so either at "c" or lower.
Regarding "global concerns", I only ask this question. Are we speaking or not about a drive that can go faster than light or not? I mean, at some point, we are indeed talking about something that is meaningful in some way. My initial point about how looking at this from a more distant point of view is that if we abstract all of this engine into some kind of black box (and in this black box I might include the wave and so on), then what theoretically happens is that there's something at point X in time 0 in a perfectly smooth plane spacetime, and then this thing is at x = 1 light hour at time = 1 second. If I seem to be speaking from a "global reference frame", think about someone watching this in a perpendicular angle from some light hours away. Now this "black box" could be spoken as a kind of a weird (exotic?) particle. And this "particle" seemingly has the ability to travel faster than light (when all things are said and done).
I'm incredibly skeptical at this, and not just for the violations you mention. Although they can be mathematically related.
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Fine. If you want to stick with "observed" entities, substitute "Graviton waves" with "Gravitational waves".
"ok lemme just juggle physics terminology until i come up with something that makes sense"
Very fine comment, all of it, but it didn't address any of my concerns, since it dealt with basic questions on space-time curvature that I had already agreed with from the start. My concern isn't about the Bubble itself being "anti GR", it's about "how do you tell spacetime itself to do this without informing it so superluminally".
This just further exemplifies your general mistake here: you do not understand GR. You do not know or understand the mathematical framework underlying it. You think you can get by using nice approachable heuristics like this but you just can't. Stop trying to poke holes in the work of people who actually do: you are not going to achieve anything above petty bickering.
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To be fair, that's a very common mistake made by laymen. The only way to truly describe real physics is by mathematics. English language is only capable of approximating them, and working off those approximations leads to many mistakes. That said, this is one of the first things you learn when you start seriously working with physics, so this is a typical layman's mistake no professional would make (or at least, they'd correct themselves quickly after checking the mathematics). Indeed, the whole point of having mathematics is to provide a "language" which can be used to unambiguously (and thus "correctly" in the very essence) describe even the most complex physical phenomena. Whether our description is actually representing reality is, of course, a subject to revision, but a mathematical model (unlike an explanation in English) can be a perfect representation of processes that actually occur. This is an useful thing to know, too. By knowing what kind of process or object a mathematical concept may represent, it's easier (for some) to understand the concept itself.
In that case, it's a classic "it's hard to describe, but it works". That's the best response you can get at an "internet forum with no LaTeX support" level. You can talk about tensors and transformations, but in the end, the best explanation would be purely mathematical, and probably full of nablas, tensor products and other terrifying symbols, to boot.
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as an aside that I hope doesn't derail the thread too much, I've wondered for a while now, what the **** is the difference between a tensor and a matrix?
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Are we speaking or not about a drive that can go faster than light or not?
Are these observable galaxies moving faster than light or not? (http://www.publish.csiro.au/?paper=AS03040)
The answer to your above question is both "Yes and no." Again, and with elaboration having been already provided, faster than light motion does not violate general relativity unless it is faster than light locally. Such motion does not occur anywhere in the Alcubierre solution as far as anyone has yet been able to demonstrate in a convincing manner.
Which brings me back to what I said a few pages ago:
Were these physicists you consulted specialists in general relativistic field equations, and did they explain to you why the Alcubierre solution is invalid? If so, then I urge you to urge them to submit their critique to a relevant journal, perhaps ApJ, so that others can examine it. If not, then you may want to reconsider your views.
It appears that the answer to this question is a resounding "No."
You may attempt to counter all you wish that "physicists don't take these papers seriously", but it's pretty difficult for me to take such a counter seriously, and it seems a number of our fellow forumites hold the same view. I would like to think that if one of these people you cite had a valid refutation of the Alcubierre solution, they would want to submit it to a relevant journal in addition to posting it on their blog, at the very least to have experts in the field review it and ensure that they are right. If it is attention that they are after, they'd get that, too. If not, then why are they posting about it on a blog at all?
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as an aside that I hope doesn't derail the thread too much, I've wondered for a while now, what the **** is the difference between a tensor and a matrix?
A matrix is simply a grid of quantities (which can themselves be almost anything -- scalars, vectors, derivatives, etc), and they may represent something physical (like pixel values in an image, or temperature on a map) or they can be totally random entries.
A tensor on the other hand (which can be expressed with a matrix!), is a geometric object which describes quantities that transform under a particular set of rules. Another popular way of thinking of tensors is that they are 'machines', which accept certain inputs and spew out outputs, again according to a particular set of rules. This is pretty vague (e.g. what rules? (http://www.tommangan.us/Tensors.pdf)), but basically what they do is allow us to describe things irrespective of choice or change in coordinate system. Thus their importance in relativity and other branches of physics.
So to be brief, a matrix is a grid of quantities, while a tensor is a grid of quantities which follow a particular structure.
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A good example of a tensor would be moment of inertia. For example: you input a force (a vector) and you get a rotational acceleration vector. A high-school level approach is to use a coordinate system connected with principal inertia axes (which diagonalizes the tensor) and then apply the force along one of those axes (usually the simplest one), which means you can use a single scalar, which is what they call "moment of inertia". Obviously, such a neat situation is rarely the case in real life. If it isn't, you're gonna work with a MOI tensor.
Tensors are used everywhere, from relativity through classical mechanics and electricity to quantum mechanics. Rank two (representable by a 3x3 matrix) tensors are particularly common.
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Are we speaking or not about a drive that can go faster than light or not?
Are these observable galaxies moving faster than light or not? (http://www.publish.csiro.au/?paper=AS03040)
The answer to your above question is both "Yes and no." Again, and with elaboration having been already provided, faster than light motion does not violate general relativity unless it is faster than light locally. Such motion does not occur anywhere in the Alcubierre solution as far as anyone has yet been able to demonstrate in a convincing manner.
There's one point in your article I don't quite get:
Superluminal recession is a feature of all expanding cosmological models that are homogeneous and isotropic and therefore obay Hubble's law. This does not contradict special relativity because the superluminal motion does not occur in any observer's inertial frame. All observers measure light locally to be travelling at c and nothing ever overtakes a photon.
Isn't that exactly what would happen with an alcubierre drive? Wouldn't our alcubierre-ship overtake photons travelling in the same direction? Or is there some curvature of space-thingy going on?
To clarify: In my imagination of an inflationary process, everything is moving away from everything else. So even if space is inflating superluminally, nothing could ever possibly overtake something else at a relative velocity higher than c.
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maybe the alcubierre drive is like a photon snowplough
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Is that possible you're right a second time in a single thread? :) Well, I dunno (I don't have a PhD in relativistic physics, or at least not yet), but it is predicted that an FTL Alcubierre drive would produce a massive burst of gamma radiation in front of the ship once it stops (something to keep in mind if we ever end up at war with aliens. Or if we ever encounter aliens, for that matter...). It seems to me that it might actually stem from photons "accumulated" in front of the warp bubble. It's probably a huge oversimplification, if it's even remotely on target, but it might be that sort of effect. It can't "fly past" photons it encounters (it'd violate relativity), and they will be "scooped up" by the bubble, that's for sure. Something has to happen to them, it makes sense that they'll travel with the bubble and accumulate in front. And once it's turned off, the space stops being curved, but the photons are still going at c... I can't see how there could not be a huge, multispectral photon flash every time the thing is turned off. It shouldn't damage the ship (dunno what the exact geometry would be, but the photons would probably not be moving towards the ship), but woe to anything that happens to be in front of it.
Isn't that exactly what would happen with an alcubierre drive? Wouldn't our alcubierre-ship overtake photons travelling in the same direction? Or is there some curvature of space-thingy going on?
Your picture of inflationary process is correct. Also, an Alcubierre ship certainly can't "overtake" photons in a traditional sense. Nothing can, in fact, it's inherent in the geometry of the universe. As such, it must warp space in such a way that it doesn't overtake photons, yet it arrives at destination in shorter time than it'd take to traverse that distance at c was it a flat region of spacetime. See the above - it seems consistent with the gamma burst that is predicted to be emitted in front of such a ship once it stops. Remember, it warps space, so everything within the bubble is affected, including light. There's also nothing stopping the light from entering the bubble.
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as an aside that I hope doesn't derail the thread too much, I've wondered for a while now, what the **** is the difference between a tensor and a matrix?
A matrix is simply a grid of quantities (which can themselves be almost anything -- scalars, vectors, derivatives, etc), and they may represent something physical (like pixel values in an image, or temperature on a map) or they can be totally random entries.
A tensor on the other hand (which can be expressed with a matrix!), is a geometric object which describes quantities that transform under a particular set of rules. Another popular way of thinking of tensors is that they are 'machines', which accept certain inputs and spew out outputs, again according to a particular set of rules. This is pretty vague (e.g. what rules? (http://www.tommangan.us/Tensors.pdf)), but basically what they do is allow us to describe things irrespective of choice or change in coordinate system. Thus their importance in relativity and other branches of physics.
So to be brief, a matrix is a grid of quantities, while a tensor is a grid of quantities which follow a particular structure.
One could rather say tensors don't transform at all, that they are fixed properties of, or extra structure on space-time. When you choose a particular way of looking at a space-time (ie. when you choose a particular set of coordinates) then you can describe them by (in general n-dimensional) arrays of numbers, and when you change the coordinates, those arrays change in a certain way.
It's exactly analogous to how the boiling point of water is either 100°C or 373 K or whatever the hell °F. But you don't say that temperatures are numbers that change in a particular way (depending on the temperature scale you're using), even though technically you could. You think of temperature as being what it is, and the numbers that change in certain way depending on the change of scale are just our way of representing it.
In other words, a representation of a tensor in GR changes when you change the space-time coordinates, and a representation of a temperature changes when you change the "temperature coordinates", but it's probably best to view both as things that just are.
As for the Alcubierre's drive, it may be a solution to the Einstein's field equations, but isn't that kind of a non-brainer? I mean, isn't every metric a solution, depending on what we are willing to take the stress-energy tensor to be? I'm sure the Alcubierre's solution makes infinitely more sense than what you'd get by just plugging in numbers randomly, but just saying it's a solution doesn't seem to mean that much.
In any case, I also don't understand how an Alcubierre's drive you can turn on and off doesn't lead to exactly the same causal consequences the superluminal travel does in SR, but I'm not a physicist and I've never studied GR, so I'll admit I'm talking out of my ass on this one.
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Isn't that exactly what would happen with an alcubierre drive? Wouldn't our alcubierre-ship overtake photons travelling in the same direction?
It depends on if we qualify this question with "traveling in the same direction (parallel) to the ship's trajectory inside the Alcubierre field or outside. If it is outside the field (flat space-time), then yes, the ship overtakes the photon. If it is inside the field, in same local space-time geometry as the ship itself, then no, the photon overtakes the ship.
Basically this comes down to an analysis of null geodesics (light-like paths) throughout the Alcubierre solution. Clark et. al (1999) (http://iopscience.iop.org/0264-9381/16/12/313) were the first to do this IIRC, though this paper is not free to view.
Added:
@Dragon: Your descriptions of radiation effects when the ship stops are qualitatively correct. :) There is a horizon-like structure in front of the ship, upon which photons leaving the ship's bow become trapped, and photons moving parallel with the ship but initially outside the field also become trapped. (But photons moving anti-parellel pass through this horizon and meet the ship, with curvature effect making the destination seem closer than it "really" is.) At the terminus of its trajectory, these photons would be released as a powerful pulse of EM radiation.
There is also a horizon-like structure behind the ship, on which photons moving parallel to the ship but initially behind and outside the field will never reach.
So, @Phantom Hoover, yes, the Alcubierre ship is sort of like a photon snow-plow, albeit a very strange one. :)
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As for the Alcubierre's drive, it may be a solution to the Einstein's field equations, but isn't that kind of a non-brainer? I mean, isn't every metric a solution, depending on what we are willing to take the stress-energy tensor to be?
Great question. Basically you are asking: "For every possible metric of space-time, does there exist a unique curvature and stress-energy tensor which satisfy the set of partial differential equations that are the Einstein Field Equations?"
I'm fairly sure the answer to this is going to be "no", but I am not certain and don't have a source to check/cite at the moment. I am also fairly sure that it will not be guaranteed that for all choice of metrics you will preserve the conditions of energy/momentum conservation, Newtonian approximation, etc. (If anyone happens to know please feel free to add to or correct me.)
To give an analogue of this situation, consider the wave equation (http://mathworld.wolfram.com/WaveEquation1-Dimensional.html). There are many (an infinite number) of functions psi that you could try to plug into it that will not be solutions. To check if it is a valid solution, you check if the second derivatives with respect to time equal the second derivatives with respect to position. If it holds true, your result will look like some sort of wave. Naturally, solutions to the wave equation are sinusoidal in form because the sine/cosine functions interchange through their own derivatives.
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As far as metrics go it's not hard to come up with something way too weird to qualify, but that's really just pedantry.
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To give an analogue of this situation, consider the wave equation (http://mathworld.wolfram.com/WaveEquation1-Dimensional.html). There are many (an infinite number) of functions psi that you could try to plug into it that will not be solutions. To check if it is a valid solution, you check if the second derivatives with respect to time equal the second derivatives with respect to position. If it holds true, your result will look like some sort of wave. Naturally, solutions to the wave equation are sinusoidal in form because the sine/cosine functions interchange through their own derivatives.
As you describe it, the wave equation is of the form Aψ = Bψ, where A and B are two differential operators, so it's obvious that plugging in random ψ's won't do you any good.
On the other hand, if you take an inhomogeneous equation, like eg. Poissons Δψ = f, you can say that any (sufficiently smooth) ψ solves a Poisson's equation. You just calculate Δψ, and set f = Δψ. Of course, that's exactly the opposite of what you usually need to do: usually you're given f, and then need to reconstruct what ψ is from knowing Δψ.
I thought the Einstein's equation is analogous; disregarding the cosmological constant and with the right choice of units, it's just G = T, where G is the Einstein tensor, and can be computed directly from the metric tensor g, and T is the stress-energy tensor.
So if a metric g_0 is given, you can just calculate what G is and then say g_0 solves the field equations for T = G. This would work if T doesn't itself depend on the metric, which in retrospect sounds unreasonable; the distribution of matter changes in time due tue the effects of among other things gravity, which in GR is nothing but the metric, so... yeah. In my defense, I've seen someone claiming to be a physicist describe it this way (ie. that anything is a solution to GR with the appropriately chosen stress-energy tensor.)
It would be interesting to know then what is known about the space of solutions, at least for a fixed space-time topology, but the answer's probably "hardly anything" besides the fact that it includes some very weird things (eg. Gödel's metric).
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I thought the Einstein's equation is analogous; disregarding the cosmological constant and with the right choice of units, it's just G = T, where G is the Einstein tensor, and can be computed directly from the metric tensor g, and T is the stress-energy tensor.
So if a metric g_0 is given, you can just calculate what G is and then say g_0 solves the field equations for T = G.
Certainly. I usually write the field equations in the form G + Λg = kT, but since G is technically determined uniquely by g, you could say G = kT if you like. The problem is there is a lot of mathematical machinery hidden within this relation. You could make up any kind of metric g, but not be able to compute G from it, or find that a corresponding G may not exist at all. For this reason you will almost never find this method described or used in the literature. Generally, people will first compute the Ricci curvature tensor R, or a more simple route if the situation makes it possible.
Exact solutions to the field equations turn out to be a pretty big deal because they are so difficult to find.
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You could make up any kind of metric g, but not be able to compute G from it, or find that a corresponding G may not exist at all.
I'm not sure I understand how could this be... Riemann tensor certainly exists and contracting from that you get the Ricci tensor, and contract again to get the Ricci scalar, which is what you need to get G. In coordinates all you need to do is calculate the christoffels and then multiply the whole lot of them, which I wouldn't recommend doing by hand, but in the end is just a routine calculation. The reverse (which is what you need to do when actually trying to solve for metric) is of course impossible except in the simplest of cases.
But this is starting to get awfully off topic, sorry for that.
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If nothing else, I want to thank you guys for reminding me why I didn't continue my physics education into graduate work.
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I'm not sure I understand how could this be... Riemann tensor certainly exists and contracting from that you get the Ricci tensor, and contract again to get the Ricci scalar, which is what you need to get G.
Ah, this is a nuance of G that often gets missed. I don't know if this will make it more clear or less, but I'll try my best here. Let's start with what we already know.
We have a pretty good understanding of what the field equations are saying: mass-energy produces space-time curvature, AKA gravitation. We describe the source of gravitation with the frame-independent stress-energy tensor T. We know this cannot be any arbitrary tensor. For one, it must have zero divergence,
∇·T = 0
which is another way of saying that momentum-energy are conserved. So far so good.
We place this on the right hand side of the general form of the field equations (with some proportionality constants k). On the left hand side we have the tensor description of the curvature, G, which we may call the "Einstein tensor". This gives us
G = kT.
"Curvature = mass/energy"
But is this G the same Einstein tensor that we form by contraction of the Riemann tensor as in the usual analysis you're describing above? The answer is no, when the field equations are written in this form they are not the same.
We know of course that G must also be a divergence-free tensor, but this is not set by T. Rather, it is a consequence of the geometry -- it is true for any smooth and Riemannian space-time. So for a valid solution, we do not have
∇·G = 0,
but rather
∇·G ≡ 0.
Thus G must have the following properties:
-reduces to 0 in special-relativistic limit
-constructed from curvature tensor and metric, and nothing else
-unique from other tensors which could be built from R and g in that it must
(i) be linear in R
(ii) Like T, is 2nd rank, symmetric, and divergence free.
Given a metric, there is, (I'm pretty sure), a unique curvature tensor for it. The curvature tensor is simply a way of describing how non-flat the metric is. If it is flat then there is no curvature, R vanishes, and we're back to special relativity. But I am also pretty sure that for any arbitrary metric, you will not find a G which satisfies the field equations.
Added: I think anything pertaining to relativity and in particular the field equations and their solutions is both sufficiently on-topic and interesting to be posted here, though granted I am not the OP. :)
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Given a metric, there is, (I'm pretty sure), a unique curvature tensor for it. The curvature tensor is simply a way of describing how non-flat the metric is. If it is flat then there is no curvature, R vanishes, and we're back to special relativity. But I am also pretty sure that for any arbitrary metric, you will not find a G which satisfies the field equations.
Oh no, I completely agree, I gave up on that a few posts back :)
And yes, given a metric, curvature not only exists but is easily if tediously calculated. The Riemann and Ricci tensors are actually defined for a structure on manifolds a lot more general than (pseudo-)Riemann metrics, it's only the Ricci scalar that requires rising and lovering of the indices (to contract the rank (0, 2) Ricci tensor). So yeah, you can always calculate what R - 1/2gR is, but I agree there's no telling if it satisfies the field equations.
As for the graduate work in physics, seeing General Relativity actually sometimes makes me wish I went for that instead of just pure math :) but i understand that going straight from "vector calculus" to full-blown differential geometry must be daunting. I'm honestly amazed by how much stuff are physicists expected to just pick up along the way.
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No worries, I was just wording things poorly earlier. Basically I was trying to explain that while you can always compute "G", it is not necessarily the right "G" for G=kT. But I had simply asserted "G might not exist at all" which, without understanding the difference, sounds really odd. :)
But anyway, I think that at least in vector calculus things remain fairly easily graspable in a visual sense. In differential geometry, there may still be a visual interpretation, but it becomes much harder to see, so everything seems quite abstract. On the physics side it never fails to amaze me how even really high level concepts, like general relativity, can be distilled down to a really simple set of statements. But of course, to fully grasp those statements so as to make use of them for calculations and predictions, the mathematics and rigor can be daunting.
On a slight tangent to the viability of the Alcubierre solution, I sometimes ponder about how it works in the sense of a toggle-able drive on a spaceship, versus having an "always on" state such as a particle.
When the ship activates its drive, it is changing the curvature around it, and so GR says the mass/energy distribution must be changing. But how is it changing? The energy being output or converted by the drive must come from some kind of reservoir within the ship, so the total mass/energy must be constant. This makes me think the solution makes the most sense in describing what is effectively a tachyon particle, rather than a spaceship drive. That is, if this type of localized anti-symmetric curvature was an intrinsic property to a particle, then it would naturally exhibit faster-than-light motion relative to non-local observers. I see absolutely no reason whatsoever to suppose such a particle actually exists, but it is an interesting way of thinking of it.
The explanation for how it works as a drive given by Alcubierre and others is that the ship's mass/energy reserves are being transformed into (rather exotic) forms which produce this style of curvature, in way consistent with GR's mass --> curvature description and without violating mass conservation. So I suppose the drive is not so much "producing" this curvature from scratch, but "redistributing" the curvature which was already present all along. It might make sense then that the weak but infinite-range 1/r2 standard gravitational field might become a very strong Alcubierre field if it is condensed to a small volume around the ship, similarly to how the gravitational field of the Earth would become equivalent to a black hole if it were compressed down to the size of a marble.
Granted, I have no idea if these thoughts actually make much sense or if someone who has studied the Alcubierre solution in detail would have words to say. ;)
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there are only a handful of ways I am aware of that come close to making anything that could in any way possibly be described as negative energy. one of those ways would be the casimir effect, if this could count as the required negative energy then the way you would "make" it would probably be via a meta-material that when energized alters it's molecular structure to have a vast number of nano-scale cavities. Another way to produce negative energies is via a rapidly spinning singularity
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I would say that as far as we can tell, the cosmological constant (dark energy) is a very good candidate for 'negative energy', in the sense that when described as a fluid it exerts negative pressure. But it has a really tiny value which only becomes relevant on large scales. How tiny is it? Let's imagine a sphere of space around the Earth whose boundary lies at the Moon's orbit. Then let's calculate "how much" dark energy lies within this sphere. Call this value Λmoon:
(http://i.imgur.com/yjUVD1m.png)
(http://i.imgur.com/zjkWOI4.png)
Taking H = Hubble's constant ≈ 68km/s/Mpc
ΩΛ = ratio of dark energy density to critical density ≈ 0.69
rmoon = moon's semi-major axis (really should have called this a rather than r, but whatever) ≈ 385000km
G = Gravitational constant ≈ 6.67x10-11m3*kg-1*s-2,
ρc is the current critical density of the universe. (Kind of fudging this, but not by much).
WolframAlpha tells us (http://www.wolframalpha.com/input/?i=%28%2868km%2Fs%2FMpc%29%5E2*0.69*%28385x10%5E3km%29%5E3%29%2F%282*6.11*10%5E-11m%5E3%2Fkg%2Fs%5E2%29) this is about one kilogram. One measly kilogram of dark energy spread through all of the Moon's orbital space! (By the cubic meter, it's on the order of 10-26 kilograms, or a few hydrogen atoms equivalence).
If it is possible to actually harvest dark energy (highly dubious), it wouldn't be easy.
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WolframAlpha tells us (http://www.wolframalpha.com/input/?i=%28%2868km%2Fs%2FMpc%29%5E2*0.69*%28385x10%5E3km%29%5E3%29%2F%282*6.11*10%5E-11m%5E3%2Fkg%2Fs%5E2%29) this is about one kilogram. One measly kilogram of dark energy spread through all of the Moon's orbital space! (By the cubic meter, it's on the order of 10-26 kilograms, or a few hydrogen atoms equivalence).
Well, if we consider that intergalactic space is commonly cited to have baryonic matter density of less than one hydrogen atom per cubic metre, that's actually quite substantial amount of dark energy. In a cubic metre of intergalactic space, there's more dark energy than baryonic matter energy!
Which, actually, explains why it currently appears as though majority of energy in the universe is in the form of dark energy.
Ah, cosmology and quantum physics, the two branches of science that regularly make me think "go home universe, you're drunk".
there are only a handful of ways I am aware of that come close to making anything that could in any way possibly be described as negative energy.
Gravitational or electric potentials? :p
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Which, actually, explains why it currently appears as though majority of energy in the universe is in the form of dark energy.
Precisely. :) Normal baryonic mass density is only ~5% of the total density, while dark energy is a whopping ~70%, or 14 times as much. The other substantial difference between the two is that dark energy seems to be uniformly distributed, while baryonic matter is certainly not. So on scales like planets, solar systems, and even galaxies, dark energy doesn't amount to much. Over cosmological distances, though, it dominates, and makes the universe expand faster.
Strange universe is strange; nobody expected this result.
That citation for the matter density by the way is actually made through the same kind of method as above: we calculate the critical density (that which would make the universe flat) via some equations, and measure what the total density is relative to this value (which turns out to be very close to 1), then chop it up into the components (baryonic matter, dark matter, dark energy, radiation).
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Yeah, a bit too strange for my liking. :) One alternate theory I once took a liking to (dunno what become of it, I've heard about it quite some time ago) rejected the Copernican principle and made universe non-uniform. It makes sense to me - afterall, cosmic background radiation isn't uniform, either. Neither was the Big Bang. Perhaps the structure of space might be as well, we know it's quite more malleable than it seems. IIRC, at least a part of this theory was disproved at some point, but dunno which. That said, I've always found it very "untidy" that there's so much stuff that is undetectable, doesn't interact and might exerts negative pressure to boot (under tension without anything to keep it that way?). Remember, a whole lot of this is very theoretical, and subtle changes to certain assumptions can produce different results. I have a feeling that somewhere, we missed something very subtle, but essential.
That said, I haven't heard anything about this theory since then, and I'm not sitting deep enough in cosmology to work on it myself.
Ah, cosmology and quantum physics, the two branches of science that regularly make me think "go home universe, you're drunk".
My thoughts exactly. :) One lets you do the weirdest things with matter, the other with spacetime itself. That said, it's fun once you get used to it, and is logical in strictly mathematical sense. As I often joke, I prefer working with quanta rather than with people. In both cases you never know what they'll do, but in the former case, you can at least calculate how much you don't know it. :)
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Ah yes, there have been alternative non-uniform or 'fractal' cosmological models, but they have more or less died out over the last couple decades with observations of the cosmic microwave background (CMB). The background is not perfectly isotropic, true, but it is isotropic to about one part in 100,000. Surveys of the distribution of galactic clusters also indicate homogeneity on scales larger than about a hundred million light years.
In other words, the slight anisotropy of the CMB is crucial to understanding the origin of structure and its evolution, but it does not mean that the Copernican principle (or more generally, the cosmological principle, which states the universe is homogenous and isotropic on large scales and naturally leads to Copernican principle) is wrong. It is rather a nice proof that it is correct. The universe began in a hot, dense, and extremely uniform state, and structure arises because of the tiny variations from perfect homogeneity collapse gravitationally over time. Expansion limits the size of collapsing regions to ~100 million light years at present, and so this is the scale in which we see the largest structures.
I also must disagree on the notion of dark energy not being detectable or interacting. If it didn't interact, we would not know that it is there, and we wouldn't be talking about it (at least not as a scientific discussion). :)
First, some historical context:
Dark energy is another name for the cosmological constant which appears very naturally in the field equations. Einstein first used it as a parameter to try to force a static solution out of the equations, because he (and pretty much everyone else at the time) thought the universe was static and eternal. But the equations show that the total mass content of the universe would cause it to collapse via gravity. Since evidently it is not collapsing, Einstein used the cosmological constant as a repulsive field which would exactly counter the gravitational collapse and preserve a static universe. Nice try, but unfortunately, such a solution is precariously unstable, like a pencil balanced on its point. The tiniest fluctuation would cause it to fail. People pointed this out, and before long Einstein abandoned the idea, especially after Hubble's galactic redshift surveys indicated the universe was indeed not static, but expanding.
If Einstein had been a bit more bold and trusted the equations he had derived, he might have predicted the non-static nature of the universe, though not necessarily in which direction (expansion or contraction).
The cosmological constant remained in the equations, but as a term most people assumed to be zero, because with available data it wasn't necessary to consider otherwise. We knew with increasing confidence that the universe was expanding, a model developed which described this as being due to a 'Big Bang', causing the prediction of the CMB, which was later discovered. The equations describe this all very well, with an expanding universe whose expansion rate slowly diminishes due to the mass it contains. Concentration lay on modeling the future behavior of expansion (is universe open, closed, or flat?), depending on how much mass there is. Out of curiosity/thoroughness, models were also worked out under the premise of a non-zero value for the cosmological constant, but nobody thought these would actually be valid.
Imagine our surprise then when surveys of distant Type-Ia supernovae indicate that the distant ones are much fainter than expected. There are several plausible explanations for this, ranging from mundane to mind-blowing. Maybe intergalactic dust is simply blocking out more of the light than we thought. Or perhaps some physics is going on which causes supernovae to be more luminous today than in the distant past. Or, maybe our assumption of an expansion rate governed by mass and a zero-valued cosmological constant is wrong.
We've tested all of these ideas, and the simple, most easily believable ones do not seem to work. The non-zero cosmological constant best fits the data. You would be right to think that this in itself would not be quite compelling enough. But now with the latest generation of CMB mapping satellites (esp. WMAP and Planck), we can very precisely determine the value of the cosmological constant, or 'how much' "dark energy" there is. This works because, thanks to the modeling work done long before, we know that the relative abundance of the various types of mass/energy (matter + dark matter, dark energy, and radiation) in the universe has observable consequences on the appearance of this background radiation. We now know that we live in a universe currently dominated by dark energy, and increasingly so as it ages.
That's the history, here's the science:
Dark energy is weird. Yep, that's the technical term. When it is described as a fluid, it is one which exerts negative pressure. That's really weird. How does a fluid exert negative pressure? What does that even mean?
If you think in context of particles of matter or radiation zipping around, bombarding the sides of a container, they exert forces due to the change in momentum upon rebounding off the surface. Force over area is pressure. There's something else going on, too. From the field equations, we find that a medium of particles zipping around has a "mass" to it which is more than what you get from simply summing the individual particle rest-masses. Why? Because each particle has a momentum, and the momentum factors in to the stress-energy-momentum tensor of the field equations. You can say it's a consequence of E=mc2. Thus, momentum acts as a source for gravitational field or space-time curvature. That's interesting. That means even a photon produces a (very) weak gravitational field. It also yields a very counter-intuitive result: in cosmology, a uniform field of radiation or moving particles, which exerts pressure, actually decreases the expansion rate, rather than increasing it. Weeeird.
The established value for the cosmological constant produces an accelerating expansion. This again follows straight from the field equations. We can, if we like, choose to describe the cosmological constant as a uniform matter/energy distribution, AKA "dark energy". In this case it enters into the stress-energy tensor, with a negative contribution to the space-time curvature, a negative pressure/momentum, and, if you want to treat it as particles, they have negative mass.
Sorry, that was a long post. Cosmology is weird. ;)
edit: Fixed where I said non-zero when I meant to say zero.