Hard Light Productions Forums
Off-Topic Discussion => General Discussion => Topic started by: General Battuta on May 29, 2010, 10:37:45 pm
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What do you think the solution is?
Points for creativity in a non-goofy way.
Supplementary question in my next post.
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We've only been looking seriously for, what, 30 odd years? Maybe 40? We've only even been really capable of looking for a century. Give it time.
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Several factors probably are at play.
1: What Black Wolf said.
2: Inhabited worlds probably aren't as common as Drake's equations suggest. Drake didn't know much about circumstellar habitable zones (and how they change as the star evolves). Also, we don't yet know how common "stable" planetary systems are.
3: Means of communication. All advanced civilizations probably aren't screaming out in all directions at all the frequencies that we've been using.
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[WAG]Other intellgent space-faring civilizations do not utilize radio or electromagnetic waves as communication.[/WAG]
They exist, we're just looking the wrong way. For example, we're (analogously) trying to find an extra hot rock in the middle of a desert filled with measurably less hot rocks. Unfortunately, we've decided to try finding it visually, with no enhancement, when instead using thermal imaging would find it comparatively quickly.
Comic related (http://xkcd.com/638/)
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Supplementary question!
Why do you think the galaxy has not been devoured by von Neumann machines, given that there's been time and opportunity for them to do so?
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Simple, obvious guess: Von Neumann machines do not actually exist.
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Simple, obvious guess: Von Neumann machines do not actually exist.
Demonstrably false! You are one! Just a very inefficient one.
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Simple, obvious guess: Von Neumann machines do not actually exist.
Demonstrably false! You are one! Just a very inefficient one.
Well then there you go. There aren't any that are efficient enough to devour the galaxy. My gut feeling is that entropy would be a problem as well.
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I don't buy it!
If meatlife can survive this long, the berserkers should be out there, man.
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How do we know we are not the berserkers?
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Maybe they just haven't had time to spread this far. We are on the arm of a spiral galaxy. The best dispersion method would be from the center out and that could take a long time.
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Maybe they did, and we're the waste products?
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Maybe we're the First Ones
TAKE THAT VORLONS
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Ah the Vorlons were not the first ones although referred to as such along with the Shadows and other races.
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Now to actually consider this topic seriously...
w.r.t. Von Neumann machines:
As far as we can tell, we're capable of interstellar travel --- although the 'we' that gets to the next star with any remotely habitable planets might be vastly different from the 'we' that begins the many-generation-long journey (unless we're in stasis during the trip). Could we expand fast enough to not be killed off when our sun burns out? If so, then it must be possible for a species to expand throughout the galaxy --- although that says nothing about the frequency of that sort of thing occurring; all it means is that it occurred at least once.
w.r.t. extraterrestrial life:
It could be that we're the only ones in our galaxy. Or perhaps, as the universe has been aging, the conditions for intelligent civilizations to form have been getting better? After all, the earth is 4.5 billions years old; that's a decent fraction of the age of the universe.
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Maybe the super-smart Von Neumann aliens have the Prime Directive and wont mess you up unless you crap in the well by waging interstellar war or something. Yeah, you know who I'm talking about.
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Ughhh, leave FreeSpace out of this.
Lol, Shivans + Trek term
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Fact: Humans are a type of von Neumann machine.
Fact: Humans could currently cause their own extinction.
Postulate: Other types of von Neumann machines could do the same.
Theory: Other types of von Neumann machines have caused their own extinction before contact.
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Possible. But it depends on them having existed in the first place.
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Fact: Humans are a type of von Neumann machine.
Fact: Humans could currently cause their own extinction.
Postulate: Other types of von Neumann machines could do the same.
Theory: Other types of von Neumann machines have caused their own extinction before contact.
Mutant strains should have been selected for by evolution and now be dominant.
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Mutant strains should have been selected for by evolution and now be dominant.
I don't know what that is, but it looks like some weird combination of verbiage, jargon, and alien syntax.
What's it mean?
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Mutant strains should have been selected for by evolution and now be dominant.
I don't know what that is, but it looks like some weird combination of verbiage, jargon, and alien syntax.
What's it mean?
It's evolution 101. Self-destructive berserker strains would have self-destructed. Mutants with better survival traits wouldn't have.
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What do you think the solution is?
Points for creativity in a non-goofy way.
Supplementary question in my next post.
It's because we've been looking for radio signals. Radio waves are slow and have a limited range. I think it's much more likely that whatever star faring civilization are out there are using something more advanced that we can't detect because we haven't discovered how to yet.
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It takes millions of years for complex life to achieve intelligence, but only a few thousand for them to destroy themselves. I think we just missed them.
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Maybe. Maybe not.
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Exactly. It took billions of years for complex life to evolve on earth and it was reset a couple times by mass extinctions and the like. We probably arent the first to emerge nor the last. I think its a statistical certainty that other life is out there.. but intelligent life like us is much more rare, may or may not be prone to self destruction, and is probably so far away they will only detect our radio waves long after we are either gone or developed the means to spread to other star systems. Hell they could be on the opposite end of the galaxy, in which case they couldnt detect us through traditional means even if they wanted to.
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Blindsight
Give it a little longer, the antibodies are coming.
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It's because we've been looking for radio signals. Radio waves are slow and have a limited range. I think it's much more likely that whatever star faring civilization are out there are using something more advanced that we can't detect because we haven't discovered how to yet.
:wtf:
They travel at the speed of light. Just like every other electromagnetic wave.
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And that's slow when we're talking possible hundreds of light-years to the nearest inhabited planet from Earth.
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And that's slow when we're talking possible hundreds of light-years to the nearest inhabited planet from Earth.
But still faster than anything but other electromagnetic waves, which are the same speed. Faster than anything else. It looked as if Kosh was attempting to say there was something better to look for that traveled faster than that.
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Like, what? Have tachyons become detectable suddenly?
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Only if you re-configure Voyager's deflector array. :p
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Mutant strains should have been selected for by evolution and now be dominant.
I don't know what that is, but it looks like some weird combination of verbiage, jargon, and alien syntax.
What's it mean?
It's evolution 101. Self-destructive berserker strains would have self-destructed. Mutants with better survival traits wouldn't have.
That presumes the "self-destruction" event isn't flamboyant enough to take said "mutant strains" out as well.
But honestly, I just don't think broadcast communication is going to be used for a very long period of time by any civilization because of gross energy inefficiency, regardless of the specific wavelengths used, regardless of even the medium used (gravitic versus electromagnetic etc.). Tightbeam transmissions, or even better, lasers, give you better bang for your buck over large distances.
Consider the wireless networks we are currently using and continue to deploy. They are only broadcast (and even that with a highly directional emitter) over the "last mile." Everywhere else, the communication is either over hard-line or again, highly directed emissions to and from satellites. The "last mile" transmissions aren't going to be detectable over the background noise of space for very far at all (I don't have exact numbers, someone could prove me wrong here). The communications to and from satellites are so directional that even if their power was sufficient to be detectable over RF background noise at interstellar distances (which I very much doubt), you would have to have a receiver at just the right spot. And, since the Earth is rotating and the satellite is orbiting, the signal is going to be rotating in direction.
So, I don't think we can infer too much from the fact that we haven't heard anything yet.
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Why do you think the galaxy has not been devoured by von Neumann machines, given that there's been time and opportunity for them to do so?
"there's been time for them to do so" --> most certainly true. The galaxy is several billion years old.
"there's been opportunity for them to do so" --> likely not true. To do so requires being able to do both interstellar travel and colonization, which probably are not as feasible as we'd like to believe. Otherwise we would expect to have the entire galaxy "devoured" already.
My guess is that it's "somewhat feasible" to travel to other star systems, but the actual "let's take this here planet and make it new home for us" is not. Life is very specially evolved to survive on its own planet. Didn't you see War of the Worlds? :P
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"there's been opportunity for them to do so" --> likely not true. To do so requires being able to do both interstellar travel and colonization, which probably are not as feasible as we'd like to believe. Otherwise we would expect to have the entire galaxy "devoured" already.
Neumanns do not (always) colonize, do not (always) terraform, and do not have anything to do with the life that created them.
Look up what a von Neumann probe is, as well as the berserker and seeder subsets.
Step 2 of your objection is therefore not a factor.
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http://en.wikipedia.org/wiki/Berserker_(Saberhagen)
:yes:
These make Mass Effect seem unoriginal :nod:
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Hmm, qwib-qwib are anti-Berserker Berserkers? I'm reminded of a certain quarian in ME2...
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Hmm, qwib-qwib are anti-Berserker Berserkers? I'm reminded of a certain quarian in ME2...
Whoa. Dude.
That's sweet.
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They travel at the speed of light. Just like every other electromagnetic wave.
Exactly, slow.
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Unless you're holding something faster out on us, they're as fast as we know. :p
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They travel at the speed of light. Just like every other electromagnetic wave.
Exactly, slow.
Nothing is faster. Fast.
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They travel at the speed of light. Just like every other electromagnetic wave.
Exactly, slow.
Nothing is faster. Fast.
Only one thing is faster. Here's video proof (http://www.youtube.com/watch?v=wI4tevra8Lg).
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I'm sure this is all totally related to Fermi's Paradox and everything.
'Fast' and 'slow' are relative terms. If you want to compare c to the speed of anything else, obviously it's fast. But on an astronomic scale, it's quite slow (takes 100000 years just to cross our galaxy, doesn't it?). Really, once you start dealing with magnitudes this large, 'fast' and 'slow' really don't mean much until they hold some relevance to the matter at hand.
Wait, why are we talking about this, again? :/
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Because Kosh was calling radio waves slow in relation to some as yet undisclosed or undetected signal we're supposed to be detecting.
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Well I get his point that they're slow when you consider the size of large scale structures in the universe. Ie, the speed of light makes for a woefully slow journey (or communication) between stars... let alone galaxies.
Neumanns do not (always) colonize, do not (always) terraform, and do not have anything to do with the life that created them.
Look up what a von Neumann probe is, as well as the berserker and seeder subsets.
True, I didn't consider that.
Dekker: :lol:
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For the moment there's absolutely nothing faster than C that doesn't involve causality violation, so lightspeed's all we've got.
Regardless, even at sublight speed, a single Neumann replicating at feasible speeds should have been able to hit every star in the galaxy by now.
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Assumig their creators would have developed intelligence, and applied it to building said machines instead of... i dunno a giant theme park or some sort of soup recepie.
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For the moment there's absolutely nothing faster than C that doesn't involve causality violation, so lightspeed's all we've got.
Regardless, even at sublight speed, a single Neumann replicating at feasible speeds should have been able to hit every star in the galaxy by now.
just because we haven't figured out a way around this doesn't mean some other intelligence hasn't, the fact that we have no idea where to even begin thinking about how to do this just shows that if someone else is using a technology like this then we would have not the slightest hope of detecting it.
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Because Kosh was calling radio waves slow in relation to some as yet undisclosed or undetected signal we're supposed to be detecting.
Exactly, there could be something out there we have yet to even discover, beyond our current perception or understanding!
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For the moment there's absolutely nothing faster than C that doesn't involve causality violation, so lightspeed's all we've got.
This is something I've never quite understood. What is this 'causality' that is somehow violated if something goes faster than light? Does this just mean that if something goes faster than light, that it just 'happens' without any visible cause?
Because if that's all 'causality' means, then "violating causality" doesn't seem like it would really mean anything. I mean, how is it different from how a supersonic jet isn't audible until the sound cone passes you (and if you were blind, you wouldn't be to tell it was coming)? It's not like it's going back in time.
@Wobble73: Fhtagn?
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For the moment there's absolutely nothing faster than C that doesn't involve causality violation, so lightspeed's all we've got.
This is something I've never quite understood. What is this 'causality' that is somehow violated if something goes faster than light? Does this just mean that if something goes faster than light, that it just 'happens' without any visible cause?
Because if that's all 'causality' means, then "violating causality" doesn't seem like it would really mean anything. I mean, how is it different from how a supersonic jet isn't audible until the sound cone passes you (and if you were blind, you wouldn't be to tell it was coming)? It's not like it's going back in time.
Indeed, you clearly have never quite understood this.
Causality violations occur when effect precedes cause.
Objects that move faster than light move backwards in time.
This would allow a faster-than-light ship to destroy itself while it was still being constructed.
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What I don't get is how moving faster than light is the same as moving back in time.
Something to do with the whole (1-v2/c2)0.5 ?
Waiiiiiiit... diagramming it. I'll get back to you :P
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For the moment there's absolutely nothing faster than C that doesn't involve causality violation, so lightspeed's all we've got.
This is something I've never quite understood. What is this 'causality' that is somehow violated if something goes faster than light? Does this just mean that if something goes faster than light, that it just 'happens' without any visible cause?
Because if that's all 'causality' means, then "violating causality" doesn't seem like it would really mean anything. I mean, how is it different from how a supersonic jet isn't audible until the sound cone passes you (and if you were blind, you wouldn't be to tell it was coming)? It's not like it's going back in time.
Indeed, you clearly have never quite understood this.
Causality violations occur when effect precedes cause.
Objects that move faster than light move backwards in time.
This would allow a ship to destroy itself while it was still being constructed.
And if it destroyed itself so that it couldn't be constructed then it couldn't destroy itself! There's the crux! It couldn't destroy itself therefore it wouldn't. It would have to exist outside it's own space-time continuum, once it had travelled faster than c. Then it wouldn't be able to ever get near it's past self.
Scuze me, I have been drinking Brandy and realise I really shouldn't be posting! LOL!
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What I don't get is how moving faster than light is the same as moving back in time.
Something to do with the whole (1-v2/c2)0.5 ?
Waiiiiiiit... diagramming it. I'll get back to you :P
If you are travelling faster than light you are doing things before anyone can perceive it, therefore you would be in the future when people in your present would be in the past.
Does remind of the joke that all Australians are actually 1 day ahead (nearly) than us in the UK! Therefore they are from the future! LOL!
EDIT** Sorry for the double post!
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What I don't get is how moving faster than light is the same as moving back in time.
Something to do with the whole (1-v2/c2)0.5 ?
Waiiiiiiit... diagramming it. I'll get back to you :P
You need to take a course on special relativity.
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Okay, it's like this.
At location A at time 0, an event happens. The effects of that event are transmitted to the rest of the universe at a maximum speed of c outward from A. An observer at location B can only know about the event after a time equal to the distance between A and B divided by c has passed.
If we now assume that the event is accompanied by an FTL pulse that B can detect, then B will know about the event before the light of the event has reached him. That means that B will be changed by the event before the event has happened in his context (context being defined as what B can observe using light speed methods only). As such, to someone observing B, it looks as if B reacts to the event before the event has happened, thus causality has been violated, as the effect comes before the cause.
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Another way is... it takes what 30-45 secs for transmissions from the moon to reach earth, thus the time lag in communication between Houston and a moon mission! If we could transmit and receive transmissions faster than light. Then that lag would disappear or at least become negligible.
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When the spacetime interval between two events is space-like (which characterizes events along an 'FTL' world-line), two different inertial reference frames may disagree not only on the time between the events (as they would in a timelike worldline), but also the order of the events.
This stands in contrast to two IRFs observing a normal, timelike worldline, wherein they may disagree on the time between two events, but never on the order of the events.
In a spacelike FTL worldline situation, IRF A may claim event 1 happened before event 2, but IRF B will claim that event 2 happened before event 1. Remember that both IRFs are equally valid.
The consequences of this are very problematic. To quote Wikipedia:
In the case of a hypothetical signal moving faster than light, there would always be some frames in which the signal was received before it was sent, so that the signal could be said to have moved backwards in time. And since one of the two fundamental postulates of special relativity says that the laws of physics should work the same way in every inertial frame, then if it is possible for signals to move backwards in time in any one frame, it must be possible in all frames. This means that if observer A sends a signal to observer B which moves FTL (faster than light) in A's frame but backwards in time in B's frame, and then B sends a reply which moves FTL in B's frame but backwards in time in A's frame, it could work out that A receives the reply before sending the original signal, a clear violation of causality in every frame.
To summarize: for a FTL (spacelike) worldline, there will always exist some IRFs which interpret the worldline as an object moving back in time. This leads to paradoxes which cannot occur, like signals being received before they are sent.
For an illustration of why this is bad, imagine that you have a radio signal you will send to detonate a bomb under your chair. You send the signal down a spacelike geometry, and it is received before it is sent, detonating the bomb and killing you. Yet this means you never sent the signal, because you are dead, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to 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hurk
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@The E: Yeah I get that much. But to me, that suggests that it doesn't matter if causality is violated. There's no paradoxes to be had.
Diagram I was making:
(http://i93.photobucket.com/albums/l77/Aardwolf001/Quick%20Illustrations/ftlspacetimeconesomething.png)
X = position (1-dimensional instead of 3, but whatever), Y = time (up is later)
Basically, to an observer (at the head of the light cone), the FTL object looks as though it has appeared and then split into two parts, moving in opposite directions. The one moving 'backwards' appears to be at positions that are getting farther away at a rate faster than the speed of light. The one moving 'forward' appears to be moving at the same speed (I think).
So what am I doing wrong? (And don't just say "not taking a course on it"; if there isn't a flaw in my logic, then whether or not I've taken a course on it is irrelevant).
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Reread my last post. Your logic is faulty.
Because you've never had any courses on the topic, you made the mistake of assuming that there existed a 'visibility cone', rather than understanding - as any SR graduate would - that the light cone represents a fundamental causal horizon.
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And thus parallel universes are born.
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Aardwolf, if you still don't get it, examine this diagram.
(http://upload.wikimedia.org/wikipedia/commons/3/3d/Minkowski_diagram_-_causality.png)
Take the origin as event 1.
Now, event 2 is going to occur somewhere on this diagram. If event 2 is on a slower-than-light, timelike worldline, it WILL fall somewhere in the absolute future: the white delta at the top of the graph. All spacetime events in this area will, one hundred percent guaranteed, occur after (or simultaneous with) event 1. However, the exact interval between event 1 and 2 is up for debate.
If event 2 falls in the spacelike grey zones, however - which any FTL worldline will - then the event does not have to occur in the future (above the X axis). It could fall into the past.
This is why FTL = time travel. In some IRFs, the object will not appear as traveling faster than light; instead, it will appear as if it is moving back in time.
From here causality violations are inevitable. And:
In the theory of general relativity, the concept of causality is generalized in the most straightforward way: the effect must belong to the future light cone of its cause, even if the spacetime is curved
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Hmm......The problem is, there is no easy way to simplify it down more than the wikipedia summary without losing important details.
The "light cone" analogy is slightly misleading. In special relativity, everything in an event's forward light cone will happen in the future. That is, it will be affected by the event. Everything outside that light cone is indeterminate, there is no way to tell if something happened before the event, after it, or at the same time. If you send an FTL transmission, you are sending something outside your light cone. That means that it is absolutely possible to send a message to a point that has already happened, thereby altering that point and the light cone it generates, affecting your own light cone.
EDIT: Here's some math explaining the tricky bits (for very mathematical values of "explaining"): http://en.wikipedia.org/wiki/Tachyonic_antitelephone
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you made the mistake of assuming that there existed a 'visibility cone', rather than understanding [...]1 that the light cone represents a fundamental causal horizon.
Wat. I know it's not just light, it's the normal propagation of all information, including gravity.
But... at any point in space-time (the observer), there should be some region of space-time which is 'visible' to that point. That is, photons emitted from within this region of space-time are arriving at the observer. The force of gravity from masses within this region should be affecting the observer. Charge and magnetism from objects in this region should be affecting the observer. In effect, I'm talking about the region of space-time characterized by the fact that the propagation of its information is immediately affecting the observer. And I'm pretty sure that under normal circumstances, this region is the same as the surface of the past light cone.
Hm... according to this diagram (http://upload.wikimedia.org/wikipedia/commons/thumb/1/16/World_line.svg/500px-World_line.svg.png), I should possibly be using the word 'hypersurface' instead of saying "region of space-time".
That said, if somehow you had an object moving with constant speed, and it's faster than the speed of light, then the curve through space-time traced by that object is a line. And that line is shallower than the slope of the cone, so as the cone moves through time, the intersection of the cone and the line goes from being the null set, to a single point, to two points which move hyperbolically farther apart in space, i.e. the distance between them would be something like c1 * (c2 + t2)-0.5
Update after seeing people have replied before I finished typing:
@Battuta: Old news. I still don't get what's bad about violating causality, other than that you've violated causality --- which is apparently bad, but I still don't get what's bad about it.
This is why FTL = time travel. In some IRFs, the object will not appear as traveling faster than light; instead, it will appear as if it is moving back in time.
An object moving back in time appears identical to, and for all intents and purposes is equivalent to an object moving forward in time in the opposite direction.
You've got to prove there would be paradoxes. Unless you can do that, the only bad thing about violating causality is that it looks really weird2.
1 I've taken the liberty of omitting that remark. It seemed like attribution error, if you ask me.
2 Also it confuses the **** out of people who were taught that it's impossible to do it :p
Note: personally I don't believe it's possible to go faster than light. But all of the arguments in favor of this seem to hinge on the fact that if causality could be violated, it would have XYZ paradoxical outcomes. Which doesn't make sense to me.
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Okay, back to the light cone analogy.
From your point of view, there are two light cones. One moving outward from you, representing things that are, with absolute certainty, in your future. Another one pointing toward you, containing everything that is definitely in your past.
Everything outside those two cones will either happen in your future, happen concurrently with you, or has happened in your past (although those will only affect you if the light cones of those events intersects with yours).
Now, if you are sending an FTL message from your current point of view ("message" including anything moving at FTL speeds), you are sending it effectively into that realm of undefinedness. Meaning that there is a very real possibility that your message will be received at a point that lies in your relative past. The reception of that message would alter the outcome of that event, thereby altering what will happen when the light cone of the event intersects with yours.
Going back to this Gedankenexperiment (http://en.wikipedia.org/wiki/Tachyonic_antitelephone), it can be shown mathematically that there are situations where it is possible to receive an answer to a message before the message that incited the answer was sent. Something like that would alter the sending of the message, thereby altering the response, ad nauseam et infinitum. There you have your paradox, because there is no way for an outside observer to tell what message was actually sent.
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@Battuta: Old news. I still don't get what's bad about violating causality, other than that you've violated causality --- which is apparently bad, but I still don't get what's bad about it.
This is why FTL = time travel. In some IRFs, the object will not appear as traveling faster than light; instead, it will appear as if it is moving back in time.
An object moving back in time appears identical to, and for all intents and purposes is equivalent to an object moving forward in time in the opposite direction.
You've got to prove there would be paradoxes. Unless you can do that, the only bad thing about violating causality is that it looks really weird2.
Wow, you're pulling a Marcov. It takes a special kind of stupid to ignore a huge post not just once but twice, the second time after it was pointed out to you.
I'll repost it for you:
To summarize: for a FTL (spacelike) worldline, there will always exist some IRFs which interpret the worldline as an object moving back in time. This leads to paradoxes which cannot occur, like signals being received before they are sent.
For an illustration of why this is bad, imagine that you have a radio signal you will send to detonate a bomb under your chair. You send the signal down a spacelike geometry, and it is received before it is sent, detonating the bomb and killing you. Yet this means you never sent the signal, because you are dead, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, 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never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal, so the bomb never detonates, allowing you to send the signal, detonating the bomb, killing you before you can send the signal
hurk
Now let me address a specific and particularly asinine misconception you have:
An object moving back in time appears identical to, and for all intents and purposes is equivalent to an object moving forward in time in the opposite direction.
Really. Is that so.
Why are you talking about objects instead of worldlines? Why are you ignoring the relative position of the ends of the object's worldline?
First off, in whose reference frame? The telltale sign of a relativity novice is their failure to identify reference frames. You keep assuming that there is a 'correct' IRF where things 'actually' happen.
In the above example, describe to me what you see from the standpoint of the hapless victim.
Before you ever touch the detonator, the explosives spontaneously detonate. You die. Just after that moment, a signal appears from the explosives, moves along the wire to your detonator, and enters the detonator, which is no longer there because it has exploded.
Yet the detonator
is the cause and the signal is the effect.
In the effect's worldline, the cause must precede the effect (otherwise the signal will never occur), yet here the signal's worldline crosses its own origin and prevents the origin event from ever occuring!
So from the standpoint of the (subluminal) signal, it should go like this:
The signal leaves the detonator (event 1), moves down a spacelike geometry, and arrives at the explosives (event 2), all in correct linear order. This is logically equivalent to the FTL ship blowing itself up while it's under construction: event 2 prevents event 1 from occurring.
And, again, to the hapless victim, event 2 precedes event 1, making event 1 impossible.
You're calling that for all intents and purposes identical?
If you accept that superluminal signals travel back in time, how do you avoid this paradox?
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In one universe you explode, in another nothing happens cos the signal transferred to another universe?
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In one universe you explode, in another nothing happens cos the signal transferred to another universe?
Well.....no. Your status is impossible to determine. Meaning that the Universe you occupy is inconsistent. Which could be bad.
Special relativity is not like Schrödinger's cat and the superimposed wave functions of Quantum theory.
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Y'know, just so you get it, I think I'll throw in a few more thought experiments just to demonstrate how trivial it is to come up with these on your own.
Which makes it all the more mind-boggling that you didn't, Aardwolf.
1. Construct a device that fires a signal down a spacelike worldline terminating back at the device 5 minutes before the firing event. The signal prevents the device from firing for 6 minutes.
PARADOX
2. You board an FTL spaceship and take a cruise, then land thirty years before you departed. You destroy the Earth with the ship's amazing future weapons.
PARADOX
3. You have an FTL accelerator that will accept billiard-ball-sized objects. The exit is placed near the entrance. You fire a billiard ball into the accelerator in such a way that when it emerges, earlier in time, it will knock its earlier self away before it enters the accelerator.
PARADOX
4. Build a machine that asks for a numerical input every ten minutes. The machine has a number 'in mind', say, 20. If the number it receives matches the number it has in mind, it adds 10. If it does not, it adds 20. After several failed attempts, you input the number 20 and get a match, so you deduce that the machine was thinking of the number '0' ten minutes ago. You signal yourself ten minutes earlier to enter 'zero' into the machine. But the fact that you did not receive that signal ten minutes earlier is the only reason you got it wrong at that time and got '20' correct at this time; otherwise the number would have been 10 now, not 20.
PARADOX
And now, here's the killer. The coup de gras, the piece d'resistance (my French is horrid.)
The Superluminal Barn Paradox. (http://www.brool.com/index.php/the-barn-paradox-and-superluminal-communication)
In this beautiful thought experiment you quite clearly get two different and mutually incompatible outcomes in two different reference frames.
In one universe you explode, in another nothing happens cos the signal transferred to another universe?
Well.....no. Your status is impossible to determine. Meaning that the Universe you occupy is inconsistent. Which could be bad.
Special relativity is not like Schrödinger's cat and the superimposed wave functions of Quantum theory.
Yep, like The_E said, this is not QM, it's special relativity. The two have yet to be fully resolved.
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Okay... I have read the posts and still don't understand this whole faster than light=back in time thing. How is it any different from seeing lightning before you hear thunder? I remember that if you accelerated something to FTL by conventional means that it would go back in time, because you need to "steal" velocity from the time dimension in order to move in the spatial ones. But that is impossible anyway because an object reaches infinite mass at the speed of light and cannot be further accelerated. So any method of FTL will be disregarding the speed/mass limit, and likely the speed/time limit as well.
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Okay... I have read the posts and still don't understand this whole faster than light=back in time thing. How is it any different from seeing lightning before you hear thunder? I remember that if you accelerated something to FTL by conventional means that it would go back in time, because you need to "steal" velocity from the time dimension in order to move in the spatial ones. But that is impossible anyway because an object reaches infinite mass at the speed of light and cannot be further accelerated. So any method of FTL will be disregarding the speed/mass limit, and likely the speed/time limit as well.
Again:
Any object moving on a superluminal, spacelike world-line will be seen as moving backwards in time by some IRFs.
And because the laws of physics must be equivalent in all IRFs, then this means that if a signal can move backwards in any one frame, it has to be able to do so in every frame.
So if observer A fires an FTL signal, but observer B sees this signal as coming back from the future, but then B fires an FTL signal back and A sees it as coming back from the future, ultimately A has received a reply to his own signal before ever sending it.
And if the signal is a trigger for a bomb under A's seat...you've got a problem.
To address the thunder/lightning example: both the thunder signal and the lightning signal emanate from event A and propagate with different speeds in your IRF. But you and every other observer, no matter what the gap between the thunder and lightning, will agree that the lightning flash preceded the thunder.
Metaphorically speaking, superluminal travel creates a situation where some observers will hear the thunder before they see the lightning, and others will see the lightning before they hear the thunder. And that's a really big problem, because it means the laws of physics are working differently in different reference frames, and that causes all of physics to disintegrate. (And if you say 'but why' to that, for god's sake, re-read the above posts.)
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... Bah, I thought I had a relatively good grasp of physics, but this just is't lining up, and I'm beggining to get a headache from working it over. So I'll just trust that people who have spent a lot more time than me thinking about this stuff know what they are talking about.
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Okay... I have read the posts and still don't understand this whole faster than light=back in time thing. How is it any different from seeing lightning before you hear thunder? I remember that if you accelerated something to FTL by conventional means that it would go back in time, because you need to "steal" velocity from the time dimension in order to move in the spatial ones. But that is impossible anyway because an object reaches infinite mass at the speed of light and cannot be further accelerated. So any method of FTL will be disregarding the speed/mass limit, and likely the speed/time limit as well.
The problem is that it's hard to find analogies for this behaviour that do not involve FTL travel, and decades of FTL in sci fi that treat FTL as being just really, really fast.
The easiest analogy to understand is the light cone one. Two cones, both centered on a single point. One cone includes everything that definitely happens after the event the point represents and everything that definitely happens concurrently with the event represented by the point. The other one includes everything that definitely happened before the event represented by the point. Everything that is outside these two cones may happen at the same time, before, or after the event. It's impossible to determine.
Now, the naive model, the one lazy SF writers use, makes both the "future" and "past" cone infinitely large, meaning that there is an absolutely reliable global causality chain. FTL travel is possible, the Enterprise can reach Alpha Centauri in an hour, and Captain Kirk can romance the hotties.
Special relativity makes this a bit more complicated. In special relativity, if something moves faster than light, it breaks out of the light cone that is caused by the start of the FTL object. This means that it enters a realm of uncertainty, where it is possible to affect things that lie in the past of the "starting" event. Once that happens, the "past" light cone of the starting event is irreversibly altered, which will affect the starting event in some way. And then, a paradox is created, because it becomes impossible to determine just what happened in the past light cone, since it gets overwritten over and over again by altered versions of the FTL object.
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What I don't get is how moving faster than light is the same as moving back in time.
Something to do with the whole (1-v2/c2)0.5 ?
Waiiiiiiit... diagramming it. I'll get back to you :P
I finally walked Aardwolf through the math on this, and I think he gets it.
I did a few Lorentz transformations to demonstrate that a 5xC signal being transmitted and bounced back to/from a ship receding at 3/5C would return to the sender 5 minutes or so before it was first transmitted.
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After a drawn out conversation on IRC, and then after me trying to describe it in writing, ... I'm confused. Bleh.
Ooo, an animation!
(http://upload.wikimedia.org/wikipedia/commons/e/e4/Lorentz_transform_of_world_line.gif)
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Now let me address a specific and particularly asinine misconception you have:
An object moving back in time appears identical to, and for all intents and purposes is equivalent to an object moving forward in time in the opposite direction.
Really. Is that so.
Why are you talking about objects instead of worldlines? Why are you ignoring the relative position of the ends of the object's worldline?
First off, in whose reference frame? The telltale sign of a relativity novice is their failure to identify reference frames. You keep assuming that there is a 'correct' IRF where things 'actually' happen.
In the above example, describe to me what you see from the standpoint of the hapless victim.
Before you ever touch the detonator, the explosives spontaneously detonate. You die. Just after that moment, a signal appears from the explosives, moves along the wire to your detonator, and enters the detonator, which is no longer there because it has exploded.
Yet the detonator
is the cause and the signal is the effect.
The biggest thing that has always confused me about this is best summed up in one question: Why does perception have to line up with the order of events?
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Whose perception?
The reason it's a problem for effect to precede cause has nothing to do with anybody's perception.
It has to do with the fact that the effect (explosion) may prevent the cause (detonator sending signal; billiard ball entering wormhole) from ever occurring.
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Okay, back to the bomb example.
Person A triggers the detonation of a bomb at Site B using a transmission that is faster than light. Person A is caught in the explosion. Person B observes the explosion from a different Site C.
Obviously, the explosion happens. To Person A, an amount of time, however miniscule, will still pass between the trigger and detonation. To person B, however, it seems as if the explosion occurs first, and the transmission moves backwards, as in your example. However, obviously the transmission occurred, or the explosion would not have taken place, and to Person A, that's also what the perception is.
I started off with a good question for the end of this, but I can't remember it right now. So I'll ask a different one. Why does it matter what Person B's perception of events is?
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Perception, in this case, is everything. Using the original example of the chair and the detonator that is shut down by the FTL pulse, the bomb "perceives" the detonation signal. It reacts (by exploding), sending the FTL pulse to the detonator. The Detonator "perceives" the FTL pulse, and shuts down. But since it's shut down, it doesn't send the detonation signal, which means the FTL pulse isn't sent out, which means the detonator works and sends the detonation signal, etc etc.
If you perceive something, you are changed by it. That is (one of) the basic rule(s) here.
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But the detonator has to perceive sending the signal before the FTL pulse arrives.
It may still be shut down by the perception of the FTL pulse, but the detonation has already occurred.
I don't get it, obviously, and I doubt I will. It just seems so contradictory. I can accept that an object can't be accelerated to c or past it, but the rest just confuses me.
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Okay, back to the bomb example.
Person A triggers the detonation of a bomb at Site B using a transmission that is faster than light. Person A is caught in the explosion. Person B observes the explosion from a different Site C.
Obviously, the explosion happens. To Person A, an amount of time, however miniscule, will still pass between the trigger and detonation. To person B, however, it seems as if the explosion occurs first, and the transmission moves backwards, as in your example. However, obviously the transmission occurred, or the explosion would not have taken place, and to Person A, that's also what the perception is.
I started off with a good question for the end of this, but I can't remember it right now. So I'll ask a different one. Why does it matter what Person B's perception of events is?
You got something wrong at a very early step in this example.
I'll see if you can spot what it is wrong in this passage:
Person A triggers the detonation of a bomb at Site B using a transmission that is faster than light. Person A is caught in the explosion. Person B observes the explosion from a different Site C.
Obviously, the explosion happens. To Person A, an amount of time, however miniscule, will still pass between the trigger and detonation.
What is the period of time that passes between Person A pressing the detonator and perceiving the blast? You assert that it is 'an amount of time, however miniscule'. What's wrong with that assertion?
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Okay, once more. Special relativity states that any form of faster than light travel can be functionally equivalent to time travel. Meaning that the traveling object may arrive at its destination before it has left.
If it does, there is a chance that that arrival may interfere with the launch of the FTL object. The Bomb/Detonator example is one such possibility; The Spaceship travelling away from Earth communicating via FTL is another (in that example, an Astronaut aboard the ship sends a message containing a question back to base. Someone on Earth answers. Under certain conditions, the answer may arrive at the starship before the Astronaut sends his message to Earth).
In both cases, the arrival of the FTL message changes the conditions for sending the FTL message in the first place (The detonator shuts down before sending the detonation pulse, the Astronaut receives an answer before asking the question, thereby possibly changing the question). Both scenarios create infinite loops of paradoxa, where it becomes impossible to determine what the exact order of events is. This, it is assumed, is bad.
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So yeah, Fermi's paradox.
I reckon Francis Drake screwed up. That, or the statistical anomaly isn't that we exist, but that in our galaxy, nobody but us exists.
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I'm surprised no one has linked this yet. (http://xkcd.com/384/)
All this FTL discussion is interesting given that my dad posited a rather flawed proposition for FTL communication only this morning, which I promptly shot down, then told him to reconfigure the deflector array to emit an inverse tachyon pulse.
Anyway, Fermi's paradox- I think the alt-text for the comic has some value.
Firstly, the amount of EM communication we could actually detect is inversely proportional to the square of the distance between Sol and the other civilization's system- if it's on the other side of the galaxy entirely, it'd be completely indistinguishable from background noise.
Second, what range of frequencies do we actually look for? Another species might use higher-intensity or even lower-intensity frequencies than those we're listening on.
Thirdly, how exactly would we determine whether something is a 'message' or not? We may be hearing some civilization's Arecibo Message right now and interpreting it as random noise, because they encode it in a way that seems totally alien to us (because, well, it is).
(I'd actually like some sources for the second and third points, since I'm really just guessing that we don't listen on every frequency from radio to gamma rays, and an answer to the second question would be quite interesting.)
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The higher the frequency of the radiation, the less likely it is that a civilization communicates with it, because it requires more energy and effort to create and direct that communication. Additionally, longer wavelengths are more capable of penetrating the interstellar medium, which tends to scatter shorter wavelengths (just like how our atmosphere scatters bluer light and not so much red light). For these reasons, radio waves are the most logical ones to look for.
However, there is also the possibility of interstellar communication via lasers, whether they function in visible light or otherwise.
Edit: This is also a good point for trying to find/contact other civilizations, because lasers are extremely rare in nature.
Aardwolf: I think Drake was just a bit too optimistic with the variables he chose. However, it is still *highly* unlikely that we're the only life in the galaxy. We know it didn't take long for life to show up on earth once conditions were favorable (and there's some evidence that life started on multiple/separate occasions). We also know that there are hundreds of thousands of low-mass stars out there, which can potentially support habitable planets for many billions of years. So that's one hell of a statistical anomaly for us to be the only life in the galaxy. The existence of other "communicative" civilizations, however, is much more uncertain. Afterall, we only have our own species to go on, and we've been a communicative civilization for less than 100 years.
unrelated edit: Heh, spellchecker wants to replace your name with "Adolf". I chuckled a little. I'm a terrible person.
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Tell your spell-checker to get a ****ing dictionary. Seriously, the only reason I came up with this name is because I was looking at some of the first non-abbreviation words in the dictionary.
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I'd just like to point out that the Drake equation postdates Fermi's paradox by about ten years and, while it is an attempt to attack Fermi's paradox, is not the root of the problem or the only way to look at it.
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Tell your spell-checker to get a ****ing dictionary. Seriously, the only reason I came up with this name is because I was looking at some of the first non-abbreviation words in the dictionary.
Aggressive much? It's not my spellchecker, and I wasn't attacking your name. Calm yourself.
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Tell your spell-checker to get a ****ing dictionary. Seriously, the only reason I came up with this name is because I was looking at some of the first non-abbreviation words in the dictionary.
Aggressive much? It's not my spellchecker, and I wasn't attacking your name. Calm yourself.
He's very sensitive about his name.
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No, I just forgot to use a smiley :p
@Shivan Hunter: I was pretty sure what it was going to be, upon reading "I'm surprised no one has linked this yet." Heheh.
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I hate to bump this off topic again, but, this whole FTL signals travelling back in time has got me in a brainlock.
I get 100% how FTL things going back in time would cause paradoxes, but I still don't understand how those things can actually make it.
With the cone of light examples, I don't get how anybody can see things that are in the future or in the past and would be able to tell them apart. I can only see things that are actually happenning. I can remember things in the past, or I can predict things in the future based on things I am observing in the present, but I can't actually SEE them, because I'm not a Tralfamadorian.
Here's the scenario I'm working off of:
Person A has an FTL radio, and Person B is exactly one light-minute away with the other FTL radio. For simplicity's sake, lets say that FTL is only 200% of c.
It's 8:00 pm when Person A calls Person B. The signal arrives at 8:00:30 (on watch A, but he doesn't know that), and Person B responds with a signal. Person A receives that signal at 8:01 by his own watch.
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Person A has an FTL radio, and Person B is exactly one light-minute away with the other FTL radio. For simplicity's sake, lets say that FTL is only 200% of c.
It's 8:00 pm when Person A calls Person B. The signal arrives at 8:00:30 (on watch A, but he doesn't know that), and Person B responds with a signal. Person A receives that signal at 8:01 by his own watch.
Here's the full math: http://en.wikipedia.org/wiki/Tachyonic_antitelephone
The actual speeds involved are very important here, not only the exact speed at which the signal moves, but also the relative movement of A and B. In this example, (if I did the math right) it means that A would have to be moving away from B at a speed greater than .8c. The faster the signal gets, the easier this effect happens, if the signal travels at 200c, the relative velocity of A and B would only have to be greater than 0.08c.
EDIT: If we take Voyager's assertion that Warp 9.9 equals roughly 21000c as true, the ship would cause paradoxa (meaning, arrive at it's destination before leaving) if the her starting and arrival points are moving away from each other faster than 9.524x10^-5c or greater (Or, in other words, 28.5 km/second, or roughly 100 million km/h, which translates to about 64 million miles per hour for our non-metric friends). While that may sound like much, it's also far below the canonical sublight speeds Star Trek vessels are capable of.
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????it. Math, my old nemesis. I had hoped we wouldn't meet again.
Seriously, I was trying to avoid weird math with variables and junk. For the sake of greater simplicity, how about A and B stay in exactly the same position relative to each other and the universe (really hard, I know). They are exactly one light-minute apart when A transmits, and still exactly one light-minute apart when A gets a response.
Oh, wait. You're saying that the signal only goes back in time if the two parts are moving relative to each other and the signal is moving significantly faster than c? So in my example with 0 relative motion, it really would come back at 8:01?
EDIT: Discussion page on the tachyonic thing linked to this: http://xkcd.com/660/
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Yes, what I'm saying is that as the speed of the signal increases, the relative speed at which A and B have to move away from each other decreases. Make the signal fast enough, and the relative speeds will stop to matter, as long as A and B move away from each other. If they move towards each other, it doesn't work (I think, not sure about that right now).
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Here's the scenario I'm working off of:
Person A has an FTL radio, and Person B is exactly one light-minute away with the other FTL radio. For simplicity's sake, lets say that FTL is only 200% of c.
It's 8:00 pm when Person A calls Person B. The signal arrives at 8:00:30 (on watch A, but he doesn't know that), and Person B responds with a signal. Person A receives that signal at 8:01 by his own watch.
There's an understandable error here. You used a simple Newtonian coordinate transformation to get from Person A to Person B. This only holds within their shared reference frame.
Specifically, you assumed that a signal traveling at 2xC would cross a distance of one light minute in thirty seconds. It could then be returned at 2xC to reach the sender one minute after it sent.
And this is true - within the reference frame of Person A (Alice?) and Person B (Bob).
But you're using Newtonian transformations, which don't reflect the way the universe actually works. Instead, you need to use a Lorentz transformation between two reference frames.
Now, you're correct to say that this shouldn't matter assuming that Alice and Bob are sending signals to each other, stationary, in the same reference frame. Keep this in mind: Alice and Bob share a common reference frame since there is no velocity difference between them.
But there's a problem.
What does an observer driving past at 3/5 lightspeed, moving away from Alice and towards Bob, see?
To this observer, Alice is receding at 3/5 lightspeed, and Bob is approaching at 3/5 lightspeed.
Let's say our observer, Charlie, sees Alice fire the 2xC signal at time 0 from coordinate 0 (we'll treat the line between Alice and Bob as a one-dimensional axis.)
In Alice's reference frame, the signal reaches Bob 30 seconds after she fires it. The coordinates of the reception, Event B, are, in Alice and Bob's shared reference frame, t = 1/2 minute, x = 1 lmin (since Bob is one light-minute away from Alice, and Alice is at x = 0 lmin.)
We can say that Event B, the signal reception, has coordinates 1/2, 1 in Alice and Bob's reference frame. Since the coordinates of Event A, the signal transmission, are 0,0, Alice and Bob agree that the reception occurred after the transmission. t = 1/2 is clearly after t = 1, right?
But what are the coordinates of the reception event to Charlie?
We need to use the Lorentz transformation to get the time of the event in Charlie's reference frame. Behold:
t' = γ(t - ux/c^2)
Gamma is going to be the Lorentz transformation constant. I'll just tell you that right here, it's 5/4.
We can treat the speed of light as '1' since that's what we're using as our velocity unit.
T = .5 minutes, the time coordinate of event B, the signal reception
U = 3/5, the speed differential between Charlie and Bob when Bob receives the signal (i.e. when event B occurs)
X = 1, since that's Bob's coordinates when event B occurs.
Work it out and we get t', the time of event B in Charlie's speeding reference frame, to be:
t' = -0.125
Remember that our time unit is in minutes, so we transform that to seconds and get...
t' = -7.5 seconds.
(The time coordinate of the original event at 0,0 transformed into Charlie's reference frame is still t = 0; I worked out the transformation to make sure.)
In your example, Alice fires a superluminal single to Bob at a speed of 2xC. It covers the distance of one light-minute in 30 seconds, and Bob and Alice both agree the signal reaches Bob 30 seconds after transmission.
But to Charlie, flying between them on a 3/5 C rocket...
...the signal reaches Bob 7.5 seconds before it is ever sent by Alice.
And remember, all reference frames are equally valid. There is nothing to say that Alice and Bob's shared reference frame is the correct one, since there is no single correct reference frame.
The upshot of this is that, for any given superluminal signal, an observer in a different reference frame can interpret that superluminal motion as motion back in time.
As a little demonstration, by the way, just change the value of T in the above equation from .5 to 1, indicating that it took the signal a full light-minute to reach Bob and therefore was only traveling at C instead of 2xC.
In Charlie's speeding reference frame, the time coordinate of the reception becomes
5/4*(1-(3/5*1)/1^2) = .5, which is perfectly sensible since it occurs after the transmission event which happens at t = 0 for Charlie.
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What General B didn't mention, because it was already mentioned several times, is that if something is valid in one reference frame, it has to be valid in every reference frame.
Let the time of the signal being sent be t. In Alice's and Bob's shared reference frame, if they compare their notes, they will arrive at the conclusion that the signal was sent at t and received at time t+x. Since this is valid in that reference frame, it has to be valid in all reference frames. But Charlie, spoilsport that he is, says "Hey, wait a minute! You guys are wrong! The signal was sent at t, and received at t-7.5 and I have the math to prove it!".
So, in conclusion: In Alice' and Bob's reference frame, Alice sends the signal, and Bob receives it some time later, which has to be true in every reference frame.
In Charlies' reference frame however, Bob receives the signal 7.5 seconds before Alice sends it, which again, needs to be true for every reference frame.
Now, the basic point here is that the order of events has to be preserved globally. In all reference frames, Alice has to send the message before anyone (including Bob) can receive it. But since Charlie disagrees, the reference frames can not be reconciled, and paradoxa appear.
A more complete summary why this has to be this way can be found here: http://en.wikipedia.org/wiki/Inertial_frame_of_reference
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Specifically, while it's okay for reference frames to disagree on how long passes between two events, they must agree on the order of the events.
Event A must precede Event B in all reference frames or causality is violated. When the signals go superluminal, order is no longer preserved in all subluminal reference frames.
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Thanks, Battuta. That long post helped me to reconcile the math quite a bit.
So now my questions are:
1. Doesn't it only appear to violate causality if Charlie is observing things at the speed of light? If Alice sent two signals, one to Bob and one to Charlie, wouldn't Charlie then get notification of sending before observing or being notified of receipt by Bob (Assuming he doesn't pass Bob before the signal would arrive)?
2. Does the scenario placing the effect before the cause still apply if Charlie is only moving at 2/3c?
3. Why does it matter if one observer sees causality violated? We see the effects of things before the causes all the time, like a bullet impact being seen before the gunshot is heard. If the effect arrives faster than the cause, what does it matter if anybody thinks it's backwards? In other words, just because you observe an effect before a cause, why should it indicate that time has flip-flopped, wouldn't it make more sense that your observation is just slow?
Extra credit: And doesn't this negate the example of the superluminal billiard ball, and the self exploding bomb paradoxes? Since the bomb and detonator occupy the same frame of reference, and the billiard ball carries it's frame with it.
EDIT: Remember, please explain as if I couldn't do the math myself if I wanted to am a total higher-math noob.
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Okay, I tried to do that, but people seem to be ignoring my posts here.
You see, SoA, the problem here is that according to the theory of special relativity, there are infinitely many reference frames in the universe. If something is true in one of these frames, it must be true in every reference frame, because the laws of physics are the same in every one of them.
The message that travels faster than light, through a combination of the actual message speed and the relative speed of the reference frames involved, travels back in time in one reference frame (Charlie's) and appears to be moving normally in another. Since according to the rules of special relativity, if something happens one way in one reference frame it needs to happen that way in every reference frame, a paradox is created, since there is one reference frame (Charlie's) that sees things differently.
So, in more general terms. If an event A happens that causes another event B to happen, every observer, regardless of where he is or how fast he is moving, needs to see A before B. If he doesn't, something is very, very wrong with the universe.
And yes, some of the examples General B chose were not examples of paradoxa caused by FTL travel, but rather examples that demonstrate temporal paradoxa in general.
And, well, the problem is that it's hard to explain this adequately without resorting to Math, so just take it as truth from those of us who can do the math, at least until someone proves that special relativity is wrong.
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And, well, the problem is that it's hard to explain this adequately without resorting to Math, so just take it as truth from those of us who can do the math, at least until someone proves that special relativity is wrong.
There. That! That's exactly what I needed.
So. My understanding is: FTL travel and the current theory of special relativity cannot coexist. And since as far as anybody can tell, special relativity is true, that makes FTL impossible.
Now I can sleep at night again, thanks.
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1. Doesn't it only appear to violate causality if Charlie is observing things at the speed of light? If Alice sent two signals, one to Bob and one to Charlie, wouldn't Charlie then get notification of sending before observing or being notified of receipt by Bob (Assuming he doesn't pass Bob before the signal would arrive)?
Well...the use of 'appear to violate causality' is a bit deceptive here. You're assuming that one reference frame describes how the events really happens, and another (Charlie's) describes how the events appear to happen. In this model, some people just see the 'appearance' of an event, like the way a distant observer can think that a flash of lightning precedes a blast of thunder, even though they both come from the same event.
It doesn't really work that way.
First off, just take a glance at the second postulate of relativity.
As measured in any inertial frame of reference, light is always propagated in empty space with a definite velocity c that is independent of the state of motion of the emitting body.
Imagine that you and I are going to have a duel. To have a fair duel, we need a signal to draw and fire at the exact same moment. We know that the speed of light is always measured the same in all reference frames, so we decide to use a flash of light from a pile of gunpowder exploding to signal the start of our duel.
Now here's the key. Our duel is going to happen on a moving train car. We are inside the car, at opposite ends, and the gunpowder is in the middle. We are equidistant from it.
We have brought in a judge, The_E, to ensure that our duel is fair.
As our train car rushes past a station, the gunpowder goes off, light is emitted, and it moves away from the pile of gunpowder towards both of us at the speed of light. We are equidistant from the gunpowder, and since the speed of light is a constant, the flash of light reaches us each at the same time.
The_E declares that the duel is fair, since he concludes that the flash of light reached each of us simultaneously. (Think of the light reaching you as Event A, and the light reaching me as Event B.)
However, JeffVader is standing on the train platform that we passed just as the gunpowder went off. Now, from his perspective, one of us (the one at the back of the train car) is moving into the light emitted from the gunpowder...and the other one (the one at the front) is moving away.
Because, remember, JeffVader measures the speed of light to be a constant! Which means the speed of the light emitted from the pile of gunpowder is not affected by the movement of the pile of gunpowder...which, to him, is moving along with the train, even though to us it was stationary.
As a result, he declares that the person moving towards the light, drawn along by the motion of the train, sees it first, and the person moving away saw it second. He declares the duel unfair!
Who is correct? The_E, who is aboard the train car with us, or JeffVader, on the platform? The_E claims the duel was fair, JeffVader doesn't, one of them has to be correct, no?
Right?
Wrong. They are both correct. Both of their versions of reality are equally true. It is equally valid to say that the duel was fair as to say that it was unfair, depending on your reference frame.
And this means that Charlie's description of what happens in the signaling scenario is, unfortunately, just as valid as Alice and Bob's.
Which leads to something very important: the first postulate of relativity.
The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems of coordinates in uniform translatory motion.
This means that if you can demonstrate that a superluminal signal can move back in time in just one reference frame, it can move back in time in all reference frames.
So if Charlie sees that signal as moving back in time, then his perception can hold true for any reference frame, and causality violations can occur to any observer. (I would struggle to do the math to prove this, unfortunately.)
On to question #2.
2. Does the scenario placing the effect before the cause still apply if Charlie is only moving at 2/3c?
Redoing the math quickly:
t' = 5/4(.5 - (2/3)*1/1)
Yes. Charlie will place event B as occuring about 12 seconds before event A.
Reduce to 1/3 C and Charlie will again see event B after event A, however.
Remember, though, what holds in one reference frame must hold in all, so really we only need to demonstrate time travel in one IRF.
3. Why does it matter if one observer sees causality violated? We see the effects of things before the causes all the time, like a bullet impact being seen before the gunshot is heard. If the effect arrives faster than the cause, what does it matter if anybody thinks it's backwards? In other words, just because you observe an effect before a cause, why should it indicate that time has flip-flopped, wouldn't it make more sense that your observation is just slow?
See the First Postulate of Relativity above. The laws of physics hold equally in all reference frames. If they didn't, then you would have one privileged reference frame that was 'more true' than others, and you'd be right back to Newton.
The analogy you brought up is a useful one, however.
When you see a bullet impact before the gunshot, you are actually describing two different events: event A is bullet arrival and event B is sound arrival. Both of these propagate from event C, which was bullet fired.
All observers will always agree that event A precedes event B, because the bullet is faster than the sound. No matter what, everyone will agree on this order. If you stood sufficiently close to event C you might see events A and B as very nearly simultaneous.
But you will never, ever, ever see event B occur before event A (assuming constant bullet and sound velocity.) The sound will never reach you before the supersonic bullet.
Observations matter when they're based on physical law.
And here's why, put in a very (I hope) simple nutshell.
If two events take place in space-time in such a way that a beam of light could travel between them, then all observers will agree on the order of the events.
If two events take place in such a way that a beam of light could not possibly have traveled between them, something very special happens.
These events could not possibly be causally connected. Nothing goes faster than light.
This means that observers can safely disagree on the ordering of the events.
But a superluminal signal could causally connect the events.
Imagine that we take the Alice/Charlie/Bob problem above. Alice has a bomb under her chair. She sends the detonation signal for the bomb to Bob via a 2xC signal, as above. Bob sends it back, to the bomb under Alice's chair. The bomb blows up, killing poor suicidal Alice.
Everything seems fine.
But for Charlie, cruising past at 3/5C - who, remember, will see the signal as reaching Bob many seconds before it is ever sent...Bob will receive the detonation signal before it is sent, and send it back to Alice and the bomb even earlier, thus detonating the bomb before Alice sends the detonation signal in the first place. It's a paradox!
You might ask why this is paradoxical. Surely Charlie's perceptions don't matter.
But remember, Charlie's reference frame is equally valid. The laws of physics have to be the same in all reference frames. The fact that Alice and Bob are stationary with respect to each other doesn't give them special privilege to be right!
Put differently: how can Charlie possibly exist in a universe where a bomb detonates for no reason? Bombs don't detonate for no reason. How can Charlie's reference frame be correct if he sees Alice blow up (event B) but never witnesses event A (Alice sending the signal) because Alice has already exploded? That universe just can't exist, it makes no physical sense.
Which leads nicely to the extra credit...
Extra credit: And doesn't this negate the example of the superluminal billiard ball, and the self exploding bomb paradoxes? Since the bomb and detonator occupy the same frame of reference, and the billiard ball carries it's frame with it.
Ah, but that frame is not privileged as the 'correct' one. What happens in one frame must make physical sense in all frames.
If something makes sense in one frame, but does not make sense in others, it is physically impossible, because the laws of physics must hold equally across all frames.
To explode, the bomb must receive a detonation signal. If there exists a reference frame wherein the bomb could have exploded before it ever received the detonation signal, thereby preventing the signal from being sent, then the whole system is impossible and paradoxical.
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Post updated a bit. If you were in the middle of reading, re-read the second-to-last section. I made it clearer why there's a bomb-under-chair paradox by linking it to the earlier Alice/Bob/Charlie scenario we worked out the math for.
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Well, my point with the extra credit was that because all events were centered within the same frame, that all other frames would be able to agree that it happened that way. If the signal to detonate the bomb had to be sent and bounced off Bob in order to detonate, then that would be at least two seperate frames involved and there you'd have your paradox.
PRE-EDIT: Your edit nicely summed up what I was about to say here, but I'll say it anyway.
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Well, my point with the extra credit was that because all events were centered within the same frame, that all other frames would be able to agree that it happened that way. If the signal to detonate the bomb had to be sent and bounced off Charlie in order to detonate, then that would be at least two seperate frames involved and there you'd have your paradox.
PRE-EDIT: Your edit nicely summed up what I was about to say here, but I'll say it anyway.
Wait, the signal's being bounced off Bob, who is in Alice's reference frame.
You could treat Bob as a radio repeater Alice is holding in her hand. They still share a reference frame, all events are still centered in the same frame, but the paradox still occurs in Charlie's IRF as he cruises past.
You're right to say that you can easily construct a reference frame with no time travel where no paradox occurs. But so long as you can construct a reference frame where a paradox does occur, the whole system is doomed to paradox, since the laws of physics have to be the same across all IRFs.
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Wait, the signal's being bounced off Bob, who is in Alice's reference frame.
Yeah, sorry, I edited Charlie to Bob as soon as I could, but I guess I was too slow.
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Wait, the signal's being bounced off Bob, who is in Alice's reference frame.
Yeah, sorry, I edited Charlie to Bob as soon as I could, but I guess I was too slow.
Well, the point still stands: even if all the events are 'centered in one frame', namely, Alice is sending a superluminal signal directly to a bomb beneath her chair, Charlie will still see that signal as going back in time and causing a paradox.
You don't have to introduce the Bob party or the radio relay to cause a paradox. A one-way signal to the bomb is enough.
Remember, there are an infinite number of possible observers for any given system, at an infinite number of possible distances and relative speeds (er, I think) - meaning an infinite number of reference frames who can observe the system. You only need one of them to see a paradox. And in any system with an FTL signal, at least one will see a paradox.
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Ok, got it. My scenario was flawed, I wasn't considering the trigger and bomb to be two seperate IRFs.
The self smacking billiard ball is nonsense though, right?
EDIT lol: Thanks for your patience Batty, E.
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The self smacking billiard ball is nonsense though, right?
No, it's not. If FTL travel is possible (for anything), then a reference frame exists in which the order of events is different than the "actual" order of events. The presence of an actual observer is not required to create a paradox.
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Ok, got it. My scenario was flawed, I wasn't considering the trigger and bomb to be two seperate IRFs.
The trigger and the bomb aren't two separate IRFs - so long as they're at relative rest, they're in the same IRF.
The problem is the superluminal signal between the trigger and the bomb. Remember Charlie? We worked out math that demonstrated that he'd see a signal from Alice to Bob arriving before it was ever sent. (And remember, Alice and Bob shared an IRF there. All objects at relative rest are in the same IRF - like you and your computer right now.)
Call the trigger Alice and the bomb Bob and you have the exact same paradox. Even if the detonator is sitting right on top of the bomb, at relative rest, in the same IRF.
The self smacking billiard ball is nonsense though, right?
Not at all. Say the billiard ball is fired into the mouth of a wormhole and moves down the wormhole at superluminal speeds. We can now consider the ball a 'signal', moving, perhaps, at 2xC. The wormhole has an exit at Bob's position.
From here, you can see that Charlie, driving past at 3/5C, sees the ball (the signal) reach Bob a few seconds before it entered the mouth of the wormhole.
Now, let's say Bob immediately fires the ball back into another wormhole, going the opposite direction...Charlie sees the ball emerge from that wormhole before it's ever fired into it, and in fact, before the ball is fired into the first wormhole.
In fact, it just so happens that the ball emerges from the mouth of the return wormhole in such a way that it knocks its earlier self away before entering the first wormhole.
Now we got a problem!
EDIT lol: Thanks for your patience Batty, E.
Actually, this is really fun. I've only had hazy intuitions on a lot of these topics, and this discussion has really helped me learn this stuff and get it down.
They say teaching something is the best way to understand it.
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Yes, this is one of the most fun threads in a very long time. Way better than religion or politics.
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So, Scourge of Ages, you're right that you can't have a superluminal transmitter paradox within a single IRF in which all the elements of the system are at rest with respect to each other.
But the law of the universe is that a given system has to be valid in every possible observer's IRF.
All the problems in the above situations are created by the fact that Charlie's perceptions of the order of events differ as he cruises past at 3/5C. And you've always got to pretend that there's a virtual Charlie watching your system.
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So between any two points, if there's FTL anything moving between them, then there's a Charlie somewhere that won't agree that anything happens how it's happenning.
In the billiard ball, Alice is the front of the ball, and Bob is the back (or vice versa), and Charlie is somewhere in between. Can that make sense?
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So between any two points, if there's FTL anything moving between them, then there's a Charlie somewhere that won't agree that anything happens how it's happenning.
Exactly. I wish I'd put it that way on page 1. :shaking:
Specifically, that hypothetical Charlie will see the FTL signal/object arriving before it departs.
In the billiard ball, Alice is the front of the ball, and Bob is the back (or vice versa), and Charlie is somewhere in between. Can that make sense?
Uh...I dunno. You may have blown my mind.
I need to go to the gym!
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Okay, one more thing.
In the Lorentz transformation, what happens if the FTL signal travels instantaneously? Wouldn't that break the equation in a divide-by-zero sort of way? And would that mean that in that instant, everything shares a common IRF, and therefore no paradox?
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No, not really. It would be an "infinity divided by infinity" error (which most definitely is NOT 1). The problem is that instantaneous travel is really, really impossible, as it would mean that the traveling object is travelling at an infinite speed.
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Thought it'd be fair game as long as we were dealing with hypotheticals anyway. Oh well, at least I got the other part down-ish.
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The problem is that that hypothetical cannot be dealt with using the math describing this problem (one of the variables involved is the multiple of the speed of light the signal travels at, if it is set to infinite, we get a division of infinity by infinity, which is indefinite).
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Okay, one more thing.
In the Lorentz transformation, what happens if the FTL signal travels instantaneously? Wouldn't that break the equation in a divide-by-zero sort of way? And would that mean that in that instant, everything shares a common IRF, and therefore no paradox?
Actually...though my first instinct was to go with The_E's response...
It looks like instantaneous travel time is often commonly assumed when working problems like this. See here. (http://www.theculture.org/rich/sharpblue/archives/000089.html)
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Yes, from a logical standpoint, you can use infinite speed. But the math (at least, the math I have seen so far, and of course, my interpretation of it) doesn't work if you set the speed of the signal to infinite, for the very simple reason that infinity divided by infinity is not defined.
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You are probably very correct.
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Interesting stuff! :)
I'd like to ask question that's probably based on false logic and thinking about it too hard would probably make one's head implode, but how would this work in the context of a rapidly rotating black hole, specifically in the ergosphere, which (if I understand correctly), is a region of spacetime that is rotating around the black hole at a velocity faster than the speed of light?
Specifically, let's imagine Anna, who is in a spaceship within the ergosphere, sending signals out in all directions. Let's assume her spaceship is immune to the tidal forces of the black hole (reasonable if the black hole is very large), and the ships engines are powerful enough to maintain its distance from the center of the hole (also reasonable as it's outside the event horizon, but the ship cannot maintain a *fixed* position and thus must orbit the hole).
Let's say there is an observer, Bob, who is far away from the hole and maintains a fixed position relative to it. Bob is monitoring Anna's signals.
Now I have a lot of difficulty trying to visualize this, but doesn't the "FTL" velocity of the ergosphere mean that some of Anna's signals would be essentially travelling backward in time relative to Bob (from some reference frames)? At the very least I imagine some funky time-dilation effects would be seen but I'm just not sure what they would be like. If anyone can better analyze this scenario, feedback would be greatly appreciated.
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Okay, after a cursory reading of wikipedia on the subject, I'd have to say that due to the weirdness of Black Holes in general, nothing special happens.
Explanation: Objects in the Ergosphere are not actually moving at FTL speeds. Their reference frames are just distorted by the distorted spacetime in the ergosphere, they are dragged around to follow the black hole's rotation. As a result, since Anna's signals are not travelling at FTL speeds inside her reference frame, and since they are not actually accelerated to FTL speed by the process of frame-dragging that is happening, no paradoxical behaviour occurs, since the signals leave the ergosphere with a speed of c or lower.
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I'd imagine it to be rather like group velocity where something can appear to be propagating at FTL speeds but it's not something that can carry information and thus break causality.