[Dragon]

I found that in most relativistic cases, it's more convenient to think in terms of momentum than mass. Strictly speaking, photons are massless, but there's more to momentum than just mass - relativistic equation for it gives you a mass-independent component of it. It makes things a bit easier to understand. Momentum also can increase indefinitely, is conserved and generally behaves much more neatly than either mass (which leads to this "mass increases with speed" hogwash you occasionally hear) or velocity (which has trouble with c "speed limit"). It also clears up things like Oberth effect, which are really darn hard to explain. Photon drive really does work like a newtonian reaction drive with an Isp of c (at least at low velocities), and if you consider momenta involved, it's pretty obvious why.

[Bobboau]

one of the ideas for how this thing works (jumping to the assumption that it does in fact work and we haven't just had good/bad luck with error or some sort of yet undiscovered systematic error) is that it pushes on the quantum vacuum virtual plasma, but it also seems to act sort of like an alcubierre drive. crazy thought(cause that's what we get with this sort of thing), what if the quantum vacuum virtual plasma basically IS spacetime and by moving the virtual particles you are moving the spacetime along with them? might this be the key to figuring out quantum gravity? maybe warps in spacetime are equivalent to suppression of virtual particles, maybe gravity is caused by normal matter suppressing the formation of virtual particles or something like that. I can barely handwave the terms, this is way beyond my level of education in this subject.

[Herra Tohtori]

And no, if you use mass-energy equivalence to get a mass flow from that 300MW per Newton figure you gave, then calculate an exhaust velocity from that you find that, as I said initially, a photon drive works exactly as you'd expect a Newtonian reaction engine with an exhaust velocity of c to.

Not... exactly, but sort of, yes.

The problem with this interpretation is the age-old question of whether photons truly have "mass" or not. They certainly have momentum, and they have energy, so you can characterize the "exhaust" in terms of those, but when you calculate a mass-energy equivalency for finding the mass flow for the "exhaust" of photons, things become a bit dodgy.


True enough, the general thrust of exhaust gets the formula F = dm/dt * v, but that doesn't need to mean it must always be expressed that way. Mass flow just doesn't make a lot of sense when you're talking about massless particles. After all, that formula is just a specialized form of the general definition of thrust (F = dp/dt) and the momentum is separated into mass flow and ejection velocity simply because that makes sense in a situation where there is a mass flow and thrust is dependent on both the mass flow and ejection velocity.


Just because "Newtonian physics" happens to end up to a "correct" solution in a situation that is thoroughly beyond the realms of Newtonian physics doesn't mean it's the best way to handle or visualize the situation.

[watsisname]

Strictly speaking, photons are massless, but there's more to momentum than just mass - relativistic equation for it gives you a mass-independent component of it.

This is a good point.  Everyone knows the formula E=mc², but this only applies to massive particles which are at rest.  The general formula is actually E² = p²c²+m²c⁴.  Right away you can see that massless particles have energy E=pc as Herra showed.  And for massive particles which are not moving, it simplifies to E=mc².

It also clears up things like Oberth effect, which are really darn hard to explain.

Actually no, the Oberth effect has nothing to do with this -- it is explained by classical mechanics without relativity.  If you tried to explain it with relativistic effects, your prediction for its strength will be orders of magnitude too small.  You really only need the definition of work to understand it, or definition of total energy (gravitational potential plus kinetic) if involving a gravitational field.  To think about it, consider a rocket which is held stationary while burning.  In this case, the work done on the rocket by the fuel is zero, by definition, since it is moving through zero distance.  All of the work is done on the propellant -- the propellant gains all the energy from combustion.  If the rocket is moving, then more work is done on it because it moves through a greater distance.  This is at the cost of putting less energy into the propellant -- it is left behind with less kinetic energy. 

@Herra:  Great posts.  I don't think it's likely that it has anything to do with dark matter (the properties of dark matter just don't really fit the bill), but dark energy might make sense, given that it's apparently a property of space-time pertaining to vacuum fields.  If the quantum thruster works by interacting (somehow?) with the vacuum energy, then that's... crazy.

I've also heard a lot of speculation that the drive might be operating via general relativistic effects, of the sort involved with Mach's Principle.  Mach's principle is the notion that the local standard of "rest" (the locally inertial frame) is determined by fields produced by the large-scale distribution of matter and energy in the universe.  Perhaps the drive is (somehow?) interacting with those fields, thereby exchanging momentum with matter "at a distance".  Which reminds of quantum entanglement sort of, and I can't help but think it can't possibly work that way because of causality issues.  Then again, this thing pretty much completely defies our understanding of basic physics...  we haven't seen such a confounding experimental result since Michelson-Morley over 100 years ago.

[Luis Dias]

So this is a real thing and you guys are all buying it? Hmm. That is interesting.

[Phantom Hoover]

The problem with this interpretation is the age-old question of whether photons truly have "mass" or not.

No, this is irrelevant. A Newtonian model of a rocket that shoots potatoes out the back at 3*10^8m/s behaves the same way as the relativistic model of a photon drive for the purposes of things like delta-v for a given mass fraction.

[watsisname]

I actually think it will end up having a mundane explanation, but whatever it is probably isn't straightforward or you'd think someone would have figured it out already.

[Bobboau]

So this is a real thing and you guys are all buying it? Hmm. That is interesting.

well, so far it's shown to actually behave more or less as advertised in experimental setups, and more than one team has repeated the results. Reality is the true measure of reality, so... so far it looks like it's real even though we have no idea how it works. There is still a (much) better than not chance we'll find that it doesn't work, but so far we haven't. and that's neat.

[headdie]

Be interesting to see what happens when we get to the point of throwing one of these into orbit because like Bobboau says until it works in reality it dosnt work... but yer this is an interesting time for the space sciences.

[Phantom Hoover]

i have a feeling there will be people calling it measurement error even if it gets mounted on a spacecraft and used to propel it, though

[Bobboau]

i have a feeling there will be people calling it measurement error even if it gets mounted on a spacecraft and used to propel it, though

>the probe seems to have reached 0.77c
>m... margin of error!

[Herra Tohtori]

I actually think it will end up having a mundane explanation, but whatever it is probably isn't straightforward or you'd think someone would have figured it out already.

Pretty much, that's what my pessimist/realist side expects.

But if it's real, it's real. Honestly I'm waiting for the peer reviewed publications, full size engineering testbeds on orbital conditions, and the eventual Nobel prizes until I believe it. But so far, so good...

A Newtonian model of a rocket that shoots potatoes out the back at 3*10^8m/s behaves the same way as the relativistic model of a photon drive for the purposes of things like delta-v for a given mass fraction.

Yeah, and it turns out that the Newtonian model is wrong at relativistic speeds and with massless objects, so why should that be used in any case?


Just because the model happens to produce the right answer doesn't mean it's correct. You can't just apply a Newtonian model to a non-newtonian situation and be satisfied by the fact that it appears to be working, and assume that nothing has changed. It's even worse of an idea to use that to "explain" something to someone with a layperson's understanding of physics. It's just going to mislead them and the next thing you get is armchair physicists posing thought experiments that have the underlying structure of "if we ignore the laws of physics, what will happen according to the laws of physics".


I could just as easily say that under Newtonian physics, spotlight rocket's exhaust mass flow is zero because photons have zero mass so the entire thing has zero thrust, and be done with it.

[Phantom Hoover]

I was mentioning it as an illustrative analogy because calculating it myself had given me a better grasp on the quantities involved by relating it to a situation I understand quite intuitively. You still don't seem to have realised that I know all of the basic facts you keep spouting and trying to correct me on them is a fool's errand.

[Herra Tohtori]

Look, I understand that you have a sufficient understanding about the subject to make analogies without assuming that they are 1:1 equivalent.


What I disagree with you about is this particular point being a good analogy.


My point is that this particular apparent point of unity between relativity and Newtonian physics is completely artificial, an artefact of how the equations are set up and happen to produce same results in certain special cases.

I think it is misleading to compare the thrust of EM radiation to a thrust of imaginary Newtonian lightspeed potato gun. While you have enough understanding of the physics to "play around" with the equations, for a layperson it creates an illusion that Newtonian physics can be used in contexts where it should not be used.

In some cases, it is acceptable to say that a more basic theory can work as a subset of more advanced theory in specific conditions. Good example of this is use of Newtonian mechanics as a special case of special relativity, when v<<c (practically v<0.1c is sufficient for most applications). This is not the case here.


In other words, you can get the correct result for the wrong reasons, and that can reinforce incorrect paradigms about physical reality. While this exercise may have helped you to look at it from a different perspective, I don't think it is a good "example" to make for someone who doesn't have necessary level of knowledge and understanding of physics to already know how the actual thing works.

[watsisname]

There are very specific cases where Newtonian formulas give exactly the right answer for a relativistic scenario, but purely by mathematical coincidence.  It's like the idiom "a broken clock is right twice a day".  Another example is that you can calculate the radius of the event horizon of a black hole from the Newtonian formula for escape speed (setting it equal to c).  But it works for totally the wrong reasons.  Newtonian physics suggests that photons emitted from the horizon go off to infinity.  They don't.  It suggests that stuff thrown upward from just inside the horizon can, at least briefly, pass through it.  Never happens.  Newtonian physics even suggests that particles falling into the hole will always miss the singularity.  Not only do they hit the singularity, it is impossible to avoid it.  I agree with Herra that it's not a good idea to explain -- even for the sake of just obtaining numbers -- relativistic phenomena with Newtonian arguments.  Even if you understand the relativity, you risk causing confusion for those who don't.  I can't tell you how many times I've seen people try to calculate relativistic stuff with Newtonian formulas and get the answer wrong by orders of magnitude!  And if you do understand the relativity, then why not use it to get the right answers for the right reasons?

Seriously though, let's move on to talking about the cool stuff again.  Vacuum fields and virtual particles and wtf is up with this experiment?

[Bobboau]

thoughts about this maybe giving us some sort of insight into quantum gravity?

[Herra Tohtori]

I concur, the cool stuff is much cooler.

For example, are vacuum fields - or in fact fields in general - actual physical entities? Or just mathematical models for things?


And what implications would it have if the vacuum field turns out to be something you can "push" against? Is there some way to measure velocity relative to this vacuum? Does it act as some sort of universal reference frame? What happens if you put two EM-drives against each other end to end so they're dealing with each other's "exhaust" flows??

[Phantom Hoover]

I agree with Herra that it's not a good idea to explain -- even for the sake of just obtaining numbers -- relativistic phenomena with Newtonian arguments.

Good thing that wasn't what I was doing!

[Bobboau]

no one cares who was doing what. no one is going to remember or care about who won this argument about semantics an hour after it's settled or an hour from now.

what happens if you put two em drives pushing against each other? does the space between them become 'less virtual' or something?

also, what are some of the arguments against mutability of the quantum foam (assume I am going to get some terminology slightly wrong)? I've heard things like because protons are the same no matter when they were made proves that the virtual plasma must be immutable. doesn't that just imply that there is a certain respect in which it has not changed? not that there is no way to change it, or that there are other ways in which it could and often does change?

[Black Wolf]

As I understand it - and I could be way, way wrong here - the argument goes that you can't push against the virtual particles in the vacuum because empty space ought to be at the absolute lowest energy state possible. If its possible to extract kinetic energy out of it, then that throws that idea right or the window.

Personally, I'm super skeptical about this, for the whole extraordinary claims, extraordinary evidence thing - we all remember the whole "faster than light neutrinos" debacle. But I'm also really, really hoping that this is legit, because it seems like it would be the first major flaw in physics since the discovery of the acceleration of the expansion of the universe. And this one also related to inexplicable kinetic energy. I'm hoping that the more data we get that disagrees with the current paradigm, the better the chances of someone coming along who can shift that paradigm. These experiments might just be the missing corner piece of the puzzle that lets somebody bust open the dark matter/energy problems. Very exciting... with that great big caveat of scepticism, of course.