Quantum Setback For Warp Drives

KentuckyFC writes "Warp drives were generally considered impossible by mainstream scientists until 1994 when the physicist Michael Alcubierre worked out how to build a faster-than-light drive using the principles of general relativity. His thinking was that while relativity prevents faster-than-light travel relative to the fabric of spacetime, it places no restriction on the speed at which regions of spacetime may move relative to each other. So a small bubble of spacetime containing a spacecraft could travel faster than the speed of light, at least in principle. But one unanswered question was what happens to the bubble when quantum mechanics is taken into account. Now, a team of physicists have worked it out, and it's bad news: the bubble becomes unstable at superluminal speeds, making warp drives impossible (probably)."

Comments

Longer lifetimes is the answer

[tjstork]

The SCI-FI buff in me holds out hope that physics will uncover a trick to FTL, but...

It doesn't really matter if we cannot travel faster than the speed of light so long as we can live long enough to get there.

Who cares if it takes 50 years to fly to Alpha Centauri if we can engineer ourselves to live for a thousand!

Re:Longer lifetimes is the answer

[Zordak]

No, it's time dilation and space contraction. If you're traveling at 0.9c relative to earth, your gamma is something like .44. So if you travel one light year, an observer on earth will see you go one light year in about 1.1 years. But from your perspective, you will have traveled only about .44 light years, and it would take something like .48 years. If you travel fast enough, you can reach even distant stars in very short times from your perspective. But you won't get a nice, tidy Galactic Federation, because people on earth will be getting very old very fast. That's the real problem with relativity. It's not that you can't get somewhere fast. Tell me where you want to go and how fast you want to get there, and we can calculate how fast you need to go (relative to the earth) to make it in that time, and it will be less than c.

In other words, we could (in theory) colonize all of the habitable planets in the galaxy in a fairly short time. But the colonies would all basically be cut off from each other. Even sending a radio message to another colony would take prohibitively long. And forget about "rescue" or "supply" ships.

Causality

[QuoteMstr]

Faster-than-light travel always causes causality paradoxes, so a priori, FTL drives are impossible unless special relativity is wrong. (That's is a bit like saying that perpetual motion machines are impossible unless thermodynamics is wrong.) The proposed mechanism behind the FTL drive doesn't matter -- it'll still cause a time paradox.

Just like we know any proposed perpetual motion machine must have a flaw, any proposed FTL drive must also have a flaw. They belong to the same class of impossible device, and deserve the same degree of consideration.

Re:Causality

[delt0r]

There have been some papers that even survived peer review on possible resolutions to this. But this is by far the biggest stake in FTL heart. Ironically this is not the biggest problem with the Alcubierre drive. Negative mass energy being one of them.

IIRC Einstein said they GR and SR may be proven wrong, but that the laws of entropy will never be broken (ie entropy is always getting bigger). I would aggree with this. ie FTL is less sci fi than "vacuum energy" or anti inertia drives.

But if I were a betting man, I would bet on light speed as the ultimate speed limit of the universe.

Re:Causality

[JerryLove]

I'm not sure perpetual motion is, strictly speaking, impossible.

Thermodynamics doesn't seem to preclude 100% efficiency, allowing motion in perpituity. Some real-universe examples:

Light on the fringes of the universe will continue travelling forever (unless we assume something new to stop it).

The electron on an atom that never falls into a star, black hole, or the like will forever circle the nucleus.

Heck: the atom itself will never stop moving.

Nor, best as we can tell, will the universe. It will be in motion perpetually (I suppose unless it all disintegrates into Hawking radiation, but then *that* will be in motion.

There are two problems with perpetual motion machines. One is the false math that you can derive infinate energy from one. That's not true at all. You could derive exactly the energy put into one.

The second is 100% effeciency, which is required for perpetual motion to obey thermodynamics, is not possible in what we would likely call "a machine"

Quantum mechancs+General relativity incompatible?

[Vellmont]

I thought we knew that combining these two theories resulted in answers we know to be nonsense. So the implication is one or both of them are wrong in some way. So I'm a little confused why we should trust results based on the combination of two theories that don't work together.

Granted I'm just a laymen, but does anyone else want to comment about the intersection of these two theories?

Re:Proof!

[geckipede]

This isn't anything new, it's an old idea being analysed more rigorously with quantum mechanics.

The problem is that in order to have a region of spacetime moving in relation to the outside universe, space has to expand behind it and contract in front, which demands negative and positive gravity in those regions. You need a large negative mass held in place in front of you, and a large positive mass behind. (We'll leave aside the problem that nobody has demonstrated the existence of negative mass, I personally don't believe it could exist precisely because it would enable FTL, but that's seperate to this point.) What you have to achieve is to have the centre of gravitation of the two masses at the centre of the edges of distortion. It means inevitably that half of the negative mass you are using has to stick out of the bubble ahead of you into normal unwarped space, and so that in order to keep generating the field ahead of you, it has to travel faster than light in its local frame. That is strictly not allowed.

Re:Quantum mechancs+General relativity incompatibl

[JustinOpinion]

First of all it's worth remembering that quantum mechanics and relativity are not 100% incompatible. In fact "relativistic quantum mechanics" has been around for a long time. Quantum theory was greatly advanced when relativistic effects were included.

But you're right that we have good reason to believe that something is wrong with either quantum mechanics or relativity (or both), since they give contradictory predictions in a certain number of extreme cases. (Quantum gravity is not yet solved...)

However we also have ample evidence that quantum mechanics and relativity are incredibly accurate and predictive theories in a vast range of circumstances. We have every reason to believe that the correct "Theory of Everything" will reduce to conventional quantum mechanics and conventional relativity in the appropriate limits. And thus we have every reason to continue using those theories to make predictions all over the place.

Now a warp bubble is one of those extreme situations where the two theories might be expected to give contradictory results, in which case only the hypothetical theory-of-everything would give the correct answers. But it is certainly still useful to ask what our current theories would predict for these extreme situations. It helps us better understand the theories. And, again, we have reasons to believe that many of the things our current theories predict (even in extreme situations) will be right. Absent the theory-of-everything, quantum mechanics + relativity will give us the "best guess" about how such objects would behave

Re:Mod parent up

[mgv]

Yeah but you need a massive amount of fuel to accelerate to C and then slow down again. About 40,000 times the size of the shuttle's boosters.

Perhaps this is why, despite our best efforts, no other civilization has contacted us. It's simply too hard to bridge the huuuuge gap between the stars.

Yes, if I was going to build a universe with all sorts of playthings in it, I'd probably separate the experiments with enough spacetime that when the odd experiment blows up it doesn't really affect any others around it.

Not that I think that the universe was actually designed, but if it was, that would be how I would do it.

Michael