[stryker]

I feel like a lot of people here misunderstand the speed of light as a cosmic speed limit. The speed of light as a speed limit is a local constraint. On a larger scale there is no such speed limit. Just because you travel faster than light on a non-local scale does not mean you break causality.

For example, light "travels" more slowly through water than vacuum. Well, actually, photons travel at c always -- it's just that light gets bounced around by water molecules, getting absorbed and reabsorbed many many times before arriving at the destination. It's like driving to a destination as opposed to running to it. If the roads are curvy enough, the runner will win because he is not constrained by roads. Could it be that the Alcubierre drive gets past the roads of empty space in a way light cannot?

For a better example, consider the expansion of the universe. Link: http://curious.astro.cornell.edu/question.php?number=575. I quote:

The fact that galaxies we see now are moving away from us faster than the speed of light has some bleak consequences, however. Astronomers now have strong evidence that we live in an "accelerating universe," which means that the speed of each individual galaxy with respect to us will increase as time goes on. If we assume that this acceleration continues indefinitely, then galaxies which are currently moving away from us faster than the speed of light will always be moving away from us faster than the speed of light and will eventually reach a point where the space between us and them is stretching so rapidly that any light they emit after that point will never be able to reach us.

In other words, vacuum fills in between us and far away galaxies faster than light can traverse it. Again, locally, the derivative of position with respect to time is c, but the universe is able to "cheat" by messing around with the definition of position. If the Alcubierre drive messes with the fabric of the universe in a similar way, I don't see any reason why it cannot succeed.

Here's a related link: http://curious.astro.cornell.edu/question.php?number=56. I quote:

Technically speaking, the speed of light limit only applies when you are in an "inertial frame" -- that is, sitting where you are, without any forces acting on you, and measuring the speed of an object that moves past a ruler and clock that you are holding in your hand. Across the large distances in the universe, however, we have a very different set of circumstances. No one is in an inertial frame, because everyone is being accelerated with respect to everyone else, due to the universe's gravitational field and the fact that the universe is expanding. In effect, the universe's expansion isn't really due to galaxies moving "through space" away from each other, but rather due to the stretching of space itself, which isn't governed by the same limits that we are.

I'm not sure whether the Alcubierre drive contradicts causality, but if it does, I highly doubt that it is because of the arguments presented on this forum.

[nessus42]

I'm not sure whether the Alcubierre drive contradicts causality, but if it does, I highly doubt that it is because of the arguments presented on this forum.

The Alcubierre drive most certainly does violate causality, and this follows from just the most basic amount of Special Relativity that you might learn if only you'd spend your time learning real things, rather than spreading disinformation.

[stryker]

I'm going to ignore your condescension. I'm also going to assume you didn't read the rest of my post, because you presented no arguments to counter my points.

I'd like to point out that the wikipedia article on the Alcubierre drive writes at length to show how this warping process does not violate causality: http://en.wikipedia.org/wiki/Alcubierre_drive. I know you feel strongly about people not understanding special relativity and I actually agree with you on that point, but you're not addressing any of the valid counterpoints that people are making. You're simply closing your ears and exclaiming "special relativity" which only applies locally (if you want an appeal to authority, I talked about this with a former physicist who worked with Stephen Hawking).

1) The universe is expanding faster than the speed of light. In other words, galaxies are moving away from us faster than c (evidenced by the growing redshift of light emitted by galaxies when they were NOT traveling away from us faster than c). That does not violate causality (as I mentioned in my post) because the topology of space itself is changing. Locally, everything still obeys special relativity.

2) Virtual particles, EPR Paradox (I know it's not actually a paradox). (http://math.ucr.edu/home/baez/physics/Quantum/virtual_partic...) My point with these phenomena is that locality and the speed of light are not very intuitive concepts. You can still have "spooky action at a distance" without violating causality.

[nessus42]

You're simply closing your ears and exclaiming "special relativity" which only applies locally (if you want an appeal to authority, I talked about this with a former physicist who worked with Stephen Hawking).

If you come up with a mathematical "proof" and I can use its conclusion to prove that 1 + 1 = 3, then I have shown your proof to be incorrect via contradiction, no matter how hard you continue to argue for it.

Special Relativity does not only apply locally. It applies to any system where information can be transmitted between two points that are in inertial frames.

The universe expanding at faster than the speed of light is neither here nor there, since you cannot use the expanding universe to transmit information between two points in at FTL speeds. On the other hand, you CAN use an FTL drive to transmit information between two points in inertial frames at FTL speeds.

QM spooky action at a distance does not let you transmit information between two points at FTL speeds.

This is all you need to do to show that any FTL scheme violates Special Relativity: Information transmitted between two points in inertial frames at FTL speeds. If you can do that, you've either disproven causality, or you've disproven Special Relativity.

[stryker]

I agree with your assessment, but you're still missing my point. Following your analogy, what I'm saying is not that 1+1=3, but that we're not really using the plus operator here.

I think we both agree that wormholes do not violate causality. By creating a wormhole, you are connecting two points in space with a shortcut. As a result, traveling through a wormhole is not even FTL -- there are two paths to the destination and the shorter path is so much shorter that you can beat light that goes on the longer path. You would still lose to light if light also took the shortcut.

That's essentially what I'm arguing -- that the Alcubierre drive somehow warps space so that if you're traveling in one, you're not actually going faster than light, but through a makeshift wormhole. It isn't FTL.

Highly unlikely to be realistic, but the premise still obeys causality.

[nessus42]

Wormholes most certainly do allow you to violate causality!

How about I just point you at the FTL FAQ?

http://www.physicsguy.com/ftl/html/FTL_part4.html

[stryker]

Can I talk to you offline? How can I message you on this forum?

I feel like I'm missing something. The FAQ defines FTL as "getting to a location faster than light" but what if light is taking a suboptimal path? Assuming wormholes exist, depending on your frame of reference, it would take light different times to get to the destination.

The FAQ says to consider the case of someone shooting a bullet through a wormhole and killing someone faster than light could have transmitted that information through the long way. I don't see how it's a problem. The light would have also traveled through the wormhole and it would have been possible to know that information. I guess my point is, with the wormhole, the "correct" distance between A and B is no longer the long way.

The way it sounds to me, it's as if I shot a bullet straight at someone at 0.99999c and it killed him, but now you're saying that violates causality because of this star that is between you and the victim and it would have taken light longer to traverse around the star than my bullet.

[cygx]

Your reasoning is flawed. The argument against the Alcubierre drive cannot stem from special relativity as it would only apply if there were a violation of local causality.

The problem is that using an Alcubierre drive, one could construct closed timelike curves, which are impossible in a 3+1-dimensional, asymptotically flat and singularity-free spacetime where the weak energy condition holds.

Also keep in mind that FTL is less problematic than many physicists think, even in special relativity - see eg 'Tachyon Kinematics and Causality: A Systematic Thorough Analysis of the Tachyon Causal Paradoxes' by Recami.

[nessus42]

My reasoning is not flawed. And the paper you pointed me on re tachyons, specifically states that we cannot use tachyons to transmit information via them.

I never stated that nothing can travel faster than light. Only that WE cannot travel faster than light, nor can we transmit information faster than light, without violating causality.

On the other hand, maybe we can violate causality.