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Warp Drive May Be More Feasible Than Thought, Scientists Say

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Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#141

Post by jloughry »

nullc wrote: post #82
Ygg2 wrote: post #15

Earlier quoted context omitted.

Seeing how if we use our warp drives to 'colonize' space, other creatures would have far more to fear from us than we from them (think Independence day with roles reversed). If a space faring species encounters non-space faring species, the one with warp is probably (not necessary) in the better position. Even without the weapons, FTL drive implies a magnitudes greater technology than anything a non-FTL race would ha…

> If a space faring species encounters non-space faring species, the one with warp is probably (not necessary) in the better position. I read a fun science fiction novella once about a interstellar space faring species that invented their warpdrive whatchamacallit before their industrial revolution, no electricity, machines, etc. Flying medieval space galleons. And they were going about successfully conquering a bunc…

'The Road Not Taken' by Harry Turtledove.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#142

Post by stryker »

nessus42 wrote: post #140
stryker wrote: post #124

Earlier quoted context omitted.

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 specia…

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.…

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.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#143

Post by enraged_camel »

teamonkey wrote: post #135
nessus42 wrote: post #109

Earlier quoted context omitted.

Special Relativity applies ANY TIME that you transmit information from one point A in an inertial frame to another point B in an inertial frame. If you can use your warp drive to transmit information from A to B, then Special Relativity applies. If you cannot use your warp drive to transmit information from A to B (e.g., from Earth to Mars), then what good is it? (I've mentioned several times in this thread that you…

> Special Relativity applies ANY TIME that you transmit information from one point A in an inertial frame to another point B in an inertial frame. Not true. It also implies uniformity of space-time in all inertial frames and the space between them. Since the theoretical purpose of the warp drive is to create a non-uniformity by definition SR does not apply.

Yeah, pretty much. I'm actually amazed that so many people (nessus included - as he seems to be the most vocal poster on this thread) do not know this.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#144

Post by bluekeybox »

bluekeybox wrote: post #114
jxcole wrote: post #84

Earlier quoted context omitted.

A lot of people responding to this thread do not understand relativity. The OP is correct, the fundamental problem is causality, relativity and FTL do not mix. The common way of saying it is that you can have two of the three: FTL, causality, or relativity. You cannot have all three. Here is a very nice explanation of why: http://thebestforumever.com/community/threads/tachyon-pistol... Also, I've never understood how…

The link had a really interesting but unsatisfactory explanation, IMO. The way I am understanding it is as follows: Man A and Man B are traveling (with combined speed of 0.866c) away from each other. Because of time dilation, to A it appears that B's clock is twice slower (1/sqrt(1 - 0.866^2) == 2). Then A shoots from some sort of weird space gun whose bullet is instantaneous. The bullet passes B at 4 seconds (relati…

For anyone reading this, the referenced explanation finally did make sense after I drew a Minkowski diagram of the two duelists. I guess this means that I finally understood special relativity... Brief summary: FTL would indeed violate causality, unless a special frame of reference is proposed (this violates special relativity) or a specific new property of Universe is established such that it would prevent certain events from occurring (aka Novikov conjecture) -- and we don't have evidence for any of the two being true.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#145

Post by michael_nielsen »

Steuard wrote: post #128
nessus42 wrote: post #35

You can't have faster than light travel without violations of causality. And while the equations of General Relatively allow for universes where causality is routinely violated, the probability that we are in such a universe seems vanishingly small. E.g., just because you can violate causality does not mean that you can go back in time and kill your grandfather. If you were to try, something would stop you, as a univ…

[Background note: I'm a physics professor with a background in string theory and relativity.] You're raising a sensible point, and most of the objections raised here are invalid. But it nevertheless turns out (surprisingly, to me!) that your argument doesn't work. This topic came up a lot during the short-lived superluminal neutrino excitement last year, and one good discussion of it by Sean Carroll can be found at:…

The issue is addressed explicitly in Alcubierre's paper where he mentions (at least twice) that there are no closed causal loops in his metric. So there's no violation of causality, and the standard intuitions from special relativity don't apply. Pretty remarkable! The lack of closed causal loops apparently follows immediately from standard results about the sort of metric construction he uses. There's no need to invoke Geroch.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#146

Post by Dylan16807 »

lmm wrote: post #121
teamonkey wrote: post #102

Earlier quoted context omitted.

Special Relativity only applies to cases where there is uniform relative movement in two or more inertial frames of reference. That is not the case with this theory - the relative movement is clearly not uniform and although I've not read the paper I'm certain that the time warp provides a non-inertial frame. TL;DR - ignore Special Relativity completely, General Relativity is the only thing that applies here.

We're not applying SR to an observer who warps, just to two observers in inertial frames of reference. Suppose we have guy A and guy B who have clocks. They're on spaceships which are both drifting away from each other (i.e. provide inertial frames) at speed c/root 2; at time t=0 they were in the same place. From SR it should be clear that in A's frame, his clock reads 100 at the same time as B's frame reads 50, whil…

Sure, B sees things happen in the wrong order, but I will maintain that causality is not actually violated until an effect actually disturbs a cause.

In other words, if B sends that message back to A, you have violated causality. If B just sits there and watches events happen out of order, then the causality is obscured but not broken.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#147

Post by lmm »

Dylan16807 wrote: post #146
lmm wrote: post #121

Earlier quoted context omitted.

We're not applying SR to an observer who warps, just to two observers in inertial frames of reference. Suppose we have guy A and guy B who have clocks. They're on spaceships which are both drifting away from each other (i.e. provide inertial frames) at speed c/root 2; at time t=0 they were in the same place. From SR it should be clear that in A's frame, his clock reads 100 at the same time as B's frame reads 50, whil…

Sure, B sees things happen in the wrong order, but I will maintain that causality is not actually violated until an effect actually disturbs a cause. In other words, if B sends that message back to A, you have violated causality. If B just sits there and watches events happen out of order, then the causality is obscured but not broken.

Maybe. I can't directly see an event disturbing a cause after the first message, but that doesn't mean there's not a way to make it do so.

Of course the cheap answer is that it violates SR (and common sense, given the complete symmetry of the situation) for A to be able to send an insta-message to B but B not able to send an insta-message to A.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#148

Post by Dylan16807 »

lmm wrote: post #147
Dylan16807 wrote: post #146

Earlier quoted context omitted.

Sure, B sees things happen in the wrong order, but I will maintain that causality is not actually violated until an effect actually disturbs a cause. In other words, if B sends that message back to A, you have violated causality. If B just sits there and watches events happen out of order, then the causality is obscured but not broken.

Maybe. I can't directly see an event disturbing a cause after the first message, but that doesn't mean there's not a way to make it do so. Of course the cheap answer is that it violates SR (and common sense, given the complete symmetry of the situation) for A to be able to send an insta-message to B but B not able to send an insta-message to A.

Well, either one can send a single message. It's the reply from a different frame that causes trouble. In fact you can have unlimited FTL messages sent in a single reference frame. It's only when you combine multiple FTL reference frames that things get wonky.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#149

Post by nessus42 »

teamonkey wrote: post #135
nessus42 wrote: post #109

Earlier quoted context omitted.

Special Relativity applies ANY TIME that you transmit information from one point A in an inertial frame to another point B in an inertial frame. If you can use your warp drive to transmit information from A to B, then Special Relativity applies. If you cannot use your warp drive to transmit information from A to B (e.g., from Earth to Mars), then what good is it? (I've mentioned several times in this thread that you…

> Special Relativity applies ANY TIME that you transmit information from one point A in an inertial frame to another point B in an inertial frame. Not true. It also implies uniformity of space-time in all inertial frames and the space between them. Since the theoretical purpose of the warp drive is to create a non-uniformity by definition SR does not apply.

All Special Relativity requires is that space be more or less flat between point A and point B. It doesn't matter if the information travels from A to B through a wormhole and then into another universe and then out the butt of Dumbo, if it ends up at B before light would, then causality has been violated.

If, on the other hand, your FTL drive bends up space all around us to be like a giant hyperspace pretzel, (1) we're going to have a lot more problems than just a little violation of causality, and (2) causality still ends up being violated via General Relativity.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#150

Post by nessus42 »

Dylan16807 wrote: post #146
lmm wrote: post #121

Earlier quoted context omitted.

We're not applying SR to an observer who warps, just to two observers in inertial frames of reference. Suppose we have guy A and guy B who have clocks. They're on spaceships which are both drifting away from each other (i.e. provide inertial frames) at speed c/root 2; at time t=0 they were in the same place. From SR it should be clear that in A's frame, his clock reads 100 at the same time as B's frame reads 50, whil…

Sure, B sees things happen in the wrong order, but I will maintain that causality is not actually violated until an effect actually disturbs a cause. In other words, if B sends that message back to A, you have violated causality. If B just sits there and watches events happen out of order, then the causality is obscured but not broken.

> Sure, B sees things happen in the wrong order, but I will maintain that causality is not actually violated until an effect actually disturbs a cause.

You maintain incorrectly. All inertial frames are equally good in Special Relativity. If there is any inertial frame that observes events elsewhere in the universe where an effect precedes a cause, then causality has been violated.

Furthermore, whenever Special Relativity is violated like that, there is always a way to get the violation to turn up in your own frame of reference, unless you constrain things so severely that what you are proposing isn't going to be useful for much.

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