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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

#161

Post by lmm »

Dylan16807 wrote: post #148
lmm wrote: post #147

Earlier quoted context omitted.

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.

But if B can't send an FTL-message to A because he's going to receive an FTL-message from A in what to him is the future, that's your causality violation right there.

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

#162

Post by SolarNet »

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…

So This may not be an FTL drive, but the way I always sort of pictured it is that the FTL moves you forward in time to the relative observer (so if you move distance x you move (c/x)^-1 forward in time). So if you send an FTL courier (assuming travel time of 2) from point A with times tA=102 tB=51 it would arrive at point B at times tA=52 tB=104 (A time 53 units in the future from tA), if it got sent back it would arrive at point A at tA=106 tB=53 (A time 53 units in the future from point B). The distance traveled would be c*53 distance (53 units of time for light to travel) in 2 units of time.

This also works for the mars/earth scenario (say they are 14 minutes apart). Traveling to the place that the light you see in 14 minutes will be coming from instantly would only ever be traveling forward in time. Forward time travel doesn't break causality(?)

This is always sort of how I envisioned wormholes work (and is indeed how they work on shows like star gate). You go in one end and come out the other, unless you can communicate through the wormhole it would take light (distance times c) years to tell people on the other end you succeeded.

But perhaps I'm missing something...

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

#163

Post by SolarNet »

lmm wrote: post #122
gizmo686 wrote: post #93

Earlier quoted context omitted.

I have very little experience with post newtonian physics, so could you explain why FTL implies time travel. Also, doesn't quantom entanglement imply that data travels faster than light?

Under SR, for two events (an event is a point in 4D space, i.e. a space and a time) with spacelike separation (i.e. outside each other's light-cones), neither comes objectively before or after the other. Different observers travelling at different speeds have different "surfaces of simultaneity", which say which events happen simultaneously. It's a theorem (or intuitively obvious) that for any two events with spaceli…

But is it really important that it looks like you arrived after you left? Can't we just know that the light from one thing took longer to reach us than the other so what we are seeing is just an artifact of FTL.

Can that prevent causality? Or does it prevent relativity?

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

#164

Post by [deleted] »

lmm wrote: post #122
gizmo686 wrote: post #93

Earlier quoted context omitted.

I have very little experience with post newtonian physics, so could you explain why FTL implies time travel. Also, doesn't quantom entanglement imply that data travels faster than light?

Under SR, for two events (an event is a point in 4D space, i.e. a space and a time) with spacelike separation (i.e. outside each other's light-cones), neither comes objectively before or after the other. Different observers travelling at different speeds have different "surfaces of simultaneity", which say which events happen simultaneously. It's a theorem (or intuitively obvious) that for any two events with spaceli…

[deleted]

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

#165

Post by stryker »

nessus42 wrote: post #152
stryker wrote: post #142

Earlier quoted context omitted.

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 destinati…

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

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.

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

#166

Post by Steuard »

michael_nielsen wrote: post #145
Steuard wrote: post #128

Earlier quoted context omitted.

[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 inv…

I'm not surprised that Alcubierre addressed this point, though I'm glad to hear that for certain. (I've done little more than glance at his original paper... I never have enough time for all the cool stuff.) But my understanding is that his metric is "eternal", in the sense that the warp bubble exists into the infinite past and future. That's obviously not practical for any actual application of the idea (as you'd need in a "warp drive" as discussed in this original article).

Instead, people want to find a way of creating a similar warp bubble where one did not exist before (presumably analogous to the way that stellar collapse creates a black hole that eventually approaches the Schwarzschild solution). So my question is, if someone does eventually find a GR solution that "creates" a warp bubble, would it be possible to patch multiple such solutions together in a way that allowed closed causal curves? (I'm imagining sending a warp "pulse" from Earth to a distant star, then having the recipient boost into a different rest frame, and then send a new warp "pulse" back to Earth, for instance.)

The answer may still be a clear "no", mind you. But it's not obvious to me what arguments you'd use to show that (particularly since we don't know what form a "warp bubble creation" metric would take, assuming it's possible at all).

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

#167

Post by Steuard »

nessus42 wrote: post #137
Steuard wrote: post #128

Earlier quoted context omitted.

[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:…

I would certainly defer to your much superior knowledge of physics. Mine is only that of an MIT student who was thoroughly indoctrinated with Special Relativity as part of MIT's general science education, and who sat in on an undergrad course on GR taught by cosmologist Max Tegmark. If I understand Bob Geroch properly (much of his paper is Sanskrit to me, but I think I understand the parts that are in English), he ac…

I've admittedly not worked through Geroch's paper thoroughly myself, so I won't make strong claims here. But based on my understanding, I'd say that his assertion is a bit stronger than what you're suggesting here. He's not just giving a sort of meta-argument that "the space-time manifold of the universe must be consistent, so some mysterious effect will always stop you from killing grandpa".

Rather, his point is that your second theory (the one that you're adding to standard relativistic physics) has already been assumed to be causal when taken on its own (it must have a well-defined initial value formulation: that's one of his postulates). What he demonstrates is that putting the two causal theories together will still preserve causality. So when he says that you won't be able to use standard SR tricks to create closed causal curves, he really honestly means that any possible arrangement of the matter/energy content that could do that would be forbidden by the fundamental laws of the theories you're using. (Not just that it "wouldn't happen to work out".)

To phrase it yet another way, Geroch would say that you could not build a time machine in such a theory, not just that you couldn't use it to change the past in a meaningful way.

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

#168

Post by teamonkey »

nessus42 wrote: post #149
teamonkey wrote: post #135

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. 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…

Causality is a consequence of the physical laws you are currently using, not a law in themselves. It just paints a logical boundary condition around what is possible using the current set of laws that you are using.

Compare Newtonian causality where there is only one timeframe everywhere to SR causality with its local timeframes to GR which supports warping of space and time. All different and not interchangable.

Wormholes cannot exist in SR, therefore applying SR's brand of causality is equally invalid. Under GR the concept of simultaneity is relaxed: the effect is only limited to the light cone of the cause.

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

#169

Post by nessus42 »

Dylan16807 wrote: post #159
nessus42 wrote: post #150

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. 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 Relati…

All inertial frames are equally valid. All inertial frames see that the effect does not munge the cause, even if it happens out of order. I know that 'causality' is a bad term for me to use here; I'm open to suggestions. But my point is that cause and effect are still intact. Cause, with effect not inside its light cone in the past, sends information toward effect. Effect, with cause not in its light cone in the futu…

> I know that 'causality' is a bad term for me to use here; I'm open to suggestions.

I think that you are looking for the term "closed causal curve" or "closed causal loop". If you use FTL to achieve this, this is often called a "closed spacelike curve". If you use GR without FTL to achieve this, it is often called a "closed timelike curve".

I'm not convinced that other sorts of violations of causality are any better than closed causal loops, since you can typically use the other violations to create closed causal loops. And even if you do have closed causal loops, you can decide not to go back in time and kill your grandfather, so if the issue is just what you chose to do, then closed causal loops are no worse in this regard.

In any case, whether or not you chose to go back in time and kill your grandfather, you won't succeed, since you did not succeed.

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

#170

Post by Dylan16807 »

nessus42 wrote: post #169
Dylan16807 wrote: post #159

Earlier quoted context omitted.

All inertial frames are equally valid. All inertial frames see that the effect does not munge the cause, even if it happens out of order. I know that 'causality' is a bad term for me to use here; I'm open to suggestions. But my point is that cause and effect are still intact. Cause, with effect not inside its light cone in the past, sends information toward effect. Effect, with cause not in its light cone in the futu…

> I know that 'causality' is a bad term for me to use here; I'm open to suggestions. I think that you are looking for the term "closed causal curve" or "closed causal loop". If you use FTL to achieve this, this is often called a "closed spacelike curve". If you use GR without FTL to achieve this, it is often called a "closed timelike curve". I'm not convinced that other sorts of violations of causality are any better…

Well the important part here is that you can pretty simply have a self-consistent universe that has mildly-limited FTL (and highly-limited time travel) if nobody makes a loop.

And I'll try to use something like 'causal loop' in the future.

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