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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

#121

Post by lmm »

teamonkey wrote: post #102
nessus42 wrote: post #64

Earlier quoted context omitted.

Nothing you just said contradicts my statement, which was, "there are frames of reference in which that information arrived at the destination before it left the origin." I suspect that you don't know anything at all about Special Relativity, in which case, there's not much more I can tell you except to go educate yourself.

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, while in B's frame, when his clock reads 50 A's clock reads 25. All this should be basic and obvious.

So suppose an event happens on A's ship at time tA=100, and he transmits this instantly (or sends a courier C on a warpship) to B at time tA=102, when B's clock reads tB=51.

From B's perspective, he's just received a message from A at time tB=51 (which is simultaneous with tA=25.5), which A is not going to send until tA=102 i.e. tB=404. Thus, B perceives a violation of causality.

(This becomes very obvious if B can send an insta-message back to A with the same technology; B sends it at time tB=54 which is simultaneous with tA=26, A receives it at tA=26, well before he sent the original message at tA=102)

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

#122

Post by lmm »

gizmo686 wrote: post #93
nessus42 wrote: post #49

Earlier quoted context omitted.

Faster-than-light travel allows you to travel backwards in time. This is an essential part of Special and General Relativity. Those who don't understand this and who are not ready to address its consequences, should refrain from speculating on what is and isn't possible.

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 spacelike separation, there is a frame (i.e. a velocity at which you could be travelling) for which those two events are simultaneous.

So consider the events A=point 1, time 0; B=point 2, time 0; C= point 1, time 1 hour. As long as points 1 and 2 are more than 1 light-hour apart, it is legitimate to say that A is simultaneous with B, or that C is simultaneous with B. And so if you have a device that lets you travel from point 2 to point 1 in less than an hour (i.e. faster than light), you leave at B and arrive after A but before C. Then from the perspective of some observers (and the point of relativity is that all observers are equally legitimate), you arrived before you left.

The collapse of quantum entanglement can be confusing, which is part of why I prefer to follow many-worlds (i.e. no collapse postulate). You can construct quantum wavefunctions such that they're consistent with SR; for two entangled but spatially separated particles, you simply have a wavefunction that's a superposition of two states. When you as an observer observe one of those particles, you entangle yourself and your own state becomes part of the superposition. Of course "from the inside" it feels like you measured the particle and got one result or the other, but the actual wavefunction is just an ordinary superposition. From this perspective it's obvious no data was transmitted - you just became entangled with this distant particle without communicating with it, which is kind of odd, but no danger of violating causality.

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

#123

Post by lmm »

sentientbicycle wrote: post #117
nessus42 wrote: post #115

Earlier quoted context omitted.

Did you read the part above where I wrote this: It is an irrefutable fact of Special Relativity that if you can transmit any information from point A to point B faster than light would travel from point A to point B in a vacuum, then there are inertial frames of reference in which the information arrived at point B BEFORE it left point A. Is there a reason why I can write this irrefutable fact a dozen times and peopl…

OOoohhhh. I totally get it now. You are an unpublished internet stranger and the Alcubierre drive has been scrutinized by some of the top minds in physics. But your criticisms are more important for some reason I don't know. Mkay, going to bed now. Edit: By the way. I did read the thing about point A and B and understand it completely. The thing is, it doesn't matter. This is why I'm absolutely sure now that you have…

I'm sure the authors of the paper know that the warpdrive violates causality.

It doesn't matter that you're warping space to move points A and B closer from your perspective. If an outside observer can see you at point A at time t, and at point B at distance d away at time <t+d, you have violated causality, and an observer moving at a suitable velocity will see you arrive at B before you left A. That is a basic, irrefutable fact of special relativity.

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

#124

Post by stryker »

nessus42 wrote: post #100
stryker wrote: post #98

Earlier quoted context omitted.

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…

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.

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.

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

#125

Post by anonymouz »

chernevik wrote: post #33
anonymouz wrote: post #24

Earlier quoted context omitted.

There are a lot of challenges we have to overcome to travel to the stars, but this is certainly not one of them. What weapon system would be "good enough" anyway? A potential other civilization could be completely primitive, or it could have faster-than-light travel for millions of years by now. To me it seems entirely absurd to consider this as one of the major problems.

Maybe not weapons development. But doesn't the whole problem bear some thought? The strategic problem postulated here would be defending planets, presumably from collisions with masses moving faster than light. Warheads aren't more than an afterthought, right? I don't see how anyone spots the inbound attack, never mind puts enough energy on it to change its vector. Which suggests that the only balance comes from dete…

Of course it may happen that becoming more visible in the universe would provoke impossible to defeat enemies against us. But the argument for this is purely hypothetical and one could similarly make an argument that sitting around on our rock and not expanding could end in us being wiped out (cf.: Every primitive culture on Earth ever wiped out by explorers/settlers).

It seems impossible to decide which way is safer with our current information, so I'm for expanding/exploring till we know more.

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

#126

Post by nessus42 »

sentientbicycle wrote: post #117
nessus42 wrote: post #115

Earlier quoted context omitted.

Did you read the part above where I wrote this: It is an irrefutable fact of Special Relativity that if you can transmit any information from point A to point B faster than light would travel from point A to point B in a vacuum, then there are inertial frames of reference in which the information arrived at point B BEFORE it left point A. Is there a reason why I can write this irrefutable fact a dozen times and peopl…

OOoohhhh. I totally get it now. You are an unpublished internet stranger and the Alcubierre drive has been scrutinized by some of the top minds in physics. But your criticisms are more important for some reason I don't know. Mkay, going to bed now. Edit: By the way. I did read the thing about point A and B and understand it completely. The thing is, it doesn't matter. This is why I'm absolutely sure now that you have…

Where do any of these top minds of physics say that this drive does not violate causality??? They DON'T! If they asserted such a nonsensical thing, they would be the laughing stock of the scientific world and would no longer be considered any kind of mind, other than a quack. The only reason that you don't see much talk about this violation of causality is that it's just a fact that all physicists know. One physicist doesn't have to tell another physicist that an FTL drive violates causality; it just goes without saying. It would be like one physicist telling another physicist that 1 + 1 = 2. It doesn't need to be said!

It doesn't matter ONE TINY BIT to Special Relativity how it appears to YOU that you got information from point A to point B. What matters is how it appears to an OUTSIDE OBSERVER. If Earth and Mars are still 8 light minutes apart from some point of view, and you managed to transmit information from Earth to Mars in only 4 minutes, then there are frames of reference in which your message arrived at Mars BEFORE it left Earth.

This is just freshman-level Special Relativity. I guarantee you, that no one has found a work-around for that, especially since it is a matter of LOGIC. It follows LOGICALLY from the assertions of Special Relativity. If you wish to deny this conclusion, then you would have to deny Special Relativity.

If on the other hand, you have used your drive to permanently shred up space and time, rapidly crashing Mars into Earth and destroying all life on both planets, leaving the mess like that, and then stating you have successfully transmitted a message at FTL speeds, then more power to ya!

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

#127

Post by twoodfin »

codehotter wrote: post #104
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?

I keep hearing about faster than light communication with quantum entanglement. It seems to be a very persistent misconception. The same "communication" as with quantum entanglement can be done in the classical universe. You can easily create two entangled envelopes. People's minds are blown just because we put the word "quantum" in there. Take two indistinguishable envelopes. Put a green card in one and a red card i…

But isn't the essential difference (and what makes the QM effect seem like spooky action-at-a-distance) that it's been proven that it isn't like the envelope example? Namely, there's no definite "redness" or "greenness" to the entangled particle before you examine it (unlike the envelope, which contains a red or green card the whole time). Whether you have a "red" or a "green" quantum part isn't decided until you look, at which point the wave function collapses in both parts simultaneously, no matter how far apart they are.

http://en.wikipedia.org/wiki/Bells_theorem

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

#128

Post by Steuard »

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:

http://blogs.discovermagazine.com/cosmicvariance/2011/09/24/...

That post includes a link to a paper by Bob Geroch that goes into the topic in detail. The very short summary is that there is no mathematical necessity for "causality" to be defined just by relativity: the union of the relativistic definition and some other well-defined causal theory can work just as well, even if that theory involved faster-than-light travel.

So thank you for raising a very good point about this topic, even if it wasn't quite accurate. And to those who've objected here: unless your objection explicitly quoted Geroch, odds are very good that they're wrong.

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

#129

Post by cygx »

nessus42 wrote: post #100
stryker wrote: post #98

Earlier quoted context omitted.

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…

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.

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.

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

#130

Post by Steuard »

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:…

That being said, I'm not certain whether there's any way to avoid causality violation if the hypothetical FTL travel results entirely from within relativity itself. (Geroch's argument, as I understand it, is based entirely on combining two different theoretical structures.)
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