I'm not the down-voter, but I think you were likely down-voted because this article/discussion is not about time travel. And your first statement is that "you can't have faster-than-light travel..." is precisely what this paper discusses -- that with certain loopholes in physics (and consistent with General Relativity), faster-than-light travel is at least theoretically possible.
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.
> [Those] who are not ready to address its consequences, should refrain from speculating on what is and isn't possible
Really? Do you hear yourself? I hope you're taking down badge numbers because I'm not sure that all these guys are qualified to address the consequences of faster-than-light travel.
Badge numbers? Are you talking about the badge numbers of the authors of the original article? If so, I'm sure they are quite aware that their proposed drive violates causality. Some scientists are willing to bite that bullet. It's some people on HN who don't seem to understand the chose-two-out-of-three principle when it comes to Relativity, FTL, and causality, and want to willfully ignore it.
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 in the other. Shuffle the envelopes so that you do not know which card is in which envelope. Have a friend take one of the envelopes many kilometers away. Your envelopes are now entangled.
If you now open your envelope, and it contains the red card, you instantly know your friend's envelope contains the green card. Did information travel faster than light? No, it did not, since in order to communicate by this method you still need to tell your friend what it means to have the green card.
You still need to send slower than light information that Red is 1 and Green is 0 which you can only do once you've measured your own envelope, after which you've destroyed the entanglement of your envelopes.
Quantum entanglement is a phenomenon where you can tell something about the other part of the entangled pair by measuring one part. Just like with the entangled envelopes, you cannot communicate anything by this since you don't know in advance what you're going to measure.
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.
No, quantum entanglement does not allow Information to travel faster than light. Classic channel is needed to know what if any unitary transformations are needed. Otherwise requiring roll of massively many sided dice. Check out No Communication Theorem.
I can't provide a coherent explanation of Relativity in one little HN box, but what I can say is that Special Relativity is pretty easy to learn and understand if you're willing to put in a bit of effort and deal with some high-school level algebra.
General Relativity is much much more difficult, but once you understand Special Relativity, a lay-person's understanding of GR gets the job done for most purposes.
Re quantum entanglement, no it doesn't imply that data travels faster than light. The versions of QM where the probability waves collapse might, but in the Many Worlds interpretation, which is probably the most popular interpretation now among physicists and philosophers of science, the probability waves never collapse, and so there is nothing to transmit.
In the Copenhagen interpretation, one might argue that the probability wave collapsing transmits information all along that wave instantaneously, but if so, this happens under the covers of the universe. There's no way for anyone to use this feature of QM to transmit usable information from one place to another at faster than the speed of 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.
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?
All observers are equally valid under SR; it would be very arbitrary to say that some of them are seeing "artifacts". Physics is based on observation, and the natural way to interpret our observations is to say that what we saw really happened. (Obviously this isn't true if some observable effect is distorting our perception, but that's not what's happening here - the observer who sees the person arrive before they left is a perfectly normal observer using perfectly normal equipment). Now if you want to say physics works differently for observers in different frames, that's fine, but it goes against relativity and all known physics.
Now assuming we accept that FTL-traveller really did arrive before they left, if that's all they do it's a matter of language as to whether they have violated causality by so doing. But it would take some very perverse physics (again, basically discarding relativity) to say they couldn't simply jump back, again going backwards in time, and arrive back at their starting point before they left it, at which you definitely have causality violation.
I don't know the specifics about quantum entanglement, but any means of making information travel faster than light that can work reliably in two different inertial frames (thus communicating four different points, two not moving relative to each other in space in each) can be used to break causality
or relativity. This is achieved by constructing a loop in which if the second pair of devices is used to disable the first you have a time paradox.
Is FTL the same as warping space? what happens in a (reduced dimensions for intuitiveness) scenario where a universe is like a line segment, and gets bent into crossing itself, allowing jumps from one point to another? Then there is a long way and a short way to send a message, kind of like travelling through vacuum is faster than traveling through water.
FTL is not the same thing as warping space in the same way that speeding is not the same thing as driving a Maserati. But if you can warp space at your will to give you the power to effectively get from point A to point B faster than a beam of light would, then you will also be able to construct "closed time-like loops", which will allow you to travel backwards in time.
I think the "faster than light" part wording seems a bit off since from what I gather the point is not to go fast to get to a location but to get there through warped space-time.
I imagine a 100 km long and 1km high brick wall, you're in the middle but want to get to the other side of the wall.
Option #1 is to travel as fast as you can 50km to one end and then 50km back to the spot on the other side.
Option #2 tunnel through the wall.
Option #1 is fast but the option #2 is faster although you didn't exceed the speed of option #1.
That's not the scenario described in the OP. The FTL drive described in the OP is a traditional "warp drive", that allows you to travel faster than light by moving the space around you faster than light, while you continue to travel slower than light within your rapidly moving space bubble.
You're still effectively traveling faster than light, however, and you'd still be able to use this to travel backwards in time.
I understand it perfectly fine for the purposes of this discussion.
Edit: I can't believe people down-vote irrefutable facts, so let me state this ONE MORE TIME:
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.
THAT'S JUST A FACT OF SPECIAL RELATIVITY.
People can bitch and moan about whatever they want to, and come up with all sorts of creative ways to try to work around this FACT, but YOU CANNOT.
I'm sorry, but I just can't find any sort of citation stating that the Alcubierre drive violates any of these principles. I also can't find any mention of paradoxes incurred by the drive and there is only one mention of where the meta-materials themselves would have to violate the light speed limit. Here is how I understand it (and correct me if I'm wrong), but it is not travelling in a vacuum, it is crunching that vacuum in front and expanding it in the back. This doesn't violate any laws because we know our universe is actually already doing this (expanding at a greater than light speed). If what you say is true then the galaxies that are currently moving away from us at those "super-luminal" (not actual super-luminal because we all know that's impossible) speeds shouldn't exist, but we know these do exist and we know why they appear to be doing so. Because of the expansion of the universe. http://curious.astro.cornell.edu/question.php?number=575 So since this is already done naturally is it not feasible to do so artificially?
But aside from all that it seems a little short-sighted to howl about how people can't do something. Time and time again nay-sayers have been proven wrong.
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 people continue to ignore it like it was never stated?
Look, I understand that people want perpetual motion machines. And I understand that people want the Earth to be the center of the universe. And I understand the fact that people want one plus one to equal three. But these things aren't going to happen.
With FTL, you have to pick two out of these three: FTL, Relativity, causality.
It is not a fact of science that you have to pick two out of the three. It is a fact of LOGIC.
Unless you've found a way to transcend the laws of logic, then we have to live within this constraint. So, if you want to have FTL, you either have to give up causality or Special Relativity. Is it conceivable that Special Relativity is wrong? Sure it's conceivable. Is it likely? NOT ON YOUR LIFE. And if you're willing to give up on Special Relativity, when why are we designing space ships based on General Relativity??? When Special Relativity goes out the window, so does GR!
So, are we going to give up on causality instead? Well, who am I to say yes or no. But if we decide that it's okay to give up on causality, then we have to bite the bullet and decide that we're okay with us being in a universe that is really, really strange. E.g., one that will thwart you somehow one way or another in any attempt you might make to transmit certain kinds of information, etc. One that might be perfectly okay with letting you visit your past self, and yet will suddenly give you a case of laryngitis if you try to tell your past self which stocks to buy. Who knows how weird it could get!
Re your space ship being in a vacuum or not: (1) It's still in a vacuum, and (2) it doesn't matter. What matters is if you can use it to transmit information from say Earth to Mars at a speed faster than a laser beam or radio signal would make it from Earth to Mars.
Re galaxies moving away from us at FTL speeds: (1) Yes, galaxies that are very far from us do move away from us at FTL speeds. Can you use this fact to transmit information between two points (e.g. two planets) at FTL speeds? No you cannot! Special Relativity says nothing at all about space itself moving at FTL speeds. It only puts constraints on information traveling from one point to another point at FTL speeds.
Go read the irrefutable fact at the top of this post again. Read it over and over again until you understand it. I can't keep repeating it over and over again for it only to fall on willfully deaf ears.
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 haven't got a clue as to what the drive actually does. Here let me explain it to you like you are 5. If you send a packet of information from point A to point B, and it goes faster than light, then yes that will screw with causality. But if instead you bring point A closer to point B, it will appear as though it is "traveling" (notice the quotes) faster than light. It is however not traveling. it isn't moving. No movement through space. Freefall. zero movement. Stillness. Think about that for a moment. Absolutely positively no movement. Complete stillness. There is a lack of motion with this drive relative to the speed of light. Mind you this isn't a wormhole because even wormholes can screw with causality. This is a space creator/destoyer. Destroys in the front, creates in the back. No movement is involved. I know it's really hard to wrap your head around but just think about it for a couple of days. think about bringing point A closer to point B instead of going through all the trouble of sending the information. Remember, doesn't move.
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!
Alcumbierre himself states that it does not violate causality. from: http://www.npl.washington.edu/av/altvw81.html > The possibilities for FTL travel or communication implicit in the Alcubierre drive raise the possibility of causality violations and "timelike loops", i.e., back-in-time communication and time travel. Alcubierre points out that his metric CONTAINS NO SUCH CLOSED CAUSAL LOOPS, and so is free of their paradoxes. However, he speculates that it would probably be possible to construct a metric similar to the one he presented which would contain such loops.
So either he's a laughing stock and nobody is talking about that or (more likely) you are wrong.
He has only stated that there are no closed causal loops for one particular use-case of his drive. He never stated either that causality was not violated from the point of view of a distant observer in a different frame of reference. Nor did he state that such a drive would not allow closed causal loops to be generated. In fact, he implied the opposite.
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.
So, I haven't looked closely at it, but he does mention that your clock remains (almost) the same as an earth clock. That implies that there is no frame of reference that shows him traveling faster than the speed of light. I'm not sure that this can be used to violate causality.
>I'm not sure that this can be used to violate causality.
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.
It's really just as simple and plain as that. You need consider nothing further once you know the above facts.
Based on this effect though, it doesn't travel from point A to point B faster than the speed of light. Point A and point B are literally much closer together. Perhaps the space time distortion makes impossible to think of it in terms of static frames of reference.
If you've somehow made A and B closer together, then you've made a wormhole, not a warp drive. Wormholes also come with "closed time-like loops", which allow for time travel.
Yes, there are parts of the universe traveling away from us at faster than the speed of light. In fact, it's quite likely that the universe is infinitely big and an infinitely large part of it is traveling away from us at faster than the speed of light.
This fact, however, does not allow information to be transmitted between two points in space at faster than the speed of light. That's the important constraint here. Causality is only violated when you transmit INFORMATION between TWO POINTS at faster than the speed of light.
