# Could a warp drive work as a time machine?

1. Nov 2, 2013

### KeplerJunior

I'm sure you've heard of the warp/Alcubierre drive that would work by expanding space behind it and contracting it in front to achieve faster than light travel. I was thinking about this the other day and was wondering if this concept could be applied to time as well to allow time travel. Would this be possible or would times one-dimensional nature not allow it?

2. Nov 2, 2013

3. Nov 2, 2013

### Staff: Mentor

We have re-opened the thread. Be sure to keep responses factual.

Personally, I am not aware of time travel directly with an Alcuiberre spacetime, but I would not be surprised to learn that someone has worked it out mathematically.

4. Nov 3, 2013

### dauto

Even with trying to find a direct solution for the field equations that leads to time travel (That would be hard to do), it is clearly possible to travel back in time using any means of FTL travel because of the relativity of simultaneity. A FTL travel connects to points whose distance is space-like, and there is always a boost that reverts the order of two events connected by a space-like distance due to relativity of simultaneity. That is how we know that FTL is not possible and Warp-drive is a pipe dream.

5. Nov 3, 2013

### PAllen

Time travel is normally taken as being able to construct a CTC - that is, you leave e1 on some time like world line and return to e0 that is earlier on that world line. With tachyons, for example, this requires the additional assumption that tachyons obey the POR (as opposed to picking out a preferred frame; if you allow POR violation, then the tachyon anti-telephone and all similar constructions need not occur).

With wormholes, it has been shown with sufficient exotic matter you can arrange a traversible one that allows CTCs.

For alcubierre drive, it is not at all obvious that the construction can be used to construct a CTC because it is normally not possible to escape and enter the warp bubble (a slight practical problem with such a 'drive' )

My knowledge is consistent with Dalespam - I've never seen a time travel construction using warp drive, nor have I seen an argument that it is impossible, and would not be surprised if some variation could allow it.

6. Nov 3, 2013

### dauto

The problem is this idea is threefold.

*Getting rid of POR is just wishful thinking.
*Sufficiently exotic matter doesn't exist.
*Wormholes probably do not exist either.

7. Nov 3, 2013

### PAllen

I'm not saying they do. I'm saying the statement that FTL = travel back in time (as normally understood - getting back to an event that is in your causal past) is not a formally correct statement. Without violating any currently known physics, it is possible, in principle, to add FTL phenomenology without producing time travel. I agree the laws you would have to propose for it are 'ugly', and there is not the slightest reason to expect it to be possible.

However, the OP asked specifically about alcubierre drive. I have read many papers on this and related spacetimes, and none that I have seen suggest the possibility of time travel. This is in contrast to Kip Thorne's wormhole constructions which definitely produce it. However, like Dalespam, I would not be very surprised if someone produced such a construction because there are many constructions allowing time travel in GR if you allow sufficient exotic matter.

These statements are factual. An opinion of mine (in, I think, agreement with you) is none of this is likely possible in our universe. However, I think it is important to distinguish what mathematical GR (or SR for known physics plus contrived FTL) allows, versus opinions on what is likely true.

8. Nov 3, 2013

### Staff: Mentor

Well said, PAllen.

9. Nov 3, 2013

### K^2

Of course Alcubierre Drive allows for time travel in some limited sense. Forget about the warp field itself. Look at the statement of the problem from perspective of nearly-flat space-time away from the ship. You have something that travels from event A to event B. The two events are space-like separated. That's the whole point of FTL travel. Now, order of any two space-like separated events is frame dependent. So if you have a ship under Alcubierre Drive depart at event A and arrive at event B in one frame, there is a frame of reference in which it departs from B and arrives at A.

That's time travel.

Granted, it's not what most people think of when they talk about time travel. There are some very strict limitations on this, and yet you can have an observer watch information carried drom a "future" event to a "past" event breaking causal relationship.

This is not directly useful to anyone at departure or arrival locations, of course, because in their frame of reference no "time travel" takes place. What would be interesting is to have a closed loop, and that requires you to do something creative with space-time. Where Alcubierre Drive can make a difference is that it's very difficult to organize a closed time-like loop under anything like reasonable conditions. If, however, you have a ship that can follow a space-like trajectory, you have opportunity for time travel in its fullest sense.

Now, if we start talking about consequences of something like this, that's where physics as we know it breaks down. Time travel is entirely fine in General Relativity. Whether solutions that allow it are plausible remains to be seen, but there is no problem with framework of Relativity itself.

Similarly, we can build a particle field theory in any plausible space-time. Including a space-time that has closed time-like loops. Id est, allow time travel. There could be serious problems with renormalization in such a theory, and I don't know if anyone has done the math on this in earnest, but we at least have an idea of how to approach the problem.

But the moment we acknowledge the fact that space-time geometry is a consequence of matter field configurations in that space-time, we no longer have a theory we can actually make a use of. So we cannot possibly do a complete description of what's going to happen if we use a warp field to guide a ship along a closed time loop.

10. Nov 3, 2013

### yuiop

If the Alcubierre drive predicts FTL travel then it appears that it also predicts reverse time travel. For example if the drive gets from A to B faster than light in one reference frame, then there is another reference frame where the drive can return to location A before it departed. Thus it seems that the drive violates causality. This is only avoidable if the POR is violated which in effect means that an absolute or preferred reference frame exists. If the POR is violated then one of the postulates of relativity is invalidated.

11. Nov 3, 2013

### bcrowell

Staff Emeritus

Everett, Allen E. (15 June 1996). "Warp drive and causality". Physical Review D 53 (12): 7365–7368. Available online here: http://exvacuo.free.fr/div/Sciences...tt - Warp drive and causality - prd950914.pdf

12. Nov 3, 2013

### Staff: Mentor

While I wouldn't be surprised to find CTC's with the Alcubierre drive, I don't think it is that obvious.

First, simply going backwards in time in some frame does not lead to a CTC. You have to turn around and go back to where you started along a second spacelike trajectory. I am not sure if the Alcubierre drive can turn.

Second, the interior of the bubble is causally disconnected from the bubble itself. So it isn't clear to me that the interior of the bubble forms a CTC even if you can get the bubble back to the same event.

Those two things make it unclear to me. I am sure that someone has worked out the math, but I haven't seen it and I don't know what the conclusion is. Neither conclusion would surprise me.

EDIT: Thanks bcrowell, you posted while I was writing this. That is exactly what was needed!

13. Nov 3, 2013

### PAllen

That isn't time travel according common usage in the literature. Time travel is taken to be a way to get from some event e1 to some event in the causal past of e1 (possibly just sending a message - but there is little difference since the message must be made of something). In the case of tachyons, it is well accepted that you don't get time travel without the assumption that the physics of tachyons is the same in all frames. If, instead, you allow tachyons to pick a preferred frame (while all other physics follows the POR), you can prevent all forms time travel using tachyons. All you need is to posit there exists at least one privileged frame in which all tachyon paths move forward in coordinate time.

An example of a minimal derivation of time travel with alcubierre or similar warp drive would be to show you can have two drives set up in an analagous way as the tachyon anti-telephoned, such that they approach close to each other at one event, and you have a way to send a message or payload from the inside of one warp bubble to the inside of another. I would not be shocked to see such a thing established, but it is also not at all obvious - there are problems getting anything into or out of a warp bubble in the constructions I've seen.

14. Nov 3, 2013

### PAllen

Great, I never saw this one. As advertised, I am not surprised either. In particular, they address the issue of getting from one bubble to another, addressing the issue I was unsure about.

15. Nov 3, 2013

### K^2

No. You cannot get this in any inertial frame. To do what you suggest, your frame of reference need to accelerate, and then you can no longer use Special Relativity to describe what's going on. So in flat-space time you still cannot get the practical time travel you are looking for.
Fair enough. If we define it that way, Alcubierre Drive does not allow for time travel in the otherwise flat space-time.

Why? Take any known closed space-time curve with space-like regions. Send a FTL ship along it. From perspective of the crew, they've traversed a CTC. QED.

Honestly, you don't need to make Alcubierre Drive more complicated than it is. If we fiat ability to build a warp drive, form there on all you have to consider is a ship that can traverse along space-like world lines. You no longer have to think about the specific geometry of the space-time near the ship. We can make the ship and the warp field small enough to fit through whatever neighborhood of the world line that's practically available. In theory. Practically, things might be very different.

16. Nov 3, 2013

### yuiop

I am curious.
Can an Alcubierre drive stop or be turned off once started? If not then it is not much use for transporting material or passengers from A to B.
Can an Alcubierre drive be detected perhaps by the warping of space that makes it work? If the drive can be detected by any means at all, then it can be used to transmit information from A to B faster than light. If it is completely undetectable and causally disconnected (i.e. can not collide with objects in this universe) then it effectively does not exist in this universe.

We do not need the Alcubierre drive to stop or turn around at B, to send material or information back to A in the causal past. All we need is to send a second Alcubierre drive from B in the opposite direction when the first drive arrives at B. If there is any way to detect a drive in this universe, then the only way to forbid information being transmitted FTL or into the causal past is to specify that the drive must always be travelling FTL. This means an Alcubierre drive can not be invented or constructed, but must have always existed.

17. Nov 3, 2013

### PAllen

To me, showing you can have closed loop Alcubierre trajectory in asymptotically flat spacetime is a significant result beyond the the main papers on this and related metrics. The paper posted shows a way to achieve CTCs with a pair of Alcubierre bubbles, but the key parts of the derivation are not what I would call obvious.

18. Nov 3, 2013

### George Jones

Staff Emeritus
I don't think I know what you mean. Are you saying that ships can move along curves that have spacelike tangent vectors?!

19. Nov 3, 2013

### yuiop

It is fairly well established in this forum, that as soon as you can transmit matter or information FTL, then it follows that you can violate causality or you violate the POR. As I am sure you are aware, if we preserve the POR, then transmission of information FTL allows us to construct causality violating situations such as the tachyonic anti-telephone.

We do not need to accelerate to a different reference frame to get the practical time travel. All we have to do is show that there exists in principle, an inertial reference frame where the FTL travel will be seen as travel back in time. The second reference frame is only there to illustrate the situation and is not actually required for practical time travel. All we need is an FTL drive and we can violate just about anything we like about relativity. As mentioned before an FTL drive that does not violate relativity would have to be so disconnected from this universe that it would it would be completely undetectable and unable to interact with anything in this universe and so effectively does not exist, like the aether.

20. Nov 3, 2013

### K^2

Define asymptotically flat. If you mean with respect to Alcubierre metric itself, then no, it's impossible and I've stated that already. You need source of significant curvature outside of the warp bubble. That curvature can be asymptotically flat itself and such solutions are known.

A FTL ship, yes. Of course.

As PAllen pointed out, the requirement here is that you can move information into past cone of an event. Alcubierre drive lets you go backwards in time, but only outside of the past light cone. That means that you cannot construct a frame of reference where the loop is closed.

I can draw you a diagram if you like.