Question regarding Warp Drives, FTL and time travel

[AcesHigh]

Well, from what I know (and I may be wrong):

1 - FTL implies in time travel to the past

2 - Theoretical Warp Drives (Alcubierre, and the new mathematical model being researched by Dr Sonny White at NASA) move the space-time around the ship, and could be used to move a ship faster than light, althought its not moving relative to the space around it (its the space that is being moved). However, even so, that would imply in time travel to the past, and that could be shown by the mind exercise of putting two portals of a wormhole, one inside the ship, another on Earth, and the gate at the ship would be a portal to the past. Therefore, some people consider warp drives/wormholes, etc, impossible, on the grounds that it would be a time travel machine, with all the causality problems

3 - HOWEVER, we also know that in the early stages of the universe, it expanded MUCH FASTER than light.

4 - if the universe expanded faster than light at the very beginning, and if faster than light implies in time travel to the past, even if it is spacetime expanding, and if time travel to the past is impossible due to causality... so what part of the equation is wrong there?


ps: even if we consider time travel to the past possible, I wonder how could something travel way further to the past than the existance of the universe and of time itself (very fast expansion would mean travelling more and more to the past, but time itself began only a few microseconds ago!)

[hanelyp]

1 - FTL implies in time travel to the past

If you don't also assume an additional restriction. http://en.wikipedia.org/wiki/Chronology ... conjecture

The chronology protection conjecture is a conjecture by the physicist Professor Stephen Hawking that the laws of physics are such as to prevent time travel on all but sub-microscopic scales. Mathematically, the permissibility of time travel is represented by the existence of closed timelike curves. The chronology protection conjecture should be distinguished from chronological censorship under which every closed timelike curve passes through an event horizon, which might prevent an observer from detecting the causal violation.

[AcesHigh]

well, yes, but if closed time like curves are forbidden by nature, then how did the universe expanded faster than light?

well, ok, apparently the mind exercise of the star gate aboard a FTL ship would only create a closed time like curve IF the ship returned to its starting point. Then it means, based on the universe own FTL expansion, that FTL is allowed only if the ship travels in one direction but does not tries to return?


and anyway, if you are travelling to the past when in FTL, how could you travel to the past in the first microseconds of the universe?

[kcdodd]

Do all FTL trajectories create a time-like loop?

[hanelyp]

Do all FTL trajectories create a time-like loop?

No.

Consider wormholes as an FTL travel method. A wormhole is created, one end kept at home while the other end is send on a sublight trajectory to a destination. As observed through the wormhole the remote end may reach the destination much faster than the light trip time. Travelers may pass through this wormhole for a round trip much faster than any ship flying through normal space. A network of such wormholes radiating outward from a hub don't create a closed timelike loop. You always return to your starting point after you left. But suppose the remote end of such a wormhole made a round trip at high sublight speed, or spent some time near light speed in a particle accelerator. One end is time dilated relative to the other, and this wormhole may be used as a time machine.

[Schneibster]

Well, from what I know (and I may be wrong):

1 - FTL implies in time travel to the past

That's a loose way of putting it. The tight way is, it's movement outside the light cone, which implies the possibility of time travel. That doesn't mean everywhere and -when is in the past; but the main barrier to travel to the past is removed if movement outside the light cone is possible.

The problem is, if this were true, the receiving point in the past would perceive mass-energy to be created from nothing, and the sending point in the future would perceive it to be destroyed into nothing, and neither is permitted by the conservation law of mass-energy. And there is no indication in the mathematics that it must be possible to have an apparent dual violation of this type. In fact the conservation laws are based on the dimensionality of the universe, and its uniformity. The conservation of mass-energy is due first to the conservation of energy which is dual under Noether's Theorem to the uniformity of experimental results over time, and secondly to the equivalence of mass and energy under Einstein's famous mass-energy conversion equation, E=mc^2. But this conservation of mass-energy only makes sense if mass-energy cannot be transmitted, in fact energy and information cannot be transmitted through time. This is the basic conflict of time travel to the past and consistency/conservation laws dictated by the dimensionality of the universe, a postulate of Special Relativity Theory. (Specifically the first postulate, "space and time form a 4-dimensional continuum.")

2 - Theoretical Warp Drives (Alcubierre, and the new mathematical model being researched by Dr Sonny White at NASA) move the space-time around the ship, and could be used to move a ship faster than light, althought its not moving relative to the space around it (its the space that is being moved). However, even so, that would imply in time travel to the past, and that could be shown by the mind exercise of putting two portals of a wormhole, one inside the ship, another on Earth, and the gate at the ship would be a portal to the past. Therefore, some people consider warp drives/wormholes, etc, impossible, on the grounds that it would be a time travel machine, with all the causality problems

More or less, yes. Most physicists think that nothing material can get through a wormhole, and neither can any message that can transmit information to the past; the physics of the inside of the wormhole would scramble any message and any material object always just enough to prevent it. This is called the chronology protection conjecture.

3 - HOWEVER, we also know that in the early stages of the universe, it expanded MUCH FASTER than light.

A common question. At that time, space was filled with a single field: the inflaton. There was no matter yet; there was no space or time yet. There was a vacuum fluctuation in the über-universe (people often call it the metaverse or multiverse, I think those sound too cute since we can know no more of it than it is capable of supporting vacuum fluctuations like the one that resulted in our universe) and it by chance had just the right value of the cosmological constant. This isn't surprising; many, many, many, in fact an infinity, of other universes were being created by vacuum fluctuations all "around" it at "the same time" (scare quotes because we don't know what "all around" and "time" mean in the über-universe). That one of them should have had this particular random value of the cosmological constant isn't even surprising- it's inevitable.

So when it was doing that expanding, there not only wasn't any "thing" (mass-energy) moving at faster than light, without any "things" or a large 4-dimensional manifold with a hyperbolic dimension, there wasn't any movement or velocity or anything else.

This expansion didn't just continue, it accelerated exponentially. In far less than 10^-43 seconds (a time so short that to probe it we would need an accelerator the size of the galaxy, not just that little ant nest in Switzerland) areas that had been only a Planck distance or less apart at the beginning were at the far ends of each others' observable universes, a hundred billion light years apart.

Then the unstable negative cosmological constant decayed. It was inevitable; a negative cosmological constant is unstable and will always decay. When it did, all the mass-energy of the universe was dumped from the inflaton into the universe, creating the Big Bang.

When you have finished considering this, you will find that some of your old questions have been answered, but you have some new questions. Come back, and if they've been nice to me here I'll answer them.

[Schneibster]

Do all FTL trajectories create a time-like loop?

No. However, the existence of FTL trajectories inevitably allows time-like loops.