# Logically consistent FTL without time travel [duplicate]

If relativity holds, than faster-than-light travel would allow for time travel into the past. However, some claim that if all FTL travel used only one frame of reference, there could be no temporal paradoxes such as going into your own past and preventing the journey from happening in the first place. This is an interesting solution because I don't want time travel in my universe. However, I want to know whether some kind of "jump" travel can be made logically consistent with our physics.

I'm not the most experienced with physics, but some potential problems I've seen so far are:

Quantum field theory has problems with any fields that allow for faster-than-light particles (as a discussion in the comments in the first answer to the first question notes)

In quantum field theory, if two points are separated by a spacelike interval, then the operators for the observables commute, showing that effects cannot go faster than light.

Conservation of energy, linear momentum, and angular momentum (Jason W. Hinson makes an argument that angular momentum, in particular, cannot be conserved with this kind of travel, would Noether's theorem mean in that case rotational invariance does not hold, and would that cause big problems?)

Is travelling in one preferred frame even possible?

There may be other problems I do not know about. What I want to know is not whether this "jump drive" is possible in our actual universe(it probably is not) but whether these problems can be overcome and a mathematically consistent set of rules created that allows for a universe with this faster-than-light travel while preventing temporal paradoxes and allowing for most phenomena to be similar to our universe(with stars, planets, life, and so on).

Edit: This question may seem similar to the one in the first link, but the difference is there seem to be some problems with the answers listed there that I have so far not found solutions to. What I want now is either information on how to get around these problems or an explanation of why no consistent set of rules can be created to do that.

• So the speed that you're travelling at defines the frame that you're in. Travelling FTL can't be used to go back in time. If you're travelling at some speed and suddenly start going FTL, you're changing your frame, therefore it doesn't matter what frame you're in before you go FTL, only how fast you're going when FTL. The first answer you linked uses 'instantaneous travel between frames' which is simply impossible (infinite speed...). Even if it were possible, then 1 person travelling at infinite speed will have the same frame as another person travelling at infinite speed Commented Feb 23, 2017 at 8:40
• It might be helpful if you specifically state the problems in those answers which you need workarounds for. Commented Feb 24, 2017 at 14:36
• You should be specific on the issues you allude to in your edit. The question should be specific in asking that! You should look into the sandbox post on Worldbuilding Meta and then the Worldbuilding Chat room. Commented Feb 24, 2017 at 16:09
• «some problems with the answers listed there that I have so far not found solutions to» what problems? Try posting comments on those answers; I’d certainly update mine if I knew of some issue. Commented Feb 24, 2017 at 16:17
• Sorry, I'm new here and don't know all of the protocols. It's very nice that you helped me. Commented Feb 24, 2017 at 17:12

Scotty where are you?

You may be looking for Alcubierre drive technology. Alcubierre

This is a warp drive rather than a jump drive. However it does deal with your requirement of no time travel allowed via the chronology protection conjecture.

No faster than light movement is involved. 'The interior of the bubble is an inertial reference frame and inhabitants suffer no proper acceleration.' The bubble is allowed to move through a flat spacetime at a speed faster than light. The issue as yet unresolved is (roughly) if the Alcubierre metric is an allowed solution of the Einstein equations under a solution to quantum gravity.

Alcubierre is mathematically consistant with the standard model. The barriers to it are the exotic matter required to create negative mass, a solution to quantum gravity that supports the chronology protection conjecture without disallowing the Alcubierre metric and a resolution to the energy requirements (which may not be that great).

For the first dark energy is the logical candidate. For the second, still being worked on and mathematical physicists are arguing both ways. The energy requirements, as above, seem to be coming down.

In short while Alcubierre may be disproved it has not yet been and the problems lie in the realms of engineering as much as science.

• Does the protection conjecture work for the Alcubierre drive, or only for wormholes? Anyway, the problem is we don't know if the exotic matter necessary for either exists, much less how it can be made. For fiction, I know you could handwave it and say "the characters have a method to obtain it," but I don't know whether there could be a method that is logically/mathematically consistent with physics similar to ours. That's the big problem-I want absolute internal consistency. If a civilization somehow had the resources, it should be able to simulate my universe without problems. Commented Feb 23, 2017 at 17:50
• Also, you mentioned that dark energy can be used for the exotic matter. I looked it up; dark energy is actually positive, not negative. So it doesn't count, and we still don't know whether negative mass/energy is real. Commented Feb 24, 2017 at 4:02
• Alcubierre doesn't get around the fact that if it ends up being FTL, it will be time travel. Even if you warp space around yourself so you're traveling subluminally according to yourself, your'e still going to pass someone's light cone, which means it will be time travel according to some possible reference points (and in relativity, all reference points are equally valid!) Apparently even Alcubierre himself has stated this. The only way it doesn't violate causality is if the universe has some gimmick law that blows up the device as soon as it realizes a logical paradox is about to occur.
– pete
Commented May 28, 2019 at 20:58

This has been discussed in a variety of answers. An obvious candidate for FTL travel is the wormhole. It is suggested you search Worldbuilding SE for questions about FTL travel and wormholes.

More information about wormholes and FTL travel can be found here. This is a good starting point.

If you have looked Jason Hinson's website about relativity you will aware of the many difficulties. Whether and how they might be resolved in a subject for cutting edge research in physics. The likelihood is that special relativity will remain unscathed and there is no FTL travel. However, nature has a way of surprising us.