# Causality and FLT! a time reference consideration

## Main Question or Discussion Point

excuse my lack of understanding! Is there anything in physics which directly suggests that if FLT was possible, you would travel back in time? does anything forbid the possibility that instead you gradually change your time reference to keep causality? With the example of a very fast train, a light is turned on in the centre and an observer outside sees the light hit the back of the train before it hits the front. However, observers on the train see it hit both walls at the same time. if FLT is possible, it may be possible to travel to the front of the train where the light hasn't hit and hence go back in time. Is it possible that instead as you approach the front of the train faster than light, you would gradually change to its true time reference where the light has hit the wall and hence you will have travelled there quicker than light could but causality wouldn't be broken. Not very good explanation but potentially you could travel across the universe nearly instantaneously with a wormhole for example, much faster than light, and a change in time reference would stop causality being broken. I hope I am making sense. Just a suggestion, tell me if it's ridiculous!

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mfb
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Is there anything in physics which directly suggests that if FLT was possible, you would travel back in time?
There is, it is called special relativity. FTL (faster-than-light) in one frame is always backwards in time in another frame.
You, going FTL, can do whatever you want, others will observe you to go backwards in time.

I don't understand your train setup. What is a "true time reference", and how do you change clocks of other observers just by traveling?

Yes, but there is a modification to SR that allows casual FTL by dropping the idea that distance from point A to point B is equal to the distance from point B to point A.

I heard (but I don't know the details) that standard SR depends only on the length of closed loops. You don't need to know the distance from point A to B. You only need to know the full length from A to B to A and that's all you need to formulate SR. This is the synchronization problem. When we know the full length |ABA| then we have to synchronize our clocks to know the distance |AB| and |BA|. In standard SR we assume that |AB| = |BA| = |ABA| / 2. But we can define it differently.

And I heard that it is possible to redefine SR in such a way that it predicts everything unchanged for slower than light speeds, but it also has absolute infinite speed. This way, FTL particles would not go backwards in time for any observer.

In standard SR we have 3 kinds of particles: normal slower than light particles that move forward in time, tachyons that move forward od backwards depending on the observer and slower than light particles that move backward in time.

In the modified setup, we have 4 kinds of particles: STL forward, FTL forward, FTL backwards, STL backwards. Moving forward or backwards in time is a property of a particle idependent of the observer.

This was a short article that I read when there was a famous incident with neutrinos that apparently moved faster than light. I'll try to find it.

mfb
Mentor
Yes, but there is a modification to SR that allows casual FTL by dropping the idea that distance from point A to point B is equal to the distance from point B to point A.
I really like the concept of spacetime that looks the same in all spatial directions. Every deviation from that would be really odd.

This was a short article that I read when there was a famous incident with neutrinos that apparently moved faster than light. I'll try to find it.
Sounds interesting.

The reason faster than light travel leads to backward in time travel can be seen with simple geometry.
Draw a space-time diagram with space on the horizontal axis and time on the vertical axis. Your axes are at a 90 degree angle to each other. Now, the axes for a frame of reference movng relative to you are not at 90 degrees; their space and time axes for an angle less than 90. The time axis tilts down and the space axis tilts up, both toward the 45 degree diagonal line, which represents the world line of light. The closer to the speed of light, the small the angle of the axes. They squeeze in toward the 45 degree diagonal.

Now, if you go from the origin to a point far to the right but only a little way up, this represents faster than light travel. Relative to you, you went slightly forward in time. But your world line has gone BELOW the moving frame's space axis. This represents backward in time travel to the moving observer.

Try drawing this out on graph paper. You will see easily what I am saying. There is no way to get out of this conclusion, if special relativity is correct. It doesn't matter how you get from one point to another faster than light (wormhole, warp drive, teleportation, magic) Those backward in time paradoxes will always arise.

[copied more-or-less verbatim from my answer to an identical question here]
Observers can not agree on the chronological order of space-like separated events, so a common sense interpretation is that they can not be causally related.
If you allow SOME space-like separated events to be causally related, you must by extension allow ANY of them to be causally related, in either direction, because there's nothing special about any pair of space-like separated events.
If you allow all space-like separated events to be causally related, you must allow time-like separated events to be causally related in reverse chronological order, by means of a third proxy event which is space-like separated from both.

[and paraphrasing my post two posts down from it]
But if you only limit causal relations between a special class of space-like separated events, you can prevent formation of such "proxies", and so prevent it from leading to reversely connected time-like separated events.