What would happen to time if we could travel faster than light?

[dimreepr]

I was pondering this when I read the aliens and FBI thread, and didn't want to steer it that far of topic.

I mean supposing an alien had an FLT spaceship, could they go home for reinforcements and get back in time for the battle? 

[Janus]

This is one of those "If we could break the rules, what would the rules be" type of question.

If FTL is possible, then there is something missing in our understanding of the universe.  But from our position, we don't know what that is.  And without knowing that, we can't even guess what the consequences of FTL would be. 

[swansont]

There a pithy saying - FTL, causality, relativity: Pick two.

IOW if you want FTL you have to give up either causality or relativity. Whatever you end up with will not be how our universe behaves.

[Genady]

1 hour ago, swansont said:

There a pithy saying - FTL, causality, relativity: Pick two.

IOW if you want FTL you have to give up either causality or relativity. Whatever you end up with will not be how our universe behaves.

I like that saying. Don't remember seeing it before and will use it often. Thank you, @swansont. Any known source?

[swansont]

1 hour ago, Genady said:

Any known source?

No, I don’t have one. If you Googled it, you’d likely find dozens of hits in various forms, all probably unattributed.

[Tutoroot]

Special relativity states that nothing can go faster than the speed of light. If something were to exceed this limit, it would move backward in time, according to the theory.

[Intoscience]

On 12/10/2022 at 3:54 PM, dimreepr said:

I mean supposing an alien had an FLT spaceship, could they go home for reinforcements and get back in time for the battle

They could be back before they set off. 

[Janus]

6 hours ago, Tutoroot said:

Special relativity states that nothing can go faster than the speed of light. If something were to exceed this limit, it would move backward in time, according to the theory.

No, not really.   I think this idea comes from a misunderstanding of time dilation.  While it is true that a clock moving relative to you would,as measured by you, tick slower and slower as it it approached the speed of light, the equation that predicts this is "undefined" for speeds greater than c (It gives a result that is the square root of a negative number. )

Now there are some setups that could be used to create causality issues in Relativity if FTL is allowed.  Basically, this involves extended systems moving relative to each other, and rely on how these systems measures simultaneity differently.  An example of such a system would be two long trains passing each other in opposite directions. One train sends an FTL signal from one car to another. That car transfers the signal a car of the other train as they pass each other.  The second train sends an FTL answer back along it length, to be transferred back to the car that originated the message.  Under the right conditions, allowing FTL transmission can result in the answer arriving at the origin before the initial signal was sent.  

This goes back to what swansont was referring to a few posts ago.

The truth is that the Theory make no predictions as to what would happen if you exceeded c, as that is beyond it range of applicability.