# Faster than speed of light = time travel ?

1. Jun 8, 2013

### collector

This is my first post so let me know if I'm posting this in the wrong section.

I read somewhere that if we are ever able to travel faster than the speed of light that fact would automatically enable us to travel back in time. I don't see how those two things are connected. Even if we travel faster than the speed of light, say 1.5c, we would still be travelling at a finite speed.
Is this true, or did I misunderstand it when I read it? How does it work?

Thanks.

2. Jun 8, 2013

### Staff: Mentor

If you can travel faster than the speed of light in some reference frame (like the earth), there is always a different reference frame where this journey goes backwards in time.
You would go backwards in time for some, not for all observers. If you turn around and go backwards (again faster than the speed of light, for some reference frame), you can get back to earth before you started. That is time travel for all observers.

I am sure we have a detailed description somewhere in the forum.

3. Jun 8, 2013

### collector

Does the observer on earth also see the person arrive before he leaves?

4. Jun 8, 2013

### Staff: Mentor

All reference frames will see the arrival before the departure.

5. Jun 8, 2013

### Staff: Mentor

This is like an FTL guy who checks into a hotel and the clerk says I'm sorry you don't have a reservation and then checks again oh there you it just came in.

6. Jun 8, 2013

### collector

So say you had a line of people (all stationary with respect to the planets) between earth and some other planet and you let a faster-than-light spaceship fly next to the line. It would go from earth to the other planet and then back again. What would the people observe? Let's say that the spaceship takes some time accelerate to it's top speed.

7. Jun 8, 2013

### Ookke

I think that's a hasty conclusion. We don't really know what would happen, but we will find out when the first FTL travel is observed.

8. Jun 8, 2013

### Staff: Mentor

That depends on details of the journey, but a backwards-moving spaceship would be a possible observation.

Well, we can give up causality, and ask how FTL travel in a Minkowski spacetime (=the framework of special relativity) would look like.