# If you could travel faster than light

1. Jul 22, 2012

Supposing that you could travel faster than the speed of light, what would you see, feel, or experience? Since an object travels slower in time as it approaches light speed, wouldn't the object, if it did travel faster than light, enter into a complete and absolute void because nothing else in the universe traveled faster than light. I was wondering if the object would just be surrounded by a blank white canvas with no way to escape. To simplify it, would there be something or nothing?

2. Jul 22, 2012

### Simon Bridge

Hi shadow15; welcome to PF -
You do realize that this sort of topic is highly speculative right? Pure speculation is not something we do here. But I don't want to just dis your first post, that would be mean ;)

You should realize that there are all sorts of problems with FTL travel ... causality violation being one important one. When we consider it we are usually thinking in terms of what an FTL object would look like to us since we do not travel FTL ourselves.

overview from special relativity:
http://www.physicsguy.com/ftl/html/FTL_part4.html
tachyon physics

Several mechanisms have been proposed to overcome the various difficulties.
Which mechanism is employed will determine what the experience of the trip is like.

eg. in the Alcubierre drive ship you'd look out your window and see the Alcubierre space-time. In general you'd see and feel whatever the geometry of the space-time bubble you were in let you. Pick your theory.

That's the problem with this speculation - if you went FTL by going through a magic portal to fairy land then you'd see fairies. (Something which has been done in science fiction incedentally...)

3. Jul 22, 2012

### bcrowell

Staff Emeritus

Please tell us a little about the level of your knowledge of relativity so that we know what level to reply at. What book or books have you read on the topic? If the answer is none, you might want to start out by reading at least the first half of Takeuchi, An Illustrated Guide to Relativity.

Using terms that physicists understand, and on whose definitions we agree, I would rephrase your question as follows: (1) In special relativity, do FTL frames of reference exist (or more precisely, can one frame be moving relative to another at greater than c), and (2) if so, what would we experience if we were in one?

We live in a universe with three spatial dimensions and one timelike dimension, which is referred to by the shorthand 3+1. In 3+1 dimensions, there is a no-go theorem developed in this paper by Vieira
http://arxiv.org/abs/1112.4187
that says that FTL frames don't exist. (They can exist in 1+1 or 3+3 dimensions.) This is independent of any other considerations such as the existence or nonexistence of tachyons. The only assumptions are the basic assumptions of special relativity (which actually do not forbid the existence of tachyons or demand that causality not be violated).

Maybe a simpler way of looking at it is this. General relativity becomes special relativity locally. Therefore if you look out the window what you see locally is simply whatever matter is around you locally -- and that matter is moving relative to you at *less* than c.

Similarly, we can say that due to cosmological expansion, distant galaxies are receding from us at velocities greater than c, so in that sense, we're FTL relative to them. This is no big deal and doesn't imbue our own frame with any special properties or cause us to see mystical trippy stuff.

Another way of putting it is that GR doesn't have global frames of reference, so you can't unambiguously even answer the question of whether distant objects are FTL relative to you. And when you restrict to local observations, GR becomes SR. So the OP's question is really an SR question, and trying to expand it to a GR question is vacuous.

4. Jul 23, 2012