# Faster than Light Travel Side Effects

1. Nov 24, 2009

### Ultrastar 1

I was doing research on faster than light travel and a question popped into my head: what would be the side effects of faster than light speed travel? Any ideas?

2. Nov 24, 2009

### Integral

Staff Emeritus
Use your imagination, write a novel if you wish since physics has no answer to that question. All physics can say is that is impossible for a body with mass to reach or exceed the speed of light.

3. Nov 24, 2009

### Rubix

well i've thought about this. If you turned around while traveling the speed of light and stayed stationary relative to yourself, you would see nothing, because there would be no photons entering your eyeball. And if you moved your head slightly forward while facing backwards you would see a picture, and it would go away when you stopped moving your head forward.

4. Nov 30, 2009

### anvesh111

.....but u know something about LHC(large hadron collider) which is an accelerator which makes the protons to travel ,speed of light

5. Nov 30, 2009

### Dmitry67

no.
It accelerates protons to ALMOST speed of light.

6. Nov 30, 2009

### Dmitry67

You forgot about the flying unicorns and dancing pink elephants.

7. Nov 30, 2009

### twofish-quant

Not true at all. I can think of several papers that have asked the question "what would tachyons look like".

What you can show is that *if* you could send information faster than the speed of light *and* our understanding of relativity is correct *then* it would be possible to send information back in time, which would cause all sorts of problems. You can also make the statement that we know of no mechanism by which information can travel faster than light.

There's no way that physics can declare FTL travel impossible, merely that it has not been observed and would cause huge, huge problems if it existed.

8. Nov 30, 2009

### Dmitry67

Correct.

Interestingly enough, Faster then Light (tachyons) is more 'compatible' with Relativity then 'Body moving at speed of light'. In some sense, moving FASTER then c is "easier" the moving at c :)

SR denies FTL indirectly (via causality), but it denies observer moving at c directly (it is not possible at all).

9. Dec 1, 2009

### Chronos

A bifurcated universe may me theoretically possible - one like ours where nothing can exceed c, and another where c is the minimum speed.

10. Dec 4, 2009

### Lsos

It's like asking "what would happen if things fell up, instead of down?" It's kind of a meaningless question because that's just not how the world works....

11. Dec 4, 2009

### Wallace

Tachyons aside, you can start to think about what an 'FTL' traveller would observer if you play around with the Alcubierre metric. The term 'metric' in this context can be taken to mean particle space-time described in terms of General Relativity. The Alcubierre metric shows how a 'warp drive' could be constructed that is compatible with GR. Crucially, it does not permit FTL travel 'locally', which is what is prohibited by SR, yet effectively achieves FTL globally.

This metric still violates the laws of physics, because in order to make space-time warp in the ways demanded by the metric, you need exotic energy sources which violate known laws of physics. None the less, it still provides a self-consistent GR description of FTL. Playing around with the paths of light rays in that space-time would answer some of the questions posed in the OP. It's still science fiction, but has a modicum of credibility and some plausibility.

12. Dec 4, 2009

### twofish-quant

It's actually not a meaningless question because you might end up in a situation in which things do fall up instead of down, and you also get to think about questions about what "up" really means. People have thought a lot about what happens if you travel faster than light, and the problem is that you end up being able to kill your grandfather. Thinking about this is interesting because it means that there are situations in which you can have two points move away from each other FTL, and you don't run into any problems unless you can exchange information.

One of the really weird ideas that I've heard is that the reason you can't travel faster than light is that if you lived in a universe where you *could* kill your grandfather, then intelligence would be impossible. For intelligence to exist you have to have a universe in which there is a past and a future. Meg Tegmark wrote a paper in which he argued that we live in a 3+1 universe because a universe with any other set of dimensions would not have a clearly defined *past* and a clearly defined *future* and without a past and a future, intelligence would be impossible.

As with all weird ideas, you have take them with a grain of salt, and think about them some more.

13. Dec 4, 2009

### twofish-quant

The other thing that happens when you do GR is that things get a lot more complex, and so you have to throw a lot of brilliant mathematicians at the problem to see what's possible and what's not. One question which I think is still open is whether or not general relativity can allow for situations in which you *can* go back and kill your grandfather. The answer is probably *no* (because it would be a total mess otherwise), but I don't think anyone has mathematically proven it (and no one has done it because the math is hard.)

This is an example of mathematical physics at work, because if you try to prove something like this, you either find a weird situation in which you can go back in time, but even if you don't, you understand more about the theory. If it does turn out that GR prohibits closed loops, it would be interesting to know *why* that is the situation.

14. Dec 4, 2009

### twofish-quant

One other thing that mathematical physicists do is to ask the question "what sort of theories of physics allow for time travel and FTL and what ones don't." After all, general relativity could be wrong, and so if you have another theory of physics, it would be useful to take that, and say "yes it does allow for killing your grandfather, or no it doesn't." If you have a theory of physics in which it is *easy* to travel faster than light, then it's likely to be wrong. I can't say that traveling FTL is impossible. I can say that it's not routine.

One theory of physics that's fun to play with is to imagine a universe in which there are *two* dimensions of time.

15. Dec 4, 2009

### Wallace

The Alcubierre metric isn't *that* complex, and isn't too complicated to analyse, although I guess that statement is a relative one, depending on ones mathematical training.

This is a really cool metric though, and it doesn't lead to close time-like curves, because there is no local FTL travel.

There are some other solutions that claim to allow closed time-like curves (such as Goedels (sp?) infinite rotating rod), but that's a different story. In any case they don't have 'what if you killed you grandfather?' paradoxes, due to the way they are constructed (I could go into the details, but it's a long story, and possibly not very interesting. If you'd like to discuss it I'd be happy to, but probably better in a new thread).

16. Dec 4, 2009

### twofish-quant

The Alcubierre metric is cool because it says that *if* you had negative energies then you can do weird stuff. Also to answer the question of the OP. If you had an Alcubiere spaceship, you'd been in a "warp drive bubble" so while you are in the bubble, nothing weird would happen.

Also there are ways to avoid the causality problem. One is to require that you need to make one trip at slower than the speed of light. Another is to say that in order to travel FTL, you need enormous energies

17. Dec 4, 2009

### Wallace

It's more fundamental than that. You can fly from here to andromeda and back in one of these bubbles and it doesn't matter how short the trip seems to you or anyone else watching, there are no causality problems. It's technically not FTL travel, even if it seems that way.

18. Oct 19, 2010

### Sagittarius A

What of faster-than-light travel and it's effects on causality? Space-time? The body of mass itself?