# Why do some think faster than c= time travel?

1. Oct 1, 2011

### Edi

The main, or one of the main, arguments I hear about neutrinos flying faster than light is that it would violate casualty aka. time travel.
The Light Cone is supposed to be broken, in that case, and impossible things happen.
But, even if neutrinos do travel faster than c - all we need to do is to re name the "light cone" to "neutrino cone" or any other name, witch would be simply .. wider.
or they dont travel at all faster. I dont know.

2. Oct 1, 2011

### ibysaiyan

Re: Why does everyone think faster than c= time travel?

Ok, so what point are you trying to establish here? There's a lot of speculation going on over the recent discovery announced at CERN. I have not been able to stay up to date on this issue( I will go through each and every post later in the thread made for this event).I never perceived that to achieve FTL would imply time travel. Let's say something goes at 310000 m/s instead of 299792458 m/s, does this mean we will be covering a distance of 1 light year in few seconds ? Obviously not...

I suppose people mean in the context of "these particles going through different dimensions. I personally think that physicists are more baffled about what repercussion/s this may have over a theory which has stood still for this long.

3. Oct 1, 2011

### Bill_K

Re: Why does everyone think faster than c= time travel?

The basis for all of special relativity is the Lorentz transformation, and it is extremely well confirmed that the fundamental speed involved is c, the speed of light, not some other speed.

Particle accelerators depend for their operation on the motion of particles with velocities approaching c. The Large Hadron Collider is an obvious example of this with γ ≈ 3500, but actually the speed record is held by the LEP (Large Electron-Positron Collider) that preceded it in the same location, which accelerated electrons to γ = 200,000, implying v = c to within a factor of about 10-10.

If a material particle exceeded the fundamental velocity c in one frame, other frames would see the same particle traveling backwards in time.

4. Oct 1, 2011

### Edi

Re: Why does everyone think faster than c= time travel?

Exactly - it would see [or detect it in any other way that is base on information traveling at maximum c ] the particle going backwards in time, but that doesnt mean it is!
I mean - what if you are "looking" at a superluminal particle with another superluminal [possibly even faster one] particle?

If we go down to a level where we are beings wich do not know of supersonic travel. The fastest thing for us is sound and we live by touch.
So something one day takes a ball and accelerates it to supersonic speed. Aimed at me. I would feel the ball before I hear it and the ball would bounce back and hits its thrower before .. the sound travels that distance. - it would seem as the ball can travel back in time and kill its thrower before the ball was thrown, but.. would it actually be true? Well of course no.

5. Oct 1, 2011

### Bill_K

Re: Why does everyone think faster than c= time travel?

Sorry, I didn't realize you wanted to discuss philosophy. I only do physics.

6. Oct 1, 2011

### San K

Re: Why does everyone think faster than c= time travel?

I wonder what "backwards" in time would mean. An event that has happened has happened, is done and over.

7. Oct 1, 2011

8. Oct 1, 2011

### MrNerd

Re: Why does everyone think faster than c= time travel?

I think going faster than light(if it can happen at all) would simply cause blackness behind you, as well as some serious frying power of the EM radiation in front of you. It doesn't seem very safe. Imagine hitting an asteroid going faster than light. That would clearly be a very dangerous situation, giving one very little(if any) time to blast/dodge it.

I suppose people might think time would go negative because of the equation t = t$_{0}$$\sqrt{1 - v^{2}/c^{2}}$

If v were to exceed c(faster than light), than the square rooted part would be negative. One could always say this is an imaginary situation, though(pun intended)

9. Oct 1, 2011

### Janus

Staff Emeritus
Re: Why does everyone think faster than c= time travel?

No. when we say that superluminal speeds would cause a particle to appear to travel back in time according to some frames, we are talking about after we take the delay due to light transmission into account.

With your ball example, just because you hear an event after the event took place does not mean that it seems that the order of events is switched for you. All you have to do is factor in delay causes by the speed of sound to determine when the events took place.

We can do the same thing with light. We can correct for how long it took for light to get to us to determine when events "really" happened. The problem that superluminal speeds cause is that even after you factor out the light delay, you can be left with frames disagreeing on causality of events.

10. Oct 2, 2011

### kmarinas86

Re: Why does everyone think faster than c= time travel?

The originating assumption here is the "spacetime metric". The idea that faster than $c$ travel implies traveling backwards in time is true if space and time are combined in the way defined by special relativity.

11. Oct 2, 2011

### zaybu

Re: Why does everyone think faster than c= time travel?

If you like a simple graphic explanation, see here: http://soi.blogspot.com/2011/09/why-ftl-violates-causality.html

12. Oct 2, 2011

### Saw

I am a layman, although I have studied SR to a good extent and think I understand it reasonably well. And, yes, I tend to believe something akin to what you suspect. Probably there is a mistake in the CERN experiment and neutrinos do not travel FTL. But if they did, causality would not be affected.

Causality is not a theory, it is reality, something that seems quite untouchable. In any case, what is not so untouchable is time. Time is a concept we invent to solve problems, like predicting whether an influence (ie a cause) will produce an effect. We feed it with actual measurements. But if in the end we get disoriented, we stop making a judicious use of the idea and end up questioning reality and causality, then there is an easy solution: please just give up the concept, stop measuring and thus causality will be restored…

Being a little more specific, it appears to me that the obvious mistake is attributing to relative time the features of absolute time. Certainly, “relative simultaneity” is what you actually get, when you measure. That is how the universe works and we have to live with it. By the way, the system works quite well and it solves most problems. But it is not “absolute simultaneity”. It does not mean the same thing, it does not contain so much information as the latter. In particular, with relative simultaneity you solve all the problems where the cause travels not faster than light. If the distance between two events is timelike or lightlike, all reference frames agree on the sequence of those events, ie which one was the cause and which the effect. If instead the distance is spacelike, there is discrepancy about the order but that is irrelevant, because those events cannot be causally connected. However, if we now assume that FTL travel is possible, then we must simply recognise that we do not know the answer to some questions. For instance, we do not know if by sending a FLT projectile from event A towards B we can prevent the latter to take place or vice versa. We do not know this because our information is not precise enough. It is not information about absolute simultaneity, which is what the pre-SR physicists expected to have: the certainty that two distant events, if simultaneous, are already unavoidable to each other, even if an instantaneous projectile is sent from one to the other, as if there were no distance in between.

Precisely this approach is very appropriate for showing the absurdity of the idea of FTL travel causing causality violations. The author prefers to talk about a hypothetical instantaneous influence, instead of purely FTL. He argues that this way the principles are the same, only the numbers change, which I suppose is right.

A possible development is as follows:

The distance between events A and B is spacelike.

Event A is that you send an instantaneous projectile from the ground to a travelling ship. According to the simultaneity line of the ground frame, you are in time to prevent event B in the ship and hence you do. That is paradoxical, because according to the ship frame, events A and B were simultaneous and according to a third frame it was even the other way round, event B took place before A.

Furthermore, the receiver in the ship fires back another instantaneous projectile. In the simultaneity line of the ship, you get the projectile… before you sent your own. That is to say, you get your punishment before your crime… And what if that kills you?

Did I make any mistake? If not, I tend to think this means too many paradoxes. There is an easier explanation. We are mixing apples with pears.

Hm… Allow me to be a little provocative, with all respect for your comment. Predicting what may happen and what may not, is it not physics? Wiping paradoxes off and polishing the concepts of the discipline, is it not physics? Conversely, is it only causality violations and the multiverse what is physics?