# Speed of light?

1. Jul 26, 2010

### smslca

Is there any situation in the world of physics that speed of light > (c=3*10^8)
or can it be? IS there any possibility?

2. Jul 26, 2010

### K^2

Phase velocity of light in some metamaterials can exceed the speed of light, but not group velocity, so you can't carry any information that way.

There are some experiments for sending information faster than light using quantum tunneling, but there is nothing that actually propagates faster than light there.

Finally, you can play some tricks with metric, but it only makes things travel faster than light relative to external observer. In local space, they are still moving at most at the speed of light.

So if you are looking for actual propagation at faster than light speed, no, it doesn't happen.

3. Jul 26, 2010

### smslca

what are these tachyons. Are tachyons correct in the theory. It is said that they can move faster than light .(read in wikipedia)

4. Jul 26, 2010

### smslca

is this Quantum entanglement related to speed of light? As said in
http://en.wikipedia.org/wiki/Quantum_entanglement

"If the distance between particles is large enough, information or influence might be traveling faster than the speed of light which violates the principle of special relativity. One experiment that is in agreement with the effect of entanglement "traveling faster than light" was performed in 2008. the experiment found the "speed" of quantum entanglement has a minimum lower bound of 10,000 times the speed of light. [5]However, because the method involves uncontrollable observation rather than controllable changing of state, no actual information is transmitted in this process. Therefore, the speed of light remains the communication speed limit."

what does it actually mean

5. Jul 26, 2010

### Nabeshin

Tachyons are pretty much complete rubbish. They're simply a mathematical possibility in the equations. To illustrate how ridiculous they are, I'll give an analogous demonstration:
Suppose we have a one-dimensional kinematics problem. A runner running with constant acceleration starting from zero velocity and some initial position. His displacement is described by:
$$x=x_0 + \frac{1}{2}at^2$$
We can solve for t...
$$t=\sqrt{\frac{2\left(x-x_0\right)}{a}}$$
So apparently, for a negative (or x0 > x ) we have imaginary time. Therefore we have given evidence for the existence of imaginary time. Might as well call these runners who move on an imaginary timeline tachyonic runners and make a wikipedia article about it.

Obviously this is ridiculous, we're just abusing equations. In my opinion, and that of a fair number of physicists I know, tachyons are much the same way (of course, my example is overly simplified but I think the point is clear).

6. Jul 26, 2010

### mgb_phys

If you 'mix' two particles and then separate them.
If you observe the state of one particle you force it to collapse into a particular state and force the other particle to collapse into the other state.
This message to collapse is faster than light (in some interpretations of QM) - but since you can't pick which state your particle collapses into you can't force the remote particle to do anything in particular.

7. Jul 27, 2010

### FawkesCa

the only theory, i know of, where something can travel faster than light (and still transmit information) is in the Big Bang Theory in regards to expansion. seems like only space itself can travel faster than the Speed of Light

8. Jul 27, 2010

### K^2

No, entanglement doesn't have anything to do with the speed of light. There is no super-luminal communication between entangled pairs.

Anything that moves faster than speed of light cannot absorb/emit photons, gluons, or gravitons. So whether or not tachyons exist is sort of a moot point, if they cannot interact with anything.