# Travelling greater than speed of light

1. Sep 9, 2010

### ABHISEK DATTA

It is known that in a medium of refractive index 'n' , the speed of light becomes c/n. So if we can make a medium of high refractive index, then the speed of light will be substantially less in that medium. Then it can be possible to accelerate particles inside that medium at speeds greater than the speed of light in that medium i.e. 'c/n'. Then what will be our observation from an stationary frame of reference ? What will be the observation from the particle's frame of reference ? Will the particle travel in time as it is crossing the speed of light in its surrounding medium ?

Also one problem in reaching the speed of light 'c' in vacuum was that - as the speed of a particle approaches 'c' its relativistic mass increases and at v=c, its mass becomes infinite. But in the case as described above the particle is able to cross the speed of light in that medium (of refractive index 'n') without reaching 'c' , and thus its mass does not become infinite.

So is time travel really possible for that particle in that medium of refractive index 'n' ?

2. Sep 9, 2010

3. Sep 9, 2010

### Staff: Mentor

No. Realize that the limiting speed in relativity is c = 3 x 108 m/s, which happens to be the speed of light in a vacuum, not the speed of light in some medium.

4. Sep 9, 2010

5. Sep 9, 2010

### ABHISEK DATTA

And, yeah its true that the limiting speed in relativity is 3x10^8 m/s. But my main question is whether the particle will be able to experience time travel or not ? You can assume that the medium of refractive index 'n' is the world for that particle, and in its world the particle is travelling faster than light.
For time travel, i.e. to travel to the future, i think its not necessary to cross 'c', but to cross the speed of light in your surroundings, even if it is c/n. By travelling greater than c/n, the particle with will travel to the future with respect to the observer inside that medium.

6. Sep 9, 2010

### Staff: Mentor

Well, you're wrong. Time travel (and it's associated paradoxes) would happen if particles could go faster than 3 x 108 m/s, not simply 'faster than light'.