# Light clock with spaceship moving vertical at .99c

1. Feb 19, 2013

### ebodet18

So what would happen if there was a space ship moving at 99% the speed of light straight up. The spaceship has a light clock, so a photon of light moving bouncing between 2 mirrors in such a way that the light moves ALMOST in sync with the space ship moving up. In theory wouldn't the light photon move very very slowly up to the top mirror and back to the bottom. Would time be almost come to a complete stop?

2. Feb 19, 2013

### FeynmanIsCool

Welcome to PF!
It seems that way but not really! - Why?
Lights speed is absolute, meaning, it always travels at the speed of light relative to EVERY viewpoint. So....If you were moving at .99 c, and you saw a photon, that photon would STILL be moving 300,000 m away from you every second. THIS is why time and space stretch in relativity. Since C was absolute, Einstein saw that if the speed of light was ALWAYS the speed of light, something else had to give (i.e space and time) In order for it to make sense that a photon would still be traveling away from you at 300,000 m/s if you were traveling .99% of c, Einstein postulated that you the observer would need to experience time much slower in order for this to occur. Hope that helps!

Last edited: Feb 19, 2013
3. Feb 19, 2013

### ebodet18

Ahh that's interesting! Thanks for the quick answer!

4. Feb 19, 2013

### FeynmanIsCool

No problem! Relativity is fascinating

5. Feb 19, 2013

### Staff: Mentor

Assuming the spaceship's light clock is oriented in the usual manner--parallel to the direction of travel--then the light would be moving just slightly faster than the mirror as the light traveled from bottom to top. So as observed by you on earth (if you could make measurements of the light's travel between mirrors) the light would cover that distance rather slowly. (The speed of light is the same, but the mirror is moving away from the light.) But during the reverse trip (from top back to bottom) the light would be moving towards the oncoming bottom mirror. So from your viewpoint on earth, the travel time going up is greater than the travel time going down.

Of course from the viewpoint of the people in the space ship, everything would work normally.
No. While the "ticking" of that moving light clock would be slow as observed by you, it only slows by a factor of about 7.