Constancy of Speed of light

[madhumadanan]

Dear PF i am new to this forum .

Just wanted to clear a doubt about relativity.

consider a inertial system on a light ray moving at c . If the light ray has to travel betwwen two points in space wont the observer on the light ray feel that the light has travelled almost instantly form the source to destination because of the length contraction observed . So how can light's speed be finite when it is almost instantaneous!:confused:

just tried to substitue the muon example with a light ray and got confused can anybody clarify that for me :surprised

[Physics Monkey]

You can't have an observer "on the light ray", this isn't allowed in relativity because light always moves (at the speed of light) in every inertial frame. A light ray doesn't have a rest frame, which is the frame an observer "on the light ray" would have to be in. Hence, no such observer can exist.

So how can light's speed be finite when it is almost instantaneous!

You answered your own question! Light is only "almost" instaneous because it moves very fast compared to our everyday experience.

Also, this is the quantum physics forum. If you have more questions about relativity, you should post in the relativity forum http://www.physicsforums.com/forumdisplay.php?f=70 . You will get a better response there.