# Light vs. time

1. Feb 10, 2010

### binbots

If time stops at the speed of light, how does light travel throught space and time? Example: The light from a star 12 light years away travels throught space and takes 12 years to get to earth. But in those 12 years the time on the star has stoped. Same goes for the light that is leaving earth and traveling to that star. No time is exchanged. Isnt the fact that light itself traveling at the speed of light mean there is no time at all, and that all things we see are now?

2. Feb 11, 2010

### torquil

These popular statements about "time stopping at the speed of light" etc. are not very precise, and lead to a lot of confusion. They don't specify which time one is talking about. In SR there are many different "times" and this must be specified. One must always specify the observer, and the relationship between the observer the phenomenon to be studied. This seems like a good intro to SR:

http://galileo.phys.virginia.edu/classes/252/

Torquil

3. Feb 11, 2010

### HallsofIvy

Staff Emeritus
In relativity you can never say "it take 12 years". You must specify a frame of reference. I believe you mean "12 years in earth's frame of reference".

What do you mean by "time on the star". Did you mean to say "time in the frame of reference of the light as it is traveling from the star to the earth"? It is true that time has stopped in the frame of reference of the light. Which is why it is better never to refer to such a frame of reference!

The fact that a certain amount of time has passed in one frame of does not mean that same amount of time has passed in another frame of reference. You cannot draw inferences from relativity while ignoring the basics: everything depends on the frame of reference.

4. Feb 11, 2010

### Matterwave

I think the second postulate of SR, the constancy of the speed of light, would prohibit us using the frame of reference of a photon. In that frame of reference, the photon would be stationary; however, the photon must travel at c in every inertial frame, therefore, there can be no such frame in which the photon is at rest.

I think this is equivalent to saying that no observer can travel at speed c.