# Light and time

1. Oct 26, 2005

### daniel_i_l

How can we say that light has speed but at the same time say that relative to us it doesn't move through time? (sorry if this is a stupid question:uhh: )

2. Oct 26, 2005

### El Hombre Invisible

I think you misheard. It isn't relative to us that it doesn't move through time - it is in its own reference frame (i.e. one moving at c relative to us).

3. Oct 26, 2005

### daniel_i_l

Isn't light justified to say that he is at rest and so his clock is normal?

4. Oct 26, 2005

### derz

Light doesn't have a rest frame. If it did, it wouldn't travel at c in every frame of reference. This would also mean that photons have rest mass, which they don't possess.

5. Oct 26, 2005

### JesseM

No, as derz said, light does not have its own rest frame. One of the postulates of relativity is that the laws of physics should work the same in all inertial reference frames, and this would be violated if you gave light a rest frame, since clearly the laws of electromagnetism could not work the same in this frame. Also, the Lorentz transformation for transforming between the coordinates of different frames moving apart at velocity v gives infinities if you try to plug v=c in.

6. Oct 26, 2005

### daniel_i_l

So if light doesn't have a rest frame, then when we say that light doesn't travel through time is that relative to us? If so relative to who is its speed
3*10^8 meters per SECOND?

7. Oct 26, 2005

### JesseM

If you have a clock that is approaching the speed of light in your frame, the length of the ticks of the clock approaches infinity. You can test this with natural clocks like particles that have a characteristic decay time--the closer to light speed they're moving, the longer the decay time as measured in your frame. So whichever author said that light doesn't travel through time relative to us, this is presumably what they meant.

8. Oct 27, 2005

### daniel_i_l

Thanks, now it is more clear.

9. Oct 27, 2005

### El Hombre Invisible

And also what I meant when I said: "i.e. one moving at c relative to us". I was not assessing the feasibility of constructing such a frame, merely where the idea of a photon being emitted and absorbed instantly comes from: i.e., for v = c, /\T' = 0.