# B Local/proper time of EM waves?

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1. Jul 7, 2017

### stoomart

This may be a dumb question, but some of these other thread got me wondering: is there a concept of local/proper time for electromagnetic waves?

I imagine the only 'clock' that could measure time (ticks) at the speed of light would be the field oscillations.

2. Jul 7, 2017

### phinds

No. There's a FAQ here somewhere on why there is no such thing as a rest Frame of Reference for EM waves (and thus the concept of time is meaningless for them).

Here it is:

3. Jul 7, 2017

### stoomart

Thanks phinds, I assumed that was the case, but the following from the Spacetime article on Wikipedia has me confused:

In other words, the spacetime interval between two events on the world line of something moving at the speed of light is zero. Such an interval is termed lightlike or null. A photon arriving in our eye from a distant star will not have aged, despite having (from our perspective) spent years in its passage.​

Is this "spacetime interval" only applicable in SR?

The idea that light does not age seems odd to me considering CMBR and red-shifting, though I suppose that has more to do with expansion/stretching than "aging".

4. Jul 7, 2017

### Staff: Mentor

The spacetime interval is a fine concept, and it works for timelike, spacelike, or null paths. But the spacetime interval is only called proper time for timelike paths.

5. Jul 8, 2017

### Orodruin

Staff Emeritus
Redshift is not something intrinsic of a light signal. It depends on the motion of the source, the path, and the motion of the observer. You cannot say "that light pulse has frequency X" without specifying an observer, at least implicitly.

6. Jul 8, 2017

### stoomart

That makes sense since distant observers would record a different frequency. Would it be accurate to say "that light originated with energy X"? If so, does this imply light doesn't lose energy while traveling through vacuum, but instead the energy is simply spread/stretched out?

I imagine a CMB wave being emitted as something like a slinky over a certain period of time from the source, which is observed as something like spaghetti over a much longer period of time until the whole string (photon) has been received.

7. Jul 8, 2017

### Orodruin

Staff Emeritus
No. Energy is also observer dependent.