# B Question about the rest frame of a photon

#### Brian E

I am not an expert!

As I understand it, Einstein considered, what a guy standing in a photon will experience if he look at his watch.

My understanding says, that the guy, and the photon does not experience the flow of time, and also no other measurable change.

If my understanding is correct, should the universe then not be a singularity, when experienced from a photon?

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#### PeterDonis

Mentor
Einstein considered, what a guy standing in a photon will experience if he look at his watch.
No, that's not what Einstein considered. He considered whether it was even possible for someone to be riding along with a photon. And he realized that it was not possible to do that, because someone riding along with a photon would see it as a standing wave, oscillating in space but not in time; but there is no solution of Maxwell's Equations in free space that describes a standing wave. There are only solutions describing waves traveling at $c$.

My understanding says, that the guy, and the photon does not experience the flow of time, and also no other measurable change.
No, that's not correct. The correct understanding is that it is impossible for an observer to ride along with a photon, and concepts like "rest frame" and "flow of time" are meaningless for photons.

#### phinds

Gold Member

I am not an expert!

As I understand it, Einstein considered, what a guy standing in a photon will experience if he look at his watch.

My understanding says, that the guy, and the photon does not experience the flow of time, and also no other measurable change.

If my understanding is correct, should the universe then not be a singularity, when experienced from a photon?
The problem with your analysis is this: A guy riding on a photon would be a rest relative to the photon. This would mean that there is an inertial frame in which a photon is at rest. But by definition, a photon travels at c in all inertial frames. Thus you have posited a frame in which a photon is BOTH at rest and traveling at c. Makes no sense.

All I've done is just say the same thing the thread you linked to

#### Brian E

PeterDonis & phinds Thank you both.

I think I understand better now.
I actually were told years ago, that photons travel at c relative to any observer, but now I get it (I think )

but that must make photons able to beeing emitted without experiencing any acceleration?

#### phinds

Gold Member
but that must make photons able to beeing emitted without experiencing any acceleration?
Yeah, I had a little trouble with that when I first learned about this, but yep, that's the way it is. They travel at c as soon as they are created.

#### PeterDonis

Mentor
that must make photons able to beeing emitted without experiencing any acceleration?
Photons are created when they are emitted, and they are already moving at $c$ when they are created. So they don't need to accelerate.

#### Brian E

Okay and thanks phinds & PeterDonis.

"Question about the rest frame of a photon"

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