# Photons and Time

It's my understanding (I might be talking senescence) that as an object approaches the speed of lights time starts to slow down, and if it achieves the speed of light time would stop.
Do photons experience time?

Dale
Mentor
2021 Award
If you take the formula for proper time (the time "experienced") and if you apply that formula to a pulse of light then you get 0. However, I am not sure if it is appropriate to call that quantity "proper time" any more.

ghwellsjr
Gold Member
It's my understanding (I might be talking senescence) that as an object approaches the speed of lights time starts to slow down,
Yes, objects can approach the speed of light as close as you want and time for that object will slow down according to the Inertial Reference Frame (IRF) in which you are specifying the speeds, a clock traveling with that object will tick more slowly than the Coordinate Time of the IRF. Note that we define time by what a clock measures.

and if it achieves the speed of light time would stop.
But no object (including a clock) can approach and achieve the speed of light so your understanding is flawed on this point. Since no clock can travel at the speed of light, time is not defined at the speed of light. It's not that time has slowed to a stop at the speed of light, it's that time is meaningless (without a definition) at the speed of light.

Do photons experience time?
Since you can't build a clock out of just photons, the answer would have to be "no". But I would rather say that time does not apply to photons. You can establish the time it takes for photons to travel from point A to point B according to a particular Inertial Reference Frame (IRF) but that time can be different in another IRF because the distance between those two points can be different in different IRF's and the speed of light is the same in all IRF's.

Does that make perfect sense to you?

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RaptorHunter
PeterDonis
Mentor
See the forum FAQ on why photons do not have a rest frame:

Since photons do not have a rest frame, they do not "experience time", since that requires a rest frame. (More precisely, applying the concept of "proper time", which is what is needed to "experience time", requires the object to have a rest frame.)

I am not sure if it is appropriate to call that quantity "proper time" any more.

I don't think it is, for the reasons given above and in the FAQ.

RaptorHunter and bcrowell
bcrowell
Staff Emeritus