# Measurement of the Speed of light

• B
Hi,

What is people measuring when they measure the speed of light (c) actually? the photons or the waves? or some kind of manifestation of light?

I ask because I was reading about photons and that they "might" have a very small mass and therefore are not travelling at c. If photons have a mass, and are traveling slower than c what exactly is being measured when they measure the speed of light?

Thanks

## Answers and Replies

PeterDonis
Mentor
2020 Award
What is people measuring when they measure the speed of light (c) actually? the photons or the waves? or some kind of manifestation of light?

They're measuring light. The different things you mention are not different possible things they could be measuring; they're just different ways of describing light in ordinary language.

I was reading about photons

Please give a specific reference.

they "might" have a very small mass and therefore are not travelling at c.

This is logically possible in the sense that you can write down a self-consistent theory of light in which photons have a nonzero mass; however, all experiments have shown that photons have zero mass.

• anorlunda
If the photon is not a strictly massless particle, it would not move at the exact speed of light, c in vacuum. Its speed would be lower and depend on its frequency. Relativity would be unaffected by this; the so-called speed of light, c, would then not be the actual speed at which light moves, but a constant of nature which is the upper bound on speed that any object could theoretically attain in space-time. https://en.wikipedia.org/wiki/Photon#Experimental_checks_on_photon_mass

and also

But some theories allow photons to have a small rest mass and one consequence of that would be that photons could then decay into lighter elementary particles. http://physicsworld.com/cws/article/news/2013/jul/24/what-is-the-lifetime-of-a-photon

PeterDonis
Mentor
2020 Award
If the photon is not a strictly massless particle, it would not move at the exact speed of light, c in vacuum.

This is more correctly phrased as: if the photon were not a strictly massless particle, then the speed of light in vacuum would not be precisely equal to the invariant speed--the speed which is the same in all reference frames. The speed of light in vacuum would instead be variable, always lower than the invariant speed, but it would be possible in principle (though not necessarily likely in practice) to find a reference frame in which a photon was at rest.

In other words, our usual terminology, which refers to the invariant speed as "the speed of light", assumes that photons are massless; if that assumption were to turn out to be wrong, the correct response would be to change our usual terminology, so that it would draw a clear distinction between the speed of light and the invariant speed.

some theories allow photons to have a small rest mass and one consequence of that would be that photons could then decay into lighter elementary particles

If there were any.

Mister T
Science Advisor
Gold Member
What is people measuring when they measure the speed of light (c) actually? the photons or the waves? or some kind of manifestation of light?

They are measuring the speed of a beam of light. That beam can be modeled as either a stream of photons or a wave.

I ask because I was reading about photons and that they "might" have a very small mass and therefore are not travelling at c.

That depends on your definition of ##c##. You either define it as the fastest possible speed or your define it as the speed of light. Right now we don't need to make that distinction because all observations and experimental results indicate that those two speeds are the same.