Why is the speed of light constant?

The subject came up the other day in the breakroom. ( I am an x-ray tech. in an inner-city trauma center). We were discussing the physics of radiation protection, and some how the following question was brought up.

If photons of light are particles (and supposedly waves too), then they have mass. Shouldn’t photons slow down as they pass through a medium such as the Earth’s atmosphere?

That’s the beauty of it - photons have no mass. At least, that’s the accepted theory. They don’t accelerate to c, because they come into existance already going that fast.

Photons don’t have any (rest mass), this is why they can travel at c, the reason they do is because they can’t travel at any other speed (in otherwords something travelling at lightspeed cannot accelarte or deccelrate).

Light does slow down in mediums like the atmospehere bu this is to do with the absorbtion and re-emission of photons by electrons in the air molecules.

How can a photon be considered a particle if it has no mass?

Right. The “constant” is the speed-o-light in a vaccuum.

Follow up question:

If the speed of light is constant, that means if you turn on a flashlight, the light is moving at the speed of light the moment it is created. How can it go from 0 to 186,000 miles a sec. without speeding up?

It was never going zero. The instant those photons came into being, they were going c.

“Particle” means that it has the property of being approximately in one place. It doesn’t mean that it has mass.

Think of particles as packets of energy (which, by E=MC[sup]2[/sup], is the same as mass). Particles such as the photon (and the theorized graviton) have energy - as well as momentum.

It doesn’t have to be intuitive.

I believe light is also a wave.

Well, not exactly a wave. We’ve all heard this before but I’ll regurgitate anyways.

Photons behave in a manner similar to both the classical wave, and the classical particle. It acts according to the wave equation like any regular electromagnetic wave and can be visuallized as a wave but also acts as a particle when knocking electrons out of atoms in a photoelectric effect experiment.

Having said that, I don’t think it accurate to say it is a wave, or is a particle. If think there are major areas of study today trying to figure out what it is. It just has properties of both.


Excatly. Photons are neither particles nor waves. They have some properties of each, and some all of their own.

Just to mess with your heads, electrons are also neither particles nor waves. In a sense, they’re “localized probabilities”.

No, that is most definitely not the cause. Absorption and re-emission results in the reflection of the light. You need to look a lot deeper than that for the cause of light slowing in a medium. Maxwell handles it easily by dealing with it as an E-M wave phenomonon. QED is much messier, involving absortion and re-emission of virtual photons, not actual photons.

A medium that is transparent to a given frequency range of electromagnetic radiation doesn’t have any energy transitions that correspond to the frequencies of the incident light.

But photons can still interact with the medium via virtual interactions in which an atom-photon energy state is formed. Since these states are not “allowed” the atom must immediately reemit the light, but these interactions still take some time and therefore the light is both slowed and refracted.

I remember a news story a while ago about some guy who had managed to slow light down until it stopped completely. Here is the story:


So are you saying that actually light hasn’t been stopped, it just looks like it has, as individual photons are still travelling at c? I don’t suppose you could try to explain in laymans terms how that works, or does it depend on horribly complicated maths?