# Explain photons to me

1. Jan 27, 2004

### Gale

Someone explain photons to me... please... i'm trying to imagine the whole massless particle thing... but i'd like a good definition of what a photon really is.

2. Jan 27, 2004

Staff Emeritus
Hey, welcome back Gale. Haven't seen you around for a while.

The photon is the quantum of the electromagnetic field. And a quantum is a little chunk (story is that when Planck was a student, the latin word quantum, meaning how much, was used for a chunk of anything, as in "Hey Hans, gimme a quantum of cheese").

So a quantum is a little chunk of action, but if you're not too cool with action, you can think of it as energy. A little bit of electromagnetic energy. Energy is only transferred in whole multiples of these chunks. Since light is a form of electromagnetic energy it is only transferred in whole number batches of these chunks; a light ray is a bunch of photons.

So light, and other forms of EM radiation like radio, microwave, and x-ray is made up of photons.

Now you can also think of light as EM waves and if you do that, experiment shows you something odd. In general waves can wave to and fro perpendicular to the direction of motion, or back and forth in the direction of motion. Waves in a rope that you swing from one end do the first, that is transverse vibration. Sound waves in air do the second, that is longitudinal vibration.

And the odd thing they discovered about light is that it has NO longitudinal vibration. Now working from the wave representation to the partical representation in the math, that turns out to mean the photon can't carry any mass. Any mass in the photon would mean a corresponding longitudinal amount of vibration, and there isn't any, up to some extremely delicate experiments.

Well it's a lucky thing the photon has no mass because relativity says, the only things that can travel at the speed of light are things that have no mass. You know that story, as a massive object goes faster and faster (relative to its starting point) its energy goes up and up and becomes infinite as the speed approaches light speed. But a massless particle has no proper energy to increase, so it can go at light speed - indeed it has to, because there can be no frame of reference in which it is truly at rest.

I'm going to stop now, and let you see if you want to ask more questions.

3. Jan 28, 2004

Doh - I was enjoying that!

4. Jan 28, 2004

### Gale

yeah, i was enjoying too. it was just getting exciting!

but ok... i guess i'll ask a few questions...

so its a quantum. understood. massless, because it must be... now my questions...

>>

...actually... i've tried replying about thirty times now, and i can't ask any questions without completely changing the subject of the thread. soo... i'd say just continue where you left off love, it was good that way. just, point of clarity what do you mean when "its energy goes up" as its speed increases, and about how a massless particle has no "proper" energy to increase?