# How do we Prove That Photons Exist?

*Most recent answer: 03/07/2015*

- lila (age 13)

jordan

We don't ever really "prove" any physical ideas, but we do sometimes come up with very strong evidence. For the existence of photons, I'll give just a little of that evidence.

0. The thermal radiation from an object doesn't obey classical statistical theory. The error is a factor of infinity. Planck showed in 1900 that the spectrum of thermal radiation could be very accurately predicted if the energy came in packets of size hf, where h is Planck's constant and f is the frequency of the light. You can see the evidence for these effects if you watch the color of the glow from a piece of metal change from dull red to white as it's heated.

1. The photoelectric effect shows that the energy from light is delivered to a metal surfaces in packets of size hf. (We now do this as a lab for undergraduates.)

2. A sensitive photodetector doesn't give a steady output but a series of random clicks, just as expected for photons. (This is something I saw repeatedly in my lab.)

3. The Compton scattering of electrons and light acts like the light comes in packets of energy hf and momentum hf/c, just like photons should do.

4. The ratio of the spontaneous emission of light to the stimulated emission (as in lasers) is well described by the photon theory.

Of course, there's much more.

I don't know much about what would be needed to make progress on the dark energy and big bang issues, but people say that these are among the tools needed:

1. General Relativity, based on differential geometry.

2. Quantum mechanics, based on linear algebra.

3. Quantum field theory.

4. Group theory.

5. Maybe String Theory, which requires all of the above.

People who understand this business would probably say I've left off some other essential parts of the background. So you've got plenty of things ahead to learn!

Mike W.

*(published on 03/07/2015)*