Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Evidence of photon existence

  1. Apr 17, 2014 #1
    Hi to everyone!

    I'm searching information about evidences of photons existence. It seems like the photoelectric effect isn't for itself a proof of photons existence. Some people tried a semi-classical discussion of this effect (Lamb - "The Photoelectric effect without photons").

    I'm looking for an experiment that shows photons existence not only in the process of emitting or absorbing them, also in the electromagnetic waves while they travel in space. I heard about an experiment where researchers made an atom of a crystal emit a high energy photon. The atoms break away from the crytal, showing that photons have momentum, because of the conservation of this physical mangnitude. This would be a proof of the fact that photons are emited in a single direction, and that their energy is not distributed in a spherical wave; otherwise the atom will rest in his original place. I searched a lot in the web, but couldn't find any reference to this experiment.

    If you know any other evidences os the existence of single photons in waves, not only when they ara emitted or absorbed I would be glad to read about them.

    Thank you very much for your help and time. Have a nice day!
  2. jcsd
  3. Apr 17, 2014 #2


    User Avatar
    Science Advisor
    Gold Member

    Welcome to PhysicsForums, JCMaxwell!

    Well, evidence is often in the eye of the beholder. One of the best proofs of the existence of the photon is the following experiment. However, it alone may not satisfy what you are seeking. It may take a series of experiments and theory taken together.

  4. Apr 17, 2014 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    In addition to the which-way experiment, there are also the photon antibunching experiment, multiphoton photoemission, etc...

    Please note that while the NAIVE version of the photoelectric effect may be explained (or more like "mimicked") by a stretched-out classical picture, the more detailed version of this phenomenon (resonant photoemission, angle-resolved photoemission, multiphoton photoemission, etc..) does NOT have ANY classical description whatsoever. There has been ZERO classical picture being offered to account for these phenomena or where they are used to describe the results. This is similar to the idea that if you squint enough, a cow will look like a sphere, but if you open your eyes wide and get closer, that sphere model completely falls apart.

  5. Apr 22, 2014 #4
    I want to thank you both for your replies, they were very helpfull. The experiment that DrChinese linked in his post looks very interesting. I will also check the other experiments that you enumerate in yout message, ZapperZ. To be honest, I only knew the "naive" version of the photoelectric effect. My physics knowledge is small at the moment, I am on my first university year. I will try to read about those details of the photoelectric effect that can't be explained without photons.

    Thank you again for your help and time.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook