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

Photons as particles

  1. Oct 21, 2012 #1
    If you had a perfectly sealed box that did not absorb energy, and was a perfect mirror on the inside, and you fired a stream of photons through a photon sized hole in the side of the box such that they would not reflect (bounce) back out through the hole, and you placed a photon detector inside the box, why do the photons disappear the instant you close the hole?

    If photons are particles shouldn't they be trapped inside the box and detectable (seeable) forever?
  2. jcsd
  3. Oct 21, 2012 #2

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    What makes you think they wouldn't stay there forever? (Well, until you destroyed them by detection)
  4. Oct 21, 2012 #3
    By simply seeing a photon (detecting if your eyeball is a photon detector in the visible range), you destroy it?
  5. Oct 21, 2012 #4

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Yes - your eye absorbs it.
  6. Oct 21, 2012 #5
    So, a photon is not a particle then...

    I am seeking something more than the typical classroom answer.

    Back to the original question -- if photons were trapped inside the theoretical perfect mirrored box, it would be light in there forever, presuming we shot visible photons in...
  7. Oct 22, 2012 #6


    User Avatar
    Science Advisor

    Your idea of what the meaning of the term "particle" in physics is, seems to be way off. In a nutshell particles can only be created or annihilated in discrete amounts. The term "particle" does not imply particles are like tiny balls or something.

    It is not a realistic scenario as the photon gas and the walls will be at different temperature and will tend to thermalize, but for the sake of the simple scenario: if you managed to realize the unphysical case of a perfectly sealed box filled with mirrors which only reflect and do not absorb or transmit anything and if the photon number is so low that interactions with the vacuum state do not take place, the photons will stay inside the box forever.

    However, please note that the assumption of that perfectly reflecting box is already unphysical.
  8. Oct 22, 2012 #7

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    One of the least efficient ways to learn is to post a bunch of incorrect statements and wait for someone to correct them. We discourage that approach here.
  9. Oct 22, 2012 #8
    In the classroom I was taught that a photon is a "wave packet"; that's certainly not perfect, but I find it better than "particle" which leads to lots of misunderstandings. However, "wave packet" also has another meaning, notably in QM; thus both terms can be misunderstood. I guess that there is just a lack of English words...

    (compare http://dictionary.reference.com/browse/particle?s=t :
    Physics .
    a. one of the extremely small constituents of matter, as an atom or nucleus.
    b. an elementary particle, quark, or gluon.
    c. a body in which the internal motion is negligible. )

    PS I forgot: Welcome to Physicsforums :smile:
    Last edited: Oct 22, 2012
  10. Oct 22, 2012 #9
    Maybe Quick Karls question simply leads back to our (current) lack of ability to comprehend the meaning of wave-particle dualism...?
  11. Oct 22, 2012 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    Current formalism of quantum mechanics has no such "lack of ability". What we have trouble doing is trying to shoehorn such formalism in outdated classical analogues. People are asking the photon to be described in ways it isn't.

  12. Oct 22, 2012 #11
    I find that there's a basic difference between what we are able to describe mathematically and what we are able to comprehend. Inevitably this will lead to an abundance of stupid and/or strange questions seen from the photons perspective :-)
  13. Oct 22, 2012 #12
    :smile: :tongue2: :biggrin:
  14. Oct 22, 2012 #13
    The most extreme idealization that you can make of this scenario is a single photon in a perfectly reflective box. That corresponds to the basic QM problem of a single particle in an infinite potential well, which is covered in pretty much every introductory textbook. The answer is that yes, once inside the particle will just keep bouncing around forever. You can see that by solving the time-dependent Schrodinger Equation for an infinite potential well.

    However, if you put a detector in the box, that means you are putting something in the box which will interact with the photon. In this case, the photon will hit the detector and be absorbed. More precisely, it will be converted into the energy which is necessary to trip your detector, whatever that may be.

    The bottom line is that the photon acts like a particle in some ways, but in other ways it does not. Specifically, it does not necessarily live forever the way that a particle does--by interacting with another object, it can be absorbed, after which there is nothing left in the box. Until that point, though, it will indeed keep bouncing around.
  15. Oct 22, 2012 #14
    Excuse me, gentlemen,

    I should have known better than to ask a question on an internet forum -- it seems always to bring out the best in people...

    Nevertheless, the question still remains unanswered, unless you consider quips, sound bites, and insults, to be answers.

    Vanadium50, if I am too far below your vast intellect, please, feel free to delete my account immediately.
  16. Oct 22, 2012 #15

    Thank you. At least I have a path to follow now...
  17. Oct 22, 2012 #16

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Asking a question is not the same as making a statement. The former usually works better if you want an answer.
  18. Oct 23, 2012 #17
    Sorry too: I thought that post #2 already already answered your question (and you failed to engage :uhh:). But now that I read it again, I realize that it may have been not that clear for you, so that you perhaps did not understand that it was a serious reply! :eek:
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Photons as particles