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Photon question -- is it a particle?

  1. Nov 25, 2015 #1
    is a photon a particle?

    Tex
     
  2. jcsd
  3. Nov 25, 2015 #2
    A photon can be considered to be either a particle or as a fluctuation in an electromagnetic field, field depending on what it it is you want to consider.
     
    Last edited: Nov 25, 2015
  4. Nov 25, 2015 #3

    Drakkith

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    There's no easy, accurate answer to that question. You either get an easy answer or an accurate answer, not both. :wink:
     
  5. Nov 26, 2015 #4

    sophiecentaur

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    Yes, you can consider a photon as being a particle in some contexts. BUT it is not a particle like a tiny bullet and you cannot make any 'mechanical' type assumptions about how it will behave. You cannot, for instance, tell 'where it is', until it has interacted with something. It can be literally anywhere, which is an odd notion until you come to terms with its abnormal particle nature.
     
  6. Nov 26, 2015 #5

    Nugatory

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    The word "particle", as used in quantum electrodynamics, has a specific meaning. A photon is a particle according to that definition.

    But there's a catch here.... The meaning of the word "particle" used in quantum field theories does not match the non-technical English-language meaning of the word, so saying that "a photon is a particle" isn't saying what you might be thinking it means.
     
  7. Nov 26, 2015 #6

    jtbell

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    The Particle Data Group (the "official" repository of particle data for particle physicists) includes data for the photon in the same table as the W, Z, gluons and Higgs:

    http://pdg.lbl.gov/2015/tables/contents_tables.html

    (in that table of contents, it's called the "gamma.")
     
  8. Nov 26, 2015 #7
    If a single photon is emitted must you be in its "line of fire" to detect it? If I was 30 feet left or right wouldn't I miss detecting it?
     
  9. Nov 26, 2015 #8

    Vanadium 50

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    Is its wavelength less than about 30 feet?
     
  10. Nov 26, 2015 #9

    Nugatory

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    What is this "line of fire" of which you speak? This thing isn't a little bullet.
     
  11. Nov 27, 2015 #10
    Let's say a photon is emitted from a source and we know the direction that's it emoted. Let's say 100 people are evenly distributed around a one mile radius sphere centered on the source.

    Clearly all 100 people cannot detect the photon. Only one (if any) will be able to detect the photon due to his position in relation to the direction of the emission.

    Can we make some general statements about this such as...

    Only the person in the direction of the emission will be able to detect the emission. This assumes the particle acts like a particle or a little bullet so that only someone in its line of fire will detect it. Or...

    Everyone will be able to detect it. This assumes it acts like a wave and everyone will be hit by the wave and thus able to detect it. If it acts as a wave its potential energy will be continually reduced by the inverse of the distance and would be nearly non existent after only a short distance ( many miles).

    So, if a single photon is emitted who among the 100 is most likely to detect it?

    Tex
     
  12. Nov 27, 2015 #11

    jtbell

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    How do we know the direction that it's emitted?
     
  13. Nov 27, 2015 #12
    let's say it is emitted in the direction of point A on the sphere using a little photon emitting gun
     
    Last edited: Nov 27, 2015
  14. Nov 27, 2015 #13

    Dale

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    Neither of these "this assumes" statements are correct. You are asking classical questions about a quantum system.


    It doesn't work like that. Photons are not bullets that can be fired from a little gun.

    If you prepare a photon state with a well defined position then the momentum will be undefined. So you could prepare states with a wide variety of wavefunctions, each with different detection characteristics.

    If you made the photon state tightly concentrated on one detector, then the probability of being detected by other detectors would be small. If you made the photon state more dispersed then the probability would be higher that other detectors would detect it.
     
  15. Nov 27, 2015 #14

    f95toli

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    That is true in general. However, there are single photon sources that can send one (and only one) photon in a particular direction (or even launch it into a waveguide). Hence, there is nothing intrinsically wrong with that thought experiment.
    In fact, on-demand single photon sources are actually sometimes (informally) referred to as "machine gun-type sources" (presumably because they send out as photons when triggered)

    Also, if it is in fact a single photon it can only be detected once. However, most sources do NOT send out single photons.
     
  16. Nov 27, 2015 #15
    If I have a powerful point or nearly point light source and surround it with a light proof sphere no light will get out. Now if I put a pinhole in the sphere only someone in line with the source and the hole will see the light. No one even slightly to one side will see the light. In fact there will only be spot of light on a distant surface.

    Therefore the photon(s) are clearly distinct and directional. Whether they are particles or waves is at dispute. Particles would explain the observation. If waves then one would have to explain how the waves AS WAVES could be so precisely contained to a ray of light. A laser is a good example of this.

    Im aware of the slit experiment. I'm not questioning that light and photons act as waves and particles. My simple question is this...is a photon a particle as it travels from source to receiver or wave?
     
  17. Nov 27, 2015 #16
    How do you avoid light diffraction?
     
  18. Nov 27, 2015 #17
    Everyone will be in line with the source and a hole: given two points in space, there always is a straight line passing through them.
    Cannot understand here.
    They are clearly distinct if you generate them in such a way, but they are not clearly directional. They don't have "nadelsstrahlung" (needle-like radiation) as even Einstein initially believed. Prove: light diffraction.
    Maybe you intended "corpuscles". Infact they are particles: as Nugatory wrote, "particle" in QM means something which has both corpuscolar and wave behaviour.
    It's easy to describe this behaviour as waves, instead. You only have to take a monochromatic wave with low enough wavelenght (low with respect to all the other dimensions involved in the experiment = geometrical optics approximation) and which is very collimated (you can obtain this making the radiation pass through distant holes made on parallele screens). Nothing particularly difficult.

    --
    lightarrow
     
  19. Nov 27, 2015 #18
    Well here the point I'm driving at. Let's just take one star...I ca seethe light front he star no matter where I move my head. Therefore light from the star permeates every cubit centimeter of space. Therefore every cubit centimeter of space is filled with "something" from the one star.

    What is the something? Jillions of photon particles or a sea of photon waves?
     
  20. Nov 27, 2015 #19
    A sea of quantum objects which are neither particles nor waves but show properties of both (depending on the conditions).
     
  21. Nov 27, 2015 #20

    f95toli

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    It is much more complicated than that. If the photons are emitted by a true single photon source they are indeed quite "particle like" (with undetermined phase). However, if the source is coherent (say a laser) it is more akin to classical source and it will be -in general- behave a bit more like a classical wave. In the the full theory of light (QED) we refer to different types of fields (and states of light). Note for most states the number of photons in not even fixed so talking about a number of photons is meaningless.

    It is important to realize that there is no mystery here. We have an extremely accurate theory for light (quantum electrodynamics, QED) meaning the "nature" of photons is fully understood, but you need to know a fair amount of math to understand the explanation: there is no simple intuitive picture.
     
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