1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why do we say photons travel ?

  1. May 18, 2008 #1
    We consider light is made up of material particles (photon), and we know photons travel at speed of light having zero rest mass so we say photons travel but how can something travel at this speed , we say electric forces are experienced by a test charge due to some other charge , the field spreads out at speed of light but it should be that nothing travels or spreads out as the information is already present there , it just gets changed/communicated to that charge , instead of saying photons travel the energy is communicated & nothing travels , energy gets communicated from one atom to other , we cannot see in absolute darkness as there is no energy to be communicated


    It should be that energy is communicated as there is change in energy when light is there but it is already present in darkness too (we can't see anything as there is no change in energy )
     
  2. jcsd
  3. May 18, 2008 #2

    Astronuc

    User Avatar
    Staff Emeritus
    Science Advisor

    We don't consider photons as 'material', which would imply mass/matter. Photons do travel from their origin to some point of interaction with matter, e.g. electron usually or if high enough energy, a nucleus.

    Consider that light travels to us from stars which are thousands to millions to billions of lightyears from us.
     
  4. May 18, 2008 #3

    DaveC426913

    User Avatar
    Gold Member

    I don't understand the above statement, but:

    Photons can be blocked, redirected and intercepted. Photons can even impart their momentum upon a massive object.

    If it walks like a duck...
     
  5. May 18, 2008 #4

    russ_watters

    User Avatar

    Staff: Mentor

    Even when light was thought to simply be some sort of wave traveling on a medium, it was still correct to say it "travels", just like any other wave travels despite the media having no net motion. That's what you are describing. Clearly, information (not to mention energy) is transmitted from one place to another.
     
  6. May 18, 2008 #5
    A photon is an electromagnetic wave as opposed to an elastic wave, such as a sound wave which propagates through some medium. Electromagnetic waves do not require a medium in which to propagate.
     
  7. May 18, 2008 #6
    Is the vacuum not a medium?
     
  8. May 18, 2008 #7

    DaveC426913

    User Avatar
    Gold Member

    No. A medium is, by definiton, a substance. A vacuum, by definition, is not a substance.
     
  9. May 18, 2008 #8
    Right. In absence of a medium, there could be a vacuum.
     
  10. May 18, 2008 #9

    Danger

    User Avatar
    Gold Member

    Actually, in the absence of a medium, there has to be a vacuum. Those are pretty much the only available options.
     
  11. May 19, 2008 #10
    The OP question is quite stimulating. What if photons don't travel at all, but we have a photon generated at the source, an electromagnetic wave travelling from source to detector and another photon generated at the detector?
     
  12. May 19, 2008 #11

    Astronuc

    User Avatar
    Staff Emeritus
    Science Advisor

    The electromagnetic wave IS the photon.
     
  13. May 19, 2008 #12
    but isn't the em wave already present everywhere in UNIVERSE
     
  14. May 19, 2008 #13
    That's what I was thinking, but I wasn't sure.
     
  15. May 19, 2008 #14

    DaveC426913

    User Avatar
    Gold Member

    What makes you say this?
     
  16. May 19, 2008 #15
    I like it. It should be explored. It sounds like it was. These types of semi-classical theories were viable until the 1970's until photon-correlation experiments definitively proved that the quantization was a property of the light itself (taken from wikipedia article on photon)
     
  17. May 19, 2008 #16
    Hello, I'm trying to reconcile this statement with an article that I read stating that "[...]experiments confirm that the photon is not a short pulse of electromagnetic radiation[...]" and also that "[...]According to our present understanding, the electromagnetic field itself is produced by photons[...]" (ephasis added)

    Can you help clarify. I don't mean to hijack this thread. Maybe it should be a separate post?
     
  18. May 19, 2008 #17
    Light is a product of oscillatory energy with associated peaks and troughs just like any other wave energy. The photon is a momentary manifestation as the wave energy accelerates to the peak, the photon then disappears then reappears as the wave energy accelerates to the trough and so on. No spatial movement of the photon - apart from which it doesn't live long enough.
     
  19. May 19, 2008 #18
    I wish it was so simple...
     
  20. May 19, 2008 #19
    It could be, I don't know much about those experiments. However I wonder what could be the meaning of "photons as travelling corpuscles" in the following situation: a radio transmitter is switched on at 300 kHz; 1 meter away a receiver gets the signal. The photon has a wavelenght of 1 km so it is "born" in the transmitter and "dies" in the receiver much before having extended on *one* single wave? It's very strange.
     
  21. May 19, 2008 #20

    DaveC426913

    User Avatar
    Gold Member

    I question whether this actually happens. If you get too close to the transmitter don't weird things happen to the reception?
     
  22. May 19, 2008 #21

    Mentz114

    User Avatar
    Gold Member

    Everything points to the EM field being a potential to produce photons. The only time light shows particle like properties is when it interacts with matter. This always happens in quanta, which must be localised at the absorber/emitter, and also exchange momentum with the matter. In the absence of interactions, there are no photons.

    Of course this may be seen as paradoxical because we can only detect a field by interacting with it, in which case it seems like a cloud of quanta.

    In modern cavity experiments with ultracool atoms or ions, it is possible to inject a small amount of EM energy into a high-Q ( monochromatic) cavity, so that we could say there is 1 photon in the cavity. If we had two tuned atoms in the cavity there is a finite chance that they will both begin to absorb the 'photon', unless the field is non-local.

    Photons are certainly very weird.
     
  23. May 19, 2008 #22
    I'm reading that photons don't decay spontaneously in empty space. I think there is an implication that photons do exist in empty space without interacting with matter.

    I'm not trying to be a smart alec, but I hope you appreciate an apparent discrepency in these statements:
    The first statement would seems to prohibit something from being called a photon inside an empty cavity.
     
  24. May 20, 2008 #23
    but shouldn't photons be already present everywhere in the universe , how it is that they travel , which will mean that they were not present in a place before but now they are present & after the event has happened (light switched off) they again vanish (but where)
     
  25. May 20, 2008 #24
    Can a photon exist in absence of interaction? Perhaps we should start a new thread about it. (My answer is no, but I'm open to every possibility).
     
  26. May 20, 2008 #25
    If transmitter and receiver are 100 meters apart does it change anything in the reasoning?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook