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

What a photon experiences

  1. Jun 13, 2015 #1
    Hi there,

    I have some simple questions concerning a thought experiment about photons.

    Set-up: There are two molecules in the universe and in between them there is empty space. A photon of light is emitted from molecule A and is absorbed by molecule B.

    From the perspective of the photon, does any time pass and has any distance been traversed?

    And if not, then does the photon experience "teleportation" between molecule A and molecule B because there is no time nor space to cross it in (according to the photon)?

    And (this is a very big IF, if the other two are answered "no" and "yes" respectively) - is it possible that the photon does not actually exist at all between massive particles?

    I appreciate any help!

    Last edited: Jun 13, 2015
  2. jcsd
  3. Jun 13, 2015 #2


    Staff: Mentor

    There is no such thing as "the perspective of a photon" in the sense you mean; there is no such thing as an inertial frame in which the photon is at rest. So the concepts of "time passed" and "distance traversed" don't make sense from the "perspective" of photons. They only make sense in an inertial frame; in any such frame, the photon is moving at ##c##, and the time it takes to cover a given distance is determined accordingly.
  4. Jun 14, 2015 #3


    User Avatar
    Science Advisor

    One of the postulates of special relativity is that the speed of light is constant in all inertial reference frames. If there were a reference frame in which a photon is at rest, then light would be travelling at zero and 3x108m/s at the same time. The notion of a "perspective of light" is self-contradictory, unfortunately.

    To put it another way, Einstein defined time as "what clocks measure". Since you can't accelerate a clock to the speed of light, time isn't defined at the speed of light.
  5. Jun 14, 2015 #4
    Hi PeterDonis and Ibix,

    Thanks for your responses.
    These questions were part of a thought-experiement in response to astrophysicist Dr. Pamela Gay's claim that photons in fact do not experience time:


    Apparently, there are a few physicists out there who think that time and distance don't exist for photons.
    But, I also understand your point of view. Thanks for responding.

    I suppose I am asking for validity for the claim from the article linked above:
    "Photons can take hundreds of thousands of years to travel from the core of the Sun until they reach the surface and fly off into space. And yet, that final journey, that could take it billions of light years across space, was no different from jumping from atom to atom."

    Last edited: Jun 14, 2015
  6. Jun 14, 2015 #5


    User Avatar
    Science Advisor

    That article says that Dr Gay says that "[p]hotons do not experience time". It then goes on to say that photons experience zero time. The first version is certainly consistent with what Peter and I said; the second is not. It's not clear exactly what Dr Gay said and what is the author's (mis-)interpretation of what she said - so I wouldn't put too much faith in it.

    There certainly are scientists who write about photons experiencing zero time. Brian Greene is the most famous of them, as far as I know. Peter is fond of pointing out that he never uses the concept in his research work, but only in his pop-sci presentations. The obvious conclusion is that he doesn't think the idea is good for much beyond selling books.

    The article's author is also waxing poetical in the bit you quoted. When it arrives at Earth, the photon has climbed part way out of the Sun's gravity well, so will be red-shifted compared to a similar measurement made on the surface of the Sun. If the photon heads out into inter-galactic space it might land on the telescope of an alien astronomer many billions of light years away, who will see it red-shifted due to the expansion of space as it travelled. Those outcomes are different for photons that were identical when emitted.

    In short, there can be changes to a photon in flight. That strongly suggests that "experiences zero time" is wrong. "Time for a photon" being nonsense (oddly enough) makes more sense.
  7. Jun 14, 2015 #6
    Thank you Ibix,

    That was a very helpful explanation. It had occurred to me earlier that should amount of space traveled over time not really effect photons because they can't experience it, then it should be the sheer number of electrons they have hopped to/from that determines any change in its composition.

    However, I hadn't thought of red-shift due to gravitational or expansionary spacial warping, and surely if photons passed directly over the space to their next electrons, then we would hardly be able to have any red-shifting or blue-shifting, for a photon hopping between some arbitrary number of atoms towards the center of the sun (blueshift) would yield the same properties as a photon having crossed the same number of atoms over many light years through expanding space (red-shift).

    Thanks a bunch.
    Good explanation,

    I have one more question, though. Although you seem convinced, and I am becoming moreso, that photons do not experience time (not zero-time) then what about distance? If they do not experience distance, nor time, for those concepts are alien to the photon, then, distance and time being the only definitive indicators of dimension, it seems we'll have to drop all dimensions altogether and conclude that dimensions do not exist (or are a null point) for photons or anything else traveling at the speed of light?

    Last edited: Jun 14, 2015
  8. Jun 14, 2015 #7


    User Avatar
    Science Advisor

    Short on time, so will be brief.

    The key point is that asking what a photon experiences is incoherent in relativity. There's simply no coherent way to "look from a photon's perspective". So there is no way to answer what distance it experiences any more than what time it experiences (or what the flowers smell like along the way, or anything else).
  9. Jun 14, 2015 #8
    Can someone please tell me how we should calculate in SR?
    Do we have to consider that we are NOT moving, and everything else comes to us (or away from us)?
    Except photon?
    Do photons have rest frame? I think from PeterDonis answer, they don't. But I need confirmation.
  10. Jun 14, 2015 #9
    Yep, yesterday I watched a youtube video about light not photon. Not the video that you link above, a different one. Light doesn't experience time because
    ##\gamma = \frac{1}{\sqrt{1-v^2}}## But if V >= 1 (can V > 1??), then ##\gamma = \infty##
    So if ##\text {length } = \frac{1}{\infty}##, length should be zero. Time should be instant. Might right, migh be wrong. Who knows?
    But this already too much for me. Still struggling with basic SR, here.
  11. Jun 14, 2015 #10


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You're not the first on this forum to be trying to understand SR, but actually to be struggling with the concept of reference frames. You might benefit from a better knowledge of classical kinematics. Reference frames are not particular to SR. Take an example from classical physics:

    You're in a car, moving towards a road sign. In your reference frame you are at rest and the sign is moving towards you. But, you also know that the sign is not moving with respect to the Earth/road and you are. There is no mystery to this. You can do calculations from your reference frame or the the reference frame in which the Earth (road and road sign) are at rest.

    And, in fact, when doing classical problems, choosing the best reference frame often makes the problems easier to solve. For example, if you have two objects moving towards each other, you could study this from the point of view (rest frame) of either object; or, the reference frame where their centre of mass is at rest - which is often very useful. Or, of course, from your reference frame as a "stationary" observer.

    SR adds a twist in that lengths and elapsed times are not the same when you move between reference frames. And, clocks synchonised in one frame will not be synchronised in another. This is why you need Lorentz.

    Now, if we go back to the classical case: what is your absolute velocity? Well, let's say the car is moving at 100km/h (relative to the road). But, the Earth is rotating and orbiting the sun. And, the sun is orbiting the centre of the galaxy. And the galaxy is on a collision course with the Andromeda galaxy. The point is that even in classical physics, there is no absolute velocity. You have to choose your reference frame.

    You're at risk of making a mystery out of a plain matter, in my opinion.
    Last edited: Jun 14, 2015
  12. Jun 14, 2015 #11
  13. Jun 14, 2015 #12
    I recently read through this online book which gave me a lot of food for thought.
    Reflections on Relativity

    >>... once a photon is emitted, its phase does not advance. In a sense, the ancients who conceived of sight as something like a blind man's incompressible cane, feeling distant objects, were correct, because our retinas actually are in "direct" contact, via null intervals, with the sources of light. The null interval plays the role of the incompressible cane, and the wavelike properties we "feel" are really the advancing quantum phases of the source.<<

    >>This image of a photon as a single unified event with a coordinated emission and absorption seems unsatisfactory to many people, partly because it doesn't allow for the concept of a "free photon", i.e., a photon that was never emitted and is never absorbed. However, it's worth remembering that we have no direct experience of "free photons", nor of any "free particles", because ultimately all our experience is comprised of completed interactions.<<
  14. Jun 14, 2015 #13


    Staff: Mentor

    That article is a pop science article. Pop science articles, even when written by scientists, are not good sources to use if you actually want to understand the science.

    This is a good illustration of what I just said above. The quote contradicts itself, because it's using vague ordinary language instead of precise mathematics.
  15. Jun 14, 2015 #14


    Staff: Mentor

    They don't.
  16. Jun 18, 2015 #15
    One thing I find myself thinking about is that because a photon is traveling at the speed of light is has zero length in its direction of travel. So would it be accurate to say that a photon only exists in two dimensions?
  17. Jun 18, 2015 #16


    Staff: Mentor

    That is not correct. The formula that might lead you to believe that (the length contraction formula) does not apply to photons.
  18. Jun 30, 2015 #17
    Is a photon thought to have size in any dimension?
  19. Jun 30, 2015 #18


    Staff: Mentor

    A photon is not an "object" in the ordinary sense, and does not have a meaningful "size". The term "photon", when used in the context of SR and GR, is really a shorthand for "a very short pulse of EM radiation", which is modeled as an object with zero invariant mass traveling on a null worldline. This model is only an approximation, which is sufficient for most relativity problems but is certainly not intended to make any claims about the fundamental nature of photons. For that you need quantum mechanics, which will tell you what I said in the first sentence above.
  20. Jul 28, 2015 #19
    I know that this is an old post, but Leonard Susskind of Stanford University cites Paul Dirac's work that internally, any particle (photon or otherwise) traveling at the speed of light can not experience any change because time is 0.

    At 43m 48s

  21. Jul 28, 2015 #20


    Staff: Mentor

    This is not wrong, exactly, but it can be misleading, because it invites the further inferences you make in your OP in this thread, all of which are wrong. Everything that I and the other responders to you in this thread have said still stands.

    The fundamental problem here, as I said before, is that you are trying to use ordinary language to analyze something that can only be properly analyzed with math. Susskind is similarly trying to use ordinary language to express something that can only be properly expressed with math.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: What a photon experiences