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Questions about Photons

  1. Nov 2, 2007 #1
    Sometimes I think I'll never understand anything about photons. In isolation things seem to make sense, but put together they seem impossible. Here's what I'm thinking.

    A photon is a particle with energy and momentum but no mass. Or it is a varying electromagnetic wave. Presumably there's a way to see it so these are the same thing. Let's go with the electromagnetic wave notion. You can't have a stable electric field without a charge to anchor it, so the photon's electric field quickly decays, which induces a magnetic field. But there is nothing to anchor this magnetic field either, so it also decays, producing an electric field. And so on and so on.

    Since the photon has momentum, there must be some asymmetry to these fields to give the photon a particular direction to travel in. I presume there must be some spatial asymmetry in each field itself, and not simply in their relationship to each other, otherwise the photon could also be traveling in the opposite direction at the same time (with opposite phase), which seems clearly nonsensical. I suspect this asymmetry could have come about from the electron configuration in the excited atom when the photon was first emitted, but I digress.

    Nothing can travel faster than c. That's a given. Now consider two photons travelling in the same direction side by side. Can one photon be "aware" of the other? I think not, by simple geometry. Form a triangle with A being the position of the left photon at time t-zero, B the position of the right photon at at t-zero, and C being the position of the left photon at t-one. Information about the right photon would have to travel along a hypotenous to meet the other photon at t-one, but that distance is longer than AC, so the first photon is gone before that information can get there. Thus there can be no information exchanged between these two photons. In a romantic sense, each photon is a universe unto itself, but I digress again. Suffice it to say that this isolation exists no matter how close or far away these two photons are.

    Now, if no information spatially separated from the photon travelling parallel to the photon can communicate with it, how can the spatially extent electromagnetic field of the photon keep up with itself??? How can the varying electric field "know" the configuration of the magnetic field? It seems it can't, so I can only conclude that all this electromagnetic varying occurs at a single point. But that destroys the notion of asymmetry, so how can the photon move at all?

    Perhaps, from the photon's perspective, it isn't moving. Again, in a romantic sense, time stops for an object moving at c. Unfortunately, looked at this way, a photon can't interact with anything or be affected by anything, and one glance in the mirror tells you this inference can't be right.

    Still, the photon cannot move at greater than nor les than c, so it must be moving with constant velocity. But objects moving at constant velocity seem at rest from their own reference frame. When moving at constant velocity, you can't tell you're moving except in relation to external objects, which the photon can't "see", and even then it would "feel" like it was the external objects that were moving. Since a photon at rest makes no sense, I conclude the photon can't seem to be at rest even from it's own frame. From all this I can only conclude that the very notion of a reference frame for a photon is nonsensical (they are called inertial reference frames, and photon's don't have inertia, do they? even though they have momentum?)

    And how can a any field be compressed into a point? Isn't that self contradictory by definition, even though it seems necessary? The more I think about this the more confused I get. Either I'm terribly wrong about something(s) fundamental, or these's a question in here that would be interesting to know. The answer might be something else, but for now I'd be happy to just know the question.

    Can anybody help?
  2. jcsd
  3. Nov 3, 2007 #2


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    I suspect your problem is that you are trying to understand what a photon is on a "intuitive level" which is probably not possible; as with many other things in modern physics it is simply too "weird". We can give a very accurate mathematicaldefinition of what a photon is (i.e. an excitation of a spatio-temporal mode) but that doesn't really help explain what it "is".
    Also, what it "is" is strictly speaking irrelevant, the concept of a photon can be used to predict the outcome of experiments and that is all that matters.

    If you are really interested and have some knowledge about basic QM you can always try reading a good book such as Gerry&Knight's "Introductory Quantum Optics"; the first two chapters covers most basic properties of quantized fields and the math is relatively straightforward.
  4. Nov 3, 2007 #3


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    i still think the most precise way to say this is that "A photon, in some observations, behaves in a manner consistent to a particle with energy and momentum but no rest mass." photons have inertial mass (or "relativistic mass") which is their momentum divided by c, but they cannot have rest mass (unless they move a teeny-bit slower than c, the wave-speed of light and E&M and all other "instantaneous" interactions, which has been postulated but we don't believe is the case).

    that's another way to look at light or other EM radiation (not "photons" per se). for different purposes, in different experiments, different observations.

    i would say, instead, that light and the other EM radiation has wave-like properties in some contexts (like interference, diffraction, antennas, ...) and particle-like properties in other contexts (most notably photo-electric effect but also what you might expect for extremely high-frequency radiation like X-rays, gamma-rays, cosmic-rays - that even low intensity radiation can do some damage you might expect only for something of high energy).

    nor even at c, unless the "thing" is the propagation of a fundamental interaction (or information which rides on one of these fundamental interactions). light travels at c because it's the EM interaction which, along with the strong and weak forces (if we could ever measure their effect at any distance to see) and gravity (or, more specifically, perturbations in the curvature of space-time from a GR p.o.v.), propagates at c.

    i have a problem with just the notion of a "reference frame" moving at c, with respect to any other inertial reference frame. i don't think such exists.
  5. Nov 4, 2007 #4
    Thanks for the replies. I'm really struggling to find ways to express my notions, which isn't surprising. I agree the idea of any reference frame moving at c is troubling, but so is the notion of no reference frame. What would "frameless" mean? Another conundrum.

    I've ordered the Optics book from Amazon. It's not cheap, but I'll give it a shot. It's difficult trying to self teach from these books - both in the work involved and in finding the time for it, perhaps the greatest challenge. And if an explanation isn't crystal clear (how often does that happen??) misunderstandings can multiply or stop the show. Since I learn best by talking things out, and few people I know are into such topics at a suitable level, the method is far from ideal, but it's the best I've got. I'm doing the same thing with Topology, after questions some things while reading up on the Poincaire conjecture. So I'm trying to learn more about that field by the numbers too.

    I suspect I'll be back. :-)
  6. May 30, 2009 #5
    How about a imaginary universe where all dimensions are collapsing at the speed of light (or maybe just a little bit faster) and photons like everything else are simply “surfing” this multi dimensional collapse? In such universe one may postulate something about quantum inertia (force necessary to move a photon at rest = 1, force necessary to move a photon travelling at the speed of dimensional collapse = 0 (This would definitely imply that this multi dimensional collapse is faster than the speed of light)….by the way, information would be allowed to travel faster than the speed of light in such a universe. Below is a question I asked in 2007 and the reply I was provided with. The crazy stuff above just came to me when I read your question.
    “How would a photon behave in a universe where all dimensions are collapsing at the speed of light?"

    Hi Luis,

    I’ll give you my best guess on this one. It’s certainly an interesting question!

    According to Einstein, the speed of light is a constant. It doesn’t matter that’s it’s a moving frame of reference. An observer within the frame or outside the frame both measure the speed as c.

    So if all dimensions are collapsing at c (we’ll assume at least the four dimensions of space-time are collapsing), then an observer within that universe would not notice the collapse just by measuring c. However, light would be extremely blue-shifted (to the point where it would be invisible). If further objects would be approaching at c, but closer objects approaching at less than c, then the observer would observe some stars, but the furthest ones would become invisible as their light gets shifted to the blue, ultraviolet, x-ray, etc.

    So if the universe was collapsing uniformly at c, one might think that photons (which still have constant velocity c) would be so blue shifted that no light is possible. This would amount to time “standing still” for such an observer. This would be true for an observer “stationary” with respect to the moving frame (or an observer “outside” the universe). But in our case, I would think that “frame dragging” would cause our observer (within the collapsing universe) to move along at c, so that photons wouldn’t be blue shifted at all! In fact, I would contend that such an observer would not be aware of the collapsing universe, and the photons would behave the same as photons in our universe.

    All of the above, of course, is argumentative and subject to change at no notice!

    Prof. James Gort "
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