Who needs photons?

  1. I don’t want to dismiss the obvious practical utility of photons as a model. But setting aside the wave nature of light (since it is complementary anyway), why do we need the intermediary, “photons” to explain the particle aspects of light?

    SR says photons don’t age because they travel at c (the limit of time dilation). But in frame c, a photon sees its path length as zero (the limit of length contraction). Equating “zero path length” to “contact” would seem sufficient to explain all of light’s particle behavior (photoelectric effect, Compton effect, even virtual photon “exchanges”) without invoking massless energy bundles.
     
  2. jcsd
  3. bcrowell

    bcrowell 5,682
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    You don't need to invoke photons if all you want to talk about is classical relativity (i.e., not quantum gravity). You do need photons if you want to talk about the photoelectric effect or the Compton effect. The photoelectric effect is one of the classical empirical proofs that light is quantized.
     
  4. JesseM

    JesseM 8,491
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    SR doesn't allow inertial frames moving at c, so it doesn't give a definition of what a photon "sees", either in terms of time or distance. The issue of a photon not having its own frame has been discussed on a lot of previous threads, do a search and you'll probably find some.
     
    Last edited: Jun 6, 2010
  5. ZapperZ

    ZapperZ 29,657
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    Please show exact references for these, especially when I mentioned earlier about the fact that no non-photon formulation has been done to explain a more detailed aspect of photoemission phenomenon (including angle-resolved photoemission, resonant photoemission, multiphoton photoemission, etc..), anti-bunding phenomenon, etc... etc...

    Zz.
     
  6. I noticed this recently:
    here: http://en.wikipedia.org/wiki/Photons

    So it seems like the standard model requires photons??? Is that correct??

    and I did not recall ever reading that. I assume this means that as a consequence of quantum field theory certain symmetries leads to photons..

    Can anyone explain that briefly...???
     
  7. bcrowell
    I agree that light emitted and absorbed by atoms is quantized (as I also accept electron orbitals and transitions). It is the nature of the transmission which I question. Photons seem fantastical and superfluous, though useful as a conceptual crutch.

    JesseM
    I’m new and still finding my way around. I will search and read, thanks. However, I necessarily begin with the bias that not having an allowable frame is tantamount to saying that nothing in such a frame (including photons) exists. The disallowance also suggests a psychological aversion to the notion of remote contact for which the Lorentz equations appear indifferent.

    ZapperZ
    The Lorentz transform equations clearly yield both agelessness and zero path length when v=c.
    However, at the bottom of this Physics Forum page is a suggested thread “Photons” in which I found the following example on my first try, from Plover 7/3/4:
    ZapperZ
    That’s quite a list. I’m sure I will spend time exploring those (by the way, I recall no “earlier mention”). It’s the “etc.. etc…” part I worry about. The list could be endless. That’s not how science should work. I should discover one incontrovertible proof that remote contact occurs without the need of a photon, then everyone else goes back and adjusts their favorite theory accordingly (with some help on the mechanism by the discoverer).

    Think about it. There we were, happily going along, confident in Newton’s Laws. Einstein came along, and after a look at Mercury’s orbit and the Eddington photos, its everyone back to adjust their model for relativity.

    Meanwhile, lists are easy. So far, my list says photons are: massless, ageless, sizeless, spinless, chargeless (electric and color) and collisionless (with each other). Maybe photons are trying to tell us something, more or less!

    What photons do appear to have is momentum and energy. But these are the very attributes most readily explained by remote contact (sans photon).
     
    Last edited: Jun 7, 2010
  8. ZapperZ

    ZapperZ 29,657
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    Please make the exact citation as you would in a publication.

    Secondly, this is a very strange thing to do, considering that the phenomena that we see light in are in OUR reference frame, i.e. not in light's reference frame. So why would you transform to light's reference frame (is this even valid in the first place?) to describe a phenomenon that we seen in another reference frame? You don't have such phenomena in light's frame, so how would you describe such a thing in the first place?

    So yes, I'm still waiting for an exact reference here before I consider that you've violated the PF Rules on speculative, personal theory. Just because you know about Lorentz transformation, it doesn't mean that you've shown it's application in describing a particular phenomenon.

    Maybe you should have done a bit more "homework" first before making your earlier claim. To make such statements while being ignorant of a whole bunch of experiments and phenomena is just puzzling.

    I've shown "incontrovertible proof" (as if there is such a thing in science) via several phenomena that have not been described using anything else other than the photon picture. It seems that the burden in on YOU to show that you can describe these otherwise. The "etc.. " part is because the list is quite long and the examples are quite in detail. The ARPES experiments, for example, is not simply something I talk about, but also something I DID (my avatar is a raw data from an ARPES measurment that I took)! And it is in the DETAILS that a non-photon picture failed! So it appears that it is you who need to do some adjusting.

    Photons are not spinless! And collisionless? You haven't heard about photon-photon collision in QED?

    So you're right here, lists are easy to make. Sometime, it is without regards to accuracy, as in this case.

    Zz.
     
    Last edited: Jun 7, 2010
  9. bapowell

    bapowell 1,811
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    Naty1,

    The electromagnetic interaction is a direct result of the local invariance (also called gauge invariance) of the QED equations of motion. This local invariance is a symmetry (specifically, the laws of QED are invariant under U(1) transformations, which are phase rotations) that holds at each point in spacetime.

    If one begins with the free Maxwell and Dirac Lagrangians and carries out a U(1) transformation on all the fields (photon and fermion fields), one finds that the free theory is not invariant. However, the Lagrangian can be made invariant if we add a specific term to the theory. This term is precisely the interaction term between matter and light -- the term involves a direct coupling of photon and fermion fields.

    These U(1) transformations are the same gauge transformations of the vector potential that are discussed in introductory electromagnetism, but viewed from the standpoint of a gauge theory like QED, we see that such transformations are at the heart of why photons interact with matter in the first place!
     
  10. JesseM

    JesseM 8,491
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    What do you mean by "in" such a frame? Physical facts are not "in" one frame or another, different frames are nothing more than different coordinate systems used to label the space and time coordinates of different physical events. You can certainly come up with a coordinate system where different events on a photon's path are labeled with the same position coordinate but different time coordinates, in which case the photon is "at rest" in this coordinate system. But this would not be an inertial frame (one where the equations of SR apply), because the "inertial" frames are a special set of coordinate systems which satisfy both the first postulate of SR (which says the equations of the laws of physics obey the same equations in every inertial frame) and the second postulate (which says the speed of light is always c in every inertial frame--obviously a coordinate system where a photon was at rest wouldn't satisfy this postulate). The fact that it is possible to find a set of coordinate systems moving at constant velocity relative to one another and satisfying both postulates is basically the physical content of the theory of relativity, it's possible to imagine different laws of physics where it wouldn't be possible to find a set of coordinate systems satisfying these requirements.
     
  11. bcrowell

    bcrowell 5,682
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    Since you haven't invoked quantum mechanics in this argument, it isn't an argument against the existence of photons, it's an argument against the existence of classical electromagnetic waves.

    Please take some time to think carefully about sorting out the relativistic issues you've been talking about from the quantum-mechanical ones. They are not related. (If they were related, we'd be talking about quantum gravity -- but we're not.) If you want to post about your doubts about photons, that belongs in the Quantum Physics forum. If you want to post about the question of frames moving at c, then that does belong in this forum, but you will find that people's patience is short, because this gets discussed here once or twice every month, and it gets tiresome to cover the same ground over and over.

    Either way, I would encourage you to start fresh threads, because you've gotten this one off to a muddled start by mixing together unrelated issues.

    On the topic of frames of reference moving at c, here is a FAQ I've written.

    FAQ: What does the world look like in a frame of reference moving at the speed of light?

    This question has a long and honorable history. As a young student, Einstein tried to imagine what an electromagnetic wave would look like from the point of view of a motorcyclist riding alongside it. But we now know, thanks to Einstein himself, that it really doesn't make sense to talk about such observers.

    The most straightforward argument is based on the positivist idea that concepts only mean something if you can define how to measure them operationally. If we accept this philosophical stance (which is by no means compatible with every concept we ever discuss in physics), then we need to be able to physically realize this frame in terms of an observer and measuring devices. But we can't. It would take an infinite amount of energy to accelerate Einstein and his motorcycle to the speed of light.

    Since arguments from positivism can often kill off perfectly interesting and reasonable concepts, we might ask whether there are other reasons not to allow such frames. There are. One of the most basic geometrical ideas is intersection. In relativity, we expect that even if different observers disagree about many things, they agree about intersections of world-lines. Either the particles collided or they didn't. The arrow either hit the bull's-eye or it didn't. So although general relativity is far more permissive than Newtonian mechanics about changes of coordinates, there is a restriction that they should be smooth, one-to-one functions. If there was something like a Lorentz transformation for v=c, it wouldn't be one-to-one, so it wouldn't be mathematically compatible with the structure of relativity. (An easy way to see that it can't be one-to-one is that the length contraction would reduce a finite distance to a point.)

    What if a system of interacting, massless particles was conscious, and could make observations? The argument given in the preceding paragraph proves that this isn't possible, but let's be more explicit. There are two possibilities. The velocity V of the system's center of mass either moves at c, or it doesn't. If V=c, then all the particles are moving along parallel lines, and therefore they aren't interacting, can't perform computations, and can't be conscious. (This is also consistent with the fact that the proper time s of a particle moving at c is constant, ds=0.) If V is less than c, then the observer's frame of reference isn't moving at c. Either way, we don't get an observer moving at c.
     

  12. It's just a matter of interpretation. You don't really need massive particles either.
     
  13. Faradave. You've presented an idea of that I entertain quit often, though make no progress with: "can action at a distance mediated by photons be equally explained by direct interaction?"
     
  14. bapowell

    bapowell 1,811
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    What does that even mean?
     
  15. I've often wondered the same thing. Since we can't "see" or locate photons in flight (without obliterating them, it seems to me that the interaction of matter with light, at points in space and time, says it all. The concept of a photon particle, flying through space from souce to sink, seems as whimsical to me as the concept of an ether.
     
  16. ZapperZ

    ZapperZ 29,657
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    You can't "see" electron either!

    The concept of photon having clumps of energy AND can be "located" is plainly obvious upon interaction (i.e. a single photon doesn't spread itself all over a detector) and in any which-way experiments. I don't know why this is so "whimsical" considering that these are very good experiments that are in good agreement with such picture. When was the last time you had such experiments for the ether for you to put them in the same category?

    Zz.
     
  17. Zz
    The citation I gave came from Physics Forums' own recommendation (https://www.physicsforums.com/showthread.php?t=33583). It stated my interpretation of photonic path length zero and time stopping exactly. So, I learned it here, doing “homework”. It’s been there since 2004! Where were the censors then? The citation is plausible, since it agrees with the transform equations when v=c. And as is oft repeated, light has no speed other than c (in a vacuum). If the equations can’t be trusted, they might come with an asterisk. But no, I don’t publish anymore and never did in a physics journal if that’s what you want me to admit.
    To explain remote contact. What a “photon” appears to do, is transfer momentum and energy from an emitting particle to an absorbing particle. If the photon has a trajectory from one particle to another (future) particle and that trajectory has zero length, the photon is telling us that it represents an actual collision in frame c ( projecting as a remote collision in frames <c).
    I’m suggesting that the kinetic theory of heat can finally be extended to explain radiation in addition to conduction, convection and the gas laws. All in terms of collisions and migrations of real (massive) particles. It would be... beauty!!
    If I wasn’t ignorant, I wouldn’t be here! I can’t understand what claims and statements are so offensive to you. In view of the fact that photons are seen as “force carriers”, I am simply asking, is the carrier really necessary?
    I’m embarrassed! You are indeed correct that photons are reported to have spin=1. I had read this before but considered the notion of a massless and sizeless “particle” having spin angular momentum so absurd that it just didn’t register. My apologies. I don’t think spin =1 really helps the photon case though. It seems all too coincidental that the emitting and absorbing electrons each have spin =1/2 and the intermediary has spin=1 as if it were connecting and summing the two!

    In any case so that my list doesn’t have a gap, lets replace "spinless" with "frameless" and "observerless". It’s still a lot of things for something real not to be.
    No. And I don’t think I want to. For example, at http://en.wikipedia.org/wiki/Two-photon_physics I found, “Two photons cannot ever collide. In fact light is quantized only when interacting with matter.”
    Let’s consider the photoelectric effect. You have incident light and if you increase intensity you get more electrons ejected but with identical energies. If you increase the energy (frequency) you get the same number of electrons but at higher velocities coming off the metal plate.

    Don’t you agree that the same effect could be achieved with an incident electron beam? More electrons in means more get ejected but faster (more energetic) electrons in mean faster electrons ejected. Photons are like electron collisions. So maybe that is what they ultimately are. But “maybe” isn’t much in the way of proof. I agree, the burden is on me. Thus, I offer the following:

    The spectroscopic character of antihydrogen is expected to be identical to that of normal hydrogen when it is ascertained soon at CERN (http://en.wikipedia.org/wiki/Antihydrogen):

    “According to the CPT theorem of particle physics, antihydrogen atoms should have many of the characteristics regular hydrogen atoms have, i.e. they should have the same mass, magnetic moment, and transition frequencies (see Atomic spectroscopy) between its atomic quantum states.”

    “Today, no conclusive spectral signature for the presence of antihydrogen could be reported, since measuring the spectrum of antihydrogen, especially the 1S-2S interval, is exactly the goal of these CERN collaborations.”

    If that is the case, I’m wrong!! About contact, zero path length, pinholes (photo-induced wormholes), remote collisions, the whole shebang!! End of story. That’s because I predict unexpected instability of antihydrogen upon absorption of light from normal atoms. Specifically, there will be signature gamma emissions indicating the remote annihilations of the emitting electrons and absorbing positrons from their respective locations. All we really have to do is wait (vigilantly)!

    In the meantime, I note that Mills and Cassidy who are leading investigators of Positronium (Ps, arguably the closest thing to antihydrogen currently available for investigation) report http://focus.aps.org/story/v16/st16:

    "Mills and his colleagues found a higher decay rate with the denser pulses - clear evidence, they say, of frequent positronium collisions, an important step toward making molecules. They were surprised, however, that the decay rate was four times as high as expected based on the simplest understanding of the collisions." I enquired about the possibility of light contamination but received no reply.
    You avatar is wonderful! From what little I could gather on Wikipedia, ARPES (Angle-resolved photoemission spectroscopy) seems like a very sophisticated and sensitive photoelectric effect measurement (I got completely lost on "reciprocal lattice" though). Congratulations on your accomplishments there and thank you for your very considerable efforts here (Disagreements aside, I mean that.)
     
    Last edited: Jun 8, 2010
  18. Good point! I'm thinking about this. By "in" I mean whatever frame in which "photons" exist . If relativity excludes the very frame in which its photons exist, shouldn't we be all the more careful in guessing what a photon might be?

    Could they be conduits (for energy, but not mass)? That is, buy being merely a "hole" big enough only for tangential contact by fundamental particles (I call them pinholes), photons could represent yet another support (and mechanism) for the notion that no massive particle can achieve c.
     
  19. Photons exist in every frame. What relativity says is that there is no intertial frame where a photon is at rest(more spesifically, photons move at c in every intertial frame). You make make a frame where it is at reast, but it would be non-inertial.
     
  20. JesseM

    JesseM 8,491
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    Again, a frame is just a coordinate system for assigning events position and time coordinates, and you are free to construct one where a photon's position coordinate remains the same at different time coordinates, relativity just says this won't qualify as an inertial frame (you are certainly free to use non-inertial frames in relativity but the equations for various laws of physics may look different in these frames than they do in their standard inertial form). Why should this suggest anything especially mysterious about "what a photon might be"?
     
  21. ZapperZ

    ZapperZ 29,657
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    There is a severe misunderstanding of SR here, and in your reply to me, so much so I have no idea where to begin. I believe you've received several replies already on why this is a rather faulty concept to begin with. Take note that while you may have understood Lorentz transformation, your application of it to the photon frame is faulty. That, in itself, is the crux of your original argument in your first post. You need to first and foremost, understand why this is wrong, because it makes the rest of your argument in this thread rather moot.

    Zz.
     
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