Are photons energy? What is energy, anyway?

Discussion in 'Physics & Math' started by origin, Aug 19, 2019.

Thread Status:
Not open for further replies.
  1. origin Heading towards oblivion Valued Senior Member

    Messages:
    11,888
    Reported for name calling.
     
  2. Google AdSense Guest Advertisement



    to hide all adverts.
  3. exchemist Valued Senior Member

    Messages:
    12,451
    I think we've both explained the mechanism, possibly more than once. See for example my post 236.

    As an electronics engineer, you must know how a receiving antenna captures energy from a radio wave. The electric vector of the radiation accelerates charged entities, transferring energy and momentum to them.

    In the case of heat, the oscillating EM field of the IR radiation causes the bonds in a molecule that have a dipole moment* (i.e. partial charge separation) to vibrate, if the frequency is the right match. And microwaves with the appropriate frequency will cause a molecule with a dipole moment to rotate. Molecules with no dipole moment are transparent to such radiation. (This is why the air is not much heated directly by sunlight: neither N2 nor O2 has a dipole moment. CO2 on the other hand does, an opposed pair of them - hence global warming etc., etc.)

    This transfers energy obviously and damps the oscillations. It does so in quantised units, because vibration and rotation are quantised, due to the wavelike nature of matter, as described in QM.


    * Strictly speaking it is molecules in which the dipole moment changes upon vibrational excitation that will absorb IR radiation.
     
  4. Google AdSense Guest Advertisement



    to hide all adverts.
  5. James R Just this guy, you know? Staff Member

    Messages:
    39,397
    arfa brane:

    Or you're not a good learner.

    Do you? What is it, then?

    You're not using the word "reify" correctly. Look it up.

    Clearly the denial phase has started, or you didn't think about what I wrote. What I wrote is the exact opposite of saying it's the same thing. I couldn't have been clearer.

    Those charged particles in your antenna are already there, long before you switch the thing on and it starts putting out electromagnetic waves.

    Every static electric charge has an associated electric field that spreads throughout the whole of space, conceptually. In any given location in space, the net electric field at any given time is the sum of the fields due to all the charges in the universe.

    Your shaking a few charges in an antenna causes electromagnetic waves in the pre-existing electric field.

    Now, in case you're wondering, that pre-existing field can have a very low magnitude. You can even approximate its magnitude to be zero for many purposes. But that doesn't mean it's not there.

    A field, by definition in physics, is something that exists throughout space, like I said. If you didn't learn that in your electrical engineering course, that's not my problem.

    I note, by the way, that you're doing a lot of nay-saying, but without making any actual arguments to support your views. If you want to tell me I'm "totally and completely wrong" about something, it might help if you explained what is totally and completely right, and why. Just a thought. (Mind you, I think you don't because you can't.)

    Which textbooks are you using? Can you cite a particular page where either one talks about "energy waves", and perhaps extract a relevant passage or two?

    Maybe the problem is that your texts are full of errors, so it's not your entirely your fault you've got the wrong end of the stick.

    Why all the words written about love, if love isn't "stuff"?

    You understand that ideas are important, I hope, even if they aren't physical things?

    In physics, equations help us to make quantitative predictions and test hypotheses. That is why you see them in physics texts.

    Bizarre. Why is exchemist's pointing out that volts/m is not Joules evidence of his complete lack of exposure to Maxwell's theories? Please explain.

    The fields do interact with one another, in the sense of the principle of superposition. Other than that, this almost sounds like you agree with me. Almost.

    There's no point re-hashing this error, which I corrected previously. Radiation is not heat, and heat is not radiation. If they were one and the same, as you claim, we wouldn't need two different words, for starters.

    Yes, quantum fields (as in quantum field theory) exist throughout the whole of space, like I said. Go look it up if you don't believe me.

    Via the electromagnetic interaction. That's what the electromagnetic interaction is: a description of how photons and charged particles interact.

    Yes. It does it using virtual photons, in the QFT picture. But I don't see any value in trying to teach you QFT from scratch. You don't even know what energy is yet.

    You're flip-flopping back and forth between classical and quantum descriptions, which makes for a certain amount of confusion.

    Here, you're given a classical explanation. But what I want to know is: if the fields of those two particles don't spread through the whole of space, then how does one particle "know" to interact with the other one?

    Insults, the last refuge of the man who has given up on the argument.

    Asking for a "mechanism", the last refuge of the pseudoscientist.

    In my last post, I gave you a physical description of what happens, according to accepted physics. What more do you want? Why did you ignore my previous, careful, explanation, in which I gave you two different pictures of the required "mechanism"?

    Maybe it would be helpful if you could tell us all what you think the "mechanism" is when a photon is absorbed by an atom, say, or when two charged particles attract one another.

    Maybe. Depends what you mean by "correspond". A photon is not temperature, any more than it is heat or energy.

    I gave you a description just before. What was wrong with that one?

    When John's height increases, John gets taller. Therefore, John must be height. I understand your argument.
     
    Last edited: Sep 15, 2019
  6. Google AdSense Guest Advertisement



    to hide all adverts.
  7. QuarkHead Remedial Math Student Valued Senior Member

    Messages:
    1,740
    Yes, but what IS a quantum field? Look, I know I have amply displayed my weak knowledge of quantum theory, but I do know very well the definition of a field in abstract algebra and I do know the definition of scalar, vector and tensor fields (say) in linear algebra.

    Take as a rough-and-ready definition of a scalar field (say) the assignment, to every point in some chosen (and specified) space, a scalar.
    It surely cannot be that a quantum field is the assignment, to every point in some space, a quantum? I can not parse this.

    Maybe a quantum field assigns a state vector to every point in some space whose eigenvalues represent the ground state except where they are not, and you call this "an excitation"? This again seems rather unlikely......

    As you see, I am out of my depth here. Please clarify
     
  8. arfa brane call me arf Valued Senior Member

    Messages:
    7,832
    Well, no you haven't. I do know how a radio antenna "captures energy" from radio waves, yes. From the perspective of solid-state physics, the transmitting and receiving antennas are "matter-fields". The mechanism you cite--an electric vector--is what the electrons in the receiving antenna respond to.

    The "underlying field" then, of the radiation field is a matter-field; specifically it's the matter-field of lots of electrons, oscillating synchronously. There is no preexisting EM field; charged particles each have an individual "charge field", each electron is an oscillating field of charged matter . . .

    But your explanation: "it's the electric vector" leads immediately to the next question, what about the magnetic vector? Why, in electronics, is it sufficient to consider only voltages and currents? Any clues?
     
    Last edited: Sep 15, 2019
  9. arfa brane call me arf Valued Senior Member

    Messages:
    7,832
    What you appear to be saying is roughly that the interaction of EMR with matter depends on both the dimensions of molecules, atoms, or other particles, and on the wavelength of the incident radiation . . .? It seems that resonance is an important feature; resonant systems in electronics certainly are (but you might already know that--it's the reason you can tune a radio receiver for instance).
     
  10. arfa brane call me arf Valued Senior Member

    Messages:
    7,832
    So you're saying the field of charged particles is the underlying field; similarly a large number of massive particles is the underlying field, or source, of a gravitational field?
     
  11. arfa brane call me arf Valued Senior Member

    Messages:
    7,832
    Superposition means you can add waves together; this is not a description of interaction as I understand that word. Maybe this is more evidence that you have some fundamentally flawed ideas about the subject of wave mechanics?
     
  12. arfa brane call me arf Valued Senior Member

    Messages:
    7,832
    How do you reconcile two particles and two fields with a single "underlying" field? Fields exist because particles generate them. You might claim there is a "quantum field" extending through all space, but measuring this field means a particle has to interact with it--no particle, no interaction.

    Photons don't interact with themselves or with other photons, period. Actually, that might not be entirely correct, but in general in normal space, if you cross a pair of light beams, neither beam is affected (by passing through the other's field).
     
    Last edited: Sep 15, 2019
  13. arfa brane call me arf Valued Senior Member

    Messages:
    7,832
    You really are a dick, James.
    What do you suppose Einstein and Bohr were up to? Or Schrodinger or Heisenberg? What does physics usually try to explain, and how does it explain it? Does it ever hypothesize some "mechanism"? I think the whole of physics is about mechanisms, even abstract unphysical ones.

    Perhaps that's why it got called quantum mechanics.
     
  14. Write4U Valued Senior Member

    Messages:
    20,069
    Perhaps this may shed some light.
     
  15. exchemist Valued Senior Member

    Messages:
    12,451
    Good. So that is settled, then.
     
  16. exchemist Valued Senior Member

    Messages:
    12,451
    Good. So now you understand that, too.

    P. S Note that the presence of some degree of charge separation, for the electric vector to couple to, is key.
     
    Last edited: Sep 16, 2019
  17. arfa brane call me arf Valued Senior Member

    Messages:
    7,832
    I don't get it. What's settled?

    What about the magnetic vector?
    Yeah, but only since about 1989 or so (my bad) . . .
    On the other hand, the electric field component induces an oscillating dipole doesn't it?
     
    Last edited: Sep 16, 2019
  18. exchemist Valued Senior Member

    Messages:
    12,451
    Electric dipole transitions are the dominant process in atomic and molecular photon absorption and emission. There is no need to worry about the magnetic vector to provide the explanation of the mechanism that you were asking for. So now you have the mechanism, and you seem to understand it. So that is settled.
     
  19. arfa brane call me arf Valued Senior Member

    Messages:
    7,832
    About the quantum fields being in the whole of space: the Higgs field is one of these fields.
    That does not mean a Higgs particle exists at every point; it does mean though, there's a probability of there being a Higgs particle, it's possibly not very big, but it can't be zero, right?

    Given how much energy is needed to generate a Higgs boson, I mean.
     
  20. arfa brane call me arf Valued Senior Member

    Messages:
    7,832
    Very reassuring! But suppose I decide I do need to worry about it. I need to settle the question: why can the magnetic vector be ignored? Or perhaps, can I explain the same effect with magnetic vectors and not worry about the electric ones? If not, why not?
    I'm pretty sure I understood it many years before I knew sciforums existed (what might have been, huh?). But thanks for all the uh, help.
     
  21. exchemist Valued Senior Member

    Messages:
    12,451
    You were the one who demanded the mechanism be explained. So I have.

    If you now say you knew it all along, that would seem to be time-wasting troll behaviour.

    If you want to go into magnetic dipole transitions, you can do that for yourself, starting here: https://en.wikipedia.org/wiki/Magnetic_dipole_transition. They are a lot weaker, but they do account for why some electric-dipole "forbidden" transitions can be seen in spectra. There are also electric and magnetic quadrupole processes, if you have a few week of study - and a brain transplant - in mind.
     
  22. arfa brane call me arf Valued Senior Member

    Messages:
    7,832
    Just reviewing:
    You might also say the electric and magnetic fields have a "self-energy". That is to say, the energy of a photon from the perspective of the fixed field of an electron is relative (the electron might be moving towards the photon, say). But if a photon is a carrier of energy, then the energy must be in the oscillating fields; photons don't lose energy since, if they did the universe would look very different.

    So what we really need is a mechanism that ensures the energy of electron acceleration is preserved when that photon is absorbed completely by another charged particle.
    I believe this mechanism is actually the mutually oscillating fields in a photon.

    What about Unruh radiation? Why does the vacuum appear to radiate photons towards an accelerating observer? How is this connected to the conservation of energy?
     
  23. James R Just this guy, you know? Staff Member

    Messages:
    39,397
    The discussion seems to be drifting now.

    Have we come to an agreement that photons are not energy, yet? Or not?
     
Thread Status:
Not open for further replies.

Share This Page