Are photons energy? What is energy, anyway?

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

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  1. James R Just this guy, you know? Staff Member

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    arfa brane:

    And you still haven't corrected yourself. I don't even get the impression that you've take any time to really consider what I've told you.

    I gave you the required explanation a couple of pages back.

    Photon absorption is an electromagnetic interaction between a charged particle of some kind and the photon. When photons hit something, they can transfer momentum and energy to that thing. If you prefer to think in terms of forces, when a photon bangs into something, it gives it a push, which can mean that it speeds up, or vibrates more, or whatever. That extra vibration can corresponds to an increase in temperature, because temperature is a measure of the average energy in one sense or another.

    None of this goes towards advancing your claim that EM waves - or photons, if you prefer - are energy.

    Of course it isn't "stuff". As I explained to you, repeatedly and carefully, energy is not a substance - not "stuff". Therefore, energy per unit time is not stuff. Therefore, energy per unit area is not stuff. Therefore energy per unit anything is not stuff. Visible light is "stuff", in the relevant sense. But visible light, as I have told you over and over again, is not energy.

    Don't think I haven't noticed, by the way, that you're refusing to even address the main arguments I have put to you on this matter. Instead, you spend your time avoiding the central issue, and trying to find new irrelevant tangents to talk about. Why is that?

    What did you think about my post about John and his height, above? Do you have any thoughts on that at all? No?

    If you look into searchlights, the photons will damage your eyes, not the energy. If you listen to loud music, it's the molecules hitting your eardrum that will damage your ear, not the energy in the sound. If you stand too close to the fire, it's the photons - or the hot air - that will burn you, not the energy.

    Energy can't burn you. Energy isn't "stuff" that can burn.

    Whatever it is that is absorbing the photon. Wasn't that clear to you?

    Not really. The antenna creates a disturbance in the pre-existing electromagnetic field - at least that's the classical picture. A field, remember, is something that exists throughout the whole of space, all the time. In the quantum picture, the antenna generates photons that propagate outwards. In the quantum field theory picture, those photons are excitations of (guess what?) the pre-existing electromagnetic field that exists throughout space.

    What error? Of course it's a concept. What else could it be? You can't pick up intensity. You can't bottle it. There's no such thing as "pure intensity", any more than there's such a thing as "pure energy". That's not an accident, of course, because intensity is based on energy, which is not stuff.

    That's funny, because when I solve the wave equation for electromagnetic waves, for instance, the solution I get gives me two oscillations of fields - electric and magnetic fields, to be precise. Which wave equation gives you oscillations of energy waves? What does an "energy wave function" look like? Can you find any examples of such a thing in any text?

    Oscillations of water are not oscillations of "water energy". They are oscillations of water. Oscillations of electric fields are oscillations of a field, not oscillations of energy.

    And what would that be?

    The idea that numbers can turn into stuff, by some magic? You think we're wrong when we say they can't do that?

    Schrodinger's equation is an equation. It is not "stuff". It's an abstraction - an idea. Don't you understand that? So is a wavefunction. A wavefunction is a mathematical construct. It is not "stuff". It's not "real".

    Putting energy into an equation or a wavefunction is a matter of scribbling down the letter "E", or multiplying by a number. The energy can't do anything to real objects. It isn't stuff.

    It's like saying you can have a "height input" into a child, which makes her grow taller. That's nonsense. It is food and nutrients going into her body - stuff, in other words - that makes her grow, not some abstract notion or measurement you invented.
     
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  3. Write4U Valued Senior Member

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    When a photon strikes a photosensitive plate, the photon registers as a particle (pixel?).
    It changes the silver-halide particles on the photographic plate, no?
     
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  5. exchemist Valued Senior Member

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    I would not say it because the word "energy" in that context adds nothing but confusion.

    "Oscillations of field energy" does not make sense. The energy does not change periodically, back and forth. But delete the word "energy" and it makes perfect sense. It is the field that oscillates and the field is a field, not energy. You can tell because the units are different. Volts/m is not the same as Joules.

    An oscillation in a field has energy of course, because something has displaced the field from its equilibrium value, which requires an energy input. To repeat yet again, energy is a property, not an entity.
     
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  7. arfa brane call me arf Valued Senior Member

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    That's because I think you aren't a good teacher: I think your ideas about energy and momentum are flawed. Sorry, but I think my education has given me a reasonable grasp of "what" energy is. I also think you're guilty of the thing you accuse me and others of: you are reifying a particular perspective or idea: energy is not stuff; energy is just a number . . .

    What you say there is the same as saying "the energy of the photons" damages my eyes; "the momentum of the molecules" damages my eardrums; "the intensity of infrared radiation" is what burns me.

    Correcting this is completely unnecessary.
    Well, that's totally and completely wrong, James. The field is NOT preexisting! The field is generated by charged particles (in the antenna--a source or collection of sources).
    I know that. I also know an "energy wave" is a solution to a wave equation; I actually do understand what both the physics textbooks I have to hand, are saying. Unlike some people.
    For instance, I also understand that, despite being mathematical and not physical, a wavefunction does seem to describe a quantum state very well.
     
    Last edited: Sep 12, 2019
  8. arfa brane call me arf Valued Senior Member

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    So why do it? Why does one find this construction \( E \psi \), in so many places? Not just in textbooks but in many online lectures? Why bother doing it if it "isn't stuff"?
     
  9. arfa brane call me arf Valued Senior Member

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    More evidence of a complete lack of exposure to Maxwell's theories, or to any electronics.
    The voltage does change periodically. You know what voltage is, right?
     
    Last edited: Sep 12, 2019
  10. arfa brane call me arf Valued Senior Member

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    James' idea, that there's a pre-existing EM field extending through "the whole of space" which is then made to oscillate, is wrong.

    The EM field in the whole of space is a collection of EM fields from all the sources that generate . . . EM fields. The fields don't interact with each other, but they do interact with other charged particles. This is one of the basic errors I refer to; or it's one of the things that does not give me confidence.
     
  11. arfa brane call me arf Valued Senior Member

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    Looking back, I think it's little telling that we almost seemed to be there with post #8, but this is sciforums.

    . . . what they are is a travelling displacement yes, but the fields 'are' also the photon. That's the misconception, that there's a field like the surface of a pond for waves to travel along. There isn't.
     
  12. exchemist Valued Senior Member

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    Are you telling me that volts/metre= Joules? Show me how you derive one from the other.
     
  13. exchemist Valued Senior Member

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    As I understand the QFT picture, that is exactly what there is. All QM wave-particle entities are modelled as disturbances in the relevant field.
     
  14. arfa brane call me arf Valued Senior Member

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    Well, leaving aside "other properties", infrared radiation is heat and heat is a form of energy means an infrared photon is a form of energy; see post #1
     
  15. arfa brane call me arf Valued Senior Member

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    No I'm not telling you that.

    And you're sure you understand what this EM field is. It's been there since the BB, type of thing, along with the gravitational field?
     
    Last edited: Sep 13, 2019
  16. exchemist Valued Senior Member

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    OK, so you are not telling me that volts/metre = Joules. Good.

    I am saying that the fact an electric field has units of volts/metre is enough to show that an electric field is not energy.

    And yet, you are telling me that my saying this is, somehow, evidence of my ignorance of Maxwell's equations.

    Take me through the logic, please.
     
    Last edited: Sep 13, 2019
  17. exchemist Valued Senior Member

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    Wrong. You can't just "leave aside other properties" and then make that statement. It is like saying a man is height - leaving aside his other properties.

    IR radiation is one means of transmitting heat. So is convection. Do you seriously argue that convection is heat? I bet you don't.
     
  18. arfa brane call me arf Valued Senior Member

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    So this field, is it the same as the one James R says exists everywhere in space?
    How do electrons and other charged particles interact with this field?
    How does this field mediate interactions between the particles themselves? Are you able to explain this?

    I think you can't because it's just wrong. There is no "pre-existing" field.
    The particles generate fields (plural); two particles interact because each has its own electric field.

    Further, in the field theory, electric charge is called U(1) charge. The U(1) (quantized) gauge field is the photon. Note--gauge field. Or perhaps that's a bit over one's head.
     
    Last edited by a moderator: Sep 21, 2019
  19. exchemist Valued Senior Member

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    "QFT treats particles as excited states (also called quanta) of their underlying fields, which are—in a sense—more fundamental than the basic particles."

    This from the opening para of the Wiki article on QFT.

    That is what I meant, in the line of mine you are quoting. The underlying field of the photon would be the EM field. So that is the field corresponding to the flat surface of your pond. In the QFT picture. (I stress that I am not any kind of expert on QFT. We chemists did not study it at university. I don't imagine the engineers did either.)
     
  20. origin Heading towards oblivion Valued Senior Member

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    Reported. Calling forum members names because they pointed out your errors is really bad form.
     
  21. arfa brane call me arf Valued Senior Member

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    Well I see that as your misinterpretation of what's meant by "underlying field". James R appears to be making the same error.
    So, I might say once more, none of this gives me confidence that I'm going to "learn" anything here. Or that anyone else is.
     
  22. exchemist Valued Senior Member

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    That's rather the conclusion I came to, over a week ago, in post 147.
     
  23. arfa brane call me arf Valued Senior Member

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    Ok, but what was asked is: how, what is the mechanism? Are you saying here that photons can transfer momentum and energy, and that's the mechanism? Not very illuminating is it?
    So also a photon can correspond to an increase in temperature? Still no description of what mechanism increases temperature though, is there?
    Now you say that although photons can transfer energy, or increase temperature, this doesn't mean "I" can claim photons are a form of energy . . . even though energy when it flows (like, in the form of heat) does both of these things. Although this seems to be evidence that photons and EM radiation generally, are forms of energy, you say no, this in no way supports that. It does support that though.

    I say you're wrong, about what heat really is and about what a photon really is. But who cares? You obviously don't, you prefer to believe I don't know what I'm talking about. Maybe I do though.
     
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