Are photons energy? What is energy, anyway?

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

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  1. dumbest man on earth Real Eyes Realize Real Lies Valued Senior Member

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    Yes, still!
    The old adage may just be true: You can lead a man to knowledge but you can't make him think.
     
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  3. paddoboy Valued Senior Member

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    Not in the least James. By the way, you do know what my position is, don't you? Or are you simply being evasive and obtuse?
    Evidence so far supports my contention, sorry James. Photon is a carrier of energy...a photon is energy...or an energetic photon, take your pick.
    We have already seen one of your supporters labeled a condescending prick James. You need to do better.
    What I said again James was that all mass is a form of energy but not all energy is mass. Or if you like, E=Mc2
    Or we could say that the energy of any matter at rest is proportional to its mass. Of course that does not include any kinetic energy, or potential energy associated with its motion or position.
    So yes all mass is energy. But a photon has zero rest mass.
    Mass of course measures the quantity of matter in a body.

    Well let's start with the simple observation that a photon has no rest mass? Again my question to you is that if a photon is purely a carrier of energy [like your bucket of water analogy] what part of the photon is the bucket? It's not the wave length or frequency James, that you erroneously said the last time you attempted to answer that question..they are just part and parcel of what a photon is. Again don't get me wrong. I'm not saying that saying it is a carrier is wrong. I'm saying that referring to the photon as a bundle of energy, or as an energetic photon, will also fit the description.

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    There you go again James. Stop your childish condescending, it does nothing for you, No energy is not a thing, and I have never said it was, so either you misunderstand me, are not reading what I am saying, or are lying. You mention not being able to bottle it. Isn't that the same as your bucket of water analogy? Or do you want to withdraw that now? Your wave on the beach analogy is equally invalid for the reason already mentioned. You know, that thing called mass?


    I don't really give a hoot whether or not you shift your position or not. All I'm about is pedantry. I don't believe I have said that you are wrong in saying a photon is a carrier of energy either. But I'm not 100% sure on that and I'm far too lazy and the issue at hand is far too trivial for me to waste my time in checking.

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    Yep, and I'm even more sure of that James. Again please forgive me for repeating, "Mass is a form of energy. All mass is energy. Not all energy is mass" or if you like, E=Mc2
    Mass and energy are equivalent.
    Sure! and that will continue when necessary. Nothing wrong with any appeal to authority, particularly when that authority is an authority in the discipline being discussed.
     
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  5. arfa brane call me arf Valued Senior Member

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    Can a field, in field theory, have energy in it?

    Sure, just say it does! Define an energy density.
    But now if you want the "number" (according to James R) to be conserved you need some laws of physics in the local frame.
    A travelling wave of electric and magnetic 'field energy' is self-sustaining because it oscillates! The amplitudes are conserved and locally the energy is too.

    Saying we don't really know what energy is is like saying we don't really know why electric fields oscillate when electrons do.
    Richard Feynman was trying to explain the concept in his own way; you really need to read the entire lecture series, or at least the rest of the lecture that James R quoted a few sentences from.

    Likewise with my quote from Schrodinger--the entire article should be read and with the understanding that the audience is laypeople. Which isn't to suggest he misleads the public, or students of physics. That is, quote-mining (said somebody), isn't going to help; instead it tends to get you stuck on some idea that might not be the whole story.

    And, finally, what do you think is wrong with this:

    Photons carry energy around and photons are not energy. (thanks James and exchemist)

    The energy a photon carries around is a number.

    Photons aren't numbers, therefore a photon isn't energy (or a form of energy).

    When infrared photons increase the temperature of a physical object, it's because numbers are being conserved . . .
     
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  7. arfa brane call me arf Valued Senior Member

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    Why comparing EM waves to waves on the surface of a pond does not cut it:

    In order to have a correspondence, the water (a matter-field), requires an antenna that generates it. The field has to generate (or be created) at a constant speed.

    Also, disturbing the water's surface to generate waves means you actually see a kind of passive response to an input "waveform" (perhaps an oscillating finger to generate a train of waves, perhaps a single oscillation). If space or "the vacuum" is not material, is there a passive response to EM waves or wavetrains?
     
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  8. paddoboy Valued Senior Member

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    In trying to make my answer to the above as brief as possible, it appears I made it too brief and my " sure is! answer maybe misconstrued by you. Yes sure is, actually part of what I linked to James, but surely you would have known that much if you had been approaching this from any position of even handiness.
     
  9. arfa brane call me arf Valued Senior Member

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    Addressing what Feynman says in that quote:

    More reading of the lecture the quote is from, leads me to the conclusion that what he means when he says nobody knows what energy is, is that it's something conserved but it doesn't explain "what happens".

    In other words, for a field quantum, the state \( |\psi \rangle \) in some Hilbert space doesn't "do anything" until it gets multiplied by some energy (a scalar), which might be considered an input (of energy). Energy in some sense lets physicists abstract the physics away, then put it back to see what happens.

    For instance a comparison between surface waves on a pond and EM waves in a vacuum. There are waveforms in each case; abstract away the physical pond and "keep" the waveforms in a mathematical formula. Wave equations turn out to be very useful, in general.
     
  10. James R Just this guy, you know? Staff Member

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    Look. This is getting tedious.

    Consider the thing called "height". John has a height. But John isn't height.

    It's useless to ask questions like "what part of John is his height", because John's height is not a "part" of John.

    It's equally pointless to say things like "All people are heights, but not all heights are people", because that's completely ignoring the basic category mistake that is in play.

    If John's height disappears so does John, but that doesn't mean that John is height.

    John has a mouth. Height does not have a mouth. This observation tells us that John can't be height. If John and height were the same thing, then height would have all the same properties that John has.

    John can affect real things in the real world. John can exert forces on things. John can make other things move. Height cannot do any of those things. This observation also tells us that John can't be height, and height can't be John.

    There's no such thing as a bottle of height. You can't bottle height. There's no such thing as "pure height". You can't see height. You can't move it from place to place. It's a concept, not a thing. Not "stuff".

    Now, with the basics under your belt, consider photons and energy. I note:
    • Photons are not energy.
    • A photon's energy is not "part" of the photon, so it is pointless to ask "which part of the photon is its energy?"
    • It is pointless to say things like "All photons are energy, but not all energy is photons" because that ignores the basic category mistake that is in play.
    • If a photon's energy is transferred to something else, the photon also disappears, but that doesn't mean the photon is energy.
    • Photons have polarisation, and frequency and a wavefunction. These observations tell us that photons can't be energy. If photons and energy were the same thing, then energy would also have polarisation and frequency and a wavefunction (which it does not).
    • Photons can affect things in the real world. Photons can exert forces. Photons can make things move. Energy cannot do any of those things. This observation tells us that photons cannot be energy, and energy cannot be photons.
    • There's no such thing as bottle of energy. You can't bottle energy. There's no such thing as "pure energy". You can't see energy. You can't move energy from place to place (other than conceptually). It's a concept, not "stuff".
    The rest is time-wasting waffle. I'll probably get back to it later on, but in terms of the silly claim that photons are energy, there's really nothing else to say. If you don't get it by now, you're not going to get it.
     
  11. James R Just this guy, you know? Staff Member

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    Nothing is wrong with that.

    What's wrong with that, mainly, is the use of the word "carries", as if energy was a substance.

    There's also a smaller error, but in comparison it's a minor quibble. That's the fact that energy has units (dimensions). It's not quite a "raw" number. But to get the basic category error across, that's the most useful way to describe it for the moment.

    No problem there. That is correct. Numbers are not "stuff". You don't disagree with that, I hope? Or do you?

    The error there is in attributing the cause of the heating up of a physical object to the action of numbers. Numbers can't affect physical objects. Basic category error. Photons, on the other hand, are physical things, so they can affect physical objects.

    When John reaches up to the high shelf to get a book down, it's not John's height that is doing the reaching of the book: it's John.
     
  12. Write4U Valued Senior Member

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    Can we say that energy is a "potential" of photons or any other energetic object?
    https://en.wikipedia.org/wiki/Potential_energy
     
  13. James R Just this guy, you know? Staff Member

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    In a rough sense, but not in the sense of "potential energy" as defined in the wiki page you quoted.

    Roughly, as an approximate starting concept, energy is a measure of the "potential to do work".

    The energy of a photon has nothing to do with "potential energy" in the technical sense used by physicists.
     
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  14. James R Just this guy, you know? Staff Member

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    arfa brane:

    Einstein's description of gravity as curvature of spacetime does nothing to advance your argument that photons are energy, so I don't intend to discuss that in this thread. We can try to clear up your misconceptions about relativity elsewhere, if necessary.

    *sigh* Clearly, you're ill-equipped to judge that. Based on what I've posted so far, anybody who had "done much of this" would be drawing the obvious conclusion by now. And, in fact, we have at least one other person in this thread who is qualified and who has come to the correct conclusion.

    You'd probably be better off just insulting me outright, rather than making snide remarks of that kind, if that's what floats your boat.

    All fine so far...

    Intensity is power per unit area, or energy per unit area per unit time. Since energy is not "stuff", it follows that intensity is on the same conceptual level as energy. We don't detect "intensity" in a radio wave. We detect radio photons - which are not energy.

    There is a number associated with a photon, which we call the photon's energy. When the photon is absorbed, something else increases its energy by the same amount.

    I'm not sure why you felt the need to introduce BECs into this discussion. A phonon, by the way, is no more "energy" than a photon is.

    There are no waves of energy. Energy is not "emitted" from anything, except in a metaphorical sense. Energy isn't stuff.

    Come on, this isn't hard. How could a number (energy) turn into an oscillating field (stuff)? Mind you, if you want to make things even more complicated, we could have a philosophical debate about whether fields are "real". The answer to that question probably hinges on one's philosophical stance towards science as a whole. But I don't think we need to go there in this thread.

    I don't see the relevance of any of this, so I'm skipping it.

    Doesn't the fact that you need to define an energy tell you something important about the energy in a field?

    There are lots of physically useful laws and statements we can make about energy. None of them help you make the case that energy is "stuff".

    I really don't understand what you're saying. A wooden log sitting on the surface of the water can act like an "antenna" for water waves. Oscillate the log and you get waves propagating outward.

    I don't understand what you're asking, or why it is relevant to your argument that photons are energy.

    Tell me: why do we talk about "EM waves" and "water waves" and "sound waves" and the like? If all waves are energy, as you seem to be trying to argue, why don't we just talk about "energy waves"?

    Specifically, it's a number that is "conserved". The "mystery", if there is one, is why that number obeys the particular rules (laws of physics) that it does.

    That's pretty nonsensical.

    When somebody multiplies a state vector by a number, that isn't an "energy input". It's a mathematical operation carried out in an abstract vector space. The operation is scalar multiplication of a vector.

    The map is not the territory. Seriously, this reification of yours is the root cause of the confusion that you're experiencing. Mathematical objects are not the same as physical objects. To pretend that numbers are stuff is to make a basic category error.

    But I've already told you this many times. The penny still hasn't dropped, apparently. And I note that paddoboy is still "like"ing your posts, which means he's still just as stuck as you are. Out of interest, what do you think of his arguments, such as they are? Do you think it's all a matter of "pedantry", like he says?
     
  15. James R Just this guy, you know? Staff Member

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    paddoboy,

    Yes, I know what your position is. Your position is that you can't tell the difference between saying "a photon has energy" and saying "a photon is energy". It's unfortunate, but that's what you're stuck on.

    See what I mean? There it is, right there, in your own words. "Take your pick" - as if the two ideas are really the same thing. It's a basic category error.

    Why don't you just call me a condescending prick, and be done with it? That would be more honest.

    All that waffle, and you didn't answer my question. Here it is again: "Explain to me how light is energy and mass is energy, but that somehow doesn't make light and mass the same thing."

    Again, I asked you some specific questions. Why are you ignoring them? Here they are again:
    "Do you agree that a water wave is not "just" energy? Do you agree that a sound wave is not "just" energy? If so, then why do you not agree that a light wave (or a photon) is not "just" energy?"

    What was wrong with the last two times I answered that exact question, in explicit and careful detail? My answer hasn't changed. Why don't you go back and read over what I said the last two times you asked the same thing, rather than pretending I didn't answer you? This is troll-like behaviour on your part.

    1. I said nothing of the sort.
    2. If you agree that wavelength and frequency are part and parcel of what a photon is, then tell me why they are not part and parcel of what energy is, if photons are energy, like you say.

    Saying that a photon has energy is very different from saying a photon is energy. John has height, but John isn't height. Get it yet?

    Are photons a thing? If they are, and you say that photons are energy, then energy is a thing. Right?

    Don't dodge this question, or ignore it. Try to answer it.

    I already admitted that the bucket analogy was not great, because it tends to reinforce your reification of energy. The water in the bucket is stuff; the energy in a photon is not stuff. So, the bucket of water analogy breaks down there.

    A better analogy is John and his height. You can't bottle John's height, just like you can't bottle energy. To ask which part of John is his height is to make the same category error you make every time you ask which part of a photon is its energy.

    What? We're over 200 pages into this thread, and now you say you're too lazy to check whether what I've been saying all this time is actually correct? Not that you should need to check. A bit of common sense ought to be enough. It's not that this is a difficult concept to grasp.

    Careful with that word "equivalent". It does not necessarily mean "the same". In the case of mass and energy, it certainly does not mean "the same".
     
  16. arfa brane call me arf Valued Senior Member

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    Really? Who'da thunk?
    But how do they affect them? What actually happens?
    > 200 posts and still nowhere near an explanation.
     
  17. arfa brane call me arf Valued Senior Member

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    That just can't be right. You're saying the intensity of visible light is not "stuff"? I can look directly into several searchlights at the same time, there will be no effect? I can listen to music at any intensity? Or I can turn on several "radiant" heaters and not burn myself standing in front of them?
     
    Last edited: Sep 12, 2019
  18. arfa brane call me arf Valued Senior Member

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    What is this "something else"?
     
  19. exchemist Valued Senior Member

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    Since that was not the question asked 200 posts ago it is not surprising, is it? By the action of their oscillating electric and magnetic fields on electrically charged matter, of course.

    I cannot believe that you as an engineer do not know already know that. You seem now to be just looking for artificial ways to keep the argument going, for its own sake. That is what Theorist does.
     
  20. arfa brane call me arf Valued Senior Member

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    Ok. Can a wooden log generate a pond full of water? That was my intended analogy. A radio antenna generates an EM field around it, right?
     
  21. arfa brane call me arf Valued Senior Member

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    Really? Why then are you making the basic error of believing that intensity is a concept?
    Why not, indeed? I did mention that a solution to the wave equation for a wave propagating in a medium, is an "energy wave". You dismissed this.
     
  22. arfa brane call me arf Valued Senior Member

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    This action "of" the oscillating fields; might you say the oscillations are a field energy? Why might you not say this?

    I'm keeping the argument going because I think you and James have a misconception (possibly more than one), about physics. That you keep both insisting that you don't and keep trying to "correct" me is why I think this.
     
  23. arfa brane call me arf Valued Senior Member

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    It is if that's what you want it to be; it's what you do with Schrodinger's eqn, for instance.
    Usually, instead of "a number", you multiply by a number of joules expressed in electron-volts.
     
    Last edited: Sep 12, 2019
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