# Are photons energy? What is energy, anyway?

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

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1. ### James RJust this guy, you know?Staff Member

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arfa brane:

Nobody said that. Quite the opposite, actually.

Energy. As exchemist said, in a metal bar, the "heat" is mostly transferred as kinetic energy from atom to atom when they bang into one another.

It can come from lots of places in lots of forms. Put a metal bar in contact with a hot object or substance, and heat energy can be conducted into it through contact. You can also heat a bar by shining light on it, in which case it gets energy from the photons.

No, it doesn't have a flow. You'll see the term "magnetic flux" often enough, but it's somewhat of a misnomer. Magnetic fields don't really "flow" in the way that, say, fluids flow.

It doesn't. Those are separate from the energy.

I guess a lot of people are making the same mistake, or else they aren't being careful to be precise about it.

The Britannica quote is a bit careless.

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3. ### exchemistValued Senior Member

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Er no, I didn't mean to say that. I reiterate: a static magnetic field does not have a flow of anything, any more than a gravitational field does, or an electric field does.

That a field can make objects - such as iron filings, charged pith balls or apples - move is beside the point. It is obviously true, since these fields exert a force on suitable classes of object. But that does not require them to "have a flow" of anything, as the example of gravity illustrates.

On EMR and energy, Origin, James and I have all had a pretty good go at explaining this to you now. I don't know what Britannica says but Wiki gets it right: https://en.wikipedia.org/wiki/Radiant_energy

Last edited: Aug 30, 2019

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5. ### paddoboyValued Senior Member

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Don't be silly James, with your ridiculous analogies.
The point has been made with references and links that show this is just pedantic nonsense with your support of course.

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7. ### paddoboyValued Senior Member

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No not at all. Just your pedantic nonsense and support for a mate.

8. ### arfa branecall me arfValued Senior Member

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I think the whole argument is pointless. It doesn't really matter whether you say a photon is electromagnetic energy, or whether you say a photon carries electromagnetic energy, or whether you think it's important.

If you want to say the photon is a carrier of EM 'energy', then a probably naive question is then, what 'carries' a photon? If you want to say the photon is a form of energy as far as QM is concerned (where the 'form' is a wavefunction) you just mean you want to change the context, to one where energy has a discrete definition.

9. ### paddoboyValued Senior Member

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It's pedantic at best.

10. ### exchemistValued Senior Member

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As I pointed out to you earlier, a wave (in water or sound, say) carries energy, but nobody would claim such a wave "is" energy. Nor would anybody ask your "naive question" of what "carries" a water or sound wave.

So why do it for a photon?

11. ### paddoboyValued Senior Member

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And of course applying the pedant argument that a photon is not energy [just the carrier] prompts the next question, what is the photon? Sort of like asking some IDer the question, and what created him/her/it.
"Electromagnetic radiation, in classical physics, the flow of energyat the universal speed of light through free space or through a material medium in the form of the electric and magnetic fieldsthat make up electromagnetic waves such as radio waves, visible light, and gamma rays."
And supplementary, a photon can take the guise of any part of the EMS/EMR, and the energy associated with any guise of that photon, depends entirely on the frequency and/or wavelength.

12. ### arfa branecall me arfValued Senior Member

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Because water and an atmosphere are material things, perhaps even a naive person might conjecture these are what carry the wave's energy (whatever naive definition of energy you might also have).

So why, indeed, ask the question: does a photon propagate in a medium?

13. ### James RJust this guy, you know?Staff Member

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My analogy was completely on point. Not my fault if you still don't get it.

The argument I have addressed by analogy is the one that says that because a photon carries energy from one place to another, therefore the photon must be energy. It's the same as arguing that if I carry a phone from one place to another, I must be a phone. Don't like that one? How about one closer to the mark, then? A water wave carries energy from the deep ocean towards the sea shore; therefore water waves are energy. See? Wrong. Water waves are made of water, not energy. When the tsunami hits, its the water that does the damage, not the energy.

Some references and links have been made to some other sources who make the same basic mistake. That's all.

Last edited: Aug 31, 2019
14. ### James RJust this guy, you know?Staff Member

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It matters if making a basic category error matters to you. Otherwise, not so much, I guess.

Photons move through space.

Energy is a property that a photon has, just like its spin or its frequency or its polarisation. For starters, the fact that photons have these other properties (that are not energy) ought to tell you that photons are not energy, right there. Secondly, energy exists in systems that have nothing to do with photons, so again we must conclude that energy is not the same thing as photons. Thirdly, there's the basic category error of confusing "stuff" with numbers (photons being "stuff" and energy being numbers).

It isn't.

There's no such thing as an "energy wavefunction". A wavefunction is not energy. This time the category error is confusing a mathematical function with a number.

Energy is well-defined in physics. It's certainly not me who is trying to redefine it to mean something completely different, here.

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15. ### James RJust this guy, you know?Staff Member

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That would be a good question for you to research more fully than you have. Or, you could ask me, or one of the other people here who knows. On the other hand, I think we've probably already said what it is earlier in the thread.

Last edited: Aug 31, 2019
16. ### arfa branecall me arfValued Senior Member

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The polarisation and frequency are because of the symmetries of the electromagnetic field, though. Not because photons are some kind of composition of frequency and spin with a magnetic and an electric component.

The other properties you cite, and you can include zero mass, are all intrinsic to how electromagnetic radiation propagates. The properties of a photon are the photon. It isn't wrong to say a photon is a form of energy (lots of textbooks do), and saying that doesn't mean you can't say it carries, or has, energy.

As for a wavefunction: since writing $E \phi$ on the LHS of an equation means whatever you have on the RHS completely defines a quantum state, the LHS is clearly a representation of this state as a form of energy; the RHS defines how this state evolves, i.e. propagates.

17. ### river

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No comment ; but could you please explain what the acronms LHS and RHS , stands for , for all of us .

Thanks . Just asking so we can all follow along .

18. ### exchemistValued Senior Member

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The question you should ask is what the system is, because energy is a property of a physical system. Remember that a physical system can embrace not only matter but also fields. And, as I am sure you know perfectly well, EM radiation, i.e. photons, consist of waves in the 2components of the EM field. Don't be disingenuous.
Again Wiki gets it right, even if other sources, written for the general public, may not: https://en.wikipedia.org/wiki/Electromagnetic_radiation

19. ### James RJust this guy, you know?Staff Member

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arfa brane:

Really? Are you still going on this? This horse is long dead but you're still here waving the beating stick.

A particular photon - one photon - has a particular polarisation and frequency. A different photon might have a different polarisation and frequency. It follows that polarisation and frequency are properties of photons. And - lo and behold! - we say things like "The photons from the laser are polarised vertically" or even "The photons have vertical polarisation."

Yes, it's wrong, for reasons I have clearly explained - many of which, I might add, you have completely failed to address.

If lots of textbooks say photons are energy, then lots of textbooks are wrong.

See what you wrote there? That "E" represents what? Energy? If so, then the $\phi$ must be something else - something not energy.

In fact, that $\phi$ is a wavefunction, as you mentioned. A wavefunction, like a photon, is not energy. In fact, we can go further if you like. A photon has a wavefunction. I note for your further edification that a photon is not a wavefunction, nor is a wavefunction a photon. Nor is a wavefunction energy.

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20. ### paddoboyValued Senior Member

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Your analogy was childish at best.

21. ### James RJust this guy, you know?Staff Member

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I note you have made no attempt to address the substance. Instead, you opted for the personal attack. Ho hum.

22. ### paddoboyValued Senior Member

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And you have not made any personal attack on my person? Or made some childish remark to one of your mates at my expense?

Please Register or Log in to view the hidden image!

If you see my attack as personal, moderate me. I am not able to do the same for you sadly.
I'm OK james ol son...I do participate in another forum now with real live experts and ubiased moderation. I have aaalso acquired more then 500 rep points in a short time...Things over there certainly are quieter as the nuts, trolls, IDers, the IDers mates are quickly and efficiently dispatched.
You have really gone downhill since the old days and your pet adversary Zarkov.

Last edited by a moderator: Aug 31, 2019

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