# Are photons energy? What is energy, anyway?

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

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1. ### arfa branecall me arfValued Senior Member

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Or its position? What would you say a dot on a screen, made by a particle, actually is? What if instead you have a device that counts particles?

Say the particles are x-ray photons and the screen is standard photographic film.

3. ### exchemistValued Senior Member

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The point is that the fact a photon needs to be absorbed in order to measure its energy does not enable you to conclude is "is" energy. Because you also have to absorb it to measure its spin and nobody says a photon "is" spin, do they?

A photon is a disturbance in the electric and magnetic fields, that has energy as one of its properties.

5. ### originHeading towards oblivionValued Senior Member

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Arf does not appear to be able to change his mind once it is made up. Hard to learn anything new with that attitude.

7. ### exchemistValued Senior Member

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Well he does seem to want to change the subject, rather than concede the point.

8. ### arfa branecall me arfValued Senior Member

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The photon is spin-1, and this means measuring photon spin isn't really "absorbing" it after all, but in order to spin polarize a beam of photons some of them are absorbed.

It's also well-known that the Schrodinger eqn doesn't include particle spin. It does handle the likes of $E \phi(x)$. where $\phi$ is a wavefunction (more properly a "matter-field" amplitude). The wavefunction doesn't describe anything physical.

9. ### exchemistValued Senior Member

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The only way one can confirm the photon has a spin of 1 is by the effect it has when absorbed or emitted. This is seen, for instance, in the selection rules for the allowed transitions in atomic and molecular spectroscopy.

Your second sentence seems to be rambling and entirely off the point.

10. ### arfa branecall me arfValued Senior Member

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When absorbed or emitted by what? And what does it tell you about polarized photons?
Seen how? What is seen?
Except it isn't rambling; it isn't off the point either: Schrodinger's eqn doesn't differentiate between spin states. Perhaps you haven't grasped what that means when you want to talk about measuring the energy or momentum (or position) of some particle.

11. ### originHeading towards oblivionValued Senior Member

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You're a funny guy. This is still about you trying to convince us that physics is wrong and a photon is energy.

12. ### exchemistValued Senior Member

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What Origin said.

I decline to get bogged down in a swamp of extraneous side-discussions, introduced by you to avoid the original point.

13. ### arfa branecall me arfValued Senior Member

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I decline to get even a little bit interested in origin's or exchemist's somewhat pedantic grasp of what electromagnetic energy is.

Physicists who say a photon is a form of energy don't get excommunicated or burned at the stake. This is for the eminently sensible reason that a photon is, in fact, a form of energy. Nothing either has said in this thread makes the slightest difference: energy is what it is (but what is it?).

Of course, it's also eminently sensible to say a photon carries energy from place to place; if it is a form of energy and it propagates, then that's a given.

The lack of meaningful responses is like the sound of a chicken squawking.

Last edited: Aug 27, 2019

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Sometimes those that see themselves as "knowledgable" must then use uneccessary pedant to illustrate that illusion.

Light itself is part of the electromagnetic spectrum, which exhibits the whole known range of electromagnetic radiation, from radio and microwaves to gamma waves, including the visible part of that radiation we call light, the quanta of the EMS being the photon.
Taking that further and noticing some more pedant, the EMR is the flow of energy at "c "
So exchemist in his way [highlights by me] has actually agreed that what you have said, and what I mentioned and highlighted above is correct.
Again, the quanta of the EMS being the photon.
And the EMR is the flow of energy at "c "

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

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By "pedantic" you mean "correct", I assume.

Which physicists say that? Do any physicists say that? (They could be wrong, too, of course.)

No. Energy is an accounting system, not "stuff". It's a common misconception.

exchemist likes this.
16. ### James RJust this guy, you know?Staff Member

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The word you're looking for in that sentence is "pedantry". A pedant is a person. Pedantry is the action. Sorry for being pedantic about this.

It was all going well until that last sentence, which is wrong. Electromagnetic radiation isn't a flow of energy. Sure, EMR carries energy from one place to another, but a flow of energy is not what it "is".

Again, sorry for being pedantic about this, but sometimes there's no avoiding it if you want to correct somebody's error.

exchemist likes this.

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No worries, as you say pedantic.
It carries energy but isn't a flow of energy????]
"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."
or even this at a stretch......
And no, being pedantic does not mean being correct.
"giving too much attention to formal rules or small details:
https://dictionary.cambridge.org/dictionary/english/pedantic
They were being unnecessarily pedantic by insisting that Berry himself, and not his wife, should have made the announcement".
or an old favourite of mine......

18. ### arfa branecall me arfValued Senior Member

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So, just getting a little pedantic here, a flow of energy is not energy being carried from place to place? That sounds just a little ridiculous.

When heat 'flows' through say, a bar of metal, what is carried from place to place (or not)?
When the atoms in a metal bar start "vibrating" more, is it because they gain energy? Where does this energy come from, and what form is it in?
Does a magnetic field have a flow? A flow of what?

If as you say, energy is an accounting system, how does energy "account for" the electric and magnetic components of a photon?

Last edited: Aug 29, 2019
19. ### exchemistValued Senior Member

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A static magnetic field does not have a flow. It results from a flow of electrically charged entities, for instance electrons in a wire (in an electromagnet) or circulating* in atoms (in a permanent magnet).

Heat is, as you know, a particular form of kinetic energy, due to the random motion of assemblies of atoms and molecules in equilibrium. Temperature is proportional to the average kinetic energy present in this form. In the presence of a temperature gradient, this kinetic energy will flow.

And finally, yes of course a flow of energy is energy being transferred or "carried" from place to place. It is "carried" by whatever system it is that possesses the energy, the energy being a property of that system: moving atoms , photons, electrons, waves on the sea, etc. This is also what James is saying. He said a flow of energy is not what EMR "is". (Causing a flow of energy to take place is something it does, as a result of this property it has, of possessing EM energy.)

*"circulating" is admittedly a somewhat loose description when it comes to intrinsic angular momentum, but the effect is the same. To be pedantic. Or (for paddo), to exhibit pedantry.

Last edited: Aug 29, 2019

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Yes pedantic and ridiculous.
Again.....
"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.

exchemist appears to not be able to make up his mind, perhaps the ridiculous pedant confuses him?
You're not attempting to shit stir exchemist, are you? Surely not!

Hypocrisy that you are noted for does nothing but enhance stupidity.
Oh, and some exchemist dramatics before I go *click* let's do that again, *click* Weeee!

21. ### arfa branecall me arfValued Senior Member

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I think what you meant to say there is the magnetic flux is zero, because the paths for the vectors are all closed paths.
But it's easy to see that a magnetic field 'induces' a flow of iron filings, or in a ferromagnetic fluid. What's the pedantic explanation there? Wait, is it because the particles of iron experience a force and accelerate?

And why, when I google "electromagnetic energy" do most of the hits say electromagnetic energy is radiation (i.e. photons)? I find it hard to believe they've all got it wrong, some are from universities and otherwise reputable sources such as sciencedirect; paddoboy posted britannica's link. Did they get it wrong too?

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No, no one got it wrong per se...just a fruitless exercise in pedantry by some to boost there failing image.

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