# 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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No, I think you misunderstand what he's saying. But that's what I think.

What I also think is, what Schrodinger is "really" saying is that all forms of energy are quantized--every quantum is a field quantum. A "matter field" quantum is otherwise called a particle, like an electron or a proton are often called particles.
That's your misconception of what "a field quantum" means, to a theoretical physicist.

3. ### arfa branecall me arfValued Senior Member

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Or the continuously varying energy "associated with" a free falling cricket ball? Which it gets from the gravitational field? This would imply that gravitational energy (a number) can somehow turn into the local acceleration of an object with another number--its mass?

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

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From Feynman's Lectures in Physics:

"It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. However, there are formulas for calculating some numerical quantity, and when we add it all together it gives “28”—always the same number. It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas."
Feynman was a smart guy. I'm with him.

7. ### arfa branecall me arfValued Senior Member

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Feynman is right, we don't know what it "is" but we do know how to calculate it, because we have working "theories of energy".

In exactly the same sense, we have theories of measurement--these tell us we can do physical experiments and record certain information. How did we work this out? What informs this (possibly misguided) enterprise?

On the other hand, you could say Feynman is "really" saying we have no knowledge of why the theories predict anything at all. Or perhaps what he means is, we have no idea what a photon really "looks like", what we have is a theory.

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No, I'm not wrong, and what you hear, as has been shown in the past [with me] is faulty at best. Again this is pedant/pedantry/pedantic, take your pick.

It is you who is muddled James.
Let me again clear it up for you....All Mass is a form of energy, but not all energy is mass.eg: kinetic energy. On the highlighted parts, Yes, light is energy...no light has no mass...yes all mass is energy, and guess what? not all energy is mass. So no your conclusion is faulty at best. Matter and light are not the same thing.

Then you are muddled, because I see him saying differently....
let me reproduce the relevant parts for your perusal...
" The simplest answer is that when a photon is absorbed by an electron, it is completely destroyed. All its energy is imparted to the electron, which instantly jumps to a new energy level. The photon itself ceases to be. In the equations which govern this interaction, one side of the equation (for the initial state) has terms for both the electron and the photon, while the other side (representing the final state) has only one term: for the electron.

The opposite happens when an electron emits a photon. The photon is not selected from a "well" of photons living in the atom; it is created instantaneously out of the vacuum. The electron in the high energy level is instantly converted into a lower energy-level electron and a photon. There is no in-between state where the photon is being constructed. It instantly pops into existance.

So the question is: where does the photon come from?

Strangely, it doesn't seem to come from anywhere. The universe must put the extra energy somewhere, and because electrons in atoms are electromagnetic phenomena, a photon is born with the required energy. In a weak-force interaction, say the decay of a neutron, that energy goes into a neutrino particle which is also instantaneously created. Each force has its own carrier particles, and knows how to make them.

That's really all we can say about it. There are many interpretations of what this and other phenomena in quantum mechanics mean on a deeper level, and whole libraries worth of books which argue points of view on the matter. But my personal philosophy is that of the famous physicist Richard Feynman, who said: "Shut up and calculate."
<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>

Dave Kornreich
Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.::::::::::::::::::::::::::::::::::::

Now James, if what you say is 100% true, with no escape clause, pedant, or room for error, what is left after the energy has been imparted to the electron, and after the photon ceases to be?If it was just a carrier [with no room for error, pedant or escape clause, the energy would be gone and this mystery carrier would be left. Like I say James, I'm not arguing any point as to which is right or wrong...a photon is a carrier of energy, or a photon is energy, or if you like an energetic photon, If one is more right then the other, great! so be it, but I'm simply saying its pedantry as are many points in science and physics.
You're the immovable object, you're the one who has the ego to defend and uphold against the big bad evil paddo boy for daring to question your say so.

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And all that above supports the valaid argument I am simply putting...pedantry.
Who ever said that science was a dogma? I have often mentioned that science is a discipline in continued progress.
Authority?? There are professional people who are expert in certain areas James. I try and take notice of them when they have reasonable arguments.
Even the professionals conflict at times James.

10. ### arfa branecall me arfValued Senior Member

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I think James R's "energy as an accounting system" is fine. It's another way of saying the total energy of an isolated system is constant, and so in an open system the total energy is conserved only if the input and output energies are equal (or, equivalent).

But his grasp of the energy in the EM field of a broadcast antenna isn't quite the goods, sorry.
If you say, test the antenna with a DC "input", an electric field will extend (at the speed of light) from the antenna. Make the electrons in the antenna oscillate, and this field is disturbed, it now has more energy. In particular it has a (time-varying) magnetic "vector potential" propagating at the same speed away from the antenna.
Where did that come from (??)

Feynman (to an electronics engineer) I think was being a little disingenuous. However, I can see a parallel: we have no real idea what information is, except we know we can choose something--a small amount of charge, say--to represent it. We know information comes in discrete chunks or bits, but we don't know why . . .

Last edited: Sep 10, 2019

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Again, supporting what I'm saying James.

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https://www.quora.com/If-it-is-true...nergy-must-all-energy-be-contained-in-photons

just some extracts to continue to illustrate the point I am making.....
Photons are not containers. They don’t contain anything. They are elementary particles (or to be pedantic, quanta of a fundamental field, the electromagnetic field.)
Photons carry kinetic energy and momentum. The amount of energy and momentum carried by a photon depends on the observer. If you run towards an oncoming photon, you measure more energy than someone not running with you, or running in the opposite direction.

Photons are particles of electromagnetic radiation ie. Energy. Photons not only contain energy, they are energy. There are different forms of energy. All energy does not have to be contained by photons such as kinetic energy or nuclear energy. All electromagnetic energy must be photons, as photons are the vehicle for this type of energy. This includes gamma rays, x rays, UV and infrared and visible light, microwaves and radio waves. Energy can be converted from one form to another, such as kinetic energy being transferred to electromagnetic energy.

13. ### arfa branecall me arfValued Senior Member

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A photon sketch:

John Cleese walks into a particle shop. Michael Palin says, "Good afternoon, sir, I see you have a photon."

John: "This photon appears to be dead!"
Michael: "No, it's just resting . . ."

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14. ### arfa branecall me arfValued Senior Member

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Ahem:
Quantum computing: Scientists stop light in a cloud of atoms

15. ### arfa branecall me arfValued Senior Member

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Photons can be "stopped" by a Bose-Einstein condensate. Is that because they still "move at the speed of light" in the medium?

16. ### Write4UValued Senior Member

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This site may contain some interesting information.
https://www.grandinetti.org/quantum-theory-light

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

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arfa brane:

It's possible. It is out of context, after all. But I don't think I'm misunderstanding, especially since I've given two possible interpretations.

I don't disagree with anything you just wrote, so it's not clear what you think my misconception is, exactly. Of course, to say that all forms of energy are quantised (okay), then that fields are quantised (okay), is not to say that energy and fields are the same kind of thing.

When a ball falls, there no numbers turning into motion or acceleration. What you're looking at there is a force acting on the ball, making it accelerate, namely, the gravitational force exerted by the Earth (a thing) on the ball (another thing). All the energy does in that process is to move out of the "gravitational potential energy" column in the Table of Energy values and into the "kinetic energy" column.

By which you mean we can write down some equations and calculate some numbers and put the numbers in a Table of Energies.

Interaction between theory and experiment? Contact between the realm of ideas and the rubber hitting the road of physical reality?

Maybe. An interesting philosophical discussion we could potentially have in different thread.

Then you're willing to give up the idea that photons are energy at last? (I won't hold my breath.)

Where did what come from? The propagating electric and magnetic fields, you mean? When those (physical) electrons were made to oscillate, they created a self-propagating disturbance in the electromagnetic field, just like when you throw a rock into a pond it creates a self-propagating wave in the pond.

How so? He was just making the same point I'm making: energy is a number - an accounting system - not "stuff". Notice how he explicitly says that energy is not "little blobs of a definite amount". But you and paddoboy are telling me that's exactly what photons are - little blobs of energy of a definite amount.

That's quite a subtle effect due to the different phase and group velocities of the waves in the condensate.

It has nothing to do with photons being energy.

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

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Are you saying that photons are energy? If you are, then you're wrong.

So, let's be clear. Is that still your position, or are you changing your tune now?

It doesn't matter how much you use that word. Photons are not energy, and shouting "pedantry" until the cows come home won't change that fact.

Let me walk you through this slowly one more time, using your own words. Please pay attention this time.
"All Mass is a form of energy..."
That's you saying mass is energy.​
"...Yes, light is energy..."
That's you saying that light is energy.
Now, if mass is energy and light is energy, it follows that both mass and light are energy, right? And if energy is one "thing", then you're saying that mass and light are the same thing - or two aspects of the same thing. Right?

But then you go on to say:
".... Matter and light are not the same thing."

So, that sounds totally contradictory to me. If mass and light are both energy, but matter and light are not the same thing (i.e. not both energy), then how does that work in the paddoboy universe? Maybe matter and mass are not the same thing in the paddoboy universe? Or what?

Explain to me how light is energy and mass is energy, but that somehow doesn't make light and mass the same thing. Is energy schizophrenic or something?

Why on earth do you think that repeating a quote I've already agreed with is going to help you in any way. You need to read the quote yourself and work out why it isn't saying what you're saying (because I disagree with you, while I agree with the quote).

And so? There's nothing there that says the photon is energy.

Again, nowhere does this say a photon is energy. In fact, it explicitly talks about an electromagnetic interaction involving an electron producing a photon.

This is a statement about energy conservation. It nowhere says that energy is "turned into" a photon. It says that a photon is created ("born") with an appropriate associated energy, so as to conserve total energy of the system. Energy moves from one column in the Table of Energies to another one, titled "photon energy".

The electron is left, in a higher energy state than it was before it interacted with the photon. Before the interaction, there was a photon (a thing) and an electron (another thing). After the interaction, there was just one thing - the electron. In the process, a number was moved from the column labelled "photon energy" in the Table of Energies into the column labelled "atomic energy", or "electron energy" or whatever you prefer to call it.

Your argument is like saying that when a water wave hits the beach, the wave itself must be "left" after it has transferred its energy onto the sand.

Or, it's like saying that when a sound wave enters your ear, causing your eardrum to vibrate, the air-pressure wave that is the sound must somehow remain in your ear.

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 you have to do, fundamentally, is to lose that notion that you have that energy is "stuff". It isn't stuff. You can't bottle it. It doesn't travel from place to place. You can't isolate it from the actual "stuff" it relates to. There is no "pure energy", separate from stuff. You're stuck on this notion that energy is stuff. Let go. It will do you good.

Those two are mutually exclusive. You can't have both. You need to decide which one you want. I'd urge you to pick the only one that makes any sense, or else you'll go through life thinking that numbers can turn into stuff.

I point out that you have made no arguments to support the idea that a photon/light is energy, other than appeals to various authorities. In contrast, I have given you many reasons to drop the assumption that a photon/light is the same thing as energy.

It's not about you daring to question my say so. Clearly you're not cowed by the thought of questioning my say so.

If you want me to shift my position on this, you'll need to give me at least one good reason why I should, that's all, because I've given you lots of different reasons why I shouldn't.

Also, in the process we've discovered that you believe that mass and energy are the same thing, too, which is also wrong, for the same reason. You have the opportunity to correct your thinking in a fundamental way, if only you will take it. But ego keeps getting in the way.

You sometimes act as if you think it is. In this thread, you have relied mostly on quotes from "authorities", that you have either misinterpreted, or else that have actually been in error themselves. If you're serious about pursuing this point, at some stage you need to stop playing "my man is bigger than your man" and start talking about the actual physics.

Suppose I were to tell you that I was a professional expert in this area. What then? Would that change your mind? Would that convince you I am correct?

Yes. And professionals can make mistakes, too.

Not if you're still saying that photons are energy.

That's exactly what I told you earlier. Fine so far. But then it all goes to pot...

Is this still the above-linked site, or is this you now? Because this stuff is wrong.

Interesting that you say that, because if photons are energy, then energy must have all the properties that photons have, like I said before. But it doesn't. Kinetic energy, for instance, doesn't have a polarisation.

Which is it? Photons are energy, or photons are a vehicle for energy? You can't have both.

Cars can carry people, but cars are not people.

Yes. You can move energy from one column in the Table of Energy values to a different column, because it's just a number.

19. ### exchemistValued Senior Member

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I see this is still going on

. I agree with all you have said, obviously. It seems to me the point about the energy in the wave on the beach or in the sound in your ear may be the clearest comparison. But we have both tried this analogy, apparently without any success.

Last edited: Sep 10, 2019
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20. ### arfa branecall me arfValued Senior Member

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According to Einstein there is no force. Besides a force is either the time derivative of momentum (whatever that is, since clearly the cricket ball doesn't do any multiplying of its mass by its velocity at any point, it just "moves"); or a force is the local derivative, the gradient of the local potential. This potential is "in the field", as physics says.

Where else might it be?
Energy and fields seem to have a relation between them, though.

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21. ### arfa branecall me arfValued Senior Member

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'sigh'. You haven't done much of this have you?

The field exists because charged particles emit it. The field is disturbed when electrons oscillate, i.e. are forced to accelerate back and forth. We force them to do this in one dimension along the length of an antenna. The antenna essentially is a container, a conducting bar of metal say, but it just has to have electrons in it that can move, more or less freely like a gas or a fluid--Einstein calls it "electric fluid" in more than one of his papers.

So there is an oscillating matter-field with millions or billions of electrons all moving synchronously. Each contributes photons with the same frequency to a classical "plane wave" emanating from this 1-dimensional antenna; electrons like to emit radiation this way, or maybe photons like to bunch together. Whatever, you get a nice plane electromagnetic wave (at least, you can detect one at some distance from the antenna), and it has an intensity.

Intensity is definitely in the "is physical" column here. And I don't think it helps to invoke surface waves on a pond. I think you need to be careful with "the field is disturbed"; it refers to the field being emitted "now", right?

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22. ### arfa branecall me arfValued Senior Member

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Hmm. If I am beginning to understand your approach, it seems then that a photon is a thing that "moves a number" when its absorbed? That's kind of difficult to reconcile with field energies.

By which I mean the energies of the electromagnetic field the photon, erm, "carries around", and of the electron's field.
I feel obliged to point out that whether a photon is "material", or a "matter particle", is apparently open to debate; in that case physics usually says it's ok to think of a massless particle as material. When a BEC absorbs a photon, it stores it as a "spin wave", i.e. the photon becomes a standing wave of electron angular momentum.

There is a bit more to it, but the photon's field oscillations are transferred to this coherent (up to a minute) spin-wave. What does "transferring energy from one column to another" do for physics in this scenario?

The BEC is initially opaque to the input source (the signal laser). Another control laser is able to change it to transparent (this is complicated). So switching the control laser off while the input photon is inside the BEC is a critical step. Success means the photon is trapped in a medium it sees as opaque and in which it can't dissipate as "heat energy" or phonons.

This photon will spontaneously reappear when the control laser is switched on; the "storage of a photon in three dimensions" depends on the coherence of a quantum state in the BEC.
So this idea of oscillating electrons "disturbing the EM field" isn't quite accurate; it should be "oscillating electrons generate an oscillating field". The energy in the electron oscillations is "emitted" as this oscillating field.

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23. ### arfa branecall me arfValued Senior Member

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But I already pointed out that this "definite amount" can change if you move relative to the source; you brought up the relativistic Doppler effect, right?

However, a single photon has a definite field amplitude, similarly an electron has a definite mass (at rest). There is a difference too between definite and discrete. Photons are discrete "particles". I don't know that I have said their energy is a definite amount. But that must be true in the electron's frame when it emits a photon.

Except, for the emitting electron the photon represents a change in angular momentum. Leonard Susskind explains this reasonably well in some online lecture. So what do electrons represent for photons?