# Are photons energy? What is energy, anyway?

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I completely understand that you might well have been exposed to people or texts careless equating electromagnetic radiation with "radiant energy" and the like.
Yeah. I completely understand what Max Planck was talking about when he suggested that black body radiation comes in discrete packets.

I get there this way: heat is energy. Heat from a hot stove element is energy "radiating" through space; this radiative heat does not require a medium, this is pretty obvious because it gets to the surface of the earth from the sun. We also are reasonably certain, nowadays, that this "form of" heat is photons of infrared radiation.

What do you believe infrared radiation is?

This is what is actually observed, experimentally. Are you going to start denying reality now?
What is "actually observed", and by which observer?

Please don't bore me to death with this. I have an explanation of the relativistic Doppler effect which describes (you know, mathematically) the changes in frequency when changing from a frame at rest relative to a source of light, to one moving towards the source.

Since, in my understanding, frequency is what 'defines' the energy, then this effect is just showing that yes, energy is also relative. That shouldn't be surprising to anyone who already understands that motion is relative; hence momentum is relative and therefore so is energy.

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No it doesn't, and I have, in fact, just given you two separate example in which energy is not conserved when you change coordinates.
Energy is not conserved when you change coordinates, because relative momentum isn't conserved (see last post). That's what I meant by conservation of energy is required; I didn't mean by changing from one frame to another, but in general.

I'll try to make my point a bit more concrete: Because the universe is expanding, light from very distant objects appears to have "lost" energy when we observe it. Does that mean the universe doesn't conserve energy?

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Spacetime certainly is not a substance, and neither is it physical, but it is real just the same for the reasons given and under the auspices of GR. Is a magnetic field real? Yes you say? Good, then for the same reasons, so to is space, time and spacetime.
I didn't say magnetic fields are real. Depends very much on what you mean by "real", of course, and I don't really want to get into that philosophical debate unless we have to.

For the purposes of the current discussion, it is sufficient, I think, to note that magnetic fields are not energy.

James R said:
"The spin, the frequency, the polarisation, the particle-like nature, the wave-like behaviour, the wave-packet, ... and so on and so forth. Basically, any of the other properties that photons have apart from energy."
Thank you James. Easily answered in that all contribute or are related to the energy which defines a photon.
Wrong. If energy comes in the form of photons, as you claim, then energy itself must always have all the properties that photons have, such as wavelength, spin, frequency, a wavefunction, etc. etc. So, as I asked arfa brane, why do we never hear about the spin of gravitational potential energy, or the wavelength of heat, or the wavefunction of kinetic energy? I expect you will ignore this question.
if a photon is only a carrier [like a bag or a box with lollies] while the lollies are analogous to the energy in your claim, what part of the photon is analogous to the bag or box?
[I understand why you cannot answer that question. ]​
I'm sure you don't understand why that question makes no sense, even though I told you why in one or two previous posts. Energy is a number we assign to the photon, or associate with it. Precisely because energy is not a substance or "stuff", we cannot say which "part" of the physical photon carries the energy. Energy is not contained in the photon. Energy does not fill up the photon. Energy is not stored in the photon.

The question you're asking is like asking "which part of a litre of milk contains the litre?", and then complaining when nobody can point you to a compartment within the milk where the "litre" will reveal itself.

And, just to remind you, your entire argument is essentially the same as if you had said "a milk carton is a litre".

James R said:
What makes one particle matter and another not? Please tell me.
Mass.
But you told me that all mass is energy.

Take two particles, then: an electron and a photon. The electron has mass (I think we agree), and it is "matter", according to your definition. But all mass is energy, so it follows that the electron is energy. The photon has no mass and it therefore not matter, according to you, but you say it is energy. So, on the basis of your own definitions and argument, we have concluded that both electrons and photons are energy, according to you.

Followed to its logical conclusion, this means that, for you, all particles are energy, regardless of whether they are "matter" or not.

What I wonder, then, is why we need two different concepts - mass and energy - if really there's just one thing: energy. In fact, if everything is energy, then what's the use of talking about different kinds of particles at all. Maybe your claim is that photons and electrons are merely different forms of energy, where each particle uses different available "handles" that are "in" the energy, somehow. I don't know.

All that's apparent to me is that your conceptual framework for particles and energy is deeply muddled.

Let me tell you are story about conditions just this side of the BB, about 10-43 seconds in fact...best estimates tell us that all the four forces were united into one superforce under the extreme conditions of temperatures and pressures...as expansion took hold, those temperatures and pressures started to drop, and at around 10-35 seconds the superforce started to decouple. This created phase transitions and false vacuums with the energy differences going into creating our very first fundamental particles, quarks, electrons.
You see where this is heading James?
No idea. What has this got to do with your claim that photons are energy?

You're the one who claims there is not a rck solid definition of matter.
Right. I think there are several possible definitions, at least.

But hey James ol son, I'll go along with your cynical methodology. Matter is that which is physical and has mass. It exists in three states, solid, liquid and gas...and as a plasma if you like. Your turn James.
I don't think your definition of "matter" is of much consequence for our discussion about photons. I will accept for the purposes of our discussion that photons are not "matter", if you like. Now, all you have to do is to show that photons are energy, which is the problem you had before you started talking about matter.

While certainly energy is not a substance [and I have never said or inferred that, that's just your cynical methodology] a photon for the reasons stated is a packet of energy, or if you prefer James, a carrier of energy.
The term "packet of energy" is ambiguous. Does your "packet" have anything to it other than the energy it contains, or by "packet" do you simply mean the totality of the energy itself? The term "carrier of energy" implies that there is something separate from the energy being carried - that there is a container for the energy.

Remember: your claim is that a photon is (nothing but) energy. You're not saying that a photon is something that contains energy, that containment being just one of its many properties. You're saying that "a photon" and "energy" are equivalent - interchangeable words for the same thing, essentially.

https://energyeducation.ca/encyclopedia/Photon
"A photon is a particle of light which essentially is a packet of electromagnetic radiation"
https://www.livescience.com/38169-electromagnetism.html
"Electromagnetic (EM) radiation is a form of energy that is all around us and takes many forms, such as radio waves, microwaves, X-rays and gamma rays. Sunlight is also a form of EM energy, but visible light is only a small portion of the EM spectrum, which contains a broad range of electromagnetic wavelengths"
It's an annoying habit of yours to repeat yourself. For example, you post the above as if it had not already been examined and responded to earlier in the thread. That is troll-like behaviour, and you should stop it.

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Aren't you a little old to be acting like a petulant child?
Let's have a look at that gutless "statement" again, and the associated bravado with regards particularly to the word petulant.
There are three possible answers to this current shemozzle...
[1] James and his audience are correct, a photon is strictly and unequivocally just a carrier of force, with no correspondence entered into, as detailed in some reputable sources.
[2] A photon is a packet of energy as also detailed in some reputable sources, and without any correspondence entered into.
[3] With reputable sources supporting either definition, the real situation is as I have taken...that is this is nothing more then an exercise in pedant, with both definitions able to be handled, understood and reasonably defined.

It should then be plain to see that the petulance most likely rests with answers 1 or 2, and the supporters of either side as Listed, are simply ego driven and not able to show the intestinal fortitude in that either is acceptable, as has been shown and to constantly argue otherwise is pedant, pedantic, pedantry....take your pick.

I didn't say magnetic fields are real. Depends very much on what you mean by "real", of course, and I don't really want to get into that philosophical debate unless we have to.
No, I inferred they are real, as is space, time, spacetime. You'l also find various opinions on whether time is real or not.

Wrong. If energy comes in the form of photons, as you claim, then energy itself must always have all the properties that photons have, such as wavelength, spin, frequency, a wavefunction, etc. etc. So, as I asked arfa brane, why do we never hear about the spin of gravitational potential energy, or the wavelength of heat, or the wavefunction of kinetic energy? I expect you will ignore this question.
I said nothing about all energy coming in the form of photons.

I'll let you examine the following again.....
"Mass is a form of energy. All mass is energy. Not all energy is mass (kinetic energy, potential energy)"

arfa brane:

Yeah. I completely understand what Max Planck was talking about when he suggested that black body radiation comes in discrete packets.
Actually, originally he only said that the "oscillators" in a black body can only have integer multiples of a "quantum" of energy (not sure if he used the word "quantum"). He didn't say the radiation itself was quantised. That was an idea that Einstein introduced a few years later.

I get there this way: heat is energy.
Ok so far...

Heat from a hot stove element is energy "radiating" through space...
Black body radiation comes in the form of electromagnetic waves, or photons if you prefer. Photons (or waves) can transfer heat from one object to another, but they themselves are not heat (because they are not energy).

We also are reasonably certain, nowadays, that this "form of" heat is photons of infrared radiation.
No. Photons are not a "form of heat", because heat is energy and photons are not energy.

What do you believe infrared radiation is?
Photons, like you say. Electromagnetic radiation, if you like. Neither of which is energy.

Since, in my understanding, frequency is what 'defines' the energy, then this effect is just showing that yes, energy is also relative. That shouldn't be surprising to anyone who already understands that motion is relative; hence momentum is relative and therefore so is energy.
Then we agree that the photon energy is relative. Good.

We can change the energy of a photon just by changing our motion relative to the photon, all the while never touching the photon itself.

It follows that if the photon is energy, as you claim, then it's a variable amount of energy that can be altered from a distance, without interacting with it through any physical process. What do you have to say about that?

There are three possible answers to this current shemozzle...
[1] James and his audience are correct, a photon is strictly and unequivocally just a carrier of force...
I have not made that claim.

I have only claimed that, whatever else the photon may be, it is not (synonymous with) energy. The fact that it has properties other than its energy ought to tell you that from the start. But obstinance gets in the way, to the point of stupidity.

What? That's the best you can do in response to what I wrote to you?

I said nothing about all energy coming in the form of photons.
What does "photons are energy" mean, if not "energy can come in the form of photons"?

All photons are energy, but energy can't possibly be photons? You're contradicting yourself. Hopelessly muddled.

Photons (or waves) can transfer heat from one object to another, but they themselves are not heat (because they are not energy).
Let's suppose that's true.
No. Photons are not a "form of heat", because heat is energy and photons are not energy.
A photon can transfer heat, but a photon isn't a form of heat?
Heat flows in three distinct ways: conduction (as in a metal or similar solid), convection (as in fluids), and radiation. That's been a scientific fact since the end of the 19th century. Radiation is a flow of energy.
This is why physicists say an electron emits a photon as radiative energy. It's why Erwin Schrodinger says the same thing.
Sorry, I just can't agree that EM radiation isn't a form of energy. Heat has been called "motion"--the average kinetic energy of a system of particles. One way to increase the energy of a particle is to get it to absorb a photon.

So a physicist says the particle has absorbed the energy, or that the photon's energy has been absorbed. If photon is not energy, what happens to it when the energy it "has" is absorbed? Why doesn't the momentum, polarization and frequency persist somewhere? Why does the whole thing disappear, whereas massive particles don't?

That is, why are the frequency and momentum of a photon independent of the energy that photon carries? Wait, they aren't! In fact a photon's energy depends on its frequency and momentum. So when the energy a photon "carries" is absorbed so is the frequency and momentum, there's nothing left.

This is because the photon is a form of energy and energy is conserved.

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It follows that if the photon is energy, as you claim, then it's a variable amount of energy that can be altered from a distance, without interacting with it through any physical process. What do you have to say about that?
Well, I can say that if you move towards a source of EM radiation, then you are interacting with that radiation, and definitely through a physical process; if you don't move towards it you are still interacting with it.
Or perhaps I can say the radiation interacts with you, or with the matter you're made out of.

What do you have to say about redshift due to the cosmological expansion?

What? That's the best you can do in response to what I wrote to you?
Take it or leave it. I'm rather tired of your obtuseness and pedant.
What does "photons are energy" mean, if not "energy can come in the form of photons"?

All photons are energy, but energy can't possibly be photons? You're contradicting yourself. Hopelessly muddled.
No, you are being dishonest and obtuse.
"Mass is a form of energy. All mass is energy. Not all energy is mass (kinetic energy, potential energy)"
I have not made that claim.

I have only claimed that, whatever else the photon may be, it is not (synonymous with) energy. The fact that it has properties other than its energy ought to tell you that from the start. But obstinance gets in the way, to the point of stupidity.
And I and many links say that you are pedantic at best, and wrong at worst. The fact that it has properties such as wavelength and frequency simply define the photon energy.
Again, I repeat.....
[1] James and his audience are correct, a photon is strictly and unequivocally just a carrier of force, with no correspondence entered into, as detailed in some reputable sources.
[2] A photon is a packet of energy as also detailed in some reputable sources, and without any correspondence entered into.
[3] With reputable sources supporting either definition, the real situation is as I have taken...that is this is nothing more then an exercise in pedant, with both definitions able to be handled, understood and reasonably defined.

It should then be plain to see that the petulance most likely rests with answers 1 or 2, and the supporters of either side as Listed, are simply ego driven and not able to show the intestinal fortitude in that either is acceptable, as has been shown and to constantly argue otherwise is pedant, pedantic, pedantry....take your pick.

Wrong. If energy comes in the form of photons, as you claim, then energy itself must always have all the properties that photons have, such as wavelength, spin, frequency, a wavefunction, etc. etc. So, as I asked arfa brane, why do we never hear about the spin of gravitational potential energy, or the wavelength of heat, or the wavefunction of kinetic energy? I expect you will ignore this question.
Ignore it? Not really....It's working around your desperation in trying to show you are right that's annoying.
I'll answer that anyway by saying in the photons FoR, nothing changes. The same situation with photons and the universe expansion....Gravitational red/blueshift is simply an observation based on your time compared to the time elsewhere, depending on the gravitational well, that affects the apparent passage of time from that other frame.
The question you're asking is like asking "which part of a litre of milk contains the litre?", and then complaining when nobody can point you to a compartment within the milk where the "litre" will reveal itself.
Invalid concept again James. The amount of milk in the container [the energy] is entirely separate from the carrie [the carton. Please show this effect with a photon? What corresponds to the carton?
Take two particles, then: an electron and a photon. The electron has mass (I think we agree), and it is "matter", according to your definition. But all mass is energy, so it follows that the electron is energy. The photon has no mass and it therefore not matter, according to you, but you say it is energy. So, on the basis of your own definitions and argument, we have concluded that both electrons and photons are energy, according to you.
I said mass is a form of energy and all mass is energy, but not all energy is mass. That hasn't changed.
It's an annoying habit of yours to repeat yourself. For example, you post the above as if it had not already been examined and responded to earlier in the thread. That is troll-like behaviour, and you should stop it.
And you need to stop misconstruing what I say and being obtuse. But of course unlike you, I have no power over how you misconstrue

Sorry, it's not wrong. What you say there is actually wrong.

As someone who has done a bit of study of the EM field (namely the discipline of electronics), I'm reasonably confident I know what I'm talking about. I know what I mean by the energy in or moving through some part of an electronic circuit. I know circuits generally radiate energy and it's called electromagnetic radiation (!). Someone telling me this radiation is not a form of energy is a) just wrong, and b) in need of further study so they can understand what it means, as clearly they don't.
I'm not sure why the sudden reversion to a post of mine from a fortnight ago, as the discussion has gone a long way since then. (6 pages of it)

But whatever, nope, it is wrong to say a photon "is" energy. I have already said what it is, but to repeat, it is a travelling disturbance in the electromagnetic field, that has energy, along with momentum, spin, frequency and polarisation. It "is" energy no more than a water wave is.

Energy does not have properties. It just has a value. Ask yourself: what properties do you think kinetic energy has? What properties do you think gravitational potential energy has? See? It's meaningless.

But I see you are an electronic engineer rather than a physicist. That could account for your point of view. I suspect the subtleties of the distinction that Origin, James and I are pointing out don't matter, in practice, in electronics. a lot. But for the physicist, and to the physical chemist to some extent, they do.

Let's suppose that's true.

A photon can transfer heat, but a photon isn't a form of heat?
Heat flows in three distinct ways: conduction (as in a metal or similar solid), convection (as in fluids), and radiation. That's been a scientific fact since the end of the 19th century. Radiation is a flow of energy.
This is why physicists say an electron emits a photon as radiative energy. It's why Erwin Schrodinger says the same thing.
Sorry, I just can't agree that EM radiation isn't a form of energy. Heat has been called "motion"--the average kinetic energy of a system of particles. One way to increase the energy of a particle is to get it to absorb a photon.

So a physicist says the particle has absorbed the energy, or that the photon's energy has been absorbed. If photon is not energy, what happens to it when the energy it "has" is absorbed? Why doesn't the momentum, polarization and frequency persist somewhere? Why does the whole thing disappear, whereas massive particles don't?

That is, why are the frequency and momentum of a photon independent of the energy that photon carries? Wait, they aren't! In fact a photon's energy depends on its frequency and momentum. So when the energy a photon "carries" is absorbed so is the frequency and momentum, there's nothing left.

This is because the photon is a form of energy and energy is conserved.
Because the momentum and spin, which are conserved quantities like energy, are also transferred to whatever it is that absorbed the photon, along with the energy. That fact alone is enough to show that a photon cannot be simply energy. What do you think happens when a water wave is absorbed? What do you think is left? Just a static medium, right? Exactly like a static EM field with zero value (pace vacuum ZPE of course), which is all that is left after a photon has been absorbed.

Energy is just a quantity, with dimension ML²T⁻². A quantity can't have properties.

But whatever, nope, it is wrong to say a photon "is" energy. I have already said what it is, but to repeat, it is a travelling disturbance in the electromagnetic field, that has energy, along with momentum, spin, frequency and polarisation. It "is" energy no more than a water wave is.
So how do you explain to yourself that physicists say quantum oscillators absorb and emit energy in discrete quantities? They also say, quite often, that this discrete quantity or quantum of energy is a photon with a frequency? When a photon "gets" energy from an electron so it can "carry" it somewhere, where does the photon come from? Is it kind of hanging around the electron waiting for some energy to add to its already existing polarization and frequency etc?

Yes I can see that's plainly ridiculous.

And yes my background in electronics, yours in chemistry, leads me to the conclusion that you still don't understand what electromagnetic radiation actually is.
And no, I don't really care. But I'd like you to either learn more about it rather than rest on what I consider is a flawed notion, or stop repeating the same confusing nonsense.

That notion's the one you and James keep repeating: "a photon is not a form of energy, but is a carrier of energy".
This is flawed because it doesn't seem to be able to explain how photons are generated or annihilated. I mean, a dumb question about the notion is: what happens to the photon after its energy is absorbed? Why does it get annihilated, and by what mechanism?

I'm fairly sure I understand why.

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Because the momentum and spin, which are conserved quantities like energy, are also transferred to whatever it is that absorbed the photon, along with the energy. That fact alone is enough to show that a photon cannot be simply energy.

I think this exemplifies what is wrong with your idea (apart from the staunch belief in it being correct). The momentum is transferred because the energy is "absorbed" completely--therefore the photon has zero energy and zero momentum. The frequency though, what happens to that? Spin of course, is angular momentum and a photon that hasn't been spin-polarized is in a superposition of spin states, right? All the photon's momentum is transferred, therefore all the energy is too.

So it actually makes sense to consider the quantum of the EM field as quantum of energy, which is transferred (transported) between charged particles. This idea is commonly trotted out in lectures (I've been to a few).

So how do you explain to yourself that physicists say quantum oscillators absorb and emit energy in discrete quantities? They also say, quite often, that this discrete quantity or quantum of energy is a photon with a frequency? When a photon "gets" energy from an electron so it can "carry" it somewhere, where does the photon come from? Is it kind of hanging around the electron waiting for some energy to add to its already existing polarization and frequency etc?

Yes I can see that's plainly ridiculous.

And yes my background in electronics, yours in chemistry, leads me to the conclusion that you still don't understand what electromagnetic radiation actually is.
And no, I don't really care. But I'd like you to either learn more about it rather than rest on what I consider is a flawed notion, or stop repeating the same confusing nonsense.

That notion's the one you and James keep repeating: "a photon is not a form of energy, but is a carrier of energy".
This is flawed because it doesn't seem to be able to explain how photons are generated or annihilated. I mean, a dumb question about the notion is: what happens to the photon after its energy is absorbed? Why does it get annihilated, and by what mechanism?

I'm fairly sure I understand why.
As an electronic engineer, you may have some instinct for what is meant by a "transition dipole". https://en.wikipedia.org/wiki/Transition_dipole_moment

That is what the electron has, on its transition between states, giving rise to the emission of a photon. This dipole excites a disturbance in the EM field- a microscopic analogue of what happens in a transmitting antenna. And so an EM wave is formed, with a particular frequency envelope* and with one quantum of electromagnetic energy, one quantum of angular momentum and a quantum of linear momentum . We call this a "photon". The transition between states also involves a corresponding change in the electron's angular momentum by one unit (which is why transitions between states with the same angular momentum are "forbidden") and results in the atom acquiring a certain amount of linear momentum, in the direction opposite to that in which the photon is emitted.

Absorption is the same process in reverse.

*not exact, due to the uncertainty principle

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That is what the electron has, on its transition between states, giving rise to the emission of a photon. This dipole excites a disturbance in the EM field- a microscopic analogue of what happens in a transmitting antenna. And so an EM wave is formed, with a particular frequency envelope* and with one quantum of electromagnetic energy, one quantum of angular momentum and a quantum of linear momentum .
I take issue right off the bat with a quantum of electromagnetic energy, a quantum of angular momentum and a quantum of linear momentum.

A photon has one quantum of energy determined completely by its frequency. The angular momentum is 'undefined' or, the photon does not have a particular polarization; it does however have a particular frequency.

I've never seen the phrase "a quantum of linear momentum". What does it mean?

Nonetheless the total momentum is conserved because that's the physics.

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No. Merely exerting force or pressure on something is not defined as "work" in physics. For "work", there must also be movement.

A rock sitting on the ground is not continuously doing work. If it was, its energy would be decreasing all the time, which might prompt the question of where all that energy is coming from, and where it is going to.[/QUOTE]

Let's come from different place. Rock falling - Earth gravity doing work pulling rock down

Imperceptible - rock gravity pulling Earth up

Rock hits ground - does the Earth move down? no because Earth absorbed? yes but imperceptible?

I go no. Now we have Earth + rock combo which can be viewed as one unit OR two. I would go two as long as the rock remains loose from the Earth. If becomes melded into the Earth one unit

Even as two units the gravity of the rock combines with Earth's gravity

I would still say rock working trying to pull Earth up but cancelled by Earth work pulling rock down. So net effect no exchange of work (energy) but both work remain present

Coffee time

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