Are photons energy? What is energy, anyway?

Not really. I disagree with the guy on quora about all particles being "pure energy". Particles are excitations in quantum fields, yes, but that's not the same as "pure energy". The guy sounds reasonable enough that I think I could probably convince him of that with a brief conversation about it. It's not that he's exactly wrong, but he's using a metaphor without realising it. Like I said, it's a common mistake, and even physicists are prone to making it. To some extent, it doesn't matter if we're a little loose with the language, but when the question is this one it is actually important to get it right.

 

paddoboy:

What I'm hearing is that you're not interested in whether you're right or wrong. As long as you walk away with your ego intact, you're content to be wrong. For you, this discussion ceased being about what is true some time ago. Now it's just about you trying to save face, at least as far as you're concerned.

None of that addresses the question of whether photons are energy (they are not). There's no problem with anything in this quote, but it's irrelevant to our discussion.

Okay so far. I wrote the same thing, above.

Nup. Wrong! Back to school with you!

Sloppy. Yes, the energy in light comes in discrete amounts. Those amounts are associated with individual photons. The mistake comes when you say that the photons are the energy, or vice versa.

All fine, but irrelevant.

Another error, even though it's a very common mistake. Energy and mass are not "perfectly equivalent". A certain amount of energy can be associated with a certain quantity of mass, but it is an error to think that mass is energy. If it was, we wouldn't need two different symbols with two different units, for starters. And besides, at this point the guy has forgotten about how he careful distinguished matter from light, earlier. He's muddled. He says light is energy. He says light is not matter. But then he says matter is energy, which would make matter and light the same thing. See? Muddled.

Nup. More sloppy thinking. You can, for instance, convert mass into photons, but that's not the same thing at all, since neither mass nor photons are energy.

From here on, it's just error compounding error.

The sun turns some of its matter into photons, and there's energy associated with both the matter and the photons, but no matter is turned into energy. How could it? Energy isn't stuff that real physical things can turn into.

I have no problem with anything there. Sounds like Dave is on the right track. Note that he nowhere says that photons are energy.

 

arfa brane:

Some people just can't stand being corrected. I'm tempted, on the basis of this incivility, to just stop interacting with you. People aren't pricks just because they disagree with you. Grow up.

No. You can't create "stuff" from numbers.

A photon, as I and others have already explained many times, is an excitation of the electromagnetic field.

Your question is no different from asking where a water wave gets its properties from. It gets some of them from the nature of water itself. It gets others from the nature of whatever it is creating the disturbance that is the wave.

Another error. A photon is an excitation of the field; it is not the field itself. Your statement is equivalent to claiming that a water wave in a pond is the pond.

What is wrong is that whoever wrote that is hopelessly muddled about the difference between heat and radiation. Apart from that, the rest is sort of okay.

You realise that wikipedia is full of errors, don't you? It's a publicly editable encyclopedia. It is only as accurate as its contributors. Mind you, apparently even the Encylopaedia Britannica gets some of these things wrong - again, it's only as good as its contributors. People make mistakes.

Oh well, indeed. Maybe somebody will help correct them, like I'm helping by correcting you. Maybe you can take what you learn here and spread the knowledge around. Maybe together we can correct the internet, bit by bit.

---
Regarding Schrodinger's Scientific American article, which you quoted, the first thing to realise is that he is writing for a popular audience, not that this excuses errors. Secondly, the article was apparently written in 1953, and Schrodinger did not have the benefit of today's understanding of quantum field theories and the like at that time.

With that in mind, let's take a look....

This is ambiguous. On the one hand, it can be read as saying that particles are a form of energy and there are other forms. On the other hand, Schrodinger might just be saying that energy can be emitted from the nucleus by many different processes. It's not clear which meaning he meant. If it was the former, then he was wrong; if the latter, then no problem.

It's an understandable mistake he is making here - one that many physicists make. In practice, it doesn't make much difference if you pretend this is true. But, technically, it's wrong. Energy and mass are not one and the same. You can't make stuff from numbers.

​

Fine.

This is okay, but only if the word "radiation quantum" is understood to mean a discrete quantity of energy associated with a photon ("radiation"). An erroneous reading would be to interpret this as saying that energy (numbers) can somehow turn into photons (stuff).

 

arfa brane:

What's confusing you?

A bucket can carry water. Are you confused as to why the bucket is not the water? (Note: this analogy is imperfect, because energy isn't "stuff" like water is, but hopefully you get the point.)

What do you find problematic about any of that? You're dismissing it, but you're not saying why.

Are you telling me that you don't care how many good arguments I make, you're unwilling to believe me because I disagree with some "big name" authorities about certain subtle points?

You need to understand that Science isn't a dogma. There are no High Priests of Science. Ideas stand or fall on their merits in science, not according to how many Nobel prizes you've been given - at least in the ideal practice of science.

What you're telling me is that even though you can't put a coherent argument together to explain how a photon could possibly be energy, you're just going to go right on believing that's what it is, because it's some kind of article of faith for you, or because it will shake your faith in the Holy Men of Physics if you ever found out that they made a mistake or two.

This is also precisely paddoboy's attitude here. He doesn't realise that Science isn't based on dogma and authority, either.

Okay. That's all good. What's the relevance?

Yes. I agree with all that.

I haven't talked about most modern textbooks. I believe that so far we have discussed what one textbook says - one that you happen to have at hand. Nor have I made any comments about a university education in physics.

I don't know where you're getting this stuff from. Not from anything I've said. How can we hope to identify errors in the literature if we don't consult it?

I wouldn't put all the blame on them. People manage to mislead themselves. Also, this stuff isn't necessarily taught. People (even teachers) aren't always as careful as they should be in how they use the language, and as a result their students sometimes take away the wrong ideas.

Electromagnetic energy doesn't have to be energy in order to have an energy associated with it. Energy is useful precisely because it's a good accounting system.

I've already told you, but maybe I wasn't clear.

We can talk loosely about different forms of energy - kinetic energy, potential energy, heat, spring energy, etc. What is really going on when we do that is that we're partitioning or assigning categories to some abstract quantity called the "total energy" of some system or other. We're saying we can associate X amount of energy with the mass in the system, and Y amount of energy with the motions that are going on, and Z amount of energy due to the relative distances between separate objects in the system, and W amount of energy with the orientation of the system in space, and so on and so forth. The usefulness of this partitioning is that when physical changes to the system occur we can move some energy from column Z, say, to column Y, and yet the total Y + Z (or X + Y + Z + W + ...) will remain unchanged if we've set things up carefully.