Are photons energy? What is energy, anyway?

Michael 345:

Kind of. The thing is, the words "energy" and "work" both have precise mathematical definitions in physics, and this statement in words doesn't quite capture the whole of those definitions. In a certain sense, English is not the first language of physics; mathematics is.

That's unacceptably vague for the purposes of doing physics.

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. But fundamentally, there's no point worrying about any of that, because you're simply using an incorrect definition of "work".

Again, a bit muddled.

Work is not the same thing as force, which is not the same thing as pressure. Words like "force" and "pressure" also have precise meanings in physics. Even though you might hear them used almost interchangeably in everyday life, they are different things. Specifically, pressure is force per unit area.

When you lift a rock up off the ground, there are possibly lots of energy transfers going on. But whether any net work is done over the entire process depends on how you define the system of interest.

It depends on what you want to define as the system of interest in this example. Are you asking whether work is done on the system that is the wall, on the system that is your body, on the system that is you plus the wall, on the system that is you, the wall and the entire Earth, on the system that consists of your muscle fibres considered separately from the rest of your body, or what? The answer can be easy or complicated, depending on the system of interest.

 

"Radiant energy" from the sun (or anything else) comes in the form of photons - or electromagnetic waves if you prefer to model it that way.

"Heat" is another word that has a very specific meaning in physics, whereas in everyday life the term is used in many different ways. Most people, for instance, probably couldn't give you a clear explanation of the difference between "heat" and "temperature". Specifically, heat is a transfer of energy from one system to another due to a temperature difference that exists between the two systems.

Heat, being a transfer of energy, is not a substance, any more than any other kind of energy is a substance. It follows that "infrared radiation" can't be "heat". No form of radiation is "made of" energy. No form of radiation is energy.

Not if you're a careful physicist. "Heat" is one of those imaginary columns in the ledger of energy; it's not a "thing". Thinking heat is a "thing" is making the reification mistake again. Moving a number from one column in the ledger to a different column is not the same as moving "stuff" from one place to another.

Of course, even physicists find it useful to skip the detail and to picture heat energy (or other kinds of energy) as like stuff that can be moved around. Probably, a lot of them never worry about that picture not being a really accurate description of what is really happening, and that is probably why a lot of textbooks get it technically wrong, too.

I should say that mental models of things are really useful in physics, because they can provide shortcuts to getting (or guessing) the right answer to a physical problem. But there is danger in thinking that the models are a description of what is actually happening. The map is not the territory.

It is very common for scientists to say things like "systems want (or like) to be in their lowest energy state". Thinking like that can be a useful heuristic, but of course most physical systems are not conscious. They don't "want" anything. Nor does the energy itself play any role in changing the state of a system from one thing to another.

Yes.

Typically, the energy of an oscillator can be partitioned into potential energy of some kind and kinetic energy. The kinetic energy is a kind of measure of the speed of the oscillator, and the potential energy is related in some way to the relative configuration of different parts of the oscillator, as determined by whatever forces are in play making the thing oscillate in the first place.

If an oscillator emits a photon, that emission typically goes along with changes in both the potential energy and the kinetic energy of the oscillator, such that the total energy of the oscillator decreases by an amount exactly equal to the energy associated with the emitted photon.

Energy in general is a useful concept because it is so often conserved. In a sense, the concept is constructed for that precise purpose.

 

Here's something to think about if you regard energy as a kind of substance that things carry around.

Consider a rocket, or a satellite or similar, moving at a constant speed through space. Let's say that you're watching that rocket from your own rocket. Taking your own rocket to be stationary, the one you're looking at is moving at speed $v$, let's say. In this situation, I think we can all agree that the kinetic energy of the rocket you're watching is $E=(1/2)mv^2$, where $m$ is the mass of that rocket.*

Now, you give your own rocket's engines a quick burn so that you're now travelling at the same speed as the one you are watching. Now, the speed of that rocket relative to you is zero. Calculating the kinetic energy of that rocket now, you find $E=(1/2)m(0)^2=0$.

The question arises: what happened to that rocket's kinetic energy when you - the observer - accelerated?

If the rocket's kinetic energy was something intrinsic to the rocket - or even something "carried" by the rocket - then it should follow that something would have to change about that rocket in order to affect its energy. But all that has changed in this example is your motion as an observer watching the rocket. What this tells us is that the kinetic energy of the rocket is not something that is intrinsic to the rocket, or even a "property" of the rocket, as such. We know this, because, without going anywhere near that rocket, without touching it or interacting with it in any way, we can change its kinetic energy.

This is just more evidence that energy isn't "stuff". It has something to do with the way we look at things. It is a number that we assign to systems, as observers.

Coming back to the argument about photons, again, we notice that, just like the case of the rocket, we can change a photon's energy by changing our motion as observers. Suppose a light source is emitting red light - "low energy" light - towards us as we are stationary. Now, suppose we move rapidly towards that light source. We observe that the light is now blue - the photons from the source now all have a higher energy than before. But we didn't interact with the photons in flight. We didn't mess with the source. All we did was to alter our motion as the observers of these photons.

If photons had an intrinsic energy, changing ourselves as observers shouldn't affect the photons. To change something about the photons, we'd need to actually touch the photons or interact with them in some way. But that is obviously not required.

Also, consider the claim that photons are energy. If that is true, then presumably each photon is a certain specific amount of energy, fixed by the nature of the photon itself. How, in that case, do we account for this apparent increase in the energy of the photons as we - the observers - move towards them? By moving ourselves, did we somehow create more energy at a distance in the photons, out of nothing? How does that work in the "photons are energy" picture?

I would hope that paddoboy and arfa brane have a ready answer to this apparent conundrum. I fully expect paddoboy, for one, to ignore the obvious problem, however. To him, no doubt this is "pedant".

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* I'm not concerned with relativistic kinetic energy here, so please don't complain about that.

 

arfa brane:

For a little while there, I thought we'd made some progress with you (even though paddoboy remains stuck), but here you are back at square one. Oh well. All I can say is that it's a pity you haven't taken away anything useful from our discussion. I guess it's more comfortable for you to remain ignorant.

I completely understand that you might well have been exposed to people or texts carelessly equating electromagnetic radiation with "radiant energy" and the like. It's understandable why there is a tendency for them to do that, too, because it is understood that electromagnetic radiation has an associated energy. Every time you come across electromagnetic radiation, there's some energy associated with it, so it's very easy to start thinking that the energy and the radiation are the same thing, even though they are quite different.

Here's another thought: suppose that photons really are energy. That would mean that energy really is photons, would it not? It would then follow that every kind of energy should have all the properties of photons. If it didn't, then photons couldn't be energy. It might be the case that not all the "available" properties or "handles" that attach to energy would be needed in every situation, but they'd still be there, in the background.

Why, then, do we never hear about the polarisation of electrical potential energy, or the wavelength of heat, or wavepackets of the nuclear binding energy?

You have been presented with many reasons why electromagnetic radiation is not the same as energy. You are yet to even suggest how it could be the same. You don't respond to objections like the ones I've put to you above (and neither does paddoboy). So, don't you think you're being just a bit presumptuous by proclaiming that you're right and we're wrong about this?

You say you disagree with me, but then you go on to give a compatible definition of "heat" to the one I gave you. So, what exactly is it that you think we're in disagreement about regarding heat?

Sure. That should help. Any good introductory text will start by defining work as something like $W=Fs$, where $F$ is a force acting over distance $s$. Then, the text will note at some point that the units of work (e.g. Newton metres) are given a special name (e.g. Joule), and that this unit is what we defined as the unit of energy.

The text will probably go on to derive basic concepts such as kinetic energy from the work then, after a discussion about conservative forces, will define potential energy. At some point it will talk about conservation of energy, and so on and so forth.

Having done all that, if the textbook later goes on to say "photons are energy" or "electromagnetic waves are energy", then what has happened is that the author has forgotten his previous definitions and derivations, which had nothing to do with photons or electromagnetism.

That's all okay, up to a point. The problem comes when you start to reify energy - when you start to think that it is energy itself that causes physical processes to happen, and things like that. If energy is a theoretical device, like you say, then how could a theoretical device possibly affect the real, physical world in any way? A theoretical device cannot be a real causal agent in the physical universe.

Part of the problem, as I've said many times, is how people talk about energy. It is common to talk as if energy is a substance, or as if systems "want" to go to the lowest energy, or whatever. That's a mental shortcut - a heuristic. If you appreciate that's what it is then there's no problem pretending - it can even be useful and save time. But in this thread, we appear to be dealing with a couple of people, at least, who have come to believe that this "theoretical device" is something that exists in the physical universe as other than a mere concept.

"Quantum" simply means "amount". A quantum of energy is a fixed amount of energy, just like the "quantum of damages" is a fixed amount of money. The word "quantum" tends to be imbued with mystical significance by those who don't know much physics (Deepak Chopra has a lot to answer for), but it's not a hard concept to get your head around.

I have no objection to any of that. Energy can help us to predict how bodies will interact. But notice: nowhere in this quote is the idea that physical things (like electrons or photons) are energy.

 

arfa brane:

This is what is actually observed, experimentally. Are you going to start denying reality now?

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.

The medium makes no difference. Change the rocket scenario to two cars driving along a road (not in a vacuum). If you're driving along at 100 km/hr, next to another car that is also going at 100 km/hr, then according to you - the observer - the next-door car has zero kinetic energy. Moreover, you can change that next-door car's kinetic energy by changing your own motion, without ever touching the other car. Which means that the other car's kinetic energy is not something that is to be found in the other car itself.

A real number, with a unit.

Because physical equations must always have the same units (technically, "dimensions") on both sides of the "equals" sign.

We need physical units because, obviously, 1 second is very different from 1 kilogram, which is very different from 1 ampere or 1 metre.

Yes, a number with a unit. As of March this year, the kilogram is officially defined in terms of Planck's constant.

What a kilogram designates, of course, is the amount of "stuff" in an object. If I say my carton of milk has a mass of 1 kilogram, I am really saying something about how much milk is in there. It would be wrong to say "the milk is mass" or "the milk is a kilogram", because obviously milk is not the same as a kilogram. One is a theoretical measurement device; the other is a physical object or substance.

For you and paddoboy to say "a photon is energy" is to make exactly the same mistake as saying "a milk carton is 1 kilogram" or "a milk carton is mass".