# 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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Well I don't think Einstein's equation says mass and energy are the same thing; I think it says mass and energy are equivalent up to a constant factor. Or maybe that's just another way of saying "are the same thing".

Last edited: Sep 4, 2019

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

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arfa brane:

Look, this is ridiculous. Just above, I gave you a concise definition of the term "reification". Granted, it's probably a new idea for you, but to use that term in a way that would give it almost the exact opposite meaning to the one I just explained for your benefit, and to do so practically in your next post, is either arrogant wilful ignorance or else bullheaded stupidity. Either way, I'm coming to the view that further discussion with you on this topic is likely to be a huge waste of my time, since you're obviously not listening to anything I have to say.

Fundamentally, energy is a property of a system of one kind of another. It doesn't so much get transferred from place to place as it gets converted from one form to another. Fundamentally, it is an accounting system. Feynman's explanation of it in his famous Lecture on Physics is a good one.

I didn't say it doesn't exist. I said it isn't a substance. Love exists, but it's not a substance.

Yes. In other words, the concept of kinetic energy is a useful way to assign a quantitative measure to particle motion,

Substitute? Motion is not energy. Energy is not motion.

What happens is that we have a number, called the total energy of the system. That number is the sum of a partition of various "kinds" of energy that we assign to the system. For example, the total energy might be the sum of kinetic and potential energy. Then, when the system changes in some way, we take some energy out of the "kinetic" column and put it in the "potential energy" column, or vice versa. Often the total energy remains constant, which is one of the things that makes energy such a useful idea.

Point is: shuffling numbers from one column to another is quite unlike transferring a physical substance from one place to another.

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

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Do you think physics professors are infallible, perfect human beings who never make mistakes?

Is it possible that there could be significant numbers of physics professors out there who have some screwy idea about energy?

Note, that it may be possible to have some wrong-headed ideas and not have it affect your daily work in the least, because getting the concept wrong doesn't actually affect the calculations you're performing using that concept, or the numbers you get out when you calculate.

Yes, photons propagate at c. No, photons are not a form of energy, for reasons I have clearly explained. Yes, photons have an energy associated with them that we can take from one column and put into another column when a photon interacts with something else.

A photon is not a field. You could say it is an excitation of a field, if you like.

And yes, I have already "admitted" that we can associate an energy density with electric and magnetic fields - make a new column in the "energy" table and write down some numbers associated with the fields in a particular region. When certain physical processes occur, we can move some numbers from one of those columns into a different column, so to speak.

They aren't equivalent. Not really. Not unless you regard "mass" as simply a number we associate with "stuff".

7. ### arfa branecall me arfValued Senior Member

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Ok, so energy isn't a physical substance, it's something that can be converted from one form to another; the "form" of a non-physical "substance" is definitely hard to think about I have to say.

Given that energy has physical units, it seems energy is something quite mysterious. Really hard to grasp.

Or maybe not, if you just accept that one of the forms energy can take, is electromagnetic energy (whatever you might also think an EM field "really" is).

8. ### arfa branecall me arfValued Senior Member

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I find it very hard to accept that the faculty of a physics school at a university all recommended that same flawed text for several years.

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Nup, wrong, sorry.
But spacetime is real as it can be bent, warped, twisted etc under the auspices of GR. As is space and time.
That maybe true, but it does nothing to show that a photon is not a packet of energy, to use one description.
Photons have no mass.
I have shown at best the point is debatable and thus pedant.
Perhaps if you took the time to read my posts thoroughly and not so dismissively, you will notice that I said earlier that mass is essentially energy, but energy is not essentially mass. Again, a photon has no mass, while your electron does.

You say you have answered my question...perhaps like you I have not read your replies thoroughly, so would it be too much trouble to answer it again. What part of the photon is not energy?
Photons have no mass and are not matter. And you saying "the definition of matter is up for grabs" [I'm not so sure it is by the way] shows that what we are discussing is ambiguous in your opinion, yet you sit firm and refuse to see the same ambiguous nature with regards to a photon, being either a package of energy, and/or a carrier of energy.
Still pedant from where I am.

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And time

11. ### Michael 345New year. PRESENT is 72 years oldlValued Senior Member

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It doesn't - it is only a concept

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

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If this is you coming around just a little, then I'm glad you're getting there. We'll wait and see how paddoboy goes.

Energy, being an abstract concept, is hard to think about. The problem is, it's such a useful concept that the idea has percolated through so that every non-expert man-in-the-street believes he has an idea of what energy is. To an extent, that's true, but one problem is that it is very easy to start thinking of energy as a "thing" that can move around and to exist almost independently of other "stuff". Worse, this kind of thinking is actively promoted by people who know nothing about what energy actually is. People talk about "spiritual energy" and "crystal energy" and "energy auras" and all kinds of things like that. In sci-fi films we see glowing "creatures of pure energy" and things like that, despite the fact that such a notion makes no sense at all.

When you pay your electricity bill, you're not paying for some energy "stuff" to be shipped from the power station to your home via wires. Really, you're paying for the coal that is burned to drive electrons through your wires.
No textbook is perfect. There are many reasons that faculties choose one book rather than another. It may be that the content is good on some topics, not so great on others. It could be that the publisher gave the faculty a good deal to use the textbook and associated resources. It could also be the flaws are considered minor and unimportant in the context of what is being taught, or even that the flaws are not noticed.

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

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The debate is not about what is "real", but about what is a substance and what isn't. Time is not a substance. As for "space", it really depends on how you choose to define that word. I don't think "spacetime" is a substance, at least not in any way that I use that concept.

And so.... what?

You're hopelessly muddled. The form of your argument in this quote goes like this: M is E, but E is not M. You're contradicting yourself.

You only had to read though one post. I gave you the post number. Why are you so lazy? Here's the quote from post #81:

You: "So what part of the photon is not actually part of this energy that it is said to carry?"
Me: "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."
You are very definite about that. Okay, please identify for me which fundamental particles in the Standard Model of Particle Physics are matter, and which are not. Are neutrinos matter? Are W bosons matter? Is the Higgs boson matter? Are electrons matter? Are quarks matter? Are gluons matter? What makes one particle matter and another not? Please tell me.

If you think that you have a rock solid definition of "matter", tell me what it is.

There is no wiggle room when you make a silly statement like "photons are energy". They aren't. There's no ambiguity in that. Energy is not "stuff". Energy is not a substance. Energy is not photons.

Either your ego is getting in the way to such an extent that you simply refuse to see the obvious, or else you're not intellectually equipped to follow the argument I have put to you. Those are really the only options at this point.

I think you're too entrenched in your position to do the honourable thing and back down on this. This discussion has evolved to the point where the most interesting question is the test of your character, or lack thereof. There's really nothing further to discuss on the matter of whether a photon is or isn't energy.

Last edited: Sep 4, 2019
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15. ### James RJust this guy, you know?Staff Member

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Quoting yourself as an authority now?

This: "while all mass is energy, not all energy is mass" is wrong.

Mass is not energy. Not if by "mass" you mean "matter". Just above, you were trying to distinguish photons from "matter", saying that photons can be energy because they are not "matter". But here you are saying that all "mass" is energy, just like all photons are energy.

So, it looks like I was right all along. You are actually making the mistake of assuming that every kind of particle is energy. Hence everything with mass is energy and photons, even though they have no mass, are energy as well.

Just to be clear then, you are saying that the only thing that exists in the universe is "energy". Would that be a fair summation of your position, given that all mass is energy as well as all the other energy that is not mass?

16. ### exchemistValued Senior Member

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Indeed. Mass is not matter any more than energy is.

Both are properties: attributes, not entities. This seems to be the basic confusion.

But it has to be admitted it is a widespread confusion. Not even scientists are always scrupulous in how they communicate these ideas, so it is not too surprising. And teachers do tend to take shortcuts at school, which is where we all learn about these things. In fact, one can read a lot about what energy really is. It is quite hard to define in a way that is not a bit circular, e.g. the standard "ability to do work" etc.

17. ### Michael 345New year. PRESENT is 72 years oldlValued Senior Member

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Energy is THE ABILITY TO DO WORK

ie affect something in a recordable way

ALL matter has the capability of having energy ie even a rock just sitting on the ground is exerting pressure on the ground, through its mass, thus is doing work. Want to stop the rock doing work? Go pick it up, use your work to transfer rock mass work ie pressure on the ground, into kinetic energy, rock position above the ground

18. ### exchemistValued Senior Member

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No Michael, a rock sitting on the ground is not doing work.

Work is F x d. A force has to be applied through a distance to do work.

Your rock has to move downward, in the direction of the force of its weight, to do work.

If what you said were true, we could power our homes for nothing by just connecting them to a rock.

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19. ### arfa branecall me arfValued Senior Member

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However, the energy of electric and magnetic fields is language that dates back a while. The phrase "radiant energy", like from the sun has been around since then.
So what did Planck really mean to say with "discrete energy"? bearing in mind he meant like from the sun or a black body. Why is it so important to understand that a photon of infrared radiation isn't a form of energy, or isn't a form of heat?

And the idea of heat being a thing that can move around is basic physics, isn't it?

20. ### arfa branecall me arfValued Senior Member

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A quantum oscillator can have a discrete energy though. Do we agree on that?

If we do, what really is this energy it has? Particularly if it's about to emit some of it? As a photon, or as you might say, in a photon?

21. ### Michael 345New year. PRESENT is 72 years oldlValued Senior Member

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So if I am pushing against the unmoving wall of the gym - is not work?

In my case it is my muscles working - the rock it is its mass, which is why it takes energy to lift it, it's mass resist

The higher you lift the rock the more kinetic energy you impart to the rock

22. ### exchemistValued Senior Member

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When you push against the wall of the gym then, provided it does not move, you are doing no mechanical work. Your muscles still get tired, but that is because of the biochemistry involved in animal muscle generating a force by contraction*.

When you lift a rock, yes of course you do mechanical work: you move the rock upward through a distance, against the force of its weight, which means you have to exert an equivalent but opposite force to do it. So applying F x d, you obviously do work. (If you simply hold a rock, however, you do not do any mechanical work).

* I won't go into the biochemistry of muscle contraction just now, as I don't understand it very well myself and there would be a risk of attracting Write4U here, to start wittering on about microtubules.

23. ### Michael 345New year. PRESENT is 72 years oldlValued Senior Member

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I know a little bit about from my years as a Registered Nurse

Anyway night here and few more bits of stuff to attend to before I go Ball in a week and will stop my post here on this matter because not want to spoil my upcoming 9 days away