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Pure energy


Farsight
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ajb: a "property of the configuration" is reasonable, but that just defers the question to configuration of what?

 

I think a useful phenomena to examine here is Compton Scattering. This is from the hyperphysics website, which I think is very good:

 

compton.gif

 

The photon gives the target electron a "kick", and is reduced in energy. The electron acquires kinetic energy and moves. If you then repeat this with the scattered photon, you tend towards a situation where you have no photon left. All you have instead is electrons that weren't moving and now are. The photon has essentially been converted into kinetic energy. Hence I think "photons are pure energy" is more reasonable than you do.

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ajb: a "property of the configuration" is reasonable, but that just defers the question to configuration of what?

 

I think a useful phenomena to examine here is Compton Scattering. This is from the hyperphysics website, which I think is very good:

 

compton.gif

 

The photon gives the target electron a "kick", and is reduced in energy. The electron acquires kinetic energy and moves. If you then repeat this with the scattered photon, you tend towards a situation where you have no photon left. All you have instead is electrons that weren't moving and now are. The photon has essentially been converted into kinetic energy. Hence I think "photons are pure energy" is more reasonable than you do.

 

You will never arrive at a condition where you have no photon left.

 

According to the definitions of standard physics, energy is a property and not a thing. And in this forum we use the definitions of standard physics.

 

Besides, if the photon is pure energy, where does the momentum come from?

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You will never arrive at a condition where you have no photon left.
You will arrive at a situation where you have no discernible photon energy left. The wavelength reduces further and further until it's no longer measurable.

 

According to the definitions of standard physics, energy is a property and not a thing. And in this forum we use the definitions of standard physics.
You don't, Swanson. You might think you do, but the mathematical physics you think of as standard pays insufficient regard to experimental physics, and rather glosses over E=mc² wherein Einstein said "All matter is made of energy". Pair production is the experimental evidence that supports this. The important point to appreciate is that you can employ a photon to make electrons move, and you can also employ a photon to actually make an electron. And a positron of course. In typical pair production the input photon is more than 1022keV so the electron and positron fly apart -you've used a photon to make an electron, and make it move too.

 

Besides, if the photon is pure energy, where does the momentum come from?
Momentum isn't something distinct from energy, which is why we talk of energy-momentum. Energy is a "distance-based" scalar measure of energy-momentum rather than a "time-based" vector measure. Hence E=hf and p=hf/c, and we divide by c, distance over time, to go from one measure to the other. You cannot reduce the energy of that scattered photon without reducing its momentum by a commensurate amount. Hence we usually refer to photon just energy. Or at least, I do. You might prefer to say that the photon is an action configuration and refer to polarization, which is fine by me. Or you might prefer to say the photon is an elementary excitation of the quantized electromagnetic field. I wouldn't disagree. But I'd still point to Compton scattering and pair production to show that the photon is as close to pure energy as we can get in everyday physics. Think of it this way: a photon's energy is a vital property that makes it the thing that it is. It isn't like the kinetic energy of an electron. Take that away, and you're still left with an electron. Take away the photon energy, and you don't have a photon any more.

 

NB: as you're aware you can take away the electron kinetic energy via Inverse Compton scattering. Or you can use a synchroton. See wiki and note this bit "Electrons traversing the periodic magnet structure are forced to undergo oscillations and thus to radiate energy". They radiate energy. Take it at face value.

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You will arrive at a situation where you have no discernible photon energy left. The wavelength reduces further and further until it's no longer measurable.

 

That will take an infinite number of scatters.

 

 

Momentum isn't something distinct from energy, which is why we talk of energy-momentum.

 

Just from a semantics point, how can energy-momentum be pure energy? It's energy-momentum. Says so right there.

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ajb: a "property of the configuration" is reasonable, but that just defers the question to configuration of what?

 

 

So I was talking about the classical electromagnetic field and how waves transport energy. In this context I simply mean that energy is a property of the electromagnetic field. The energy depends on "how the electromagnetic field is arranged". Energy is not something that exists "in its own right", but is always a property of some arrangement of fields and particles.

 

The photon gives the target electron a "kick", and is reduced in energy. The electron acquires kinetic energy and moves. If you then repeat this with the scattered photon, you tend towards a situation where you have no photon left. All you have instead is electrons that weren't moving and now are. The photon has essentially been converted into kinetic energy. Hence I think "photons are pure energy" is more reasonable than you do.

 

This is a "limiting process" in which you need an infinite number of scatterings, as already pointed out. This cannot be a real physical process. In a physical experiment I can imagine you can get to the point where the photon is no longer detectable to you, but this is not the same as the photon truly vanishing.

(Not that I really know how one could set up such an experiment)

 

... wherein Einstein said "All matter is made of energy"

 

 

But what did Einstein really mean by this? I expect he said it rather informally and as part of some popularisation. You should be more worried about what Einstein formulated and calculated. And then don't forget we have had 100+ years now to refine our understanding.

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So I was talking about the classical electromagnetic field and how waves transport energy. In this context I simply mean that energy is a property of the electromagnetic field. The energy depends on "how the electromagnetic field is arranged". Energy is not something that exists "in its own right", but is always a property of some arrangement of fields and particles.
That's back to front. The fields and particles depend on "how the energy is arranged". You used the word configuration yourself. Now test it against low-energy proton-antiproton annihilation to gamma photons. You started with quarks and gluons and the strong force plus electromagnetism, you maybe saw some residual-strong-force pions for a nanosecond, then you finished up with electromagnetic photons. The arrangement of the fields and particles has changed beyond recognition. But energy was conserved. See the second paragraph of this post by Cygnus on the original thread?

 

Energy is the attribute that alters the state of being, and can't be thought of as a particle or a wave. Energy can produce a wave in the fabric of space but the wave itself is not the energy. The wave is the result of energy changing the shape of space. So you see, the cause for the change and the resultant shape are not the same.
Now look again at my post responding to what energy is:

 

http://www.scienceforums.net/topic/59122-energy/page__pid__621381#entry621381

 

This is a "limiting process" in which you need an infinite number of scatterings, as already pointed out. This cannot be a real physical process. In a physical experiment I can imagine you can get to the point where the photon is no longer detectable to you, but this is not the same as the photon truly vanishing.
It's real enough, think it through. And the photon truly vanishes in pair production when the electron and positron fly apart. And when you decelerate an electron you still have an electron, but now you've got a photon too.

 

But what did Einstein really mean by this? I expect he said it rather informally and as part of some popularisation. You should be more worried about what Einstein formulated and calculated. And then don't forget we have had 100+ years now to refine our understanding.
He meant what he said. Matter is made out of energy. And photons aren't matter. I'm not worried about what Einstein formulated and calculated, because Does the Inertia of a Body Depend upon its Energy-Content? is straightforward. But I am worried about that 100+ years. Physics has stalled and funding is under threat (here's the latest), some understanding seems to have been lost, and we see unfounded speculations and myths promoted as bona-fide physics whilst real bona-fide physics struggles for attention.

 

Just from a semantics point, how can energy-momentum be pure energy? It's energy-momentum. Says so right there.
When there's no motion. It's an early-universe / black hole thing. Don't worry about it. Edited by Farsight
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That's back to front. The fields and particles depend on "how the energy is arranged". You used the word configuration yourself.

No, actually. The word "configuration" does not appear in ajb's post.

 

It's real enough, think it through. And the photon truly vanishes in pair production when the electron and positron fly apart. And when you decelerate an electron you still have an electron, but now you've got a photon too.

 

If it has an infinite number of discrete steps, can it be a real process?

 

Oh, and your last attributed quote was mine, not ajb's.

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No, actually. The word "configuration" does not appear in ajb's post.
It was in a previous post of his.

 

If it has an infinite number of discrete steps, can it be a real process?
Of course it can. But stop nitpicking, Swanson. Go look at the photoelectric effect or something.

 

Oh, and your last attributed quote was mine, not ajb's.
Apologies. Fixed.
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But I am worried about that 100+ years.. we see unfounded speculations and myths promoted as bona-fide physics

I think you have correctly described the present situation. However, one can extend the period backwards to include Einstein and Heisenberg also, on the same grounds. The real problem is that it has become impossible to distinguish between speculations and bona-fide physics due to the complexity of both the mathematical equations and the logic used.

 

 

...real bona-fide physics struggles for attention.

This would require clarification: What is bona-fide physics? Should we start again from independent matter, space and time?

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That's back to front. The fields and particles depend on "how the energy is arranged". You used the word configuration yourself. Now test it against low-energy proton-antiproton annihilation to gamma photons. You started with quarks and gluons and the strong force plus electromagnetism, you maybe saw some residual-strong-force pions for a nanosecond, then you finished up with electromagnetic photons. The arrangement of the fields and particles has changed beyond recognition.

 

So we have energy conservation (in a specified inertial frame). The initial configuration has the same energy as the final configuration. I don't see what I have said is back to front.

 

Physics has stalled and funding is under threat...

 

The funding of science and mathematics in the UK is very worrying. Personally I am finding it very hard to find a position. But this is for another thread really.

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