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Jeff Root
2014-Jun-17, 03:02 PM
Radio signals consist of huge numbers of very low-energy
photons. They can be detected when they shove around the
electrons in a conductor in an organized way. Huge numbers
of them are required because each photon has such a tiny
amount of energy that it can't do much shoving. As a result,
we tend to see the wave properties of radio rather than the
particle properties.

Does QM describe the interaction of radio photons with the
electrons in an antenna as each photon acting on a single
electron, or as each photon acting on the whole antenna?

-- Jeff, in Minneapolis

korjik
2014-Jun-17, 03:21 PM
Should be a particle-particle scattering, I think. No matter how it is set up, it will be an unimaginably huge, unsolvable problem in QM to do the whole antenna.

Jeff Root
2014-Jun-17, 04:11 PM
Are you saying that QM isn't designed to analyze this kind of
interaction? That QM is specifically intended for describing the
interactions of small numbers of particles, and doesn't treat
large, complex entities such as antennas? Or can QM give a
reliably accurate qualitative description?

-- Jeff, in Minneapolis

John Mendenhall
2014-Jun-17, 06:19 PM
Jeff / try here:

http://en.m.wikipedia.org/wiki/Virtual_particle

Antennas are specifically discussed. Spoiler: it'll give you a headache.

EigenState
2014-Jun-17, 07:30 PM
Greetings,


Does QM describe the interaction of radio photons with the
electrons in an antenna as each photon acting on a single
electron, or as each photon acting on the whole antenna?

Radio frequency detectors are built using lumped element circuitry to achieve some predetermined resonance conditions. I seriously doubt that anyone would attempt to treat such systems within the framework of quantum mechanics if only because there is no pragmatic need to do so.

In the microwave domain, the lumped element circuitry is replaced with distributed element circuitry. Again there is no pragmatic need to treat such a system within the framework of quantum mechanics.

Best regards,
ES

Jeff Root
2014-Jun-17, 08:01 PM
I shouldn't have mentioned QM in the title or the post.
It was my assumption that QM would provide the answer
since the question is about photons and electrons, which
are the subject matter of QM. I'll re-word the question:

Does each individual radio-frequency photon interact with
an individual electron in an antenna, or does each photon
only interact with the antenna as a whole?

-- Jeff, in Minneapolis

EigenState
2014-Jun-17, 08:40 PM
Greetings,


Does each individual radio-frequency photon interact with
an individual electron in an antenna, or does each photon
only interact with the antenna as a whole?

I see no reason to treat the problem quantum mechanically. Classical electrodynamics works just fine. Thus the individual rf photon does not enter the picture--just Maxwell's equations and the Lorentz force.

Best regards,
ES

Jeff Root
2014-Jun-17, 09:07 PM
That reply is worse than useless.

All electromagnetic radiation consists of photons, regardless
of frequency. Radio-frequency photons interact with electrons
in antennas, transferring energy to or from them. Does each
photon interact with an individual electron?

-- Jeff, in Minneapolis

korjik
2014-Jun-17, 09:18 PM
That reply is worse than useless.

All electromagnetic radiation consists of photons, regardless
of frequency. Radio-frequency photons interact with electrons
in antennas, transferring energy to or from them. Does each
photon interact with an individual electron?

-- Jeff, in Minneapolis

That reply is the way it is done. No one cares about an individual RF photon for almost any purpose.

To answer your question directly: It does both, depending on how you look at it. Trying to do it as particles gives you about 1030 equations with about 1030 unknowns to cover all the interactions. That is why no one does that, and classical theory is used.

EigenState
2014-Jun-17, 09:33 PM
Does each
photon interact with an individual electron?

Asked and answered:


... the individual rf photon does not enter the picture--just Maxwell's equations and the Lorentz force.

Jeff Root
2014-Jun-18, 01:08 AM
That reply is the way it is done. No one cares about an
individual RF photon for almost any purpose.
I care.

That's sufficient.



To answer your question directly: It does both, depending
on how you look at it. Trying to do it as particles gives you
about 1030 equations with about 1030
unknowns to cover all the interactions. That is why no
one does that, and classical theory is used.
The reason I ask whether individual photons interact with
individual electrons is so that I'll know whether individual
photons interact with individual electrons, not so that I
can calculate anything. How many equations are involved
and how complex they are is irrelevant.

-- Jeff, in Minneapolis

korjik
2014-Jun-18, 03:31 AM
I care.

That's sufficient.


The reason I ask whether individual photons interact with
individual electrons is so that I'll know whether individual
photons interact with individual electrons, not so that I
can calculate anything. How many equations are involved
and how complex they are is irrelevant.

-- Jeff, in Minneapolis

why would you think an RF photon hitting a free electron in a circuit is different than any other photon hitting an electron?

Jeff Root
2014-Jun-18, 07:45 AM
why would you think an RF photon hitting a free electron in a
circuit is different than any other photon hitting an electron?
I don't know. I'd have to guess, and after guessing incorrectly
that quantum mechanics would be the area of physics which
addresses my question, I don't want to make another guess
in this thread so soon. So you tell me why I would think that
a radio-frequency photon hitting a free electron in a circuit is
different from any other photon hitting an electron.

-- Jeff, in Minneapolis

Hornblower
2014-Jun-18, 01:25 PM
My educated guess is that freely flowing electrons in a wire are in some sort of quantized states and that an electron would absorb an RF photon of just the right energy and be nudged into another state. The fact that QM can safely be ignored in radio engineering in ordinary broadcast wavelengths tells me that such quantum states, if any, are vanishingly slight in energy level differences.

John Mendenhall
2014-Jun-18, 02:11 PM
I don't know. I'd have to guess, and after guessing incorrectly
that quantum mechanics would be the area of physics which
addresses my question, I don't want to make another guess
in this thread so soon. So you tell me why I would think that
a radio-frequency photon hitting a free electron in a circuit is
different from any other photon hitting an electron.

-- Jeff, in Minneapolis

Easy, Jeff. I think Hornblower's summary guess above sounds reasonable. As do the reasons for not doing the calculation that way.

Now, having read more about antennas in the last two days than I ever wanted to know, I wonder what the receiving antenna on US subs looks like electrically. RF at 13 hertz?

Geo Kaplan
2014-Jun-18, 04:02 PM
Now, having read more about antennas in the last two days than I ever wanted to know, I wonder what the receiving antenna on US subs looks like electrically. RF at 13 hertz?

Subs are forced to use such extremely low frequencies because of skin depth's frequency dependence. At high frequencies, very little radiation would occur (the seawater would absorb most of the energy). The skin depth is large enough at low frequencies to permit radiation. The drawbacks, of course, are the need for ridiculously long antennas and the low bandwidths/data rates forced by a low carrier frequency.

John Mendenhall
2014-Jun-18, 06:39 PM
Subs are forced to use such extremely low frequencies because of skin depth's frequency dependence. At high frequencies, very little radiation would occur (the seawater would absorb most of the energy). The skin depth is large enough at low frequencies to permit radiation. The drawbacks, of course, are the need for ridiculously long antennas and the low bandwidths/data rates forced by a low carrier frequency.

Yes, I get 3250 miles for 1/4 wave at 13 hz. Hm, gotta load that antenna a bit. And it'll still be long. Ain't technology wonderful?

Sorry, off topic but fun. Jeff, you ok with answers?

Jeff Root
2014-Jun-18, 07:22 PM
John, Hornblower,

No, I'm quite surprised by Hornblower's suggestion. I've never
come across anything elsewhere that implies such a mechanism.

I also see a possible problem with it that if the differences in
energy levels are vanishingly slight, the electrons should be in
a wide range of different states, so why would they react to a
radio wave in concert? Without some suporting evidence, I
really doubt the idea has any validity.

-- Jeff, in Minneapolis

EigenState
2014-Jun-18, 11:03 PM
Greetings,


My educated guess is that freely flowing electrons in a wire are in some sort of quantized states and that an electron would absorb an RF photon of just the right energy and be nudged into another state. The fact that QM can safely be ignored in radio engineering in ordinary broadcast wavelengths tells me that such quantum states, if any, are vanishingly slight in energy level differences.

Compare kT at 300K to h\nu at 1GHz.

Best regards,
ES

Hornblower
2014-Jun-19, 12:55 PM
John, Hornblower,

No, I'm quite surprised by Hornblower's suggestion. I've never
come across anything elsewhere that implies such a mechanism.My bold. Neither have I.


I also see a possible problem with it that if the differences in
energy levels are vanishingly slight, the electrons should be in
a wide range of different states, so why would they react to a
radio wave in concert? Without some suporting evidence, I
really doubt the idea has any validity.

-- Jeff, in Minneapolis
My bold. I don't know whether or not there is any validity. Let me repeat that this was an educated guess, and that I included the caveat "if any". In response to the lack of evidence I would say "I don't know" rather than "I doubt it." We know that the electrons in an isolated metal atoms widely separated in a vacuum are in quantized states, and that the spectrum of a hot dense system of the same atoms is consistent with quantum mechanics. I just went from there to imagine some hypothetical possibilities that would be unobservable with our technology.

EigenState
2014-Jun-19, 05:03 PM
Greetings,



We know that the electrons in an isolated metal atoms widely separated in a vacuum are in quantized states, and that the spectrum of a hot dense system of the same atoms is consistent with quantum mechanics. I just went from there to imagine some hypothetical possibilities that would be unobservable with our technology.

The question pertains to exciting an electron in a macroscopic metal into the conduction band. For a conducting metal, quantum mechanical considerations afford a system with a very high density of states asymptotically approaching the conduction band. Those high-lying energy states carry population resulting from the Pauli Exclusion Principle. The point to which I alluded previously was that kT \gg h\nu at reasonable operating temperatures and the applicable frequencies such that thermal excitation to the conduction band would overwhelm excitation by absorption of individual rf photons.

Best regards,
ES

Jeff Root
2014-Jun-19, 07:15 PM
Compare kT at 300K to h\nu at 1GHz.


The point to which I alluded previously was that kT \gg h\nu at
reasonable operating temperatures and the applicable frequencies
such that thermal excitation to the conduction band would
overwhelm excitation by absorption of individual rf photons.
I had gotten as far as figuring out that k is the Boltzman
constant relating energy to temperature: 1.381 x 10-23 J / K

-- Jeff, in Minneapolis

John Mendenhall
2014-Jun-19, 08:17 PM
Greetings,



The question pertains to exciting an electron in a macroscopic metal into the conduction band. For a conducting metal, quantum mechanical considerations afford a system with a very high density of states asymptotically approaching the conduction band. Those high-lying energy states carry population resulting from the Pauli Exclusion Principle. The point to which I alluded previously was that kT \gg h\nu at reasonable operating temperatures and the applicable frequencies such that thermal excitation to the conduction band would overwhelm excitation by absorption of individual rf photons.

Best regards,
ES

Got it. Thanks.

Grey
2014-Jun-20, 05:43 PM
The second answer here (http://physics.stackexchange.com/questions/18823/how-do-we-visualise-antenna-reception-of-individua-radiowave-photons-building-up) seems to give a pretty good discussion of photons interacting with an antenna from a quantum mechanical perspective (and Jeff, you were not wrong that this would be a question for quantum mechanics; as soon as you bring up photons at all, you are addressing matters from a quantum theoretical perspective).