Understanding Photon Absorption in Radio Wavelengths: 3 Key Questions Explored

[Ponderer]

Hi. I have three questions. I'm not asking anyone to prove it, no debating :), but am merely interested in your interpretation of QM regarding the absorption of photons in the radio wavelength region. Although I'm sure your answer would remain the same regardless of wavelength.

Question 1:
If an antenna absorbs a photon of energy, then is it true that just one particle such as the electron will adsorb the energy? Of course there's secondary effects where the particle will transferring energy to neighboring particles, but let's ignore that here.

Question 2: Does the particle that absorbs a photon suddenly jerk? If so, then I would be interested in how fast you think the energy is absorbed.

Question 3: We have a transmitting antenna. The signal is a sine wave, at say 100GHz. The power is at a level such that the amount of emitted energy comes to one photon every 100 wavelengths on average. We have a receiving antenna a few wavelengths away. So assuming the temperature is low enough relative to the signal, and our oscilloscope is capable of seeing the signal, will the signal appear as spikes, but the probability of a spike appearing at any given moment depends on where the signal is at in the sine wave signal? In other words, if we know where the peak of the sine wave would normally be if we were to boost the signal, then I'm assuming that's where the highest probability of a spike/jerk would be, correct?Here are a few notes from Wikipedia on a particle emitting radiation. They say the electron "jerks" when it emits. I didn't see the opposite effect, so I'm assuming that an electron would also jerk when absorbing a photon.

http://en.m.wikipedia.org/wiki/Radiation_resistance
Wikipedia says "Radiation resistance is caused by the radiation reaction of the conduction electrons in the antenna"

http://en.m.wikipedia.org/wiki/Radiation_reaction
which is redirected to Abraham–Lorentz force which says "In the physics of electromagnetism, the Abraham–Lorentz force (also Lorentz-Abraham force) is the recoil force on an accelerating charged particle caused by the particle emitting electromagnetic radiation. It is also called the radiation reaction force or the self force."

and "The force is proportional to the square of the object's charge, times the so-called "jerk" (rate of change of acceleration) that it is experiencing. The force points in the direction of the jerk. For example, in a cyclotron, where the jerk points opposite to the velocity, the radiation reaction is directed opposite to the velocity of the particle, providing a braking action."There's Abraham–Lorentz–Dirac–Langevin equation, which is both fully quantum and relativistic.Thank you very very much! I greatly appreciate it!

 

[Khashishi]

For 1, no, the whole antenna absorbs the photon, not just one electron. For the case of a 1m large antenna, how could, for example, a 1m wavelength wave be absorbed by a single electron? If a single electron could absorb the photon, then why do we need such a large antenna? (As far as I know, QM is nonlocal. Interactions don't occur in a specific point in space-time but rather over some overlap between two wavefunctions.)

For 2, Heisenberg uncertainty makes it impossible to define the jerk. There isn't an exact time when the absorption occurs.

For 3, you'll need to amplify the signal somehow to see the spikes. If you use a photomultiplier tube, you might get results like
http://www.hamamatsu.com/resources/pdf/etd/PMT_handbook_v3aE-Chapter6.pdf

 

[Ponderer]

Thanks for the reply. I was surprised about answer 1 because I vaguely recall over the years hearing physicists say the photon is absorbed by *one* particle but guess not.

I think I underdressed what you're saying in answer 2. We can't know when the spike occurs. Only the probability. But when the spike does occur, we can detect it since you said in answer 3 that we can.

When you say Heisenberg uncertainty makes it impossible to define the jerk, I understand that to mean the math doesn't specify how fast the particle absorbs the photon.

Again thanks for the reply. Guess that's that unless someone else has different answers.

 

[phinds]

When you say Heisenberg uncertainty makes it impossible to define the jerk, I understand that to mean the math doesn't specify how fast the particle absorbs the photon.

No, it doesn't mean that the math DOESN'T specify how fast, it means the math CANNOT specify how fast

 

[Ponderer]

No, it doesn't mean that the math DOESN'T specify how fast, it means the math CANNOT specify how fast

Hm interesting. What if we view the spike on an oscilloscope? Of course with sufficient amplifiers and low enough temperature relative to the photon's energy. I mean, the scope is limited by technology, but there's fast specialized oscilloscopes. Maybe there's a maximum duration the total absorption can take. Surely it must be faster than the time it takes for one wavelength lol.

 

[phinds]

Hm interesting. What if we view the spike on an oscilloscope? Of course with sufficient amplifiers and low enough temperature relative to the photon's energy. I mean, the scope is limited by technology, but there's fast specialized oscilloscopes. Maybe there's a maximum duration the total absorption can take. Surely it must be faster than the time it takes for one wavelength lol.

The HUP is not a measurement problem, it's a fundamental limit of nature. You would likely find it interesting to look up the Wikipedia article on it.

 

[Ponderer]

The HUP is not a measurement problem, it's a fundamental limit of nature. You would likely find it interesting to look up the Wikipedia article on it.

It's been awhile since reading about HUP. I never thought to think of the spike in terms of HUP because I thought a photon was instantaneously absorbed. That's always bothered me. Good to know that's not the case. Btw I generally think in terms of experiments rather the Quantum Mechanics math with the exception of a few dozen equations such as E = hf. That's why I used the oscilloscope example. It still would be interesting to see what the spike looked like on scope.

 

[Khashishi]

Alright, I think my answer was not a full answer. If you don't measure the time a photon was absorbed, then there isn't a specific time that it was absorbed. But if measure the time, perhaps using something like a photomultiplier tube, then you collapse the wavefunction around that specific absorption time. But that doesn't tell you what the jerk is. In order to specify the jerk, you have to specify how you measure the jerk. If you don't actually measure it somehow, it doesn't have a specific value.

 

[Ponderer]

Alright, I think my answer was not a full answer. If you don't measure the time a photon was absorbed, then there isn't a specific time that it was absorbed. But if measure the time, perhaps using something like a photomultiplier tube, then you collapse the wavefunction around that specific absorption time. But that doesn't tell you what the jerk is. In order to specify the jerk, you have to specify how you measure the jerk. If you don't actually measure it somehow, it doesn't have a specific value.

Now for me those are words to ponder upon!
You know for radio wavelengths we could use something like a Josephson junction to measure the signal that's going on in the receiving antenna. It would be interesting to see how the act of measuring it would affect the signal. Assuming it would.

 

[bhobba]

For 1, no, the whole antenna absorbs the photon, not just one electron.

That is NOT what QED says.

This whole issue of speaking about single photons is fraught with danger because in QFT a Fock space is used:
http://en.wikipedia.org/wiki/Fock_space

As per the original post you are using a lot of classical analogies for a situation where it doesn't apply eg in QM particles do not suddenly 'jerk' or anything like that. In fact what's going on when not observed the theory says nothing about.

At the lay level the best book on QED I know is Feynman - QED - The Strange theory Of Light And Matter. Note - even though Feynman says in that book you do not need to unlearn anything later - that isn't true - his explanation for photons propagating in a medium is incorrect, for example. The real explanation though is quite advanced:
https://www.physicsforums.com/threads/do-photons-move-slower-in-a-solid-medium.511177/

Unfortunately the question you are asking about a QM explanation to what's going on in an antenna is also like that in that no simple picture exists. As a matter of fact I don't know of any textbook account of it because really Maxwell's equations are more than adequate ie I don't think any new phenomena like in photons traveling through a medium emerges - but I could be wrong. That said the quantum explanation of conduction is rather interesting with the emergence of quasi particles that behave like actual particles - that of course would be going on in antennas - but I don't think its peculiar to an antenna.

Thanks
Bill

 

[zonde]

I was surprised about answer 1 because I vaguely recall over the years hearing physicists say the photon is absorbed by *one* particle but guess not.

Photons can interact with (and be absorbed by) quasiparticles too. See http://en.wikipedia.org/wiki/Quasiparticle
It depends on energy of photon. At different energy levels there would be different possible interaction.

 

[Ponderer]

That is NOT what QED says.

This whole issue of speaking about single photons is fraught with danger because in QFT a Fock space is used:
http://en.wikipedia.org/wiki/Fock_space

As per the original post you are using a lot of classical analogies for a situation where it doesn't apply eg in QM particles do not suddenly 'jerk' or anything like that. In fact what's going on when not observed the theory says nothing about.

At the lay level the best book on QED I know is Feynman - QED - The Strange theory Of Light And Matter. Note - even though Feynman says in that book you do not need to unlearn anything later - that isn't true - his explanation for photons propagating in a medium is incorrect, for example. The real explanation though is quite advanced:
https://www.physicsforums.com/threads/do-photons-move-slower-in-a-solid-medium.511177/

Unfortunately the question you are asking about a QM explanation to what's going on in an antenna is also like that in that no simple picture exists. As a matter of fact I don't know of any textbook account of it because really Maxwell's equations are more than adequate ie I don't think any new phenomena like in photons traveling through a medium emerges - but I could be wrong. That said the quantum explanation of conduction is rather interesting with the emergence of quasi particles that behave like actual particles - that of course would be going on in antennas - but I don't think its peculiar to an antenna.

Thanks
Bill

Okay now I'm confused. If we're watching the net electrical current in an antenna (not necessarily any individual electron), then will a sudden jerk be seen? Of course a jerk will be seen as a spike by the oscilloscope.

 

[bhobba]

Okay now I'm confused. If we're watching the net electrical current in an antenna (not necessarily any individual electron), then will a sudden jerk be seen? Of course a jerk will be seen as a spike by the oscilloscope.

For a single photon it is doubtful it will be above your systems noise, or even discernible from noise. You need collective behaviour and Maxwell's equations are enough for that.

Thanks
Bill

 

[Ponderer]

For a single photon it is doubtful it will be above your systems noise, or even discernible from noise. You need collective behaviour and Maxwell's equations are enough for that.

Thanks
Bill

Maybe it is impossible. I was hoping a josephson junction at low temp at a frequency well into the GHz.

 

[davenn]

Okay now I'm confused. If we're watching the net electrical current in an antenna (not necessarily any individual electron), then will a sudden jerk be seen? Of course a jerk will be seen as a spike by the oscilloscope.

there is no need for a "sudden jerk" as you put it
the incoming photo(s) EM wave have frequency and energy. The energy and freq is going to be imparted to the electron(s) and they are going to oscillate at that freq

For 1, no, the whole antenna absorbs the photon, not just one electron. For the case of a 1m large antenna, how could, for example, a 1m wavelength wave be absorbed by a single electron? If a single electron could absorb the photon, then why do we need such a large antenna?

That is NOT what QED says.

Agree with you Bill and it isn't what an antenna does

Khashishi, you have a misunderstanding of the function of the size of the antenna. Take a 1/2 wave dipole ... the antenna size isn't made as big as possible to capture as many photons ( EM waves) as possible. It is made to a be a wavelength/ 1/2 wavelength (and several other variations) of the freq of the photons (EM waves) This means the antenna will be resonant at the given freq and allows for the maximum generation of the EM wave ( and RF current) from the antenna element to the transmission line and on to the receiver.
( the opposite for transmission)

For a dish antenna, and to some extent a yagi array, the dish ISNT resonant at the freq of interest it is just a reflector to focus the incoming photons ( EM wave) onto the resonant part of the antenna system ... a dipole or monopole etc

and as Bill said, you really don't need QM to explain induction of current into a wire in an oscillating EM field ... Maxwell's work covers it wellcheers
Dave

 

[Ponderer]

there is no need for a "sudden jerk" as you put it
the incoming photo(s) EM wave have frequency and energy. The energy and freq is going to be imparted to the electron(s) and they are going to oscillate at that freq
Agree with you Bill and it isn't what an antenna does

Khashishi, you have a misunderstanding of the function of the size of the antenna. Take a 1/2 wave dipole ... the antenna size isn't made as big as possible to capture as many photons ( EM waves) as possible. It is made to a be a wavelength/ 1/2 wavelength (and several other variations) of the freq of the photons (EM waves) This means the antenna will be resonant at the given freq and allows for the maximum generation of the EM wave ( and RF current) from the antenna element to the transmission line and on to the receiver.
( the opposite for transmission)

For a dish antenna, and to some extent a yagi array, the dish ISNT resonant at the freq of interest it is just a reflector to focus the incoming photons ( EM wave) onto the resonant part of the antenna system ... a dipole or monopole etc

and as Bill said, you really don't need QM to explain induction of current into a wire in an oscillating EM field ... Maxwell's work covers it wellcheers
Dave

That's interesting. So if one photon worth of energy is only absorbed on average say every 1000 wavelengths, then there's is something that keeps the electrical current of all those electrons in the wire oscillating back and forth that entire time until the next photon is absorbed? Thanks

 

[Ponderer]

That's interesting. So if one photon worth of energy is only absorbed on average say every 1000 wavelengths, then there's is something that keeps the electrical current of all those electrons in the wire oscillating back and forth that entire time until the next photon is absorbed? Thanks

That would bring up another question. How do these photons know there's going to be another emitted photon? It seems maybe we would have some time machine? Because on the scope we see the sine wave still going but yet we haven't emitted another photon, yet. What if it's still oscillating in anticipation that we are going to admit another photon, but we change our mind and don't emit another photon.

 

[bhobba]

That would bring up another question. How do these photons know there's going to be another emitted photon?

Did you read the link on a Fock space? Exactly how did that suggest to you the above was a sensible question to ask?

Thanks
Bill

 

[Ponderer]

there is no need for a "sudden jerk" as you put it
the incoming photo(s) EM wave have frequency and energy. The energy and freq is going to be imparted to the electron(s) and they are going to oscillate at that freq

Out of curiosity, what about the Wikipedia article that gives reference to jerks observed in real life?

 

[bhobba]

Out of curiosity, what about the Wikipedia article that gives reference to jerks observed in real life?

Have you actually studied QM? You do know that the kind of questions such as jerk etc are meaningless in QM? Jerks occur in real life because its classical - not quantum.

Thanks
Bill

 

[Ponderer]

Did you read the link on a Fock space? Exactly how did that suggest to you the above was a sensible question to ask?

Thanks
Bill

Thanks for the link. It's difficult for me to see how that relates to real experiments, which is essentially what I'm mainly interested in. So I assume somewhere down the line I'm in error. Was my error in misunderstanding a post that an oscilloscope would see a sine wave under the conditions I've outline in previous posts? Sorry, I'm honestly trying to see how an oscilloscope would see it, given the frequency is high enough with respect to temperature, and of course an appropriate amplifier. I know the Wikipedia article did not say exactly what it is except to describe it as a sudden acceleration. So I'm assuming an oscilloscope would see that as a spike, not a sine wave, given the transmitter power is low enough. Thanks.

 

[bhobba]

Thanks for the link. It's difficult for me to see how that relates to real experiments, which is essentially what I'm mainly interested in.

Can you explain that difficulty? Why do you think its a sensible experimental question to ask when the number of photons is a superposition?

You asked right at the start 'Does the particle that absorbs a photon suddenly jerk? If so, then I would be interested in how fast you think the energy is absorbed.' Its not an experimental question. In fact, from the very foundations of QM its a nonsense question - which leads me to believe you need to become antiquated with it to formulate meaningful statements.

Thanks
Bill

 

[Ponderer]

Can you explain that difficulty? Why do you think its a sensible experimental question to ask when the number of photons is a superposition?

You asked right at the start 'Does the particle that absorbs a photon suddenly jerk? If so, then I would be interested in how fast you think the energy is absorbed.' Its not an experimental question. In fact, from the very foundations of QM its a nonsense question - which leads me to believe you need to become antiquated with it to formulate meaningful statements.

Thanks
Bill

It's an experiment. What's so wrong with that? I've been given two different interpretations as far as I see.

Again, If we have a transmitter at least a few wavelengths away from a receiver, the frequency is high enough with respect to temperature so that an amplifier can see the signal at the desired level. The level would be such that one photon or photon packet would be absorbed by the antenna every so often. The transmitter would be a nice pure sine wave signal. I'm just asking if the oscilloscope would see a spike? I'm not asking about fock space or anything. Just interpretations of QM that predict the real life experimental results.

 

[bhobba]

The transmitter would be a nice pure sine wave signal.

And I am asking you, with a transmitter driven by a source so weak you observe individual photons, why you would see a nice pure sine wave?

You need to connect that with what a quantum field is as per the fock space link. In particular the picture is not of photons being emitted - its of a quantum field that is a superposition of a zero photon state, a one photon state, a two photon state etc - that is a fock space. So exactly how do you link that with a nice pure classical sine wave?

What you are doing is intermixing classical and quantum concepts - you can't do that.

Thanks
Bill

 

[Ponderer]

And I am asking you, with a transmitter driven by a source so weak you observe individual photons, why you would see a nice pure sine wave?

You need to connect that with what a quantum field is as per the fock space link. In particular the picture is not of photons being emitted - its of a quantum field that is a superposition of a zero photon state, a one photon state, a two photon state etc - that is a fock space. So exactly how do you link that with a nice pure classical sine wave?

What you are doing is intermixing classical and quantum concepts - you can't do that.

Thanks
Bill

Good question. I'm referring to the AC source. So for example if it's 1mV, and an oscilloscope is fully capable of seeing a 1mV source, the circuit net resistance is 100Kohm, antenna radiation resistance is 10ohms, that comes to a radiated power of 1e-15 watts. If we want less power we can increase the resistance or decrease the voltage, but I'm just saying the voltage from the AC source would be a pure sine wave.

Do you have results for such an experiment? Thanks

I am asking a question about an experiment. I'm not mixing any theories. It's an experiment. If one feels Quantum Mechanics cannot predict the experimental results then that's a valid answer.

 

[bhobba]

Good question. I'm referring to the AC source. So for example if it's 1mV, and an oscilloscope is fully capable of seeing a 1mV source, the circuit net resistance is 100Kohm, antenna radiation resistance is 10ohms, that comes to a radiates power of 1e-15 watts. If we want less power we can increase the resistance or decrease the voltage, but I'm just saying the voltage would be a pure sine wave.

What I am saying is that if you have an em field so low you observe quantum effects it can't be described in the classical language you are using - it would, for example, be way way below 1mv.

An no - I know of no experiments like that - in fact I believe it would be way way beyond current technology.

But what I can tell you is you would need Quantum Field Theory and anything you have read about that is likely wrong unless it comes from a QFT textbook - for example a source would not emit individual photons. And for atoms in the receiving antenna particles would not 'suddenly jerk' - that would require them to have well defined momentum and position - which they do not.

Thanks
Bill

 

[Nugatory]

that comes to a radiated power of 1e-15 watts...

... or about 1e10 photons per second at wavelengths in the tens of centimeters. The discrete nature of the incoming radiation will be completely invisible with steps this small (we might as usefully try to show that water is made up of molecules by looking for discrete steps in the damage done by ocean waves in a storm) so the quantum mechanical prediction is that the quantum mechanical effects will not be observed.

 

[Ponderer]

What I am saying is that if you have an em field so low you observe quantum effects it can't be described in the classical language you are using - it would, for example, be way way below 1mv.

An no - I know of no experiments like that - in fact I believe it would be way way beyond current technology.

But what I can tell you is you would need Quantum Field Theory and anything you have read about that is likely wrong unless it cones from a QFT textbook - for example a source would not emit individual photons. And for atoms in the receiving antenna particles would not 'suddenly jerk' - that would require them to have well defined momentum and position - which they do not.

Thanks
Bill

Okay given the net resistance of 100kohm and antenna radiation resistance of 10ohms, when would be the total voltage to radiate 1e-15 watts? Not that it changes the experiment.

Why do you feel it's beyond present technology. Albeit my question never placed such a limitation. Let's do some math.

Net series circuit resistance is 100Kohm. Antenna R is 10 ohms. Frequency is 100GHz. One photon per wavelength comes to h*f^2 ~= 7e-12 watts. Circuit current = V / R = 1e-3V / 100e+3ohms = 1e-08A. Antenna radiated power comes to I^2 * R = 1e-15W. According to the equations the 1mV source would emit 1e-15W / 7e-12W equivalent of one photon every 1000 wavelengths. That's way to low of power than what's necessary I think. If we bump the voltage to 10mV it comes to one photon every 10 wavelengths.

And the receiver signal voltage depends what the signal looks like. A spike would be higher in voltage than a sine wave if the scope had sufficient bandwidth. So let's do the lowest voltage signal, a sine wave. As shown above the power (energy per wavelength of time) at 100GHz is 7pW. If the load, which could very well be our amplifier, is 100Kohm, then the voltage comes to V = sqrt(P*R) = 0.8mV. That's actually a huge signal. I've worked with signals down to 1e-16 volts, albeit low frequency. Sure, it's not an inexpensive experiment because of the low temperatures. Let's see what low temperatures it would require. Voltage noise = sqrt(4 k T B R). Typically I like set the bandwidth to about half the frequency to get a sufficient signal, but we can use full bw for now. So T = 0.8mV^2 / (4 k 100GHz 100Kohm) = 1K. Seems very reasonable with modern technology. What do you think? And that's using a sine wave signal for the absorbed photon. I'm guessing it's spike, meaning the energy is focused in a much small amount of time, which means the signal will be higher, which means we can use higher temperatures, or lower frequencies. Yes 100GHz is high. So we could lower the temperature for lower frequency.

 

[Ponderer]

... or about 1e10 photons per second at wavelengths in the tens of centimeters. The discrete nature of the incoming radiation will be completely invisible with steps this small (we might as usefully try to show that water is made up of molecules by looking for discrete steps in the damage done by ocean waves in a storm) so the quantum mechanical prediction is that the quantum mechanical effects will not be observed.

Please see the math presented in my previous post. It shows the details, which is that it is very reasonable with modern technology

So may I ask what's the purpose of saying modern technology can't do it? Are you saying it is completely impossible to view such signals?

 

[bhobba]

Okay given the net resistance of 100kohm and antenna radiation resistance of 10ohms, when would be the total voltage to radiate 1e-15 watts? Not that it changes the experiment.

Didn't you see Nugatory's post?

You are intermixing quantum and classical - you can't do that.

Thanks
Bill