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3 photon questions

  1. Jun 9, 2015 #1
    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.

    Wikipedia says "Radiation resistance is caused by the radiation reaction of the conduction electrons in the antenna"

    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!
  2. jcsd
  3. Jun 9, 2015 #2


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    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
  4. Jun 9, 2015 #3
    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.
  5. Jun 9, 2015 #4


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    No, it doesn't mean that the math DOESN'T specify how fast, it means the math CANNOT specify how fast
  6. Jun 9, 2015 #5
    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.
  7. Jun 9, 2015 #6


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    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.
  8. Jun 9, 2015 #7
    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.
  9. Jun 9, 2015 #8


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    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.
  10. Jun 9, 2015 #9
    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.
  11. Jun 9, 2015 #10


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    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:

    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/ [Broken]

    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 travelling 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.

    Last edited by a moderator: May 7, 2017
  12. Jun 9, 2015 #11


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    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.
  13. Jun 10, 2015 #12
    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.
    Last edited by a moderator: May 7, 2017
  14. Jun 10, 2015 #13


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    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.

  15. Jun 10, 2015 #14
    Maybe it is impossible. I was hoping a josephson junction at low temp at a frequency well into the GHz.
  16. Jun 11, 2015 #15


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    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 well

    Last edited: Jun 11, 2015
  17. Jun 12, 2015 #16
    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
  18. Jun 12, 2015 #17
    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.
  19. Jun 12, 2015 #18


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

  20. Jun 12, 2015 #19
    Out of curiosity, what about the Wikipedia article that gives reference to jerks observed in real life?
  21. Jun 12, 2015 #20


    Staff: Mentor

    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.

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