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There seems to be two forms of radiation process:

  1. "Quantum raditation": radiation during a quantum state change, one photon is emitted.

  2. "Classical radiation": radiation of accelerated charge, a lot of photons are emmited.

Questions:

  1. What is (or is there) the essential difference between the two form of radiation process in more advanced physics like quantum field theory?

  2. How to view the second form in a quantum way? Like see the accelerated charge as a series of quantum state changes and each of which may emit a single photon like the first form does - then in this way the second forms is just a classical view of a series quantum state changes.

  3. Examples of radiation process forms that between the two "extreme forms"? When talking about light detectors or LEDs, forms one is used. When talking about antenna, forms two is used. No form in between? What is the radiation form for thermal radiation?

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  • $\begingroup$ Other distinctions are possible : coherent vs incoherent, polarised vs randomly polarised, monochromatic vs thermal, wave-like vs particle-like etc $\endgroup$ Feb 18 '20 at 2:16
  • $\begingroup$ @sammygerbil Yes, the question focus on one possible difference as described. $\endgroup$
    – jw_
    Feb 18 '20 at 3:13
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    $\begingroup$ The point I am making is that the distinction which you have made isn't a fundamental one. There is nothing special about it compared with other distinctions. In fact your distinction contains elements of some of those I have mentioned. And in all of the distinctions there is a whole spectrum of cases between the extremes. $\endgroup$ Feb 18 '20 at 3:52
  • $\begingroup$ @sammygerbil I didn't make it clear. It should be the difference between the radiation process and the theory behind, not the radiation itself. $\endgroup$
    – jw_
    Feb 18 '20 at 4:42
  • $\begingroup$ Doesn't JD Jackson's book have a section on semi-classical radiation...which. should bridge the gap? $\endgroup$
    – JEB
    Feb 18 '20 at 4:58
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To start with there exists two frameworks of mathematically describing radiation. The classical framework with Maxwell's electrodynamics, and the quantum framework, with the quantum equations ( Dirac, Klein Gordon , started with Schrodinger) and with Quantum ElecroDynamics (QED).

1."Quantum radiation": radiation during a quantum state change, one photon is emitted.

This belongs to the quantum mechanical framework and can be described by solutions of the QM equations as transitions in energy states.

  1. "Classical radiation": radiation of accelerated charge, a lot of photons are emitted.

This belongs to the classical framework, which can be shown to emerge from the quantum mechanical.

The Quantum mechanical framework was needed and therefore formulated because of data that could not be predicted by classical electromagnetism : spectra of atoms, photoelectric effect, black body radiation. QED is the underlying framework for all electricity and magnetism phenomena.

What is (or is there) the essential difference between the two form of radiation process in more advanced physics like quantum field theory?

The classical electrodynamics emerges from QFT , similar to how mathematically thermodynamics emerges from statistical mechanics. The classical theory is useful in predicting macroscopic behavior of matter to electricity.

How to view the second form in a quantum way? Like see the accelerated charge as a series of quantum state changes and each of which may emit a single photon like the first form does - then in this way the second forms is just a classical view of a series quantum state changes.

The quantum form is QED, it is always a particle interaction. The emission of radiation from a charged particle in the table of elementary particles is given by the calculation of feynman diagrams, derived from QED, see the diagrams here, for example.. In this link there is a description in term of photons.

Examples of radiation process forms that between the two "extreme forms"? When talking about light detectors or LEDs,

This is basic quantum mechanics, transition between bound states by photon emmission.

forms one is used. When talking about antenna, forms two is used. No form in between?

This is classical electrodynamics, dealing with zillions of quantum particles, the electrons. They build up the fields but it is much simpler to deal with the classical electrodynamics.

What is the radiation form for thermal radiation?

It is black body radiation, one of the main proofs that quantum mechanics is needed even for bulk processes.

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  • $\begingroup$ Thanks for the detailed answer, but the question need a more advanced answer (I didn't make this clear), this answer seems to be targeted for people who has physics up to general college level. For example, some links to example of a complete QED treatment of accelerated charge - then make it clear classical and quantum radiation are essentially the same - then question 3 does't make sense? This link astro.utu.fi/~cflynn/astroII/l3.html seems still to be classical. $\endgroup$
    – jw_
    Feb 18 '20 at 7:57
  • $\begingroup$ The mainstream physics theories accept that everything is quantum mechanical at the micro level. As dimensions and values relative to h (heisebergs constant, grow the classical theories' mathematcs are simpler . Free particles radiating are easier to describe with the classical system, but a calculation with photons has to use QFT as the brehmstralung calculation I linked. Look at a bubble chamber event. physics.stackexchange.com/questions/423985/… . The quantum mechanics calculations $\endgroup$
    – anna v
    Feb 18 '20 at 11:49
  • $\begingroup$ are essential to predict the probability of seeing such an event. The tracks of the particles though are computed from space measurements using classical physics, because the small ionisation bubbles that show up the tracks are the quantum interaction of the particles as they traverse the chamber, but are irrelevant to the curvature which is computed with classical equations. $\endgroup$
    – anna v
    Feb 18 '20 at 11:53
  • $\begingroup$ I have answered your 3. It is frameworks, the quantum for the led, and the classical for the antenna, not forms as you imagine them. $\endgroup$
    – anna v
    Feb 18 '20 at 11:55
  • $\begingroup$ Now I read Wiki Bremsstrahlung, there is indeed a QM treatment there, maybe the topic is really Bremsstrahlung. $\endgroup$
    – jw_
    Feb 26 '20 at 9:02

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