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Planck and Einstein's theories.

  1. Jun 19, 2010 #1
    Hi all,

    I've just started reading upon quantum physics and came across Max Planck. I believe he was the one to state that the oscillating atoms or molecules that emit radiation could only occupy quantum states, whereby photons with energy according to E=hf are emitted when an atom changes from one quantised energy state to another.

    But, if Einstein has simply extended Planck's concept of quantization, are there differences between Einstein's and Planck's hypotheses? Is it that Einstein extended Planck's concept to electromagnetic waves, primarily to light, as a stream of photon? Or, are there no differences?

    Also, if we have come to model light as a stream of photons, would this support the particle nature of light? If so, photons are considered to be particles, right?

    Thank you, all!
     
  2. jcsd
  3. Jun 20, 2010 #2

    tom.stoer

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    Planck tried to explain the spectrum of black body radiation. Therefore he used a trick to discretize the energies carried by the photons, namly E=hf. The question no is why the photon energy should be quantized.

    Afaik Planck did not require the phton energy to be quantized always, instead he argued that the emission of the photons by the atoms is enough to explain black body radiation. Quantization of energies was already known from atomic spectra, so this was a reasonable assumption.

    Einstein went further in his explanation of the photoelectric effect and in his re-derivation of the black body radiation. Instead of using a non-qantized (classical) el.-mag. field that appears to be quantized due to quantized emission by photons, he introduced the idea of a quantized el.-mag. field.
     
  4. Jun 20, 2010 #3

    alxm

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    This is actually a common misconception. (Just to quote Wikipedia: "Contrary to popular opinion, Planck did not quantize light; this is evident from his original 1901 paper") Planck did not assume light was emitted in quanta when he derived his distribution. Nor was he looking at atomic spectra - Actually, like many physicists at the time, it seems he wasn't fully convinced that atoms existed. He merely states that radiation is emitted by a discrete number of 'oscillators'.

    It was Einstein, later, who suggested this interpretation of the Planck distribution, not Planck. This fitted with his work on the photoelectric effect, so you got E = hf in two ways.
    Then Einstein's work on the photoelectric effect suggesting 'light quanta' inspired Bohr to explain atomic spectra in terms of quantized energy levels.

    Are you saying Einstein developed quantum field theory??
     
  5. Jun 20, 2010 #4

    tom.stoer

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    Readung carefully what I wrote you will see that this is just my statement - except that I am talking about atoms instead of oscillators.
    I said that Planck didn't qantize light but assumed that it's quantized due to the the emission only. That's where he got the light qanta E = hf from. I didn't say that he looked at spectra, but that it was reasonable to assume this quantization because spectra where already known.

    Einstein did even more, namely he interpreted the black body radiation in terms of absorption plus spontaneous and induces emission.

    I am not sure about that.

    No; I am saying that Einstein made the step from quantized emission to quantized el.-mag field. By quantization I mean that E=hf becomes no a property of the el.-mag. field instead of a property of the emission process. That does not mean quantum field theory. Just as Bohr "quantized" the electronic spectra w/o quantizing the "electron field". Just a first stept into this direction.

    Quantization of the el.-mag. field in terms of quantum field theory took much longer.
     
  6. Jun 20, 2010 #5

    alxm

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    Well, that's what Bohr told Arrhenius to convince him to give the Nobel to Einstein.

    Ah I follow you know. Yes that's a correct description of the transition from Planck to Einstein.
     
  7. Jun 20, 2010 #6

    tom.stoer

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    I knew that it took some years to convince the nobel committee; and that the Nobel prize for the photoelectric effect was somehow a trick because for some reason SR and GR seemed to be unacceptable for the "not so wise old men".
     
  8. Jun 21, 2010 #7

    alxm

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    Yes, there's a book by Elzinga about it all which I read. He went and dug in the archives.
    There's a (fairly typical) myth that SR and GR were 'too difficult' (or some such) for the committee to understand. That's of course not true and if you look at who was on the committee, e.g. Arrhenius, Osséen) it was never really plausible.

    By the time Einstein got the prize, it was a bit too early for GR. The real bone of contention was SR, to which their initial reaction was along the lines of "Isn't this just a reinterpretation of Lorentz' work?", an opinion which unfortunately stuck among some influential members even beyond the point (~1916) where they'd definitely shown electrons had relativistic mass.

    So the photoelectric effect prize was perhaps better described as a compromise. Or a diplomatic solution.
     
  9. Jun 21, 2010 #8

    tom.stoer

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    Yes, something like that.
     
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