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As suggested by one of the commentators on my last question, I am going through Bohr's Nobel prize lecture in order to understand how quantum mechanics was developed.

The lecture describes Planck's observations on radiations. I'd like to know how it was concluded from these observations that energy can only change in discrete amounts and also the derivation of Planck's formula.

The lecture then goes on to Einstein's idea about the restricting condition for vibrational energies of atoms and the specific heats of crystals. Can anyone describe exactly what Einstein's idea was about the specific heats of crystal, how that accentuated the concept of discrete energy variation and how it led to further development of QM?

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2 Answers 2

Planck's motivation to consider discrete energies came from the fact that the classic theory with continuous energies did not match the observed radiation out put from (or inside of vacant) "black bodies." Only by assuming energies are discrete, and inserting that into the mathematics, could he come up with a black body radiation equation that matched the experiments. A good description of the mathematical development is in Eisbergs's "Fundamentals of Modern Physics."

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Planck's work was in resolving the ultraviolet catastrophe. In a classical system every degree of freedom should contain the same amount of energy, but the number of degrees of freedom increases as the frequency increases and there should be an infinite amount of energy in short wavelength radiation. Planck proposed that energy can only be transferred in discrete units (of $h\nu$) and this prevents the short wavelength modes being excited and keeps the energy finite.

Einstein pointed out that similar reasoning applies to the excitation of vibrations in a crystal lattice (or indeed any system in which it is possible to excite vibrational modes).

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Actually, by degree of freedom I mean the number of independent ways in which a system can posses energy, so I feel a bit skeptic about accepting the degree of freedom tending to infinity. –  danny gotze Jan 1 '13 at 16:13
    
; How does this discrete energy change proposition prevented this? –  danny gotze Jan 1 '13 at 16:43
    
The Wikipedia article I linked covers both your questions. Have a scan through it, and if there is anything you're not sure about ask here and I'll help if I can. –  John Rennie Jan 1 '13 at 18:40
    
Which Wikipedia link ? –  danny gotze Jan 2 '13 at 15:57
    

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