The Origins of Quantum Physics

[Master Wayne]

In the book "The Quantum Story", by Jim Baggott, we find the following:

"[...] Many physicists, including Planck and Bohr, dismissed the light-quantum. They preferred to think of quantization as having its origin in atomic structure, retaining Maxwell's classical wave description for electromagnetic radiation."

I don't get this. How could they believe that light was emitted and absorbed in a discrete fashion, but that light itself wasn't discrete? If atoms were emitting quanta of light, how could light not be quantized?

 

[Simon Bridge]

Welcome to PF;
Look up "semi-classical model". There were able to believe that because of how they worked their quantum theory in terms of small masses on a spring. Anyway: the math worked.

 

[Master Wayne]

Thanks for the warm welcome, Simon. I looked up what you suggested and found something relevant on Wikipedia:

"Even after Compton's experiment, Niels Bohr, Hendrik Kramers and John Slater made one last attempt to preserve the Maxwellian continuous electromagnetic field model of light, the so-called BKS model. To account for the data then available, two drastic hypotheses had to be made:

1. Energy and momentum are conserved only on the average in interactions between matter and radiation, not in elementary processes such as absorption and emission. This allows one to reconcile the discontinuously changing energy of the atom (jump between energy states) with the continuous release of energy into radiation."

It seems like there's my answer right there, but I still don't understand it. Can someone elaborate on this?

 

[kith]

How could they believe that light was emitted and absorbed in a discrete fashion, but that light itself wasn't discrete? If atoms were emitting quanta of light, how could light not be quantized?

It could simply be a property of the atoms, that they can only absorb or emit electromagnetic waves like that. This was probably the maintream view among physicists until the early 20s when Compton discovered that photons don't scatter like classical electromagnetic waves at high energies (Compton scattering vs. Thomson scattering). I think your quote refers to this early period and not to the BKS theory, which was kind of the last effort to make the classical wave theory consistent with the experiments by abandoning really fundamental physical principles like energy and momentum conservation.

It's quite interesting that Bohr disliked the photon idea so much that he was willing to give up energy conservation AND later on worked so constructively on the complete theory which included photons. Einstein never abandoned his view about how quantum mechanics should look like.

It's also interesting that the final disproval of classical wave theories has not happened until the 70s. I'm no expert in this, but there's a review article by Thorn et al. which lies on my reading pile. ;-) Maybe you're interested in it: http://people.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf

 

[Master Wayne]

Kith, thanks a lot for your thorough answer, it was very helpful! But I think I would still have to take a look at the BKS theory to understand exactly how they managed to reconcile discrete emission/absorption with continuos radiation. As to the article you pointed out, I was really surprised to find out that the photoelectric effect can be explained without resorting to light quanta. I'll certainly be taking a better look at this. Thanks again!

 

[Simon Bridge]

It should not be surprising that even very bright people can hold to ideas that turn out to be incorrect. Everybody is wrong sometime and scientists have more opportunity to be wrong than most people: it's their job.

The semi-classical treatment in quantum optics is usually part of a post-graduate course.

 

[sheaf]

Kith, thanks a lot for your thorough answer, it was very helpful! But I think I would still have to take a look at the BKS theory to understand exactly how they managed to reconcile discrete emission/absorption with continuos radiation. As to the article you pointed out, I was really surprised to find out that the photoelectric effect can be explained without resorting to light quanta. I'll certainly be taking a better look at this. Thanks again!

Yes indeed, when I was taught QM at university in the 1980s, the photoelectric effect was presented to us as the proof of the quantum nature of light. However I also was shocked to learn that it's not actually a proof. I first heard about the semiclassical treatment that Simon Bridge discussed when I read this thread:

https://www.physicsforums.com/showthread.php?t=474537

The final "proof" came with the photon antibunching experiments not so long ago. I was so fascinated by this that I bought a book on quantum optics!