# How exactly do we know that light is quantized?

First off, I don't seriously doubt that light is quantized. I'm just curious to know the proper historical development of this non-trivial fact of reality (well, now it's trivial, to some people at least).

I've always believed that the cornerstone pieces of evidence for the quantization of light were Planck's blackbody radiation curve, the photoelectric effect, and the Compton effect. But not too long ago I learned that both the photoelectric effect and the Compton effect have semi-classical explanations - i.e. functional explanations that assume light is not quantized. [1] [2] However, Planck's blackbody radiation curve still seems to be perfect evidence for the failure of the classical theory of radiation.

My question is the following: exactly how did we learn that light is quantized, and rule out all [semi-]classical explanations? Please don't give me the standard "Planck showed that the quantization hypothesis led to the correction of the UV catastrophe... the photoelectric effects shows us that the energy of light is proportional to its frequency not its intensity... Compton scattering shows that light follows the same relativistic kinematics as a point-particle...". I've read that already and given links that mention why the latter two don't necessarily provide "indisputable evidence". Certainly there are hundreds of other historically important papers that are going under the rug here.

References:

• Those are some reliable references... Nov 22 '16 at 6:58
• So you are essentially asking How exactly do we know that light are particles? Nov 22 '16 at 7:52
• Individual photons can describe all phenomena of light. Even waves are best described with millions of coherent photons. Nov 22 '16 at 10:24
• Your second link doesn't say that photons aren't particles and whoever wrote you're first reference doesn't understand QFT or photons, I'm sorry to say. They even say that their claim is that photons are classical wave packets. Their whole theory is based on the fact that photons don't interact with each other, but in fact ATLAS recently detected light-by-light scattering. Nov 22 '16 at 11:33
• Your two citations don't show what you think they show. The first cites a famously not peer-reviewed paper by Lamb/Scully and treats the photo-electric effect as a semi-classical interaction of classical EM with quantum atoms...but the photoelectric effect happens for the free electrons in metals which are as far from electrons in polarizable atoms as it gets. I wrote the answer in the second and explicitly state that the "classical Compton effect" vanishes in the low-intensity regime, which the quantum Compton effect certainly does not since it's a single-photon interaction. Nov 22 '16 at 13:55

Well, you are mixing things in your question, you say:

. I'm just curious to know the proper historical development of this non-trivial fact of reality

This is historical:

The Nobel Prize in Physics 1921 was awarded to Albert Einstein "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect".

No semiclassical explanations at the time , because the link you give is not historical, it is decades after the detection of the photoelectric effect and its interpretation as the photon being a discrete particle.

You yourself give the historic path.

Now there is a solid quantum mechanical theory called the standard model of particle physics, where the photon has its niche as a point particle on par with electrons and neutrons in the Lagrangian .

This model has been validated by innumerable data, and the chase for physics beyond the standard model goes into making photons into string excitations, not continuous classical electromagnetic waves.

Now wavepackets, that your first reference states, are necessary in the Quantum Field Theory of describing nature, which works with quantum "events" in effect. So I do not see anything revolutionary:

Do we count indivisible photons or discrete quantum events experienced by detectors?

One could ask this for all elementary particles, and certainly they are quantum entities, not classical particles .

Certainly there are hundreds of other historically important papers that are going under the rug here

Do you have any links? I do not think they exist, which is the reason Einstein got his Nobel.

In conclusion, any semiclassical arguments have to face all the bulk of data of the self consistent current model that describes particle data, i.e. embed them in the new format, because these are data too , in addition to the photon being an elementary particle.

Is it not possible to derive BE distribution for photons in a blackbody cavity without assuming the quantum nature of light. You have to assume $p=\hbar k$ and $E=pc=\hbar k c$ (the relativistic dispersion relation itself is applied to particles and not to waves) which holds for the quanta of light. They behave the same way as a collection of bosonic quanta should behave under a given set of constraints.