Light is quantized when observed

Galina Weinstein has a new paper on The 1905 Relativity Paper and the "Light Quantum", with historical info on Einstein:

Before submitting his 1905 special relativity paper, Einstein had submitted a paper on what came to be called his light quantum hypothesis – the only one of his 1905 papers he considered truly revolutionary: "On a Heuristic Viewpoint Concerning the Generation and Transformation of Light", sent to the Annalen on March 17th, 1905, and received by the Annalen a day afterwards.9

Einstein wrote Conrad Habicht in May 1905 about this paper, "It deals with the radiation and energy characteristics of light and is very revolutionary".10

This paper extended the range of application of Planck's 1900 quantum hypothesis. In order to explain his law of black body radiation, which had been well-verified empirically, Planck was forced to assume that oscillators interacting with the electromagnetic field could only emit and/or absorb energy in discrete units, which he called quanta of energy. The energy of these quanta was proportional to the frequency of the oscillator: E = hf. But Planck believed, in accord with Maxwell's theory, that the energy of the electromagnetic field itself could change continuously.

This is all well-known, but but just what was Einstein saying in 1905 that was different from what Planck already said in 1900? Weinstein has some correspondence that throws some light on the matter:

In 1906 Planck's assistant, Max Laue, wrote Einstein on obtaining the preprints of the 1905 light quanta paper,

"When at the beginning of your last paper, you formulate your heuristic standpoint to the effect that radiant energy can be absorbed and emitted only in specific finite quanta, I have no objections to make; all of your applications also agree with this formulation. Now, this is not a characteristic of electromagnetic process in vacuum but rather of the emitting or absorbing matter, and hence radiation does not consist of light quanta as it says in §6 of your first paper; rather, it is only when it is exchanging energy with matter that it behaves as if it consisted of them."

Laue ended his letter to Einstein by saying: "By the way, I have never discussed your heuristic point of view with my boss. It is possible that there are differences of opinion between him and me on this question."14

But indeed the boss did agree with his assistant.

I am guessing that most people assume that Einstein was right and Planck was wrong, but that is not the case. Quantum mechanics teaches that light is quantized when observed, meaning when emitted or absorbed. Light in a vacuum has wave properties, and is definitely not a particle. So Planck's 1900 view is quite close to our modern quantum mechanical view. Einstein's 1905 heuristic was an interesting proposal, but it never led to the revolution that he hoped.

In fact, Planck was the first scientist to notice Einstein's relativity paper. Einstein's paper on relativity, received by the Annalen der Physik at the end of June 1905 was already in print by 26 September. And as early of November 1905 Planck had reported favorably on it.

Planck was a big help to Einstein's career. Planck apparently did not notice the much more important relativity paper of June 5, 1905 by Henri Poincaré.

Here is an xkcd cartoon. Quantum mechanics must be the most important and also misunderstood scientific theory in history. Even when big-shots talk about it, they babble nonsense about philosophical implications.

A current SciAm article explains that, while modern physicists talk about particles all the time, the electrons and photons are really not particles.

Physicists routinely describe the universe as being made of tiny subatomic particles that push and pull on one another by means of force fields. They call their subject “particle physics” and their instruments “particle accelerators.” They hew to a Lego-like model of the world. But this view sweeps a little-known fact under the rug: the particle interpretation of quantum physics, as well as the field interpretation, stretches our conventional notions of “particle” and “field” to such an extent that ever more people think the world might be made of something else entirely.

The problem is not that physicists lack a valid theory of the subatomic realm. They do have one: it is called quantum field theory. Theorists developed it between the late 1920s and early 1950s by merging the earlier theory of quantum mechanics with Einstein's special theory of relativity. Quantum field theory provides the conceptual underpinnings of the Standard Model of particle physics, which describes the fundamental building blocks of matter and their interactions in one common framework. In terms of empirical precision, it is the most successful theory in the history of science. Physicists use it every day to calculate the aftermath of particle collisions, the synthesis of matter in the big bang, the extreme conditions inside atomic nuclei, and much besides. ...

Many physicists think that particles are not things at all but excitations in a quantum field, the modern successor of classical fields such as the magnetic field. But fields, too, are paradoxical.
If neither particles nor fields are fundamental, then what is? Some researchers think that the world, at root, does not consist of material things but of relations or of properties, such as mass, charge and spin.

There is nothing new here. From the earliest days of quantum mechanics, Bohr and other talked about a wave-particle duality. An electron is not exactly a wave or a particle, but a quantum that has properties of both, as well as other mysterious properties. The plural is quanta. The term is popular, as I see that Simons Science News Is Renamed Quanta Magazine.