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Whatever Happened to Solid State Physics? J. J. Hopfield in Annu. Rev. Condens. Matter Phys. 5:1 (2014).  What the paper says!?

In this essay, Hopfield describes his vision of solid state physics and how it emerged into other fields (broadly into condensed-matter physics) and how this brought him, in various steps, to neural networks. It is full of wisdom and interesting testimonies, such as Feynman being emotionally unable to read the BCS paper or Wigner asking a trick question to discredit this theory (BCS) because it was not his.

on what is a physicist:

Being a physicist is a dedication to the quest for this kind of understanding.

The kind of understanding being that «the world is understandable, that you should be able to take anything apart, understand the relationships between its constituents, do experiments, and on that basis be able to develop a quantitative understanding of its behavior»

My upbringing had focused my interest particularly on the physics of the world around me: not the physics of the nucleus or the cosmos, but rather the physics of the daily world and its technologies.

On his choice of research problem:

I picked one [problem] having to do with the radiative lifetime of an exciton in a crystal, where the conflict was within theory itself. Naive theory yielded either zero or infinity depending on how it was applied, neither of which seemed to make sense. It became my problem, and Overhauser never worked on it at all.

On the success of his main pure (or traditional) physics input:

The polariton, a new solid state physics particle, was invented to resolve the paradoxical situation. The single paper written from the 1958 thesis is still highly cited (as is the single author) thanks to the existence of lasers, the polariton condensate, and modern photonics.

On the ability of finding one own's problem:

Acknowledging one’s own abilities, style, and weaknesses is ever so useful.

There is a nice observation (in a footnote, footnote 3) that:

The role of quantum mechanics in biology is central but trivial. Making and breaking covalent chemical bonds between molecules immersed in a solvent is intrinsically a quantum mechanical problem involving electrons. Yet for most purposes, the fast motions of electrons and many fast vibrational motions can be integrated out (adiabatic), producing force fields that drive the slower, more collective coordinates that are the useful descriptors of biological processes. Although there have been elegant philosophical discussions of issues such as whether consciousness is necessary for quantum mechanical measurement and wave-packet collapse, they contribute little to our physical understanding of the biological core processes such as self replication, processes billions of years older than consciousness in animals with highly developed nervous systems.

Most of the text relates his personal experience with solid-state physics before it branched into other fields, the specificities of this particular field, and how it brought him to biological physics and then to neuroscience:

On solid-state physics being separate from the rest of physics:

theorists like Shockley, Bardeen, Anderson, and Kittel, whatever their training, were finding their problems from within solid-state physics and separated themselves from nuclear, particle, or astrophysics. By contrast, an earlier generation of theorists, like Bethe, Block, Wigner, and Pauli, had worked across a broader span of undifferentiated physics and, when solid-state physics emerged as a field (and some detail began to matter), ceased to contribute to it.

This passage illustrates particularly well the calibre of Hopfield as a physicist:

Solid state physics was one haven for physicists who were drawn to physics by a desire to understand the world around them on a personal level. Why is iron ferromagnetic? Why does a wire point pressed to a galena (PbS) surface result in a rectification contact? (Such point-contact diodes were an essential feature of crystal set radios.) Why is ZnS a phosphor? Why do transparent NaCl crystals turn colored when exposed to ultraviolet light? Why is copper a good conductor of electricity and iron a poor one? Why do pencil marks conduct electricity?

On the emergence of biological physics:

In the 1960s, when a physical scientist looked at a biological process it appeared purposeful and almost miraculous. How could such a process as cell division or thinking be merely the simple laws of classical physics in a system without design? For the physicists’ explanation of the mysteries and seeming miracles of life processes will not involve Planck’s constant in any profound fashion.

A conceptual difference in biology, the concept of "function":

The one singular conceptual addition to the science was the notion of function: that there is a small subset of properties that is of great importance to biology, and that evolutionary choices have shaped biological systems so that they function well. The term function is peculiarly biological, occurring in biology and in applied sciences/engineering, which are pursued to benefit humans, but not relevant to pure physics, pure chemistry, astronomy, or geology.

When a physicist really understands something, he can explain it to another physicist in such a way that the other physicist will feel the result is obvious. For the budding field of biological physics, such understanding is the ultimate holy grail.

He was not the only one to look for problems beyond traditional physics. He names three although only Leon Cooper seems to be very famous: (the other two are George Feher and Ivar Giaever (Nobel prize with Josephson): {{quote|Leon Cooper turned from superconductivity theory to the theory of learning in neurobiology, interacting with experimental neurobiologists. I mention these three as examples of very successful solid state physicists who already in the mid-1960s were finding their next problems in biological systems

On a hardcore approach of physics to biology, unusual matter interesting on its own: {{quote|In the early 1970s, Hans Frauenfelder turned from using nuclear physics as a probe of local solid state environments to the study of local field environments in myoglobin. He was a particularly effective advocate of what he named biological physics, believing that biological matter was so unusual that its properties should be studied for their own sake, and that such studies were unfettered by questions of biological relevance. Of course, in the long run truly unusual properties will often be present as a result of evolutionary importance, bringing biology back in by the rear door.»»

A difference between biological physics and biophysics:

The insistence that physicists should ask their own questions of biological systems, and should be writing for physicists not for biologists, became part of the intellectual divide between biological physics and the older discipline of biophysics

A notable detail on his transition from one field to the other:

So hemoglobin provided me a simple entry from condensed matter physics to biological matter physics.

More details on how he turned to neural systems:

My entry into biological information processing at the level of the nervous system was entirely accidental.

Francis O Schmitt recruited him for his Neuroscience Research Program, for which he wanted to

add a physicist to the group, hoping to bring someone who would interact with his subject and perhaps help it to become more complete as a science. He had gotten my name from relativist John A. Wheeler, who (for reasons that I have never grasped) had always been one of my staunch supporters.

Interestingly, we find again the insightful Wheeler.

Hopfield got hooked:

How mind emerges from brain is to me the deepest question posed by our humanity.

First paper with neuron, and most quoted, owes much to spin-glass:

Eventually, my knowledge of spin-glass lore (thanks to a lifetime of interaction with P.W. Anderson), Caltech chemistry computing facilities, and a little neurobiology led to the first paper in which I used the word neuron. It was to provide an entryway to working on neuroscience for many physicists and is the most cited paper I have ever written.

Nobody from the Molecular Biology department sees Hopfield as a physicist:

[...] in the Molecular Biology Department, which was interested in expanding into neurobiology. Although no one in that department thought of me as anything but a physicist.

Still served as president of the APS, and now, Nobel prize for physics.

I had by then strayed too far from conventional physics to be courted for a position in any physics department. So I was quite astonished in 2003 to be asked by the American Physical Society to be a candidate for vice president. And, I was very happy to be elected and ultimately to serve as the APS president. I had consistently felt that the research I was doing was entirely in the spirit and paradigms of physics, even when disowned by university physics departments. I saw my election primarily as a symbolic act by the membership, saying “this too is physics”—or perhaps “this too is solid state physics.”

A beautiful conclusion for an illuminating text and a reply, ten years in advance, to critics of his work not being physics:

I am gratified that many—perhaps most—physicists now view the physics of complex systems in general, and biological physics in particular, as members of the family. Physics is a point of view about the world.