The March issue of Physics Today is now available. It contains a piece by Steven Weinberg based on a banquet talk he gave to a group of postdocs. He describes his own memories of his time as a postdoc, writing that “Many of us were worried about how difficult it seemed to make progress in the state that physics was in then.” This was the heyday of S-matrix theory and he comments:
Some people thought that the path to understanding the strong interactions led through the study of the analytic structure of scattering amplitudes as functions of several kinematic variables. That approach really depressed me because I knew that I could never understand the theory of more than one complex variable. So I was pretty worried about how I could do research working in this mess.
He describes envying the previous generation of 10-15 years before his time, that of Feynman, Schwinger, Dyson, and Tomonaga, thinking that all they had had to do was sort out QED, and speculates that they in turn had envied the generation before them since quantum mechanics was even easier. I can see that he’s trying to provide encouragement, ending with
So the moral of my tale is not to despair at the formidable difficulties that you face in getting started in today’s research… You’ll have a hard time, but you’ll do OK.
Postdocs in high energy physics these days do need some encouragement, but I also think they need some recognition from their elders that they’re facing a different situation than that faced by earlier generations. Coming into a field that has not seen significant progress in about 30 years is a different experience than what Weinberg or previous generations of particle physicists had to deal with. High energy physics is now facing some very serious problems, of a different nature than those of the past, and I think these deserve to be mentioned.
In the same issue, Weinberg makes another appearance, in an exchange of letters with Friedrich Hehl about the torsion tensor in GR. Hehl takes him to task for his comments in a previous letter that torsion is “just a tensor”, pointing out that it can be thought of as a translation component of the curvature. Weinberg responds that he still doesn’t see the point of this:
Sorry, I still don’t get it. Is there any physical principle, such as a principle of invariance, that would require the Christoffel symbol to be accompanied by some specific additional tensor? Or that would forbid it? And if there is such a principle, does it have any other testable consequences?
Actually, Weinberg is rather well-known for taking the point of view that GR is just about tensors, and that their geometrical interpretation is pretty irrelevant, so it’s not surprising that he doesn’t see a point to Hehl’s comment. In his well-known and influential book on GR, he explicitly tries to avoid using geometrical motivation, seeing this as historically important, but not fundamental. To him it is certain physical principles, like the principle of equivalence, that are fundamental, not geometry. There’s a famous passage at the beginning of the book that goes:
However, I believe that the geometrical approach has driven a wedge between general relativity and the theory of elementary particles. As long as it could be hoped, as Einstein did hope, that matter would eventually be understood in geometrical terms, it made sense to give Riemannian geometry a primary role in describing the theory of gravitation. But now the passage of time has taught us not to expect that the strong, weak and electromagnetic interactions can be understood in geometrical terms, and too great and emphasis on geometry can only obscure the deep connections between gravitation and the rest of physics.
This was written in 1972, just a few years before geometry really became influential in particle physics, first through the geometry of gauge fields, later through geometry of extra dimensions and string theory. I recall seeing a Usenet discussion of whether Weinberg had ever “retracted” these statements about particle physics and geometry. Here’s an extract from something written by Paul Ginsparg, who claims:
back to big steve w., when he wrote the gravitation book he was presumably just trying to get his own personal handle on it all by replacing any geometrical intuition with mechanial manipulation of tensor indices. but by the early 80’s he had effectively renounced this viewpoint in his work on kaluza-klein theories (i was there, and discussed all the harmonic analysis with him, so this isn’t conjecture…), one can look up his research papers from that period to see the change in viewpoint.