Life is evolutionary. Sometimes fundamental changes are needed in order to adequately incorporate changing paradigms in physics, like MOND or gravitation theory, for example. This is an idea that was made apparent when I recently read an interesting and amusing review on high energy physics written by A. De Rujula, Archeology and evolution of QCD, arXiv:1610.07466v.1[hep-ph].
De Rujula’s talk is relatively informal and discusses some of the theoretical and experimental history of discovery on charmed quarks and of the quark model in general. He intersperses his description of the physics with personal asides that I found interesting and humorous.
To begin with, he tells the story of the origin of the idea of quarks – fractionally charged sub-particles which form the basis of composite particles, such as protons and neutrons. He provides evidence to disparage the official history that they were invented by Gell-Mann and Zweig. In the process he raises some interesting background – such as the initial rejection of Gell-Mann’s paper by scientific journal editors or referees. He also makes humorous reference to competing theories (and theorists) at Harvard and Princeton.
What caught my attention in particular was his discussion on p. 8 (Section 10) of his 1975 paper on the sub-structure of hadrons (composite particles like protons, neutrons and pions). He began with a somewhat sarcastic sounding “once upon a time,” to describe physicists who subscribed to the “bootstrap” concept of hadrons – as in “pulling oneself up by one’s own bootstraps.” In this concept, which was a prevalent notion in the late 60’s-70’s, there are no real sub-particles in hadrons – just different combinations of angular momentum states or other dynamical combinations of equally elementary particles, with the hadrons being the most elementary particles. These elementary hadrons could have a variety of excited states giving rise to the various observed “particles” or “resonances”. Another term used for this was “nuclear democracy”.
De Rujula and his colleagues subscribed to the alternative idea that there are distinct sub-particles (quarks, gluons) with a “simple underlying force law” – which came to be called QCD – Quantum Chromodynamics. This was in analogy to the then well established theory of Quantum Electrodynamics (QED).
De Rujula then shows how the experimental evidence supported the quark model and in particular how the quantum of charm came to be found and accepted.
I have to admit I was a supporter of the “bootstrap” concept when I did my Master of Science thesis on particle physics: Dispersion Relations in Quantized Field Theory, Dalhousie University, 1968.
The idea that we might be at the elementary rock bottom of nature when observing hadrons, with different particles being different excitations of this set of particles seemed appealing. The notion was supported by Geoffrey Chew, a noted physicist at the University of California at Berkeley at the time, e.g.: Chew, Geoffrey; Frautschi,S. (1961). Principle ofEquivalence for all Strongly Interacting Particles within the S-Matrix Framework. Physical Review Letters. 7 (10): 394-397 (Reference found from Wikipedia entry on Geoffrey Chew).
In a sense any particular state of matter was just a varied combination of every other state of mater – matter hoisted itself by it’s own bootstraps…
Since the time that Dr. De Rujula talks about, the evidence has overwhelmingly supported the quark model and as well the notion that “atomic like” configurations of quarks give rise to a detailed structure of particles and resonant states.
This in turn implies that the world of sub-nuclear particles is composed of ever finer structure with more particles to be discovered – like an eternal Russian nesting doll. This in turn suggests an underlying fractal like structure to matter-space-time – another concept which appeals to me.
Conventional wisdom suggests this process stops at the “Planck scale” – the distance or energy that is the simplest combination of Planck’s constant with other fundamental units of Physics that produce a minimum length or maximum energy. Interestingly, the original bootstrap concept has evolved and been applied to theories of physics at and beyond this scale – evolving into what has become known as string theory. But that could be the subject of at least another blog!
What struck me about De Rujula’s article was his comments to the effect that proponents of the bootstrap clung to the theory even when the evidence was available that supported the quark model.
What this shows is that physicists, like all humans generally, will cling to a conventional paradigm, even as evidence mounts to contradict it.
As I mentioned in an earlier blog (MOND versus Dark Matter), this seems also to be the case for the “search” for dark matter particles, even when there is considerable evidence pointing to a needed change in the established paradigm of gravitation theory. Fundamental changes are needed that can adequately incorporate the predictive success of evolving theories, like MOND.