Atoms and quarks, two 20th-century revolutions

One aspect of Albert Einstein's heritage seems to have been overlooked in the many centenary celebrations of his annus mirabilis. The 20th century began with the confirmation of the revolutionary finding that matter was not continuous but made of atoms and molecules. It ended with a second revolutionary finding that matter is made of even tinier objects called quarks. The similarity between the two revolutions has been missed. Einstein played a crucial role in the first. A number of physicists were crucial to the second.

By 1966 Richard Dalitz and I were both already convinced that matter was made of quarks¹ as we led the discussion on this topic at the International "Rochester" Conference on High Energy Physics in Berkeley, California. We could not understand why quarks were not generally accepted until well into the 1970s. Unexplained regularities in the hadron spectrum, simple surprising relations in hadron reactions, the preponderance of three-meson final states in proton–antiproton annihilation at rest, relations between the electromagnetic properties of mesons and baryons, and the 3/2 ratio of the magnetic moments of the neutron and proton—all converged on the same conclusion: Mesons and baryons were built from the same elementary building blocks.

The answer for the delayed acceptance of quarks seems to be that people who do not understand history are condemned to repeat it. The missed history lesson was that Nature always reveals to us a new level of the structure of matter—new smaller building blocks—long before we have any theory of why those particular building blocks exist and what the elementary forces and interactions are that hold them together.

In describing the first revolution, Abraham Pais points out that the debate on the reality of molecules was finally settled because of the extraordinary agreement in the values of Avogadro's number, N, obtained by many different methods.² Matters were clinched not by a determination of N but by an overdetermination of it. From subjects as diverse as radioactivity, Brownian motion, and the blue in the sky, it was possible by 1909 to state that a dozen independent ways of measuring N yielded results that lay between 6 × 10²³ and 9 × 10²³.

Of course, there was no indication of what these molecules were, or their masses and interactions. No one had ever seen a single molecule, and an understanding of molecular physics had to wait for many new experiments and a completely new theory.

The parallel is striking between the development of our understanding of the structure of matter at two different levels—its molecular structure and its quark structure. Quarks were just as real in 1966 as molecules were in 1910 after Einstein's remarkable demonstrations of their reality in his work on Brownian motion and critical opalescence—the blueness of the sky. Three is the Avogadro's number of hadron physics; it might be called the Goldberg–Ne'eman number since Haim Goldberg and Yuval Ne'eman were the first to suggest the revolutionary proposal that the proton and neutron could be constructed from elementary building blocks with baryon number 1/3. (See Ne'eman's obituary in PHYSICS TODAY, August 2006, page 72.)

But none are so blind as those who do not want to see. Members of the physics establishment refused to look at the clear message that Nature was sending in experiments that confirmed the value of the Goldberg–Ne'eman number. Instead they followed the path of those who refused in 1910 to abandon the continuity of matter. They followed a similar path based on wrong theoretical ideas and looked for a "final theory of hadron interactions" before they understood that smaller building blocks were already evident.

The analogue of Einstein here was first and foremost Dalitz, who has never received the credit he deserves for showing the reality of quarks in hadron spectroscopy (see his obituary in PHYSICS TODAY, July 2006, page 65). Evgeni M. Levin and Leonid L. Frankfurt showed quarks' reality in reaction cross sections; Hector Rubinstein showed their reality in nucleon–nucleon annihilation, and he also led the work that pointed the way to dual resonance models and later to string theory.

In 1966 we had no idea what quarks really were or how they interacted. We had to wait until experiments had shown individual "partons" in elastic scattering, just as Jean Baptiste Perrin had to wait until 1926 to receive the Nobel Prize for his work on Brownian motion. We had to wait for the theoretical developments of asymptotic freedom and quantum chromodynamics. But it was already clear in 1966 that quarks were the real basic constituents of hadrons and not simply mathematical objects.

This letter was written when Dalitz and Ne'eman were still with us, and I had anticipated their reactions. Today I think it is appropriate to dedicate this letter to their memory.

References

1. 1. H. Lipkin, Nature 406, 127 (2000) [MEDLINE].
2. 2. A. Pais, "Subtle Is the Lord . . .": The Science and the Life of Albert Einstein, Oxford U. Press, New York (1982).