Lipkin replies: The Dirac equation marked the end of an era when the trail to new physics was blazed by theorists. It was followed by a new era during which trails were blazed by experimenters, with theorists trying to explain puzzling “who-ordered-that” results: beginning with the puzzling number 137, the anomalous magnetic moments of the proton and neutron, and the discovery that the muon did not behave like Hideki Yukawa’s meson. Another era began many years later with the discovery of neutral currents, charm, and the rise of the Standard Model.
My letter referred to the period between the Dirac equation and the rise of the Standard Model. I therefore do not discuss other periods. However, I note that the conclusion that matter is not continuous but consists of atoms and molecules was settled once and for all because of the extraordinary agreement in the values of Avogadro’s number obtained by many different experimental methods. 1 Scientific progress did not result from experiments designed to check theory.
P. A. M. Dirac’s goal was to find a description of the electron consistent with both relativity and quantum mechanics. The unexpected spin-off was a remarkable combination of “who-ordered-that” theoretical consequences: the spin and magnetic moment of the electron, the existence of the positron, and all the correct descriptions of electron–positron annihilation and pair creation.
No theorist has since found anything comparable to the Dirac equation. Remarkable and even great theoretical achievements cited in this set of letters are simply not in the same league.
At Princeton University in 1946, I saw all the great theorists—you name them, they were there—completely at a loss about the infinities that plagued quantum electrodynamics (QED). Niels Bohr insisted that quantum mechanics applied only at the atomic scale. The new theory needed at the nuclear scale would be as different from QM as QM was different from Newtonian mechanics. David Bohm tried hard to find such a theory. But so far QM still holds far below the nuclear scale.
Then one great theorist, Willis Lamb, decided that new experimental input was needed, and measured the Lamb shift in an incredible tour de force. I remember the colloquium describing his plans and thinking that he was crazy. Nobody could make that complicated experiment work. But he did. The significance of this work was emphasized this year by President Clinton’s award of one of 12 national medals to Lamb.
The most exciting result immediately following World War II was that the Lamb shift was indeed finite and measurable. Its completely unpredicted value started Hans Bethe, Richard Feynman, and others on the way to a new predictive formulation of QED.
Despite the great respect many theorists held for this new formulation, Feynman deprecated it as “bookkeeping,” not physics. He regarded the conserved vector current as his major discovery in “real physics.”
Tsung-Dao Lee and Chen Ning Yang deserve the highest praise for their proposal that parity was violated in the weak interaction and for pushing the experiment of C. S. Wu. But this is not “theory.” This is phenomenology, analyzing the latest puzzling “who-ordered-that” data and pointing directions for further experiments. They had no theory. The initial “who-ordered-that” experimental parity violation in kaon decay that started the tau–theta puzzle was not explained by the V – A theory and was not understood by theorists until many years later, when it became clear that kaons and pions were not elementary bosons but were made of quarks.
Unfortunately, the great advances made by phenomenologists in pushing back the frontiers of knowledge have generally been undervalued. Another example of great phenomenology was the 1975 six-quark, six-lepton model of Haim Harari, who introduced and named the top and bottom quarks. The six–six model fit the data, explained all perplexing puzzles while nothing else did, and told experimenters what to look for next.
I began my physics career with an experiment, the first test of whether relativistic positrons obeyed the Dirac equation. But such tests did not yield clues to the new theory that Niels Bohr said would replace quantum mechanics. Now we are back at another level looking for clues to new physics beyond today’s Standard Model. Collaboration between theory and experiment is certainly needed. But let us not forget the crucial role of phenomenology.
