"A Who's Who of the Unseen"

This summer, under France and a bit of Switzerland, proton collisions of unprecedented force will offer fresh insight into the nature of matter. You’ll find ­photos of the Large Hadron ­Collider in the article “The Making of a New Collider”; and in the article “The New Collider”, Jerome ­Friedman discusses its importance. Friedman won the 1990 Nobel Prize in Physics for particle accelerator experiments confirming the existence of quarks, the elementary particles that make up protons and neutrons. This work was essential to the standard model of particle physics, which ­Friedman thinks the LHC can help physicists complete. He adds that “if history is a guide, the LHC will also turn up complete surprises, phenomena not antici­pated by any theoretical speculation.”

History is a guide. It’s also an echo chamber. In the November 1939 issue of Technology Review, MIT physics professor Philip M. Morse argued for more experimentation in particle physics, lest theory go untested. (The polymath Morse went on to organize the Anti-Submarine Warfare Operations Research Group, which helped the U.S. Navy destroy German U-boats; after the war, he turned operations research into a wide field of study.)

Morse began “Ultimate and Indivisible” with a graceful nod to the distant past:

It seems to have begun with Democritus, this idea of matter’s being composed of fundamental, indivisible atoms. Centuries after the time of the great Greek, Dalton used the newly found facts of chemistry to give substance to the speculation. Today we are sure that all matter and all energy are built out of a few kinds of fundamental particles. But we are not sure just how many kinds of particles there are. Nor are we quite sure whether they are really particles and not waves. The investigations by means of which we have reached both our present surety and our present doubts have brought to light particles that would puzzle Democritus, and to language terms that often torment the reader. A survey of this development really resolves itself into a who’s who of the unseen.

After reviewing what was known about particles, Morse admonished physicists to put all unproven theories to the test:

Some evidence seems to indicate that when a neutron is transformed into a proton, another particle is given off in addition to the electron. This other particle has been called a neutrino. The Italian physicist Fermi (another Nobel prize winner, now living in this country) developed a theory of this transformation which predicted the existence of such a particle and indicated that it should have no electric charge but should possess a spin and have a mass much smaller than an electron. Such particles would be quite difficult to detect, and at present only one set of experiments seems to prove their existence.

Several years ago the Japanese physicist Yukawa studied the theory of the non­electrical forces between nuclear particles and showed that a corpuscle of radiation might be associated with these forces, just as the photon of light is associated with electrical forces. His studies could not determine whether the predicted particle had spin or electric charge, and his paper ends on an apologetic note after he calculates that the particle should weigh about a hundred times the electron mass, or about 1/20 the proton mass. A few recent observations of the constituents of cosmic rays seem to indicate that this intermediate particle actually does exist. It has been called the meson, or mesotron.

Thus Democritus’ conception of the economy of nature in its use of building materials for the universe seems to be finally demonstrated, although the picture is not so simple as one might hope. Two kinds of particles, protons and neutrons, make up all atomic nuclei; one other kind, the electron, makes up the outer structure and completes the atom as chemists know it. There is the ephemeral positron, closely related to the electron, and there is the photon, which carries ­electromagnetic energy. There are, in addition, possibly the elusive neutrino and the welterweight meson.

… The amazing feat of [Paul] Dirac in predicting the positron before its discovery, and the possibly successful prophecies of Fermi and Yukawa concerning the neutrino and the meson, constitute a heartening victory for the modern quantum theory. Nevertheless it is dangerous to become overconfident in the infallibility of purely theoretical analysis and so to neglect experimental science. Many other predictions made during the past ten years have been proved wrong. No theory, no matter how beautiful its equations, has any more value than Democritus’ original speculation until its predictions have been verified by the hard-boiled experimenters.