N4. Pauli Letter Proposing What Came To Be Called The Neutrino

By the late 1920s, physicists had been conditioned by the successes of relativity and quantum mechanics to expect the unexpected. Some were willing to abandon the law of energy conservation in the nuclear domain. Wolfgang Pauli in 1930 made a different suggestion that was at once conservative and bold. In order to “save” the law of energy conservation, he postulated an entirely new particle, uncharged and until then unseen, which might exist within nuclei and be emitted along with an electron in beta decay. The letter in which he made this suggestion is a document of considerable interest. It reveals much about the approach of a creative twentieth-century physicist to problems of nature. Pauli’s letter¹ is as follows.

Open letter to the radioactive group at the regional meeting in Tübingen.

Physical Institute of the Eidgenossischen Zürich

Technischen Hochschule December 4, 1930

Zürich Gloria St.

Dear Radioactive Ladies and Gentlemen:

I beg you to receive graciously the bearer of this letter who will report to you in detail how I have hit on a desperate way to escape from the problems of the “wrong” statistics of the N and Li⁶ nuclei and of the continuous beta spectrum in order to save the “even-odd” rule of statistics and the law of energy conservation. Namely the possibility that electrically neutral particles, which I would like to call neutrons, might exist inside nuclei; these would have spin ½, would obey the exclusion principle, and would in addition differ from photons through the fact that they would not travel at the speed of light. The mass of the neutron ought to be about the same order of magnitude as the electron mass, and in any case could not be greater than 0.01 proton masses. The continuous beta spectrum would then become understandable by assuming that in beta decay a neutron is always emitted along with the electron, in such a way that the sum of the energies of the neutron and electron is a constant.

Now, the question is, what forces act on the neutron? The most likely model for the neutron seems to me, on wave mechanical grounds, to be the assumption that the motionless neutron is a magnetic dipole with a certain magnetic moment μ (the bearer of this letter can supply details). The experiments demand that the ionizing power of such a neutron cannot exceed that of a gamma ray, and therefore μ probably cannot be greater than e(10^–13 cm).

At the moment I do not dare to publish anything about this idea, so
I first turn trustingly to you, dear radioactive friends, with the question: how could such a neutron be experimentally identified if it possessed about the same penetrating power as a gamma ray or perhaps 10 times greater penetrating power?

I admit that my way out might look rather improbable at first since if the neutron existed it should have been seen long ago. But nothing ventured, nothing gained. The gravity of the situation with the continuous beta spectrum was illuminated by a remark of my distinguished predecessor in office, Mr. Debye, who recently said to me in Brussels, “Oh, that’s a problem like the new taxes; one had best not think about it at all.” So one ought to discuss seriously any way that may lead to salvation. Well, dear radioactive friends, weigh it and pass sentence! Unfortunately, I cannot appear personally in Tübingen, for I cannot get away from Zürich on account of a ball which is held here on the night of December 6-7. With best regards to you and to Mr. Baek,

Your most obedient servant, W. Pauli

Despite his smokescreen of banter, Pauli undoubtedly meant this letter to be taken seriously. With unerring instinct, he pinpointed two of the most important puzzles of nuclear physics at the time—the apparent nonconservation of energy in beta decay and the seemingly wrong spin of certain nuclei—and pointed out that a new lightweight neutral particle might resolve both puzzles. At the same time, he recognized the speculative nature of his suggestion, and chose this informal method of publicizing it.

It was just two years later, in 1932, that an actual neutron (not Pauli’s hypothesized particle) was discovered. Two years after that, in 1934, Enrico Fermi named Pauli’s particle the neutrino (little neutron) and developed a theory for its creation and destruction that formed the basis for all subsequent theories of fundamental particles. Not until 1956 did Frederick Reines and Clyde Cowan detect the neutrino for the first time (although by then no one doubted its existence). Pauli was right: Observing the particle was not easy. For many years the neutrino was assumed to be, like the photon and graviton, massless. By 2000, physicists were seeing evidence that it might have mass, albeit much less than the mass of an electron. Now we know that there are (at least) three kinds of neutrinos (or three “flavors”) with masses still not measured but believed to be less than one millionth the mass of an electron (and definitely not zero).

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