Ettore Majorana

Ettore Majorana (, ; born on 5 August 1906 – likely dying in or after 1959) was an Italian theoretical physicist who worked on neutrino masses.

Mathematically gifted, Majorana began his university studies in engineering in 1923, but switched to physics in 1928 at the urging of Emilio Segrè.

He was very young when he joined Enrico Fermi's team in Rome as one of the "Via Panisperna boys", who took their name from the street address of their laboratory.

Majorana's first papers dealt with problems in atomic spectroscopy.

His first paper, published in 1928, was written when he was an undergraduate and it was coauthored by Giovanni Gentile, Jr., a junior professor at the Institute of Physics in Rome.

This work was an early quantitative application to atomic spectroscopy of Fermi's statistical model of atomic structure (now known as the Thomas–Fermi model, due to its contemporaneous description by Llewellyn Thomas).

In this paper, Majorana and Gentile performed first-principles calculations within the context of this model that gave a good account of experimentally-observed core electron energies of gadolinium and uranium, and of the fine structure splitting of caesium lines observed in optical spectra.

Majorana earned his Laurea in physics at the University of Rome La Sapienza in 1929.

In 1931, Majorana published the first paper on the phenomenon of autoionization in atomic spectra, which he called "spontaneous ionization"; an independent paper in the same year, published by Allen Shenstone of Princeton University, called it "auto-ionization", a name first used by Pierre Auger.

This name, without the hyphen, has since become the conventional term for the phenomenon.

In 1932, he published a paper in the field of atomic spectroscopy concerning the behaviour of aligned atoms in time-varying magnetic fields.

This problem, also studied by I. I. Rabi and others, led to development of an important sub-branch of atomic physics, that of radio-frequency spectroscopy.

In the same year, Majorana published his paper on a relativistic theory of particles with arbitrary intrinsic momentum, in which he developed and applied infinite dimensional representations of the Lorentz group, and gave a theoretical basis for the mass spectrum of elementary particles.

Like most of Majorana's papers, written in Italian, it languished in relative obscurity for several decades.

Experiments in 1932 by Irène Joliot-Curie and Frédéric Joliot showed the existence of an unknown particle that they suggested was a gamma ray.

Majorana was the first to interpret correctly the experiment as requiring a new particle that had a neutral charge and a mass about the same as the proton; this particle is the neutron.

Fermi advised him to write an article on the topic, but Majorana did not.

James Chadwick proved the existence of the neutron by experiment later that year, and he was awarded the Nobel Prize for this discovery.

Majorana was known for not seeking credit for his discoveries, considering his work to be banal.

He wrote only nine papers in his lifetime.

"At Fermi's urging, Majorana left Italy early in 1933 on a grant from the National Research Council. In Leipzig, Germany, he met Werner Heisenberg. In letters he subsequently wrote to Heisenberg, Majorana revealed that he had found in him, not only a scientific colleague, but a warm personal friend."

The Nazis had come to power in Germany as Majorana arrived there.

He worked on a theory of the nucleus (published in German in 1933) which, in its treatment of exchange forces, represented a further development of Heisenberg's theory of the nucleus.

Majorana also travelled to Copenhagen that year, where he worked with Niels Bohr, another Nobel Prize winner, and a friend and mentor of Heisenberg.

"In the fall of 1933, Majorana returned to Rome in poor health, having developed acute gastritis in Germany and apparently suffering from nervous exhaustion. Put on a strict diet, he grew reclusive and became harsh in his dealings with his family. To his mother, with whom he had previously shared a warm relationship, he had written from Germany that he would not accompany her on their customary summer vacation by the sea. Appearing at the institute less frequently, he soon was scarcely leaving his home; the promising young physicist had become a hermit. For nearly four years he shut himself off from friends and stopped publishing."

During these years, in which he published few articles, Majorana wrote many small works on geophysics, electrical engineering, mathematics, and relativity.

These unpublished papers, preserved in Domus Galileiana in Pisa, have been edited by Erasmo Recami and Salvatore Esposito.

Majorana's last-published paper, in 1937, was an elaboration of a symmetrical theory of electrons and positrons.

He predicted that in the class of particles known as fermions, there should be particles that are their own antiparticles.

Solution of the Majorana equation yields those particles, now referred to as Majorana fermions.

There has been speculation that at least some part of the "missing mass" in the universe, which cannot be detected except by inference from its gravitational influence, may be composed of Majorana particles.

On 25 March 1938, he disappeared under mysterious circumstances after purchasing a ticket to travel by ship from Palermo to Naples.

The Majorana equation and Majorana fermions are named after him.

In 1938, Enrico Fermi was quoted as saying about Majorana: "There are several categories of scientists in the world; those of second or third rank do their best but never get very far. Then there is the first rank, those who make important discoveries, fundamental to scientific progress. But then there are the geniuses, like Galilei and Newton. Majorana was one of these."

Majorana was born in Catania, Sicily.

Majorana's uncle Quirino Majorana also was a physicist.

In 1938, at the age of 32, he became a full professor of theoretical physics at the University of Naples independently of the competition rules, without needing to take an examination because of his "high fame of singular expertise reached in the field of theoretical physics".

Majorana did prescient theoretical work on neutrino masses, an active subject of research.

Reportedly, Majorana had withdrawn all of his money from his bank account prior to making a trip from Naples to Palermo.

In 2006, the Majorana Prize was established in his memory.