# Einstein's Nobel heritage

Einstein's theories of relativity are the foundation for much of modern physics - small wonder that there is a sizeable number of Nobel prizes related to relativity. Here's a list with brief descriptions of the most important ones:

## 1921 - Albert Einstein

Ironically, while relativity has led to so many Nobel prizes, it only played a minor role in Einstein's own. To be sure, it is prominently featured in the laudatio by Svante Arrhenius, however, the Nobel committee's brief prize announcement is more vague, referring to Einstein's "services to Theoretical Physics" with explicit mention given only to his finding the law of the photoelectric effect.

## 1933 - Paul Dirac (jointly with Erwin Schrödinger)

Dirac's prize was the first of many given for work on the connection between special relativity and quantum theory. He was the pioneer of *relativistic quantum mechanics*, formulating what is nowadays called the Dirac equation, the first equation for the quantum behaviour of relativistic matter particles. Using his equation, he discovered a fundamental relativistic quantum phenomenon: the fact that, for every species of relativistic particle, there must be a kind of mirror image, a species of corresponding antiparticles. In a world in which electrons exist, which carry negative electric charge, Dirac's equation demands the existence of anti-electrons, particles with the same mass as electrons, but a *positive* electric charge.

## 1936 - Carl D. Anderson (jointly with Victor F. Hess)

What, at first sight, appeared to be a stumbling stone for Dirac's theory - where were those anti-electrons he postulated? - later turned into a triumph. Among the particles of cosmic rays, a highly energetic particle radiation reaching the earth's surface from space, Carl Anderson discovered traces of anti-electrons. Diracs anti-particles, with the same mass as electrons but the opposite electric charge, really do exist! Today, antiparticles are a basic feature of all models of particle physics, and anti-electrons are now commonly called *positrons*.

## 1949 - Hideki Yukawa

The force that bonds protons and neutrons together to form atomic nuclei has a strictly limited range: while it keeps the nucleus stable, even a neutron flying by outside, a trillionth of a metre distant, is out of range and will not feel any influence. At the time Yukawa thought about this strange situation, physicists already knew of carrier particles and their role concerning elementary forces: forces are transmitted by particles. For instance, on a quantum level, the electric repulsion between two two electrons is explained by the exchange of photons flitting back and forth. The emission and absorption of these photons by the electrons is the way that the influence is transmitted from one electron to the other. Yukawa found an explanation for the short-range nuclear force that is directly linked to the fact that the carrier particle in question has a non-zero (rest) mass. He was able to derive this directly from a relativistic quantum equation for massive particles called the Klein-Gordon-equation.

## 1951 - John Cockcroft and Ernest T. S. Walton

Cockcroft and Walton bombarded atomic nuclei of the element Lithium with fast protons, thus creating helium nuclei in the first controlled transmutation of one species of nucleus to another. Summing up the energies before and after the transmutation, they managed to test directly the equivalence of mass and energy postulated by Einstein: the helium nuclei that result have a slightly lower mass than that of proton and lithium nucleus combined, and this difference in mass leads to a kinetic energy of the resulting nuclei that is higher than expected by non-relativistic physics, exactly following Einstein's prediction.

## 1955 - Willis Eugene Lamb and Polykarp Kusch

Lamb and Kusch performed precision measurements, establishing the reality of two effects that ordinary relativistic quantum theory à la Dirac cannot explained: what's now called the Lamb shift and a deviation of the electron's magnetic properties from Dirac's prediction. These measurements contributed to the eventual development of relativistic quantum feld theories, concretely: of quantum electrodynamics, the relativistic quantum theory of the electromagnetic field.

## 1959 - Emilio Segrè and Owen Chamberlain

In relativistic quantum theories, for every species of particle, there is a species of antiparticles. Segrè and Chamberlain received their prize for the discovery of anti-protons, the antiparticles of protons, one of the two species of particle atomic nuclei are made of.

## 1963 - Eugene Wigner (jointly with Maria Goeppert-Mayer and J. Hans D. Jensen)

At the heart of special relativity is the relativity principle, in brief: observers that are in motion relative to each other are nevertheless on an equal footing; the physical laws are exactly the same for each of them. In physics, such equality is called a symmetry. Whether or not a physical theory, be it a model of electromagnetic phenomena, fluid dynamics or a theory of heat, is consistent with the relativity principle can be examined in a general framework that analyzes the theory's symmetries. Wigner was the first to apply this framework to quantum theory, laying the foundation of modern relativistic quantum feld theories.

## 1965 - Shin-Itiro Tomonaga, Julian Schwinger, Richard P. Feynman

The development from earlier relativistic quantum mechanics to relativistic quantum field theories has already been mentioned. In these quantum field theories, not only the matter particles, but also the forces acting between them follow quantum laws. The distinction between matter and forces becomes blurred: The action of a force is represented by the exchange of particles, the corresponding carrier particles. Tomonaga, Schwinger and Feynman were the first to formulate such a theory of relativistic quantum forces for the simplest case, that of the electromagnetic force, creating what is known as quantum electrodynamics. This was the starting point leading to the formulation of the more general quantum field theories of the standard model of particle physics and to more relativistic Nobel prizes which are not included in this list as they do not add any fundamentally new cross-links with relativity.

## 1974 - Antony Hewish (jointly with Martin Ryle)

The discovery that won Hewish his prize, although not a consequence of relativity, is nonetheless an important step for relativistic astrophysics. Together with his graduate student Jocelyn Bell-Burnell, Hewish discovered the first pulsar, opening up the field of observational astronomy of neutron stars.

## 1978 - Arno Penzias and Robert Wilson (jointly with Pjotr Leonidovich Kapitsa)

Penzias and Wilson won their Nobel prize for the first detection of the cosmic background radiation, an afterglow from the early, hot days of the universe. With their discovery, they confirmed a prediction made by Ralph Alpher and Robert Herman in 1948 on the basis of the relativistic big bang models.

## 1983 - Subramanyan Chandrasekhar and William A. Fowler

Chandrasekhars work on the stability of White Dwarfs, the final states of low-mass stars, was the beginning of a journey that would lead scientists to stellar black holes. The Chandrasekhar mass named after him is the maximal mass for which the inner pressure of the White Dwarf can resist further collaps. For remnants with higher mass, the collapse continues, forming a neutron star or even a black hole.

Fowler won the prize for his research on the origin of the chemical elements in the universe. Part of that work concerned another prediction of the big bang models of relativistic cosmology, namely that of the formation of light elements in the early universe.

## 1993 - Russell A. Hulse and Joseph H. Taylor

Hulse and Taylor discovered the first binary pulsar: a binary in which a pulsar and a companion star orbit each other. Their observations of this pulsar, called PSR1913+16, led to the first indirect detection of gravitational waves.

## 2002 - Riccardo Giacconi (jointly with Raymond Davis Jr. and Masatoshi Koshiba)

Giacconi won the prize for his pioneering work in X-ray astronomy, in part for the first detection of objects that, to the best of our knowledge, are black holes.

## 2006 - John C. Mather and George F. Smoot

Mather and Smoot received their prize for their contributions to the COBE satellite mission, in particular for precise measurements of the black body nature of the cosmic background radiation (confirming an important prediction of the big bang models) and for detecting the tiny fluctuations in the background radiation which are the first seeds for the large scale structure we can observe in the universe today.

## Further Information

For background information on Einstein's theories of relativity and their numerous consequences, check out Elementary Einstein.