ZELDOVICH, YAKOV BORISOVICH

(b. Minsk, Russia [later Belarus], 8 March 1914;

d. Moscow, USSR, December 2, 1987), theoretical physics, astrophysics, cosmology.

Zeldovich was one of the leading physicists of the twentieth century. He made fundamental contributions to the physics of chemical catalysis and kinetics, physics of combustion and hydrodynamics of explosive phenomena, nuclear energy physics, astrophysics and cosmology, and physics of elementary particles.

Early Work. The son of Boris Naumovich Zeldovich, a lawyer, and the former Anna Petrovna Kiveliovich, a translator, Yakov Zeldovich spent his younger years in Petrograd (later named Leningrad, USSR), entered elementary school in 1924 directly into the third grade, and graduated from high school in 1930. Immediately after that he enrolled in a training program for laboratory assistants run by the Leningrad Institute of Mechanical Processing of Mineral Resources. In 1931 Zeldovich began his work as a laboratory assistant at the Institute of Chemical Physics (ICP) of the Academy of Sciences of the USSR. Zeldovich came to the ICP in 1931 right after high school and worked there until 1948, many years after becoming a Doctor of Sciences. He studied adsorption, catalysis, phase transitions, hydrodynamics, theory of combustion and detonation with application to rocket ballistics, and nuclear chain reactions.

Remarkably, he started his scientific research work without having any college degree, graduate or undergraduate. The theorists of the ICP spotted the very talented youth right away and helped him to comprehend the foundations of theoretical physics, outside the formal setting of a college classroom. Thus Zeldovich received an undergraduate education without formally enrolling in an undergraduate college program. (He did attend some classes at Leningrad University from 1932 to 1934, but did not graduate.) In 1936 Zeldovich proceeded straight to receiving his PhD in physics and mathematics, and three years later he obtained the highest scientific degree of the USSR—Doctor of Sciences (equivalent to the German Habilitation)—in physics and mathematics.

During the period from 1939 to 1943, Zeldovich wrote several papers on the mechanism of fission of heavy nuclei and especially on a theory of the chain fission reaction of uranium. Especially important were four of these papers, written in collaboration with Yuli B. Khariton (1904–1996). All the papers of this series form the foundation of the modern physics of nuclear reactors and nuclear power. Zeldovich was a theoretical physicist; however, he was equally at home in laboratory discussions of experimental techniques or in technology-oriented studies of shock waves and explosive phenomena. As a specialist in combustion and detonation, he was involved in the efforts of assuring the survival of the USSR as a nation in the Second World War. His role in the creation of new weapons for the country was invaluable. He worked on the burning of solid fuel in the special legendary Katyusha launchers, and on other projects with important technological and defense applications. From 1943 to 1963 Zeldovich participated in the development and building of atomic (and later hydrogen) weapons.

His contributions to science and technology in the USSR during the war and immediately after it were noticed by the government: He became one of the most decorated of Soviet scientists. His awards included three Gold Stars, the Lenin Prize, four State Prizes, and several orders.

In addition to his work in other fields of physics, from 1947 to 1963 Zeldovich kept working in the area of nuclear physics and the physics of elementary particles. In the early 1960s Zeldovich began turning his attention to astrophysics and cosmology. He devoted almost the last twenty-five years of his life (1963–1987) to these branches of science.

Chemical Physics and Hydrodynamics. Zeldovich began his scientific work in the Catalysis Laboratory of the ICP. His first scientific research was devoted to adsorption and catalysis. Some of the ideas in his publications were far ahead of their time. Only many years later were analogous ideas rediscovered and used. Such rediscoveries involved his ideas of absorption and catalysis, among others.

In the mid-1930s Zeldovich began his research in hydrodynamics, heat transfer, and turbulence. He introduced the notion of the rate of decay of temperature inhomogeneities, which plays the same role in the distribution of temperature in matter as does the rate of energy dissipation in the velocity field in a viscous fluid. Twelve years later Aleksandr Mikhailovch Obukhov independently introduced this quantity as a governing characteristic of the local temperature field in developed turbulent flows. Obukhov’s theory proved to be of exceptional practical importance because temperature fluctuations determine the dispersion of light in the atmosphere. Another important result in this period is the similarity laws in the development of ascending convective. These ideas were later developed by other authors; in the early twenty-first century, they were widely used by geophysicists in studies of atmospheric and oceanic convection.

Starting in the mid-1930s, Zeldovich turned his attention to hydrodynamics and thermal processes in shock waves, and to magnetohydrodynamics (magnetic field generation in the motion of a conducting fluid). In 1942 Zeldovich published a fundamental theoretical study which played an enormous role in the development of physical and chemical kinetics. This paper was devoted to calculations of the rate of formation and subsequent growth of vapor bubbles in a fluid which is in a metastable (superheated) state. It turned out that the technique developed in this paper may be transferred almost without change to a large number of kinetic problems in which the slow decay of nonequilibrium states is considered.

Zeldovich was one of the founders of the Soviet school of specialists on combustion and detonation. This school received worldwide recognition. His own studies in this field were diverse and multidirectional. They included many purely theoretical aspects and also such applied topics as combustion of gases and solid rocket fuels, condensed liquid explosives and powders, combustion of premixed fuel compounds, and diffusive combustion. In particular, together with David Al’bertovich Frank-Kamenetskii (1910–1970), Zeldovich found the structure of a laminar flame: the authors separated the flame into a narrow zone of chemical transformation situated near the region of maximum temperature, and a wider zone of heating. In this last zone, the chemical reactions can be ignored. They also found the temperature and concentration distributions in the flame. Finally they found the simple analytical formula for the velocity of flame propagation. This formula is known as the Zeldovich-Frank-Kamenetskii formula. The simple theory of flame propagation was generalized by Zeldovich and other scientists to complex chemical transformations with chain reactions, with many stages and many separate reaction zones.

Particles and Nuclei. . In 1939 Zeldovich began his very fruitful studies of nuclear fission of uranium. Four important papers of this series were written in collaboration with Khariton. It was shown in the first two papers (published in 1939 and 1940) that chain fission reactions by fast neutrons in pure metallic natural uranium, as well as self-sustained chain reactions in a homogeneous natural uranium light water system, are impossible. The third paper (1940) was devoted to the role of the delayed neutrons in a chain reaction and the need to account for them. The theory of nuclear reactor control uses the significant conclusions outlined in this paper. The fourth paper (1941) deals with the problem of the critical size of a sample of uranium-235 in the fission of nuclei by fast neutrons. It was shown that for the self-sustained chain fission reaction in a sample of uranium-235 surrounded by a neutron reflector, the critical mass is about ten kilograms. The theory developed in the paper allows calculations of the critical mass of uranium-235 dissolved in light water. This and other works by Zeldovich had very important practical applications.

At the same time, this was the beginning of his work in the theory of nuclei and in the physics of elementary particles. One of the most significant contributions of Zeldovich to the theory of nuclei was the indication of the possibility of existence of nuclei at the threshold of stability, due to the large numbers of neutrons. In 1960 Zeldovich predicted the existence of a new helium isotope, helium-8, with an excessive number of neutrons. This helium-8 isotope was soon discovered experimentally.

In the early 1950s Zeldovich began his research into the theory of elementary particles. The most important contribution to this field was one idea proposed by Zeldovich in 1955, in collaboration with Simon Solomonovich Gershtein (b. 1929). The idea consisted of the following statement: if there is a so-called vector current in β-decay, then it can be made a conserved one. Subsequent experiments confirmed this prediction. In the paper published in 1955, Zeldovich and G. M. Gandelman pointed out that the very precise measurement of the magnetic moment of an electron can be considered as a method for determining the limits of applicability of quantum electrodynamics at small distances. After that, the idea was developed and became the classical method for finding the limits of applicability of quantum electrodynamics.

In 1966 Zeldovich and Gershtein estimated the upper limit for the rest mass of a muonic neutrino. This limit is of the order of 100 eV. It was obtained from cosmological considerations: if the rest mass is greater than this limit, then the total mass of all relic neutrinos which survived after the big bang would be more than the mass of all forms of matter in the universe, which is incompatible with the astronomical observations. This and Zeldovich’s other works from that period marked the beginning of the modern physics of cosmological elementary particles. Also Zeldovich’s pioneering ideas in the theory of the physics of vacuum were very important for the development of modern cosmology.

Astrophysics and Cosmology. . During the last twenty-five years of Zeldovich’s life, his main scientific interests were astrophysics and cosmology. Zeldovich himself emphasized that his first scientific love was the theory of combustion. Astrophysics and cosmology were his last loves. His work in these new fields was very successful. The beginning of the time when Zeldovich concentrated on astrophysics and cosmology corresponded to the time of the second revolution in astrophysics, the epoch of Sturm und Drang, when the habitual fixed conceptions were broken and replaced by new ones. Many wonderful discoveries were made at that period, and many revolutionary ideas were proposed. Zeldovich himself participated in this revolution very actively; he was one of its leaders and creators.

Astronomy became an “all-wavelength” science, which meant that astronomers started to obtain information from all spectral bands of electromagnetic radiation from the universe: radio, infrared, optical, ultraviolet, and high energy. Astronomers began using satellites and spacecraft, new technology, and fast computers. It became clear that the observable universe was born in the grand quantum explosion (big bang) that occurred about 13.5 billion years ago. The Cosmic Microwave Background (CMB) electromagnetic radiation had been discovered. This radiation was born in the hot epoch of the early universe. The radiation cooled down because of the expansion of the universe, to its current temperature of 2.7 K. Observations of CMB are able to probe directly into the very early universe. Very unusual celestial objects were discovered during the epoch of the second astronomical revolution: radio stars, radio galaxies, quasars, infrared sources, radio pulsars, x-ray sources, and others. The question “What is this?” became central for theoreticians. Zeldovich played a leading role in constructing proper models in attempts to understand the nature of various objects. He was one of the founders of relativistic astrophysics, which addresses phenomena where relativistic effects in gravitational fields are crucial, and the release of energy in stormy processes is often enormous.

Zeldovich studied the properties of black holes in detail, including the amazing physical processes that occur in their vicinity. These unusual celestial bodies were predicted by the general theory of relativity. They emerge when the mass of an object is compressed so much that the gravitational field becomes enormous. Light itself cannot escape from this region, nor can anything else. Zeldovich and his collaborators demonstrated that the compression of a (nonrotating), nonsymmetrical body produces a black hole, which very rapidly becomes perfectly symmetrical. Any deviation from sphericity in the gravitational field must be radiated outward (by gravitational waves) during the formation of the black hole. This radiation creates gravitational waves. The emerging boundary of a black hole, the event horizon, is spherical and only spherical. By contrast, the collapse of a rotating mass leads to a rotating black hole. Thus the gravitational field of a (uncharged) black hole is completely determined by its mass and angular momentum. This seminal work had enormous influence on experts within the field. Another important contribution to black hole physics was Zeldovich’s theoretical discovery of the possibility of quantum radiation of waves (for example, electromagnetic ones) by a rotating black hole. This work stimulated many subsequent studies.

In 1964 Zeldovich put forward the idea of observing black holes using the radiation from gas moving in their gravitational fields. Independently, an analogous idea was proposed by Edwin E. Salpeter (b. 1924). In 1964, just after quasars had been discovered, Zeldovich and his coworkers conjectured that the main engine of a quasar can be a supermassive (millions and billions of solar masses) black hole with gas accretion onto it. Quasars, located at the centers of big galaxies, are among the most powerful sources of energy in the contemporary universe. On the basis of data from one of the quasars, 3C273, the authors gave the first estimates of the mass of the central black hole: a hundred million solar masses.

In 1966, Zeldovich, and his collaborators, predicted that black holes (as well as neutron stars) could act as extremely powerful sources of x-radiation because of the physical processes in their vicinity. This situation occurs if there is an ordinary (normal) star quite close to the black hole in a binary system, and the matter from the upper layers of the normal star comes to the very vicinity of the black hole. The x-ray emission of such matter, which heats up, makes the black hole visible. The first black holes of stellar mass were discovered a few years later as x-ray sources in binary stellar systems.

Also in 1966, Zeldovich and his coworkers predicted the possibility of the generation of primordial small black holes in the early stages of the evolution of the universe. The same hypothesis was independently proposed by English physicist Stephen Hawking (b. 1942).

Despite his having made many discoveries in other areas of astrophysics, Zeldovich’s major field of study in astrophysics was cosmology. In the 1940s George Gamow and his coauthors had hypothesized that the beginning of the evolution of the universe was hot. At the beginning of his work in cosmology in the 1960s Zeldovich proposed an alternative to the hot model, called the cold model. He always supported the search for observational methods of testing cosmological models. In 1964 collaborators of Zeldovich showed that weak Cosmic Microwave Background electromagnetic radiation (a relic of the hot universe) could be observed in the centimeter and millimeter region of the spectrum. This radiation was discovered in 1965 by Arno Penzias (b. 1933) and Robert Wilson (b. 1936). Zeldovich was one of the first who proposed to use this CMB radiation as a powerful tool for the investigation of the evolution of the universe. In 1968 Zeldovich, with coworkers, solved the problem of hydrogen recombination in the course of the expansion of the hot universe. During this process, the hot plasma of the universe was converted into neutral gas. After this epoch, the formation of the first celestial bodies from initially small fluctuations of matter density in space began. It was the beginning of the formation of galaxies and their clusters, the formation of the large-scale structure of the modern universe. In a series of publications, Zeldovich and his collaborators showed what types of CMB spectrum distortions had been formed at various stages in the evolution of the universe, and specified different types of physical processes that could be responsible for these distortions, among which were the decay of unstable particles, dissipation of rotational and irrotational motions of matter, quantum evaporation of primordial black holes of small masses, and matter/antimatter annihilation.

The study of the formation of the large-scale structure of the universe from the initially small fluctuations of the matter in space is probably Zeldovich’s most important achievement of in cosmology. In 1970 he proposed a so-called pancake theory. Before his work was published, cosmologists commonly accepted the picture of the quasispherical compression of the protoclusters of galaxies during the last stages of their formation under the action of the self-gravitational forces. Zeldovich showed that, under some conditions, this picture was highly improbable. He proved that compression along only one direction must dominate. As a result flat structures, pancakes, must form.

In the subsequent publications with his collaborators, Zeldovich indicated that pancake formation must lead to shock waves. The gas density must increase in cooling zones, which leads to the formation of separate galaxies. The theory predicted the formation of the characteristic cell structure with gigantic voids, which are free from galaxies. This structure with voids has been discovered in observations, and some details of the theory have been confirmed by numerical simulations.

Another important work was published by Zeldovich with astrophysicist Rashid Alievich Sunyaev (b. 1943) in 1972. They showed that relatively cold photons of CMB after scattering by the hot electrons in the intergalactic gas of some galactic clusters must increase their energy and be transmitted into another spectral band. This leads to a decrease in the number of photons of CMB in the typical radio band of the spectrum. As a result of this scattering, the intensity of the CMB radiation in this spectral band must decrease. This theory predicts that the intensity of the CMB radiation in the direction of the galaxy clusters with hot intergalactic gas must decrease. This phenomenon was later observed and named the Zeldovich-Sunyaev effect. This effect, together with observations of hot gas in the cluster of galaxies, allows one to determine the absolute linear size of the galaxy cluster. The analogous effect enables one to find the peculiar velocity of the cluster with respect to the CMB radiation. These phenomena became powerful tools of observational cosmology.

Zeldovich and his collaborators, and later his followers, predicted and analyzed the tiny variations (anisotropy) of the intensity of the CMB radiation at different directions in the sky, which reflect the tiny fluctuations of the primordial hot matter distribution in space in the past epoch before galaxy formation. This anisotropy contains information about the physics of the early universe, as well as information about the parameters of the modern universe, including information about mysterious invisible “dark energy” and “dark matter”—which (as became clear at the end of the 1990s) is what most of the universe is made of. The CMB anisotropy was discovered in the 1990s, and later on many ground-based telescopes, balloons, and space missions performed such observations.

In his last years Zeldovich published on the problem of the birth of the universe and its early evolution. He emphasized the role of quantum phenomena in enormous dynamic gravitational fields at the very beginning of the universe. He was interested in the physics of the so-called L-term in the Einstein equations of general relativity. Later called the problem of dark energy, this exotic form of energy corresponds to a repulsive cosmic force acting at very long distances. At the very beginning of the twenty-first century the problem of dark energy became one of the most important and mysterious problems in physics and cosmology.

Zeldovich’s works inspired new projects not only among theoreticians but also among experimental physicists and observational astronomers. Many of his predictions have been confirmed, and thus played a leading role in the development of science. Zeldovich was also an outstanding teacher. He was a professor at the Institute of Engineering Physics in the 1940s and a professor at Moscow State University from 1965 to 1987; he was a founder of a school of world-famous physicists and astrophysicists, and an author of widely read texts at all levels. Zeldovich was elected a corresponding member of the Academy of Sciences of the USSR in 1946 and was elected a full member of the academy in 1958. He was also a member of many other academies and scientific societies.

It should also be mentioned that during almost all periods of his life, he was not able to travel beyond the Iron Curtain, and his personal contacts with foreign scientists were very restricted. This created additional difficulties for him in his scientific work. Such obstacles make his outstanding achievements in many different branches of science even more remarkable. It is hard to believe that a single person did all this research. The famous British physicist Stephen Hawking wrote to Zeldovich after first meeting with him: “Now I know that you are a real person and not a group of scientists like the Bourbaki.”

BIBLIOGRAPHY

A detailed scientific bibliography of Zeldovich is in his Selected Works, vol. 1, number 9 (below).

WORKS BY ZELDOVICH

With Aleksandr Solomonovich Kompaneetz. The Theory of Detonation. New York: Academic Press, 1960.

With M. A. Rivin and David Al’bertovich Frank-Kamenetskii. Impul’s reaktivnoi sily porokhovykh raket. Moscow, 1963. Translated as Impulse of Reactive Force of Solid Propellant Rockets. Ohio: Wright-Patterson Air Force Base, 1966.

With Yurii Petrovich Raizer. Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena. Edited by Wallace D. Hayes and Ronald F. Probstein. Translated by Scripta Technica. New York: Academic Press, 1966.

With Yurii Petrovich Raizer. Elements of Gasdynamics and the Classical Theory of Shock Waves. New York: Academic Press, 1968.

With Igor Dmitrievich Novikov. Relativistic Astrophysics, vol. 1, Stars and Relativity, edited by Kip S. Thorne and W. David Arnett. Translated by Eli Arlock. Chicago: University of Chicago Press, 1971.

With Igor Dmitrievich Novikov. Relativistic Astrophysics, vol. 2, The Structure and Evolution of the Universe, edited by Gary Steigman. Translated by Leslie Fishbone. Chicago: University of Chicago Press, 1983.

With Aleksandr Andreevich Ruzmaikin and D. D. Sokolov. Magnetic Fields in Astrophysics. New York: Gordon and Breach, 1983.

With Grigory Barenblatt, V. B. Librovich, and G. M. Makhviladze. The Mathematical Theory of Combustion and Explosions. Translated by Donald H. McNeill. New York: Plenum, 1985.

Selected Works of Yakov Borisovich Zeldovich, vol. 1: Chemical Physics and Hydrodynamics, edited by J. P. Ostriker, G. I. Barenblatt, and Rashid Alievich Sunyaev. Translated by A. Granik and E. Jackson. Princeton, NJ: Princeton University Press, 1992. This volume and the next include his most essential works.

Selected Works of Yakov Borisovich Zeldovich, vol. 2, Particles, Nuclei, and the Universe, edited by J. P. Ostriker, G. I. Barenblatt, and Rashid Alievich Sunyaev. Translated by A. Granik and E. Jackson. Princeton, NJ: Princeton University Press, 1993.

OTHER SOURCE

Sunyaev, Rashid Alievich, ed. Zeldovich: Reminiscences. Boca Raton, FL: CRC Press, 2004.

Igor Novikov