###### Generally considered one of the most influential physicists in history, **Albert Einstein’s** (1879–1955) groundbreaking theories reshaped the scientific community’s view and understanding of the universe. He developed the special and general theories of relativity and won the Nobel Prize for Physics in 1921 for his explanation of the photoelectric effect.

**Ezra Newman** is professor emeritus at the University of Pittsburgh. He was a prominent contributor to the golden age of general relativity (roughly 1960-1975). In 1962, together with Roger Penrose, he introduced the powerful Newman/Penrose formalism for working with spinorial quantities in general relativity. In 1963, Newman and two coworkers discovered the NUT vacuum, an exact vacuum solution to the Einstein field equation which has become a famous counterexample to everything. In 1965, he discovered the Kerr/Newman electrovacuum, one of the best known of all exact solutions. Newman has continued to make important contributions. Some of his most interesting recent work has involved the problem of reconstructing the gravitational field within some region from observations of how optical images are lensed as light rays pass through the region.

**Simply Charly: You’re principally known for your work in general relativity. Can you begin by telling us what general relativity is?**

Ezra Newman: There is both a short, correct, and easy answer, which leaves out the essence of a good answer, and a longer, more difficult answer that gets to the core of what the theory is about.

At the simple level, one could say general relativity is a theory of gravity that supersedes Newton’s Theory of Gravity and that it makes slightly more accurate predictions than Newton’s theory did. It is a correct answer and gives some hint of the subject.

At the deeper level, general relativity is a revolution of physical thought. It is about the geometry of our world. Something like 22 centuries ago, Euclid organized the available knowledge into a book called *Elements*. For 20 centuries, it was believed that it was both the geometry of our world and, also, that it was the only possible geometry. Immanuel Kant said, in essence, that “God geometrizes according to Euclid’s ‘Elements’.” Then, in the early 1800s, with the intellectual awakening throughout Europe happening virtually simultaneously and independently, three mathematicians—Gauss in Germany, Bolyai in Hungary, and Lobachevsky in Russia—realized that other geometries existed. It seems as if Gauss and his student Riemann were among the first to raise the question of what was the geometry of our world. A variety of scientists, among them the great Hermann von Helmholtz, tried to tackle this issue. The mathematician Hermann Minkowski, in his fundamental work on Einstein’s Special Theory of Relativity, came close to realizing the proper framework for exploring the issue of the geometry of our world—but he died prematurely before doing anything more. It was Einstein, who, after struggling for almost ten years with the problem, finally developed the General Theory of Relativity—the theory of the geometry of our world. This geometry was not a fixed geometry, as was Euclid’s, but varied depending on the matter content of the surrounding space. As a by-product, though that was the issue that provoked his investigation, the theory did produce an explanation of the phenomena of gravity; gravitational forces were the manifestations of the curvature of space and time. In addition, the theory produced a picture of the universe in the large, i.e., it opened the subject of cosmology to rigorous scientific investigation.

**SC: Ever since Einstein revealed his theory of general relativity in 1915, it has had a remarkable run as the most widely accepted theory of gravitation. Although Einstein was certain that his theoretical principles were correct, he proposed a few experimental tests to confirm his predictions. Can you briefly describe these tests?**

EN: There were three classical tests of general relativity.

1. According to Newton’s theory of gravity, planets moved around the sun in orbits that are ellipses. The point on the ellipse that is closest to the sun is referred to as the perihelion.

It had been observed by astronomers in the mid-1800s that the perihelion of the planet Mercury was not fixed (as it should be by Newton’s theory) but moved forward, i.e., it advanced with each circuit of the sun—albeit by a tiny amount. There was thus a disparity between observation and theory. Einstein’s theory predicted this small effect, known as the advance of the perihelion of Mercury. The measured perihelion advance agreed with the theory.

2. In the early 1800s, it was shown, via Newton’s theory, that the gravitational effects of the sun would bend star-light that passed close to the limb of the sun. Einstein’s theory also made the same prediction, but it differed by predicting that the effect would be twice as much as that of Newton’s theory. In 1919, in several expeditions, all timed to coincide with a solar eclipse (needed to block out the direct solar sunlight), this prediction was confirmed (with questions raised of its accuracy). Since then, it has been reconfirmed with extreme accuracy. This particular confirmation made Einstein a household name. The New York Times carried the story on the front page.

3. The third of the classical tests concerned a prediction of general relativity that time would evolve at different rates in regions of strong gravitational fields. This is a difficult effect to measure directly, but a simple indirect test was available. The color of the light emitted by atoms in a strong gravitational field would differ from that of the light from the same type of atoms in a weaker gravitational field. This was first observed by a shift in the spectral lines (the specific colors emitted) from the dwarf star Sirius B, becoming the so-called «gravitational redshift» effect. It has since been reconfirmed by very accurate laboratory experiments.

**SC: With the advancement of technology, our tools have become better at measuring and predicting with greater accuracy. What modern-day tests has general relativity recently faced?**

EN: There are a variety of modern tests; they range from great technical improvements in the classical tests to the development of a major astronomical tool for exploring the universe in the large. Some of the classical solar system tests have been implemented for the dynamics of double star systems, the binary pulsar for example. This later case has been used to confirm the existence of gravitational radiation via the change in the pulsar orbital parameters that is predicted by the theory.

The classical test of the bending of light near the sun has been extended to the bending of light near other astronomical bodies and, in fact, is no longer considered as a test. It has become a tool itself to study astronomical parameters, as it is used to measure the mass of astronomical objects.

Another very large and important subject, not yet completed, is the construction of detectors to measure gravitational radiation. They involve huge government installations costing millions of dollars—and again they will not be used primarily for testing the theory but rather as a new window for observing our universe. The theory itself has essentially gone into the mainstream and is now being used as another probe, in the same way, light-waves are used to gather information.

The default picture of the universe with its big bang and Hubble-expansion has been a prediction from the theory of relativity. Observations and existence of the Cosmic Microwave background are thus further confirmations. The theory, with the associated observations, is a tool for understanding the details of the big bang cosmology.

**SC: Einstein was often confounded by ideas that other researchers derived from his theories which, in some cases, led him to revise his work. In one particular case, Einstein introduced what he called a cosmological constant to fix what appeared to be a flaw in general relativity. Can you describe the circumstances of this problem?**

EN: Early in the history of general relativity, Einstein understood that the theory could be applied to the large-scale structure of the universe, i.e., to cosmology. However, he could not construct a static universe (which was believed at the time to be the type of our universe) from the theory. He modified the theory by adding into the equations a new term known as the cosmological constant. Shortly after that, it was discovered that our universe was not static but was expanding, and the cosmological constant was not needed. Einstein called the addition of the cosmological constant to be “the biggest blunder of his life.” He dropped it from the theory.

More recent observations of the distance relations with supernova and the cosmological redshift has strongly indicated that the universe is not only expanding but also is accelerating. This observational result is now most simply explained by the existence of the cosmological constant. The cosmological constant has thus been added to the standard model of cosmology.

**SC: The incompatibility of Newtonian gravity and special relativity led Einstein to General Relativity. In what ways is General Relativity being reconciled with quantum mechanics?**

EN: Though there is debate and disagreement, I certainly do not believe that general relativity and quantum theory have been reconciled (or are even close to being reconciled)—my feeling (without having taken a survey) is that most physicists who are concerned about this issue would agree with me. But to be fair and honest about the question, one must say that there certainly are excellent researchers who believe they are close to this reconciliation.

In any case, I do consider this problem, i.e., the problem of uniting the two most important and remarkable contemporary physical theories into a single unified theory, to be the single most important conceptual issue in physics today. It is a problem that theorists have been struggling with for close to 90 years. And it is difficult. One of the principal difficulties is with quantum theory—a magnificent theory—that to many makes no sense.

**SC: For a time, general relativity fell out of favor as a respectable field of inquiry because of its remoteness from laboratory experiment. Can you tell us how and when it regained its legitimacy?**

EN: This is a very interesting question—with no simple, quick answer. Some of my answers are guesswork while other parts are easily defended.

From the beginning of general relativity in 1915, because of its mathematical complexity and the scarcity of experimental tests, it did not really enter the mainstream of physics. For a while, it remained an active research field for a small group of mathematicians and mathematical physicists. In the mid and late 1920s, the discovery and development of quantum theory did draw much of the interest away from relativity. The predictive power of Quantum Theory and its direct contact with laboratory physics kept it at the very top of mainstream physics—and monopolized the talents of the best of the theoreticians. After the war, (the early to mid-1950s), there was a sudden worldwide explosion of reawakened interest in general relativity. A group in Warsaw working with an old colleague of Einstein’s, Leopold Infeld, began working; while in Hamburg, the group around Pascual Jordan developed; in Princeton, the group around John Wheeler (already well-known for his work in nuclear physics) turned to relativity; at Syracuse University, Peter Bergmann, a former student of Einstein’s, began a relativity group; in London, Hermann Bondi started a relativity group. Precisely what provoked this international renaissance, is, at least for me, conjecture. Most of the main problems of quantum theory had been dealt with—while its very serious difficulties sent people looking elsewhere for problems and solutions. The war years were over—a freedom to look for new issues was in the air—friends, students, and colleagues of Einstein’s were now available, after war work, to think of the issues and ideas raised by Einstein and his work. In any case, it was fascinating for me to watch and live through this period—this leap of scientific interest in general relativity.

**SC: Along with Roger Penrose and numerous others, you were a major contributor to what is known as the Golden Age of Relativity, which took place roughly between 1960 and 1975. Can you describe some of the work that took place during this period in extending Einstein’s work?**

EN: The main issues raised and worked on during the so-called Golden Age were (1) the relationship of general relativity with quantum theory and (2) the theory of gravitational radiation. Though there was a great deal of insight into the problems of quantum gravity—beautiful mathematics and clarification of the issues—unfortunately, a solution to the quantum problem was not attained. On the other hand—and the main reason it was called the Golden Age—was the incredible success with the issues of gravitational radiation. Though the following is an over-simplification of a complex set of interactions, it was essentially the beautiful physical and mathematical insights of the British cosmologist, Hermann Bondi, that broke open the field. Bondi showed that the Einstein equations of general relativity predicted the existence of gravitational radiation with an associated loss of mass and momentum from interacting matter sources. Then, the scientific interactions among the different groups mentioned earlier, (the groups from Warsaw, Hamburg, London, Princeton, and Syracuse) amplified, extended, clarified, and generalized the work of Bondi. It was the work from this period that eventually led to the development of gravitational wave detectors, a major ongoing research field at present.

**SC: The late physicist John Archibald Wheeler was an undeniable force in theoretical physics and a colleague of both Einstein and Niels Bohr. He was also a great champion of general relativity. What was his impact on this field?**

EN: Though John Wheeler (who already—prior to his work on relativity—was a well-known nuclear physicist) was truly a major force in General Relativity, one must add in the name of Peter Bergmann to complete the pair that reinvigorated relativity in the United States in the 1950-60s. Both men were inspiring teachers who created schools of students who followed their lead. Though it is hard to name specific individual discoveries of either man, it was their inspired leadership that was their greatness. Their students and their students’ students populated relativity for many years—to the present. Wheeler had a wonderful way with words that captured people’s imaginations. For example, he invented phrases and words, among others, the terms black holes, geons, and superspace that have been staples ever since. Bergmann, more quietly, with his students, developed the theory of pathological mechanics that became the major tool in almost all the attempts at quantum gravity. Their legacy lies in the students they taught and inspired.

**SC: Are there any intractable problems posed by general relativity that have not yet been answered?**

EN: As we discussed earlier, the relationship between general relativity and quantum theory is still the most intractable and basic issue in theoretical physics. Other troubling issues are the questions raised by the existence of the so-called dark energy and dark matter—what precisely are they? Another intractable problem has been the origin of the “big bang”—what was there “before?” What, if any, is its relationship to the issues of quantum theory?

**SC: What projects are you working on currently?**

EN: Now the question has become personal—and thus more difficult to answer. I have been retired for 15 years and taken the opportunity to have a good time doing research in general relativity that is fairly far from the beaten path. I am working strictly with the Einstein equations; I am not modifying them or introducing any new physics. I am simply exploring (some very complicated) mathematical details of the theory that has been overlooked for many years. It has turned out to be a very fertile area for research, allowing us (myself and colleagues) to publish about 25 scientific papers over the last five years. Basically, we are studying (sources) arbitrary mass distributions confined to a finite region and their gravitational effects at large distances. More specifically, we study specific properties, at these large distances from the source, of families of light rays (called null geodesic congruences) that originate at the source. From the large-distance properties of these null geodesic congruences, we have been able to deduce detailed properties of the motion of source itself. Our results, which contain a variety of predictions and show agreement with the basics of classical mechanics, have given us a great deal of satisfaction.

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