But there was a sense among scientists that the only thing this complex theorizing was good for was to predict minute deviations from Newton’s grand theory. Experimentalists held sway in the physics of that time, and they felt that general relativity would never play a role in mainstream science.
As a result of this skepticism, the science of general relativity gradually became stagnant and sterile. By the mid 1920s Einstein had turned most of his attention to what would become a futile quest for a unified field theory that would combine gravitation and electromagnetism, and many other relativity researchers followed suit. With only a few exceptions, most work in general relativity during the next thirty-five years was devoted to abstract mathematical questions and issues of principle, and was carried out by a small band of practitioners. Science historian Jean Eisenstaedt has called this period the “low water mark” for Einstein’s theory. A classic illustration of the attitude toward Einstein’s theory at the end of this period is the advice given in 1962 to a newly minted graduate of the California Institute of Technology, about to head for Princeton to do graduate work. A famous Caltech astronomer advised him that he should absolutely not work on general relativity when he got to Princeton, because it would never have anything useful to contribute to physics or astronomy. Luckily for many of us, the student, Kip Thorne, ignored the advice.
As Kip headed east for the ivy covered walls of Princeton, Caltech astronomers were on the verge of announcing the discovery of strange objects that they called “quasistellar radio sources,” or quasars. These were very distant, very energetic sources of radio waves that defied explanation in terms of conventional physics. A few people started to wonder if general relativity might help to provide an explanation, and convened a special conference on quasars, bringing together astrophysicists and general relativists. Held in Dallas in December 1963, this historic conference would come to be known as the First Texas Symposium on Relativistic Astrophysics. Within a few years, other discoveries pointed to a definite role for general relativity in astrophysics. In 1965 came the detection of the cosmic background radiation left over from the big bang. In 1967 radio astronomers discovered the first of many pulsars, now understood to be rapidly spinning neutron stars. And 1971 saw the discovery of a compact, powerful X-ray source orbiting a normal star, the first black hole candidate. You needed general relativity if you wanted to begin to understand these phenomena.
These discoveries helped bring about a renaissance for general relativity, in which it would begin to rejoin the mainstream of physics and astronomy. This was aided by advances in technology, such as atomic clocks, lasers and superconductors, and by the development of the space program, which would provide the tools to perform new high-precision tests of Einstein’s theory, putting it on a solid experimental foundation. After all, if you needed to use a relativistic theory of gravity to understand quasars, pulsars and the cosmic background radiation, it would be very good to know if Einstein’s theory was the correct one to use. For despite Einstein’s fame, by this time there were alternative theories of gravity competing with general relativity for primacy. One of these, known as the Brans–Dicke theory, named after Princeton University’s Robert Dicke and his student Carl Brans, made a credible claim that it was just as viable as Einstein’s theory. This competition sparked a major effort to carry out new and better experimental tests to determine if Einstein was right or not.
Other factors helped set the stage for this rebirth of general relativity research. Ironically, one of these may have been the death of Einstein himself in April 1955. No other topic in physics was so closely tied to a single, towering individual. It was not uncommon in those days for the few practitioners of general relativity to make a pilgrimage to Princeton to describe their work to the great one, and, hopefully, to receive his approval. On the occasion of the centenary of general relativity in 2015, the French mathematical physicist Yvonne Choquet-Bruhat wrote a charming reminiscence of her visit to the Institute for Advanced Study in 1951 as a 27-year-old postdoctoral researcher. She visited Einstein several times during that year, explaining her mathematical work on the existence of solutions of Einstein’s equations, and listening to Einstein describe his work on unified field theory. He pronounced himself very pleased with her work, which actually turned out to be one of the major milestones of the field. His work on the unified theory ultimately went nowhere. But after Einstein passed from the scene, the field was somehow free to make its own way forward.
Another factor may have been an emerging sense of community among the small group of specialists in relativity. Beginning with a conference in July 1955 in Bern, Switzerland to commemorate the fiftieth anniversary of special relativity, regular international meetings on the subject were organized. By 1959, at the third such meeting, held in Royaumont, France, leaders of the field formed the “International Committee” on general relativity, to aid the organization of future meetings, to disseminate lists of published papers in the field and to provide information on research groups around the world. This would ultimately evolve into the International Society on General Relativity and Gravitation, with elected officers, annual dues and its own scientific journal.
The fact is that science is more than a body of knowledge. It is also a community of researchers who advance the field by sharing knowledge, collaborating and competing with one another, and even correcting each other, so that scientific facts can be established and progress can be made. Some historians of science believe that the emergence of this community of relativists in the late 1950s made it possible for them to respond nimbly and effectively to the new astronomical discoveries of the 1960s.
By the time of the 1979 centenary of the birth of Einstein, the relativistic renaissance that had begun in the 1960s was in full swing. The outpouring of books commemorating that birthday attested to the vigor and excitement of research in the field. A “toolbox” of techniques for solving Einstein’s complicated equations in a wide range of situations had been developed, and researchers were turning to computers to help solve the more complex problems. Many experimental tests of general relativity had been performed. Some exploited new technologies to do improved versions of “classic” experiments, such as measurements of the bending of light using radio waves from quasars rather than using visible light from stars. Others were new tests never envisioned by Einstein, such as the “Shapiro time delay,” an excess delay in the propagation of radar signals as they passed near the Sun on their way to track the orbits of planets or spacecraft.
Black holes were an accepted and understood part of the theory, and observational evidence that they actually exist was mounting. A model for the basic structure and evolution of the universe was in good shape, and cosmologists were beginning to explore what might have happened in the first trillionth of a second after the big bang. The nature of quasars ironically remained a mystery after twenty years of study, while a pretty good theory of pulsars was in hand.
The centenary year itself was inaugurated by a stunning announcement. At the ninth Texas Symposium on Relativistic Astrophysics, held in Munich in December 1978, Joseph H. Taylor of the University of Massachusetts described the latest result from a remarkable new testing ground for general relativity that he and his student Russell Hulse had discovered in 1974. This new arena for experimental relativity was a pulsar in orbit about a companion star, colloquially called the “binary pulsar.” Taylor reported how observations of the orbit of the pulsar since 1974 had led to the first confirmation of one of the most important predictions of Einstein’s theory: the existence of gravitational waves. Nineteen years later, Taylor and Hulse would win the Nobel Prize in Physics, the first ever bestowed for work related to general relativity. If general relativity seemed to be triumphant in 1919, it was surely triumphant now.
But there were some clouds on the horizon, and there would be more to come. None of them posed a direct threat to the supremacy of Einstein’s theory—as of 1979 it had not failed a single experimental tes
t, and that perfect record continues to today—but they suggested the possibility that Einstein might not have had the final word on gravity.
The first cloud was theoretical. In addition to developing special and general relativity, Einstein was one of the pioneers of quantum mechanics. And although Einstein ultimately found himself at odds with some of the interpretations of quantum physics, he would have been the first to admit that it was spectacularly successful in its ability to account for measurable effects in the subatomic world. And even some of the “spooky” probabilistic effects that he frequently railed against (“God does not play dice!”) have been shown by recent experiments to be true and unavoidable. Quantum mechanics rules everything from the practical (chemistry, semiconductors, MRI, nuclear energy, the workings of your smartphone, the list is almost endless) to the exotic (quarks, the Higgs boson, quantum computers, …). The basic forces of the inner world—the strong force, which governs atomic nuclei, the electromagnetic force, which governs charged particles and light, and the weak force, which governs some forms of radioactive decay and is intimately linked to the elusive particles called neutrinos—are all understood today using quantum mechanics. This notion that “the quantum” rules everything is so pervasive in physics that it is almost an act of faith that the weakest of the forces, gravity, must also somehow be “quantized.”
It is more than just faith, however. When treated as a pure, exact theory, general relativity actually seems to sow the seeds of its own destruction. The most common example of this is the black hole, which, as we will see later in this book, contains a “singularity” at its center. A singularity is a point in space where the warpage of spacetime, the density, the pressure and the energy all become infinite. Similarly singular behavior occurred at the instant of the universal “big bang,” according to standard general relativity. But the appearance of infinities in the predictions of a theory has frequently been interpreted as a signal that the theory itself is breaking down. For example, when experiments carried out in 1911 indicated that the atom consists of a tiny positively charged nucleus surrounded by orbiting negatively charged electrons, scientists realized immediately that there was a problem. The revolving electrons should emit electromagnetic radiation, lose energy and spiral inward toward the nucleus, emitting along the way a potentially infinite amount of energy. This was theoretically untenable, and of course it also contradicted experiment. The quantum mechanical model of the atom devised by Niels Bohr came to the rescue by requiring that electrons stay on fixed orbits. They would only radiate light when they made a “transition” or a jump from one orbit to an adjacent orbit of lower energy. And every atom had a “ground state,” an orbit of lowest energy, from which no further jumps were permitted. The more fully developed quantum mechanics of Erwin Schrödinger and Werner Heisenberg refined this picture in terms of probabilities and the uncertainty principle.
In the case of the black hole or big bang singularities, the hope was that quantum mechanics might come to the rescue in a similar way, for example by somehow preventing things from falling all the way to the black hole singularity, the way the atomic ground state of an atom keeps the electron from falling all the way into the nucleus.
But therein lies the rub. After almost a hundred years we still do not have an acceptable quantum theory of gravity. This is not for a lack of trying, and there is a sizable, worldwide research effort attacking this problem from a dizzying array of directions, with arcane names like canonical quantum gravity, superstring theory, causal set theory, loop quantum gravity and the AdS/CFT correspondence, to name just a few. We won’t burden you with a list of reasons why this problem is so difficult, but instead we will skip to the bottom line: general relativity as Einstein formulated it in 1915 cannot be the quantum theory of gravity. Whatever that theory turns out to be, it will be different from general relativity. The questions are how different, and can we ever detect those differences? One viewpoint asserts that quantum gravity becomes relevant only between the big bang and about a tenth of an atto-yottosecond (or 10−43 seconds) later, or at energies a million billion times higher than the levels achieved by the Large Hadron Collider in Geneva. Inside a black hole, quantum effects would kick in only at similarly incredibly short distances from the singularity, and in any case, since the event horizon that surrounds the singularity prevents the escape of any information about what is going on, why would we care? Consequently, according to this viewpoint, we will never be able to perform an experiment capable of detecting a quantum gravity effect. This raises a scientific conundrum. If you have two theories and there is no conceivable, practical way ever to perform an experiment to test them, how do you decide which one is correct? Elegance? Simplicity? A democratic vote? Faith? And is that science?
Our viewpoint on quantum gravity is agnostic. As you will discover in this book, we will be mostly concerned with experiments that can be performed in a finite amount of time, with a finite (if sometimes rather large) budget. As a result, the book will not contain a detailed description of the different attempts to develop a theory of quantum gravity, a topic with a long and complicated history of its own. We must admit, however, that the scientific community knows so little about how to make general relativity compatible with the quantum that it is entirely possible that some future experiment might stumble on an effect in conflict with the predictions of general relativity. If so, such an experiment or observation could point the way forward toward a theory of quantum gravity. For now, we can only continue to test Einstein’s theory to higher precision and in new regimes.
Another cloud on Einstein’s horizon is called dark matter. Most physicists and astronomers are now convinced that only about 4 percent of the mass of the observable universe is made up of the normal matter that we know and love because we are made of it: protons, neutrons, electrons and other elementary particles that comprise what physicists call the Standard Model. About 23 percent is called “dark matter.” (We’ll get to the remaining 73 percent in a moment.) The evidence supporting dark matter is compelling. The velocities of stars and gas around spiral galaxies, the velocities of galaxies within clusters of galaxies, and the bending of starlight around galaxies and clusters are all too large to be explained by the mass of ordinary matter making up the visible galaxies themselves. There is additional mass around these objects that does not emit light, but that attracts gravitationally, hence the name “dark” matter. Observations of the pattern of tiny fluctuations in the intensity of the cosmic background radiation confirm the presence of dark matter. Even the formation of galaxies themselves, beginning about a million years after the big bang, cannot be properly understood unless there is dark matter, using its gravitational attraction to tug the ordinary matter into the blobs that ultimately collapsed and coalesced to form the stars and galaxies that we observe.
A leading candidate for dark matter is an elementary particle that is not part of the Standard Model; by suitably tweaking that model, particle theorists have come up with numerous plausible candidates. If so, millions of these particles should be passing through your body every second as you read this book, and thus a sensitive enough detector ought to be able to sense some of them. But despite almost forty years of experimental effort carried out by physicists around the world, no detection has been made. To some, this has become a bit of an embarrassment, and so they have turned to an alternative approach: why not modify gravity itself? Although, as you will discover in this book, general relativity has been confirmed in many different arenas, it has not been well tested on the very large distance scales of galaxies, clusters of galaxies, or the observable universe as a whole. So perhaps a suitable modification of general relativity would solve the dark matter problem. So far, while most of these “modified gravity” theories have not been very successful, a future theory could be, with its predictions confirmed by future observations, especially ones that focus on these large distance scales.
What is the remaining 73 percent? Enter the final cloud on the
horizon: “dark energy.” In 1998, astronomers studying very distant supernova explosions were forced to conclude, from the pattern of their data, that the expansion of the universe is speeding up, not slowing down. Given that we have known since 1929 that the universe is expanding, your intuition is perhaps telling you that the expansion should be slowing down. After all, mass makes gravity and gravity attracts, so, just as the Earth’s gravity causes a ball thrown in the air to slow down and come back down, the universe should make its own expansion slow down (whether it continues to expand but ever more slowly or halts and starts to contract is a separate question). And indeed, Einstein’s general relativity predicts unambiguously that the expansion of the universe should slow down or decelerate, not speed up. So when this accelerated expansion was detected, it was a major shock.
But then theorists got to work coming up with ideas to account for this new phenomenon. One set of ideas was dubbed “dark energy” by University of Chicago cosmologist Michael Turner, in analogy with dark matter—the adjective is rather prophetic, because we are still pretty much in the dark about both of them. If you ascribed to this substance the right gravitationally repulsive or “anti-gravity” properties, and if you let it contribute about 73 percent of the total mass and energy in the observable universe, then it fits all the observational data beautifully. In fact the cosmological model based on this, called the “Lambda-CDM” model, has proven to be remarkably successful in explaining a diverse array of data on the universe at the largest scales. Here, the Greek capital letter lambda (Λ) refers to dark energy, while CDM refers to cold dark matter. But trying to understand what dark energy is from deeper principles of quantum mechanics and particle physics has led to a plethora of competing models, which are very hard to distinguish among through experiment or observation.
Is Einstein Still Right? Page 2