It is well known how Albert Einstein first catapulted to fame in 1919, when expeditions led by Arthur Eddington and Frank Dyson recorded the bending of starlight around the Sun during an eclipse, confirming predictions he had worked out a few years earlier. That the theory could explain the mysterious precession of Mercury’s perihelion lent it further immediate credence.

A new book, Testing Einstein: One Hundred Years of Experimental Relativity, draws on top experts in the history of physics to present a much longer history of efforts to measure general relativity’s effects. It is edited by Daniel Kennefick, a historian at the University of Arkansas, and Brian Odom, NASA’s chief historian, and is available open-access from MIT Press.

This broader history captures relativity’s complexity and the subtlety of its consequences, which placed strenuous demands on measurements and often led to indecisive results. Further complicating the experimental history is that it took place against the backdrop of general relativity theory’s trajectory of fame, neglect, renaissance, and ultimate transformation into an essential foundation for studying cosmology, black holes, and other extraordinary astrophysical phenomena.

Eclipse expeditions and redshift measurements

Although the 1919 observations were widely hailed as vindicating Einstein, at that moment many scientists resisted seeing the matter as settled. Kennefick’s book No Shadow of a Doubt carefully reexamined the observations, upholding their conclusiveness. Testing Einstein places them in the middle of a longer history of similar observations that often failed to produce clear evidence.

A chapter by Jeffrey Crelinsten, author of Einstein’s Jury, traces earlier attempts to detect light bending following Einstein’s articulation in 1907 of his equivalence principle, positing the indistinguishability of physics in gravitational fields and accelerating reference frames. The first such effort involved an unsuccessful reexamination of photographic plates originally made in search of a planet inside Mercury’s orbit. Later observations were stymied by weather, war, and photographic difficulties, and results that were obtained did not show deflection. This history foregrounds figures who spent years working to test general relativity, including Erwin Finlay Freundlich, a close ally of Einstein who encountered bad weather during the 1919 eclipse, and Lick Observatory Director William Wallace Campbell.

Meanwhile, some observers attempted to measure a slight lengthening of the wavelengths of light from the Sun, a prediction of the equivalence principle known as the gravitational redshift. A chapter by Klaus Hentschel traces observations of such a shift in spectral lines from 1890 at Johns Hopkins University, well before general relativity. After 1907, it was measured by figures such as Freundlich; John Evershed, who was studying the solar surface from Kodaikanal Solar Observatory in India; and Charles St. John, who was investigating the solar spectrum at Mount Wilson Solar Observatory. However, measurements of the shift were divergent and had various plausible explanations, making it difficult to convincingly isolate a relativistic effect.

Crelinstein’s and Hentschel’s chapters, and another by Kennefick, trace these observations well into the post-1919 period. During a 1922 eclipse expedition in Australia, observers from Lick and the University of Toronto obtained results corroborating the 1919 results, and about this time St. John accumulated enough evidence to overcome at least his own initial skepticism about the gravitational redshift. As the 1920s progressed, reactions to new observations clustered into camps of those already convinced Einstein was correct and hardline opponents who accepted measurements of light bending but sought to attribute it to other factors.

Starting around 1930, fewer attempts to measure light bending were made, and expeditions that did take place sought to achieve more precise measurements, with uneven success. Freundlich, notably, finally measured light bending in Sumatra during the 1929 eclipse after fifteen years of trying. Finding the effect somewhat stronger than predicted, he began arguing against general relativity. But Kennefick points out that by this time, as most scientists accepted the theory, there was less motivation to make new measurements, especially given the difficulties involved. This was also a time when physicists’ interest in actually working on general relativity was waning.

Precision measurement in a high-tech era

The years between the 1920s and the 1950s were what physicist Jean Eisenstaedt referred to as the “low water mark” of general relativity theory. The late 1950s and 1960s were a period that physicist Clifford Will has called the subject’s “renaissance.”

Circa 1960, a series of new tests of general relativity began to be made, spurred by the availability of new technologies and methods. A chapter by physicist Brandon Brown reconstructs an experiment by Harvard University’s Robert Pound and graduate student Glen Rebka that measured the gravitational redshift, not as light emerged from the Sun’s gravity well but rather over the height of a twenty-two-meter tower in Harvard’s physics laboratory building.

Differences in the Earth’s gravitational force over that vertical distance are so small that Pound and Rebka had to measure a frequency shift of about one trillionth of one percent. This was made possible by the discovery of the Mössbauer effect in 1958, involving the nearly recoilless emission and absorption of photons of essentially identical frequencies by nuclei fixed within crystal lattices.

Brown recounts Rebka and Pound’s labors to build the experiment and analyze its data, as well as a competing but less precise effort at the UK’s Atomic Energy Research Establishment. He also notes controversies surrounding the press attention Pound and Rebka received, including a front-page story in the New York Times published before the experiment was even conducted.

Testing Einstein mentions but does not detail another effort in the early 1960s by Irwin Shapiro and others at the MIT Lincoln Laboratory, already documented by Benjamin Wilson and David Kaiser. Shapiro devised a test of relativity involving the slowing of light in gravitational fields. Using the new ability to reflect radar pulses off Venus, he was able to precisely measure the time delay caused by their passage near the Sun. A recent article by Kaiser notes that the first calculations of time effects on proposed GPS satellites were also made around this same time.

The sensitivity of GPS to general relativity was enabled by the precision of atomic clocks, which were also at the heart of another experiment, the Gravity Probe A rocket flight, detailed in a chapter by Connemara Doran and David DeVorkin. In the late 1960s, Robert Vessot, a researcher at the Smithsonian Astrophysical Observatory, proposed the experiment along with Harvard’s Norman Ramsey and MIT’s Daniel Kleppner. Supported by NASA, it launched successfully in 1976, recording the faster passage of time high above the Earth.

The general relativity renaissance and the astrophysical turn

Historians are still researching how the renaissance in general relativity unfolded. A chapter by Clifford Will offers a first-hand account, and another by Paul Halpern looks at John Wheeler’s turn to relativity in the 1950s. In addition, Alexander Blum, Roberto Lalli, and Jürgen Renn update a portrait they have been marshalling for over a decade. They now assert the renaissance’s origins can be found in unsuccessful efforts during the low-water period to move beyond general relativity. These encompassed Einstein’s and others’ doomed development of a unified field theory, the nascent development of quantum gravity theories, and a “British” effort to develop a cosmology not based on general relativity.

Blum, Lalli, and Renn suggest the crucial link was British cosmology, and particularly a largely forgotten theory advanced by E. A. Milne, which in turn prompted Princeton University’s Robert Dicke and graduate student Carl Brans to formulate a new theory of gravitation in 1961. While the Brans-Dicke theory overlapped significantly with general relativity, it yielded differing quantitative predictions that could in principle be measured. The theory became an important motivation for newly precise tests and a touchpoint in new discussions in astrophysics and cosmology that Blum, Lalli, and Renn call the “astrophysical turn” in general relativity.

This turn arose specifically in connection with general relativity’s usefulness in theorizing about newly discovered objects, notably quasars, hypothesized objects, notably black holes, and the Big Bang model of cosmology that became dominant after the discovery of the cosmic microwave background in 1964. Testing Einstein does not focus much on the turn, but the turn is connected to two noteworthy topics the book does address.

Kennefick notes that the last professional eclipse measurement took place in 1973, to test the Brans-Dicke theory, just as precise long-baseline radio observations of quasars occulted by the Sun made eclipse expeditions obsolete. In 1974 Russell Hulse and Joseph Taylor discovered a double system of a pulsar and a neutron star, and a chapter by radio astronomers Paolo Freire and Norbert Wex examines how this extreme gravitational environment opened up a series of observational possibilities in general relativity. These included finding an orbital precession effect 35,000 times as pronounced as Mercury’s, as well as a decrease over time in orbital period, offering the first evidence of gravitational radiation—well before LIGO’s more direct gravitational wave detection in 2015.

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William Thomas
American Institute of Physics
wthomas@aip.org

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