Was Einstein Right?

Some historians trace the start of the modern era not to the turn of the 20th century, nor to the onset of World War I, but to May 29, 1919.

On that date, said physicist Clifford Will, a total eclipse of the Sun blanketed the Southern hemisphere. In those few brief moments of darkness, teams of British astronomers stationed in Brazil and West Africa were able to measure the bending of starlight predicted by Albert Einstein's nascent theory of general relativity.

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It was the first experimental proof of a theory that, as Will said, "has changed forever the way we think about space, about time, and about the universe as a whole."

Will, the James McDonnell professor of physics at Washington University in Saint Louis, was the third speaker in the 2005 Penn State Lectures on the Frontiers of Science, titled "Einstein's Legacy." His talk on February 5 at University Park traced the history of efforts to confirm Einstein's profundity.

"When Einstein invented his special theory of relativity in 1905, he was a pretty obscure scientist," Will said. "Even with general relativity in 1915, he was still really known only in science circles." When the 1919 results were announced in London in November of that year, he instantly became an international celebrity. "For the rest of his life, he was the symbol of genius."

Ups and Downs

By the mid 1920s, however, Einstein had turned his attention away from relativity to search for a unified theory of matter. "The field went into pretty rapid decline," Will said. "It was thought that general relativity was too difficult for ordinary mortals to understand." More important, the theory had few observable effects.

Too, "Einstein himself seemed rather blasé about experimental consequences," Will said. On receiving the telegram announcing the eclipse measurements, he was famously unimpressed. "I do not believe that the main significance of the general theory of relativity is the prediction of some effects," he wrote later, "but rather the elegance of its formation and the simplicity of its conception."

Lacking experiment to test and balance the theory, relativity became "a sterile field, a scientific backwater," Will said. "That began to change in the 1960s. Today it's one of the hottest fields in physics."

One key element in this renaissance, Will said, "was a revolution in astronomy." The rapid-fire discoveries of quasars in 1961, cosmic background radiation in 1964, pulsars in 1965, and the first black-hole candidate in 1971 revealed a whole new universe, riddled with objects of extremely high energy and density. "Understanding these sources required general relativity," Will said.

Also important was a new approach to thinking about the theory. "New textbooks written in the 1970s made it simple enough to teach, to the point where today it is taught to undergraduates."

Finally, technological leaps beginning in the 1960s enabled huge improvements in precision measurement with tools like atomic clocks, lasers, and radar. Together, these gains have allowed increasingly precise tests of Einstein's theory.

Ways of Knowing

Bending light. In Einstein's description, space is curved by the effects of gravity. Light passing through space necessarily follows this curve, and the stronger the force of gravity, the more the light is bent. Thus, bending should be largest for light passing close to a massive body like the Sun.

The 1919 expedition used an eclipse to mask the solar brightness and observe this phenomenon. As Einstein had predicted, the Sun's gravity pulled nearby stars into slightly different positions compared to where those stars are seen at other times of the year when the Sun is far away from them.

The discovery of quasars—strong, sharp, radio sources—and the development of radio interferometry during the 1960s allowed much more precise measurements of this bending, Will noted. By measuring the angle between two quasars (known as 3C273 and 3C279) that pass near the Sun in October each year, astronomers have confirmed that the observable bending agrees with general relativity to within a few parts in 10,000.

Orbit of Mercury. Mercury's path around the Sun is elliptical, shaped like a racetrack oval. According to Newton, such an orbit should be fixed in space. Mercury's, however, precesses; that is, the entire ellipse rotates slowly around the Sun. Very slowly—only 570 arc seconds, or less than two degrees, per century—but still enough to be a problem for celestial mechanics.

In the 1850s, the French astronomer Urbain Le Verrier explained this precession as a result of disturbances caused by the gravitational pull of nearby planets. Once these effects were toted up, however, there remained a gap of about 40 seconds of arc per century between Newton's prediction and the observed value. For 50 years this discrepancy remained a pebble in the shoe of astrophysics. Scientists even posited a hidden planet, Vulcan, to explain it. When Einstein finished the calculations of general relativity in 1915, however, he found that the predicted additional effects of his theory on Mercury's orbit exactly matched the 40-second gap.

The Space-Time twist. General relativity also predicts that massive rotating objects should drag space-time around themselves as they rotate. This effect, known as "frame dragging," is akin to what would happen if a bowling ball were set spinning in a tub of molasses. By bouncing laser beams off reflective satellites in space, Will said, scientists have recently managed to make the first tentative measurements of frame dragging. Last April, NASA launched the Gravity Probe B satellite, with an experiment intended to capture this effect much more precisely—by measuring tiny changes in the direction of spin of four orbiting gyroscopes. The probe's first results are expected in early 2006.

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Gravitational waves. Sometimes thought of as ripples in space-time, gravitational waves are slight disturbances in the fabric of the universe caused by the motions of matter. The problem, said Will, is that these ripples are so small they've been impossible to measure.

In 1974, Russell Hulse and Joseph Taylor, then at the University of Massachusetts, discovered the first binary pulsar: a system of two rapidly spinning neutron stars locked in orbit around each other. Measuring the timing pattern of pulses coming from one of these stars over a period of years, Hulse and Taylor deduced that the two are rotating faster and faster in a smaller and smaller orbit—evidence for an energy drain caused by emitting gravitational waves.

The result earned a Nobel prize in 1993. Since then, Will noted, "There's been a world-wide effort to detect gravitational waves using giant laser interferometers." The two such detectors in the U.S., know collectively as the Laser Interferometer Gravitational Wave Observatory, or LIGO, have perpendicular arms 2.5 miles long. A much larger, space-based observatory, the Laser Interferometer Space Antenna, or LISA, is scheduled to launch in 2012.

The Verdict

Was Einstein right? "Yes," Will concluded. "Over a hundred years, general relativity has passed every experimental test put to it with flying colors.

"The truly amazing thing to me is that a theory that was invented purely on aesthetic criteria, with no regard for experiment, turned out to be so right in the end."

Clifford M. Will, Ph.D., is the James McDonnell professor of physics at Washington University in Saint Louis, cmw@wuphys.wustl.edu. The lecture reported above was given as part of the 2005 Penn State Lectures on the Frontiers of Science, "Einstein's Legacy: A Centennial Celebration." Will's book, Was Einstein Right?, won a 1987 Science Writing Award from the American Institute of Physics.

Last Updated February 10, 2005