Experiments That Proved Relativity

The Michelson-Morley Experiment

In 1887, Albert Michelson and Edward Morley performed one of the most famous experiments in the history of physics. Their goal was to detect the luminiferous aether, a hypothetical medium through which light waves were believed to propagate, similar to how sound waves propagate through air. If the Earth moves through this aether, the speed of light measured on Earth should differ depending on whether the light travels parallel to or perpendicular to the direction of Earth motion.

Michelson and Morley used an interferometer that split a beam of light into two perpendicular paths, reflected them back, and recombined them. Any difference in the speed of light along the two directions would produce a shift in the interference pattern. Despite extraordinary sensitivity, the experiment detected no shift at all. The speed of light was the same in every direction.

This null result was deeply puzzling to physicists of the time. Various explanations were proposed, including the hypothesis by George FitzGerald and Hendrik Lorentz that objects moving through the aether contract in their direction of motion by just the right amount to cancel the expected effect. Einstein took a more radical approach: he proposed that there is no aether, and that the speed of light is truly constant in all inertial reference frames. This became the second postulate of special relativity. The Michelson-Morley result, while it did not directly prove relativity, eliminated the leading alternative and provided the empirical foundation for Einstein 1905 theory.

Time Dilation: Muon Decay and Atomic Clocks

One of the most direct tests of special relativity is the observation of time dilation in cosmic ray muons. Muons are subatomic particles created when cosmic rays strike the upper atmosphere at altitudes of about 15 kilometers. Muons have a mean lifetime of about 2.2 microseconds. Traveling at close to the speed of light, a muon should travel only about 660 meters before decaying, yet large numbers of muons reach the surface of the Earth. The explanation is time dilation: because the muons move at roughly 0.998 times the speed of light, their internal clocks run about 16 times slower than clocks on the ground, giving them enough time to reach the surface.

In 1971, Joseph Hafele and Richard Keating placed cesium atomic clocks on commercial airplanes that flew around the world, first eastward and then westward. They compared the airborne clocks to reference clocks that remained at the U.S. Naval Observatory. The eastward-flying clocks lost about 59 nanoseconds relative to the ground clocks, and the westward-flying clocks gained about 273 nanoseconds. These results matched the predictions of both special relativity (kinematic time dilation due to the airplane speed) and general relativity (gravitational time dilation due to altitude) to within the experimental uncertainty.

Modern tests of time dilation are far more precise. Optical atomic clocks can detect the difference in the rate of time between two clocks separated by only 30 centimeters of height in Earth gravitational field. These clocks confirm gravitational time dilation at levels consistent with general relativity to better than one part in 10^{18{\/sup}, making them among the most precise measurements ever performed.}

The Deflection of Starlight

General relativity predicts that light passing near a massive object is deflected by the curvature of spacetime. Einstein calculated that a light ray grazing the surface of the Sun should be deflected by 1.75 arcseconds, exactly twice the value predicted by Newtonian gravity (which treats light as particles affected by gravitational attraction).

In 1919, Arthur Eddington led two expeditions to observe a total solar eclipse, one to the island of Principe off the coast of West Africa and one to Sobral in Brazil. During the eclipse, the positions of stars near the Sun could be measured and compared to their positions when the Sun was elsewhere in the sky. The measured deflection was approximately 1.7 arcseconds, consistent with Einstein prediction. The announcement of this result made Einstein world-famous overnight.

The 1919 measurement had significant uncertainties. Over the following decades, increasingly precise measurements confirmed the prediction. Radio observations of quasars passing behind the Sun, which can be made without waiting for an eclipse, have confirmed the deflection to an accuracy of about 0.01%. Very long baseline interferometry (VLBI) measurements of radio sources have verified the general relativistic prediction to about one part in ten thousand.

The Precession of Mercury Orbit

Long before Einstein developed general relativity, astronomers knew that the orbit of Mercury does not close perfectly. Its point of closest approach to the Sun, the perihelion, advances slightly with each orbit. Most of this precession is caused by the gravitational pull of other planets, but after accounting for all known Newtonian effects, an unexplained residual of about 43 arcseconds per century remained.

This anomalous precession had been known since the 1850s, when the astronomer Urbain Le Verrier first calculated it precisely. Various explanations were proposed, including a hypothetical planet called Vulcan orbiting closer to the Sun than Mercury. No such planet was ever found.

When Einstein completed his general theory of relativity in November 1915, one of the first things he calculated was the orbit of Mercury. His equations predicted an additional perihelion precession of exactly 43 arcseconds per century, matching the observed anomaly with no free parameters. Einstein later wrote that this result gave him heart palpitations. The precise agreement between theory and observation was strong evidence that general relativity correctly describes gravity in the solar system.

Gravitational Waves: LIGO and Beyond

General relativity predicts that accelerating masses produce ripples in spacetime called gravitational waves. For nearly a century after Einstein first predicted them in 1916, gravitational waves remained undetected due to their extremely small amplitude. The first indirect evidence came in 1974 when Russell Hulse and Joseph Taylor discovered a binary pulsar (PSR B1913+16) whose orbital period was decreasing at exactly the rate predicted by general relativity due to energy loss from gravitational wave emission. This discovery earned them the 1993 Nobel Prize.

Direct detection finally came on September 14, 2015, when the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves from the merger of two black holes roughly 1.3 billion light-years away. The detected signal, lasting about 0.2 seconds, matched the predictions of general relativity for the inspiral, merger, and ringdown of two black holes with masses of about 36 and 29 solar masses. The detection was announced in February 2016 and earned Rainer Weiss, Barry Barish, and Kip Thorne the 2017 Nobel Prize.

Since 2015, LIGO and its European counterpart Virgo have detected dozens of gravitational wave events, including mergers of black holes, neutron stars, and mixed systems. Each detection provides a test of general relativity in the strong-field regime, where gravity is at its most intense. So far, every observation has been consistent with the predictions of general relativity.