Testing General Relativity

Mercury's Orbit (1915)

Mercury's elliptical orbit slowly swivels around the Sun. After accounting for the tug of every other planet, astronomers were left with a discrepancy of about 43 arcseconds per century that Newton's gravity could not explain. On 18 November 1915 Einstein calculated what his new theory predicted: 43 arcseconds. No adjustment was needed.

Starlight at an Eclipse (1919)

The theory predicted that starlight grazing the Sun would be deflected by 1.75 arcseconds. The stars near the Sun are only visible during a total eclipse. On 29 May 1919 two British expeditions, to Sobral in Brazil and the island of Príncipe off West Africa, photographed them. The results, announced in London on 6 November, favored Einstein, and made him famous around the world within days.

Einstein had published a first prediction of light bending in 1911. A German expedition to the Crimea to test it in 1914 was stopped by the outbreak of war.

Clocks and Gravity (1960 onward)

In 1960 Robert Pound and Glen Rebka measured the change in frequency of gamma rays climbing a 22.5-meter tower at Harvard, confirming that time runs faster higher up. In 2022 a team at JILA in Colorado detected the same effect across a single millimeter, inside one cloud of ultracold strontium atoms.

Delayed Signals and Dragged Space

• Shapiro delay. Radar signals passing near the Sun take slightly longer than they would in flat space. Tracking the Cassini spacecraft in 2002 confirmed the prediction to about 0.002 percent.
• Frame dragging. A spinning mass twists spacetime around it. NASA's Gravity Probe B, launched in 2004, measured the effect of the rotating Earth on orbiting gyroscopes and found agreement with the theory, to within about 20 percent for this very small effect.

The Binary Pulsar (1974)

Russell Hulse and Joseph Taylor discovered two neutron stars orbiting each other. Over the following years the orbit shrank at the rate expected, to within a fraction of a percent, if the pair were losing energy as gravitational waves. It was the first evidence that the waves were real, and earned the 1993 Nobel Prize.

Gravitational Waves (2015)

On 14 September 2015 the two LIGO detectors recorded a ripple in spacetime from a pair of black holes merging about 1.3 billion light-years away. See Gravitational Waves.

The Shadow of a Black Hole (2019)

The Event Horizon Telescope, eight radio observatories working as one Earth-sized instrument, released in 2019 the first image of a black hole, from data taken in 2017: the object at the center of the galaxy M87, 55 million light-years away, with the mass of 6.5 billion Suns. The size and shape of its shadow matched the theory's prediction.

The Scorecard

Every one of these tests could have failed. That is what makes them tests. Physicists keep looking for a discrepancy, because a crack in general relativity would be the first clue to whatever deeper theory lies beneath it.