In 1919, the English astronomer Arthur Eddington travelled to the island of Pr铆ncipe off the coast of west Africa to see if he could detect the lensing of light predicted by general relativity. His plan was to observe a bright cluster of stars called the Hyades as the sun passed in front of them, as seen from Earth. To see the starlight, Eddington needed a total solar eclipse to blot out the glare of the sun.
If Einstein鈥檚 theory was correct, the positions of the stars in the Hyades would appear to shift by about 1/2000th of a degree.
To pinpoint the position of the Hyades in the sky, Eddington first took a picture at night from Oxford. Then, on 29 May 1919, he photographed the Hyades as they lay almost directly behind the sun during the total eclipse that Pr铆ncipe experienced that day. Comparing the two measurements, Eddington was able to show that the shift was as Einstein had predicted and too large to be explained by Newton鈥檚 theory.
Advertisement
Following the eclipse expedition, there was some controversy that Eddington鈥檚 analysis had been biased towards general relativity. Matters were put to rest in the late 1970s when the photographic plates were analysed again and Eddington鈥檚 analysis was shown to be correct.
Eddington鈥檚 result turned Einstein into an international superstar: 鈥淓instein鈥檚 theory triumphs鈥� was the headline of The New York Times. From then on, as more consequences of his theory have been discovered, general relativity has become entrenched in the popular imagination, with its descriptions of expanding universes and black holes.
In 1959, the American physicists Robert Pound and Glen Rebka measured the gravitational red-shifting of light in their laboratory at Harvard University, thereby confirming the last of the three classical tests of general relativity.
鈥淪pace tells matter how to move and matter tells space how to curve鈥�
Read more: Instant Expert: General relativity
Gravity before Einstein
In 1686, Isaac Newton proposed an incredibly powerful theory of motion. At its core was the law of universal gravitation, which states that the force of gravity between two objects is proportional to each of their masses and inversely proportional to the square of their distance apart. Newton鈥檚 law is universal because it can be applied to any situation where gravity is important: apples falling from trees, planets orbiting the sun, and many, many more.
For more than 200 years, Newton鈥檚 theory of gravity was successfully used to predict the motions of celestial bodies and accurately describe the orbits of the planets in the solar system. Such was its power that in 1846 the French astronomer Urbain Le Verrier was able to use it to predict the existence of Neptune.
There was, however, one case where Newton鈥檚 theory didn鈥檛 seem to give the correct answer. Le Verrier measured Mercury鈥檚 orbit with exquisite precision and found that it drifted by a tiny amount 鈥� less than one-hundredth of a degree over a century 鈥� relative to what would be expected from Newton鈥檚 theory. The discrepancy between Newton鈥檚 theory and Mercury鈥檚 orbit was still unresolved at the beginning of the 20th century.