Eddington Observes Solar Eclipse to Test General Relativity

May 29, 1919: Eddington Observes Solar Eclipse to Test General Relativity

One of Eddington’s photographs of the May 29, 1919, solar eclipse. The photo was presented in his 1920 paper announcing the successful test of general relativity.

When Albert Einstein published his general theory of relativity (GR) in 1915, he proposed three critical tests, insisting in a letter to The Times of London that if any one of these three proved to be wrong, the whole theory would collapse.

  • Advance of the perihelion of Mercury
  • Deflection of light by a gravitational field
  • Gravitational red shift

Once he had completed his theory, Einstein immediately calculated the advance of the perihelion of Mercury, and he could hardly contain himself when GR produced the correct result. The next classical test was the deflection of light by a gravitational field, first performed by Sir Arthur Eddington in 1919.

Born to Quaker parents in December 1882, Arthur was just two years old when he lost his father to a typhoid epidemic that ravaged England. As a child, Eddington was enamored of the night sky and often tried to count the number of stars he could see. Initially Eddington was schooled at home, but when he did start attending school, he excelled so much in mathematics that he won a scholarship to Owens College in Manchester at age 16. He graduated with first class honors in physics, and promptly won another scholarship to attend Trinity College at Cambridge University.

Eddington completed his M.A. in 1905. First, he worked on thermionic emission at the Cavendish Laboratory, and then tried his hand at mathematics research, but neither project went well. He briefly taught mathematics before re-discovering his first love: astronomy. Eventually he found a position at the Royal Observatory in Greenwich, specializing in the study of stellar structure. By 1914 he had moved up to become director of the Cambridge Observatory; a Royal Society fellowship and Royal Medal soon followed.

During Eddington’s tenure as secretary of the Royal Astronomical Society, Willem de Sitter sent him letters and papers about Einstein’s new general theory of relativity. Eddington became Einstein’s biggest evangelist at a time when there was still considerable wartime hostility and mistrust toward any work by German physicists. He soon became involved in attempts to confirm one of the theory’s key predictions.

Since the masses of celestial bodies would cause spacetime to curve, Einstein predicted that light should follow those curves and bend ever so slightly. Isaac Newton had also predicted that light would bend in a gravitational field, although only half as much. Which prediction was more accurate? Scientists feared that measuring such a tiny curvature was simply beyond their experimental capabilities at the time.

It was Britain’s Astronomer Royal, Sir Frank W. Dyson, who proposed an expedition to view the total solar eclipse on May 29, 1919, in order to resolve the issue. Eddington was happy to lead the expedition, but initially the venture was delayed. World War I was raging, and the factories were too busy meeting the country’s military needs to make the required astronomical instruments. When the war ended in November 1918, scientists had just five months to pull together everything for the expedition.

Eddington took nighttime baseline measurements of the positions of the stars in the Hyades cluster in January and February of 1919. During the eclipse the Sun would cross that cluster, and the starlight would be visible. Comparison of the baseline measurements of a star’s position and the corresponding measurements made during the eclipse, when that star was just visible at the limb of the sun, would determine whether Einstein or Newton was right.

Then Eddington set sail for Principe, a remote island off the west coast of Africa, sending a second ship to Sobral, Brazil—just in case the weather didn’t cooperate and clouds obscured the view. It proved to be a smart decision. Eddington’s team was dismayed when heavy rains and clouds appeared on the day of the eclipse, although the skies cleared sufficiently by the time of the event to allow them to make their measurements. The Brazilian team had their own challenges: The tropical heat warped the metal in their large telescopes, forcing them to also use a smaller 10-centimeter instrument as backup.

Once the two teams had analyzed their results, they found their measurements were within two standard deviations of Einstein’s predictions, compared to twice that for Newton’s, thus supporting Einstein’s new theory. News of Eddington’s observations spread quickly and caused a media sensation, elevating Einstein to overnight global celebrity. (When his assistant asked how he would have felt had the expedition failed, Einstein is said to have quipped, “Then I would feel sorry for the dear Lord. The theory is correct anyway.”)

Not everyone immediately accepted the results. Some astronomers accused Eddington of manipulating his data because he threw out values obtained from the Brazilian team’s warped telescopes, which gave results closer to the Newtonian value. Others questioned whether his images were of sufficient quality to make a definitive conclusion. Astronomers at Lick Observatory in California repeated the measurement during the 1922 eclipse, and got similar results, as did the teams who made measurements during the solar eclipses of 1953 and 1973. Each new result was better than the last. By the 1960s, most physicists accepted that Einstein’s prediction of how much light would be deflected was the correct one.

Eddington succumbed to cancer in November 1944 after a long illustrious career. In addition to his many scientific contributions, he once penned a lyrical parody of The Rubaiyat of Omar Khayyam about his famed 1919 expedition:

Oh leave the Wise our measures to collate
One thing at least is certain, LIGHT has WEIGHT,
One thing is certain, and the rest debate –
Light-rays, when near the Sun, DO NOT GO STRAIGHT.


Gravitational Waves

Gravitational Waves[1]

The Laser Interferometer Gravitational-Wave Observatory, or LIGO, launched the era of gravitational wave astronomy in February 2016 with the announcement of a collision between two black holes observed in September 2015.

The scientific collaboration that operates the two LIGO detectors netted a second merger between slightly smaller black holes on December 26, 2015. (A third “trigger” showed up in LIGO data on October 12, 2015, but ultimately did not meet the stringent statistical significance standard that physicists generally insist on.)

Instead, scientists focused on sharpening theoretical estimates of how often various events occur. In particular, they are eager to see collisions involving neutron stars, which lack sufficient mass to collapse all the way to a black hole. Neutron star collisions are thought to be plentiful, but would emit weaker gravitational waves than do mergers of more massive black holes, so the volume of space the LIGO detectors can scan for such events is smaller.

LIGO scientists are also looking for signals from individual pulsars—rapidly rotating neutron stars that are observed on earth as pulses of radio waves. A bump on a pulsar’s surface should produce gravitational waves, but so far, no waves with the right shape have been picked up. This absence puts a limit on the size of any irregularities and on the emission power of gravitational waves from nearby pulsars such as the Crab and Vela pulsars, said Michael Landry, head of the Hanford LIGO observatory, and could soon start putting limits on more distant ones.

A few hints of possible excitement to come: LIGO data taken through the end of January, 2017 produced two short signals that were unusual enough to exceed the experiment’s “false alarm” threshold—signals with shapes and strengths expected to show up once a month or less by chance alone. Both LIGO collaboration members and astronomers at conventional telescopes are investigating the data to determine whether they represent real events.

LIGO is not the only means by which scientists are searching for gravitational waves. Some scientists are using powerful radio telescopes to track signals emanating from dozens of extremely fast-rotating pulsars. A specific pattern of correlations between tiny hiccups in the arrival times of these pulses would be a signature of long-wavelength gravitational waves expected from mergers of distant supermassive black holes.

[1] See Gabriel Popkin, “Gravitational Waves: Hints, Allegations, and Things Left Unsaid,” in APSNEWS (36, 3, March 2017, p. 1)