On November 6, 1919, at a meeting of Britain’s scientific elite, Arthur Eddington (not yet Sir) stood beneath a 30-foot portrait of Sir Isaac Newton and presented the first evidence that gravity bends starlight, confirming Einstein’s theory of General Relativity and disproving Newton’s theory of gravity, which had been the gold standard of science for over two centuries.

Images taken during the 1919 solar eclipse by Eddington and his colleagues proved the light of distant stars passing near the Sun is deflected by its gravity, as sketched here.

The green lines show the actual paths of light from two stars that bend as they pass the Sun, which are only visible on Earth (blue) when the Moon (gray) blocks the Sun (yellow). The lower starlight passes twice as far from the Sun’s center as the upper starlight, and therefore bends half as much. The two black stars are at the apparent positions we observe when the Sun bends their light. When Earth moves to the other side of the Sun, we observe the stars in their true positions (shown in green).

Eddington was a well-known proponent of General Relativity. After he presented his eclipse images, a reporter asked him if it were true that only three people in the world understood General Relativity. After a lengthy pause, Eddington replied: “I’m just trying to think who the third person is.”

In various situations, the bending of light by massive bodies — now called gravitational lensing — can magnify light intensity, produce multiple images of one source, grossly distort images, and shift their apparent position.

In the 100 years since its first discovery, gravitational lensing has become a powerful tool for revealing the hidden universe.

This image on the right shows an Einstein Ring. The central yellow galaxy, 5 billion light-years away with a mass of 5 trillion Suns, focuses light from a blue galaxy that is 10 billion light-years away. The blue galaxy’s true shape is distorted into a nearly circular blue arc.

Gravitational lensing has also revealed exoplanets, planets orbiting other stars. The OGLE chart below plots the light intensity versus time of a distant source as an intervening star and nearby planet pass through our line of sight to the source.

Lensing by the star triples the intensity of light reaching us from the remote source (at day 43), while the planet increases intensity by about 20% just before day 53. The intensity enhancement improves with better alignment of source, lens, and Earth — the greatest observed enhancement exceeds several hundred times.

Galaxies and galaxy clusters are often highly asymmetric lenses that produce multiple images of one object. The picture on the left in the image below shows two images (“A” above and “B” below) of a quasar 9 billion light-years away, produced by a lensing galaxy 4 billion light-years away, seen near B. The emission spectra of A and B are nearly identical, confirming that these are two images of one object.

Quasars are intensely radiating supermassive black holes. As shown in the right-hand graphs, radiation from this quasar varies in intensity (vertically) over time (horizontally). Changes in B lag similar changes in A by about 400 days, revealing a travel time difference between the light focused at A versus B.

Remarkably, gravitational lensing even reveals mass that is invisible. The famous Bullet Cluster image is compelling evidence for the existence of dark matter. It shows two galaxy clusters 150 million years after they collided. The larger cluster circled in green and the smaller cluster circled in yellow, with total mass 1000 trillion Suns, passed through one another at 6 million mph, 3.4 billion light-years from Earth.

Being mostly empty space, the galaxies were almost unaffected. The two plasma clouds (ionized gas shown in pink) in which the galaxies were embedded interacted strongly, reducing their speed and producing the wedge-like shock front on the right. The purple clouds represent dark matter, reconstructed by computer analysis of gravitational lensing of even more remote sources. The plasma mass far exceeds the mass of visible stars, but their sum is not nearly enough to account for the observed lensing. An invisible mass far greater than the visible mass must account for this amount of lensing.

Since the dark matter aligns with the galaxies and not with the plasma, we know it too was unaffected by the collision, proving that dark matter interacts with normal matter and with other dark matter only gravitationally — an essential insight only gravitational lensing can provide.

Best Regards,

Robert

January 2020

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