Gravitational lensing, demonstrated by a solar eclipse that happened over a century ago, has since enabled scientists to observe more of the Universe than was previously possible
DR KATIE MACK (@AstroKatie)Katie is a theoretical astrophysicist. She currently holds the position of Hawking Chair in Cosmology and Science Communication at the Perimeter Institute for Theoretical Physics.
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On 8 April 2024, a slice of land across North America was treated to the rare and wondrous sight of a total solar eclipse. The awe-inspiring event saw the Sun completely blotted out by a perfectly positioned Moon, temporarily turning day into night.
Millions of people watched eagerly, but from a scientific perspective, the spectacle simply couldn’t compare to the paradigm-shattering impact of the solar eclipse on 29 May 1919.
After the event, the New York Times ran a story on it with the headline: “Lights All Askew In The Heavens; Men of Science More or Less Agog Over Results of Eclipse Observations.” (The women of science couldn’t be reached for comment.) The headline wasn’t an overstatement. Astronomers had observed a dramatic warping not only of our understanding of gravity, but of space and time.
From an astrophysical point of view, solar eclipses aren’t particularly significant – they’re just the momentary alignment of the Sun, Moon and Earth, having little effect on anything other than a few minutes of highly localised day/night confusion.
But they do give astronomers a special opportunity to see things that are usually outshone by the light of the Sun. Such things as the hazy solar corona, for example, or any nearby stars. And it was the nearby stars that astronomers found all askew back in 1919.
It all relates to Albert Einstein’s theory of General Relativity. Published in 1915, it was a fundamental re-formulation of how gravity works. Instead of a force between objects, General Relativity says that gravity is actually the bending of space in the presence of massive objects such as stars, planets and galaxies.
An apple falls toward the ground not so much because of a force between the apple and Earth, but because Earth is bending the space around itself.
As it happens, when space is bent by something with mass, it’s not just other massive objects whose paths are altered – it’s light, too. This effect is called ‘gravitational lensing’ and it provided some of the first experime
