On 8 April 2024, a slice of land across North America will be treated to the rare and wondrous sight of a total solar eclipse. Total eclipses are awe-inspiring to experience, as the Sun is completely blotted out by a perfectly positioned Moon, turning day temporarily into night.
Millions of people are eagerly awaiting this eclipse, but from a scientific perspective, it’s unlikely it will come anywhere near the paradigm-shattering impact of the eclipse of 29 May 1919.
After the event, the New York Times ran a headline that began, “LIGHTS ALL ASKEW IN THE HEAVENS; Men of Science More or Less Agog Over Results of Eclipse Observations.” (Women of science could not be reached for comment.)
The headline wasn’t an overstatement. Astronomers had just observed a dramatic warping not only of our understanding of gravity but of space and time itself.
From an astrophysical point of view, solar eclipses aren’t particularly significant – they’re just the momentary alignment of the Sun, Moon, and Earth, at certain points in their orbits, predictable centuries out, 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 outshined by the bright light of the full Sun, such as the hazy solar corona and stars close to the Sun in the sky. It was this that astronomers found all askew back in 1919.
Albert Einstein’s theory of general relativity (GR), first published in 1915, was a fundamental re-formulation of how gravity works. Instead of a force between objects, GR says that what we experience as gravity is actually the bending of space in the presence of massive objects like stars, planets, and galaxies.
An apple falls toward the Earth not so much because of a force between the apple and the Earth but because the Earth is bending the space around itself.
The apple is following its natural path through space and time and finds that path curving in an Earthward direction. The Earth’s path is also very, very slightly moved, but it’s not a noticeable effect.
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. Just like a beam of light can be bent by passing through a piece of glass, its path through empty space can change if the space it’s passing through is curved.
This effect is called gravitational lensing, and it provided some of the first experimental confirmation that Einstein’s weird bendy-space theory actually works.
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The original papers Einstein published on general relativity included a set of predictions that could be used to confirm or rule it out. One of them, an explanation for an apparent discrepancy in the orbit of Mercury, was a definite point in GR’s favour.
But the second was something that would take new observations to check: a prediction that the presence of the Sun should bend any starlight passing by it.
This effect should be small, but detectable: the angle of deflection for a beam of starlight just grazing the edge of the Sun should be only a tiny fraction of a degree. The problem is, that close to the Sun, seeing any stars at all is impossible. Except, of course, during a total solar eclipse.
The total solar eclipse of 1919 allowed astronomer Arthur Eddington and his colleagues to photograph and measure the positions of stars around the disk of the Sun during those few moments of darkness.
When they returned with confirmation of Einstein’s calculations, it was widely hailed as a resounding success for GR and immediately made Albert Einstein a household name.
These days, gravitational lensing is one of astronomy’s most effective multitools, opening up incredible new avenues for cosmic exploration. Because lensing can magnify as well as shift or distort images, it allows us to see objects that are more distant than our telescopes would otherwise be able to detect.
It allowed us, in 2022, to observe the most distant single star ever seen. The star, named Earendel, was spotted in a Hubble Space Telescope image as a distorted smudge near a galaxy cluster, the image having been bent and magnified by the cluster’s mass.
The ancient star is so far away that its light has taken nearly 13 billion years to reach us.
Sometimes, it’s the lens itself that’s the target. More than 200 exoplanets (planets orbiting other stars) have been detected through gravitational microlensing. When a nearby star passes in front of a more distant star, it can lens it and make it appear temporarily brighter.
If the closer star has a planet and the alignment is just right, a tiny blip of extra brightness appears when the planet adds to the lensing.
Even invisible things can be revealed by gravitational lensing. Because the lensing effect depends only on mass, not any other properties of the lensing object, we can use lensing to map out the presence of dark matter, or, due to the extreme space distortion around them, light-swallowing black holes.
A huge part of science is the process of creating tools to better observe the natural world. But sometimes, the cosmos builds the tools itself. It’s up to us, as we stare, more or less agog, into the heavens, to figure out how to use them.
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