Cobra Effect · Relativity
Gravity bends light
Why starlight bends around the Sun, and how a 1919 eclipse proved it.
7 cards, read aloud in 3:35, with a test and sources.
On 29 May 1919 the Moon covered the Sun, and two teams of astronomers were waiting.
One British team was on the island of Principe, off West Africa. The other was in Sobral, in northern Brazil. For a few minutes the sky went dark enough to photograph stars close beside the Sun. They were testing a prediction Albert Einstein had published in 1915. If he was right, those stars would be in the wrong place.
Einstein said the Sun’s mass curves the space around it, and light follows the curve.
Starlight grazing the Sun on its way to the Earth would be bent slightly toward it. Looking back along the bent light, we would see the star shifted a little outward, away from the Sun. Normally the Sun’s glare hides those stars. An eclipse was the only chance to see them. Even light, with no mass at all, has to follow that curve.
The shift Einstein predicted was tiny, just 1.75 arcseconds at the Sun’s edge.
That is about the width of a coin seen from nearly three kilometres away. A calculation using Newton’s gravity, treating light as tiny particles, gave only half as much. Einstein’s extra half came from gravity curving space itself. So the eclipse could decide between the two.
In November 1919 the results were announced in London, and they sided with Einstein.
The teams compared their eclipse photographs with pictures of the same stars taken at night, when the Sun was elsewhere in the sky. The stars near the Sun had moved outward, by roughly the amount Einstein predicted. Newspapers around the world carried the news, and Einstein became famous almost overnight. The measurements were rough, and some doubted them, but later eclipses agreed.
Radio telescopes now measure the bend far more precisely.
Each year the Sun passes close in front of certain distant quasars, which shine brightly in radio waves. Telescopes spread across continents track how far each quasar seems to shift. The results match Einstein’s figure to better than one part in a thousand. Gravity bends radio waves and every colour of light by exactly the same amount.
A whole galaxy can bend light so much that one quasar appears four times.
The Einstein Cross is a quasar billions of light years away, sitting almost exactly behind a much nearer galaxy. The galaxy’s gravity bends the quasar’s light along four different paths toward us. So telescopes see four copies of the same quasar, arranged in a cross around the galaxy. When the line up is perfect, the image spreads into a complete ring.
So when you look at a distant galaxy, ask what its light passed on the way.
Astronomers use the bending to weigh things they cannot see. Clusters of galaxies bend the light behind them into long arcs, revealing far more mass than their stars and gas can account for. That missing mass is called dark matter, and bent light is one of the best ways to map it. A star passing in front of another can brighten it for a few weeks, and that has revealed more than a hundred planets.
Sources
- Eddington experiment, Wikipedia. The two expeditions of 1919, what their photographs showed, the arguments over the results, and the repeat measurements since.
- Gravitational lens, Wikipedia. How galaxies and clusters bend the light behind them into arcs, rings and multiple images, and how astronomers use it to map dark matter.
- Tests of general relativity, Wikipedia. The bending of light alongside the other classic tests of Einstein’s theory, and how precise each has become.
Nearby ideas
- Falling feels like floating. Why falling feels like floating, and how that led Einstein to gravity.
- The constant speed of light. Why light is measured at the same speed by everyone, however they move.
- Gravity slows time. Why clocks run slower in stronger gravity, and why GPS has to allow for it.
- Space itself can ripple. Why colliding black holes shake space, and how LIGO finally felt it.
- Where not even light escapes. Why some stars collapse into places light cannot leave, and how we found them.
- Moving clocks run slow. Why moving clocks run slow, and how muons and airliners proved it.
- Mass and energy are the same thing. Why mass and energy are the same thing, and why a gram holds so much.