One hundred years ago this month, the astronomer Arthur Eddington and a colleague went to an island off the coast of Africa to photograph a total eclipse of the sun. But they had no time to glance at the “marvelous spectacle.” They were too busy taking photographs, on glass plates, of some stars behind the eclipsed sun. (In broad daylight the sky is too bright to see the stars.)
They were testing a prediction made in 1916 by Albert Einstein that all masses distort spacetime, space and time not being independent in the grand scheme of things, and that distortion actually is gravity. A star field behind the sun would be magnified as if the sun were a lens – a gravitational lens. Comparing the positions of their images on plates taken at night weeks earlier they confirmed Einstein’s prediction, making him an instant celebrity.
If you could squeeze the sun until its radius is 3 kilometers, a cubic millimeter would have a mass of about 18 billion tons. Passing light rays would not just be bent, but sucked in. The gravitational field at its surface would be so strong that nothing inside could possibly get out. Not even light. It would be a black hole. Fortunately, we cannot squeeze the sun down, but more massive stars can collapse under their own weight, and then they suck in material, even swallowing whole stars, and grow. Our Milky Way galaxy has a black hole in its center with a mass of about a hundred million suns. The galaxy called M87 has a much more massive black hole, 6.5 billion suns, a monster as big as our solar system.
On April 10, the Event Horizon Telescope published an exciting image of that black hole. If no light can escape, how can it be seen? It cannot, not really. But that amazing image was made by cleverly combining signals from eight microwave telescopes spread over Earth, all recording signals from M87 at the same time for a few hours. Combining them, the resolution was a thousand times better than that of the largest single telescope. The microwaves emitted by hot matter surrounding it are converted to a visible image. The blurry “shadow” in the middle making it look like a donut results from the (much smaller) black hole sucking in matter, never to escape.
