Black holes are, by definition, invisible. Light that falls into one never comes back. You cannot photograph them, probe them, or measure them with any conventional instrument.
And yet in 2019, scientists published a picture of one. In 2022, they published another. And from those images, they calculated the black hole's size to remarkable precision.
Here's how.
The Shadow Trick
A black hole itself emits no light. But the superheated gas swirling around it - the accretion disc - glows brilliantly. And the black hole, sitting at the centre, carves out a dark region: a shadow.
While light cannot escape from the interior of a black hole, it is possible for light to escape from the region surrounding the event horizon, depending on its trajectory. The result is a murky no man's land just beyond the point of no return - which appears to observers as a shadow.
Measure the shadow, and you can calculate the size of the black hole that cast it.
The Earth-Sized Telescope
The problem is scale. The shadow of the black hole at the centre of the galaxy M87 - 6.5 billion times the mass of our Sun - spans roughly 40 billionths of a degree across the sky as seen from Earth. That's like reading a newspaper on the surface of the Moon.
No single telescope on Earth can achieve that resolution. So scientists built a virtual one.
The Event Horizon Telescope (EHT) created a virtual Earth-sized telescope by linking existing observatories around the globe using novel systems - creating a new instrument with the highest angular resolving power ever achieved.
Radio telescopes on multiple continents observe simultaneously. The data is combined using a technique called interferometry, and the result is a resolution equivalent to a telescope the size of Earth itself.
Reading the Image
The black hole shadow has a precise mathematical definition that depends only on the geometry of the black hole itself - not on what is happening to the matter around it.
Due to the curved trajectories of light rays, the black hole shadow as seen by a distant observer appears approximately 2.6 times larger than the event horizon itself. Scientists account for this magnification when converting shadow size to actual black hole size.
The M87 black hole came out at roughly 40 billion kilometres across - larger than our entire solar system.
The Bigger Point
Scientists didn't discover the size of a black hole by going near one. They measured a shadow, cast across 53 million light-years of space, using a telescope assembled from pieces scattered across seven continents.
It's the longest ruler ever built. And it works.