Science & Technology

Why Have We Still Never Seen a Black Hole?

We have images of exoplanets hundreds of light years away, stars in the midst of going supernova, and galaxies at the edge of the universe. You'd think, by now, that we had snapped pictures of every type of cosmic object that might be out there. And yet we're still waiting for the first images to return from the Event Horizon Telescope, the biggest attempt yet to directly image the environment around a black hole. That's right: the goal is only to capture what's around a black hole. We'll never have an image of a black hole itself because imaging a black hole is technically impossible. Here's why.

Related Video: Global Telescope May Finally See the Event Horizon of Our Galaxy's Black Hole

A Shadow in the Dark

The idea of black holes has been on the scene since Albert Einstein published his general theory of relativity, which says that the more massive an object is, the more it warps the fabric of the universe, known as spacetime. That warping produces a gravitational pull that makes surrounding objects orbit or even collide with their more massive neighbors. At the time, Einstein was perturbed by one pesky detail: According to his theory, if you compressed a star's mass into a small enough space, the laws of physics would break down to create a singularity, a place where density and gravity reach infinity. It wasn't even a year later that other scientists confirmed that this was no rounding error — massive stars actually collapse into this very thing all the time.

The peculiar forces at work in a singularity give rise to a sphere-shaped region of space with a gravitational pull so strong that not even light can escape. This is what we know as a black hole, and that definition hints at the reason we'll never technically be able to image one: it doesn't emit or reflect light. Because light is the only way we can see something, seeing a black hole should be impossible.

Of course, this is just a technicality; shadows don't emit light either, but you can still "see" them because of the contrast with the light around them. And that's how we plan to directly image a black hole: by directly imaging the light-emitting stuff around it. See, the border around a black hole that defines where you can and can't escape is known as the event horizon. Gas and dust that are pulled toward, but not past, the event horizon turn into an accretion disk that spins around the black hole at such tremendous speeds that it heats up and releases X-rays and gamma rays that telescopes can see. That's what the Event Horizon Telescope is after.

Incredibly Small and Incredibly Far

The Event Horizon Telescope is a truly momentous achievement: It combines 15 to 20 radio telescopes positioned all around the globe, spaced up to 12,000 kilometers apart, and points them all at the exact same object. This essentially gives us a telescope the size of our entire planet and allows for the kind of precision that could spot a fly on the moon.

That's good because contrary to popular belief, even the supermassive black holes at the centers of galaxies are relatively tiny. Take the one at the center of the Milky Way, known as Sagittarius A*, for example. It's our best bet for a direct image since it's the largest one in our sky. But that's not saying much: Although it's 4 billion times the mass of our sun, its diameter would fit into the distance between the sun and Earth. Combine that with the fact that it's 27,000 light years away, and you begin to understand why directly observing it is such a challenge.

But that's exactly what astronomers working on the Event Horizon Telescope say they've already done; it's just a matter of compiling the data and using special algorithms to translate it into a clear picture. Soon, we should have preliminary images that show the size, shape, and surrounding environment of our local supermassive black hole. It won't technically be the "hole" itself, but like a shadow on the wall, it should tell us a lot of what we need to know. (Update on April 4, 2018: the ESO has announced that it will hold a press conference on April 10 to announce a "groundbreaking result" from the Event Horizon Telescope. Stay tuned!)

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Hear the whole story about the quest to image a black hole in "Einstein's Shadow: A Black Hole, a Band of Astronomers, and the Quest to See the Unseeable" by Seth Fletcher. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer December 7, 2018

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