Space Matter: On Black Holes and the Quest to Observe an Event Horizon

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Space Matter: On Black Holes and the Quest to Observe an Event Horizon

Space Matter is a weekly column that delves into space science and the mechanics of spaceflight. From the latest discoveries in the universe around us to the fits and starts of rocket test flights, you’ll find analysis, discussion and an eternal optimism about space and launching ourselves into the cosmos.


This month, scientists are going to look at the event horizon of a black hole through a telescope of the first time. Let’s talk about why that’s important.

Einstein’s theory of relativity first predicted black holes, which are compressed areas where matter is packed in so tightly that it warps the very fabric of spacetime. Black holes are so massive (which is different from size: size is how large something is in terms of volume, mass is how much matter is packed into a certain container, in this case a black hole) that even light cannot escape them; the escape velocity of a black hole is above the speed of light, which is theoretically impossible. Black holes reflect no light; they are the perfect black. The only thing they emit is Hawking radiation (named after Stephen Hawking), which is entirely theoretical and also theoretically impossible to directly detect.

How, then, do we know they exist?

Like dark matter and dark energy, we can detect black holes through their effects on the space around them. Black holes form when giant, massive stars at the end of their lives collapse in on themselves. Basically, the stars are so massive that their own gravity overwhelms their ability to hold a structure. Black holes consume their surroundings, eating up matter around them, which is how they grow. And that’s part of how we can detect them.

All Sam Neill jokes aside, the event horizon of a black hole is the area of visible light and matter surrounding a black hole. It’s the area where the black hole’s gravitational effects are so strong that no light or matter can escape. Everything in the event horizon of a black hole is doomed. It will eventually be consumed. However, to an observer that’s far away from the black hole, it will appear as though nothing is happening in the event horizon because of time dilation. Time (which is relative) appears to move more slowly in the event horizon of a black hole than outside it. The further away you are from a black hole, the more drastic the time dilation gets, to the point where it takes an infinite amount of time (from a far away perspective) for the black hole to actually consume the matter within its event horizon. If you’re IN an event horizon, you won’t notice time is moving slowly, because for you, time is moving at a normal speed. It’s all relative.

All the matter that has accumulated in the event horizon waiting to be consumed by the black hole is what we call the accretion disk, and this is what informs our pop culture vision of black holes: a black area in space surrounding by a gorgeous, light-filled spiral of destruction. While a black hole and its event horizon aren’t visible, the accretion disk, which is also (and perhaps better) known as a quasar, is. Quasars release all kinds of energy into the universe, from x-rays to gamma rays, which we can detect.

Like most galaxies, NGC 4639 has a quasar at its center. Photo: ESA/Hubble & NASA CC BY 2.0

ln February 2016, scientists made history when they announced that they had detected gravitational waves dating back to the earliest history of the universe. These waves, the existence of which had been theorized by general relativity, are created when two black holes merge. This was a revelation for a number of reasons. We’ve observed black holes via the emissions from their accretion disks before, including the supermassive black hole at the center of our own galaxy, but their existence is still technically theoretical. The direct observation of a predicted wave, theorized as a product of black holes, is another step toward actually proving their existence.

Additionally, it has repercussions for the future of cosmology, or the science of the origin of the universe and how it works around us. By studying cosmology, we understand more about our existence, why we’re here, and how we came to be.

And all of THAT is why it’s so exciting that, starting April 5, astronomers and cosmologists will, for the first time, look through the Event Horizon Telescope to directly observe the event horizon of a black hole: the supermassive black hole at the center of the Milky Way called Sagittarius A.

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Sagittarius A is the supermassive black hole at the center of the Milky Way. Image: X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI

The Event Horizon Telescope is not actually one telescope. Instead, it’s a network of nine different radio telescopes/telescope arrays around the globe that work together to create one giant, precise, sensitive radio telescope. This can be accomplished through the technology of very-long-baseline interferometry (VLBI), in which multiple radio telescopes around the world are pointed at one space object. By using radio telescopes from around the world linked together, it’s the functional equivalent of having a telescope the size of the entire planet pointed at the center of our galaxy.

Astronomers are planning on seeing the quasar, or accretion disk of dust and gas, that surrounds the black hole. But what they’re also hoping to see is the event horizon: a bright crescent surrounding a black area of space. They also have high hopes that they may actually be able to see the event horizon’s shadow against the accretion disk. If they are able to capture these images, this will go a long way to moving event horizons from the theoretical to the proven. It’s just a matter of (heh) time.

And cue all the Sam Neill jokes.

Top photo: NASA, ESA, and D. Coe, J. Anderson, and R. van der Marel (STScI)

Swapna Krishna is a freelance writer, editor and giant space/sci-fi geek.