Black holes

Washington Post

Black holes have been a mystery to us for most of history. Physicists didn’t start thinking about them until 1916 and we didn’t have the tools to begin to understand them until 1958. Now, in 2019, we have our first actual photo of one and the Internet is abuzz with excited discussion. What are black holes? How do they form? Where is the closest one? What happens beyond the event horizon? Unlike the physics surrounding them, these questions are relatively simple to answer and they’ll surely come in handy when your coworkers inevitably bring up the new photo that’s making the rounds.

A Hole In The Void?

When we peer out into space, whether you’re using a backyard telescope or your naked eye, no matter how hard you search, you aren’t going to find a black hole. At least, you won’t find one directly. Though they live at the heart of most galaxies and take the place of some massive stars after they die, black holes are notoriously difficult to view. Other than being enigmatic, what are black holes and why are they so difficult to observe?

To put it simply, black holes are inescapable “sinkholes” in space. A lot of times, they’re visualized as a sort of cosmic whirlpool, but contrary to what these images imply, the hole itself isn’t rotating. Beyond the visible limits of black holes, nested deep inside the event horizon, is something called a singularity. All black holes have them, and it’s the mass of the singularity that determines the scale of the black hole. Like any object, the singularity pulls things toward it relative to its density and mass. The more massive the singularity, the harder the pull and the more matter gets drawn in toward the black hole. The closest visible cousin to a black hole’s singularity is a neutron star. One teaspoon of matter from a neutron star weighs ten million tons. To escape the gravitational pull of a neutron star, a ship would have to be traveling at about half the speed of light.

For an object only nine miles across, the pull of neutron stars is incredibly strong, but it has nothing on the singularity of a black hole. Singularities are so dense that their immediate gravitational field requires an escape velocity (the speed necessary for an object to escape its pull) beyond the speed of light. As a result, the singularity of a black hole is hidden from view, because any light generated by it or the material being pulled toward it can’t escape. Until very recently, the only way astronomers had been able to observe black holes was by the jets of ionized radiation ejected into space.

Neighborhood Sightseeing

If you’re looking to find a black hole yourself, your best bet is to look toward the heart of our galaxy on a clear summer night. Between the constellations Sagittarius and Scorpius, you’ll see a misty bright spot. That bright spot is the central bulge of the Milky Way, known as Sagittarius A, and the dark web obscuring its view is dust and debris from the galaxy’s spiral arms. In the center of that bright cluster, 26,000 light-years away, lies the Milky Way’s resident singularity, the supermassive black hole Sagittarius A* (pronounced Sagittarius A-star). While you won’t be able to see much of them in the visible spectrum, black holes are best observed through x-ray telescopes, such as NASA’s Chandra X-Ray Observatory.

When the Event Horizon Telescope gathered its data on the supergiant elliptical galaxy M87, Chandra also took some photos of the heart of the galaxy. While astronomers still don’t know how the supermassive black holes at the centers of galaxies form, their smaller-scale siblings, stellar black holes, form as the last hurrah of a dying star. Not all stars transition into an afterlife as a black hole. Stars the size of our sun inflate to red giants before sloughing off gas and dust and becoming planetary nebulae. Only the most massive stars will swell to supergiants before going supernova and either becoming an ultra-dense neutron star or an even denser black hole. The extra-large variety of supermassive black hole, such as the one found in the center of M87, the product of galactic mergers. Galaxies often collide, but sometimes, they hit head-on, rearranging orbits and combining central black holes to form behemoths like the one photographed by the Event Horizon Telescope.

Taking The Plunge

The movie Interstellar broached the subject of orbiting a black hole. What happens when you get close to one? While the movie’s answers are speculative and simplified for ease of understanding, the real physics gets pretty bizarre. In short, the closer you get to a black hole, the stronger the pull of gravity gets. The term “spaghettification” is a fun way of explaining the effect of exponentially increasing tidal forces. If you were to dive feet-first into a black hole, your legs and ankles would get stretched away from your head as the gravity pulled ever-harder on them with the top half of your body lagging behind. You would be turned into a human noodle, long and thin.

Outside the event horizon, which is the point of no return where gravity becomes too strong for even light to escape, Einstein’s theories of relativity are on full display. The velocity and acceleration of matter around the black hole cause time dilation, seemingly slowing time the closer you get to the event horizon. It’ll be a long time before we have to worry about getting sucked into a black hole, but if you ever find yourself piloting a starship in your dreams, be sure to give those cosmic behemoths plenty of space.