Editor’s note: This is part of a series called “The Day Tomorrow Began,” which explores the history of breakthroughs at UChicago. Learn more here.
Black holes are regions in space where an enormous amount of mass is packed into a tiny volume. This creates a gravitational pull so strong that not even light can escape. They are created when giant stars collapse, and perhaps by other methods that are still unknown.
Black holes fascinate both the public and scientists—they push the limits of our understanding about matter, space and time.
Scientists at the University of Chicago and across the world have made many discoveries about our universe with the help of black holes, but there’s a lot we still don’t know about these extraordinary cosmic phenomena.
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What is a black hole?
Black holes are made of matter packed so tightly that gravity overwhelms all other forces.
When you pick up a bowling ball, it’s heavy because the matter is densely packed. If you packed more and more mass into the same tiny space, eventually it would create gravity so strong that it would exert a significant pull on passing rays of light.
Black holes are created when massive stars collapse at the end of their lives (and perhaps under other circumstances that we don’t know about yet). One of the first steps toward the discovery of black holes was made by University of Chicago professor Subrahmanyan Chandrasekhar, when he realized that massive stars would have to collapse after they ran out of fuel for the fusion reactions which keep them hot and bright.
The universe is full of black holes. In the past decade, scientists have detected the signals of their collisions and taken images of the light from the gas swirling around them—and this has helped us learn many things about the universe. For example, black holes have helped us test Einstein’s theory of general relativity, which describes how mass, space, and time are related to one another. Scientists think they can tell us much more about these and other essential rules of the universe.
And on a more personal level, the supermassive black hole at the center of our own Milky Way galaxy may have played a role in how Earth came to be here!
What do black holes look like?
Black holes themselves are invisible—they emit virtually no light and so cannot be seen directly. But we have developed several ways to find them anyway.
By looking for the stuff that’s falling in. If material is falling into a black hole, it travels at such high speeds that it gets hot and glows very brightly, and we can detect that. (That’s how the Event Horizon Telescope took its famous first images of black holes.) Scientists hope to use this method to learn a lot more about how and what black holes “eat.”
By seeing their gravity pulling on other things. We can find black holes by watching the movements of visible objects around them. For example, a black hole’s gravity is so strong that nearby stars will orbit around them, so we can look for stars behaving strangely around a patch of “empty” space. From this, we can calculate exactly how heavy that black hole must be. That’s how Nobel Prize winner Andrea Ghez and her team detected the supermassive black hole at the center of our own galaxy.
By detecting the gravitational ripples when they collide. We can also detect black holes by detecting the ripples in space-time created when two of them crash into each other. From that signal, we can tell how massive the black holes were, how far away they were, and how fast they were traveling when they collided.