Black holes form from massive stars

Even if Matthew McConaughey hadn’t been in it, Interstellar still would have been a nerd heartthrob sensation. The movie incorporated real scientific theory — backed by California Institute of Technology physics professor Kip Thorne — into its storyline in order to depict the harsh realities one might actually encounter during space exploration. In the movie, Matthew McConaughey saves the human race by traveling into a black hole, communicating with his daughter in the past through mysterious means, and then somehow escaping the black hole. Okay, so maybe not everything was realistic.

Despite the fictitious components of the movie’s ending, black holes are real phenomena, and their properties pose daunting questions to the field of modern physics. Black holes were first postulated by Albert Einstein in his paper The Foundation of the General Theory of Relativity as the byproduct of a collapsing star.

A star is essentially a ball of immensely hot gas. The high temperature of a star causes the gasses within it to fuse together to form other atomic elements, one of which is iron. According to Einstein’s famous equation E=mc2, the mass loss associated with fusion can be converted into energy. Iron, however, does not release energy when produced through fusion. Instead of transforming into energy, it sits in the star’s core as mass.

According to Isaac Newton’s theory of gravitation, the gravity a body exerts increases as its mass increases. Einstein’s theory predicted that the gravity of a star could become so massive that eventually, its mass could not stably resist its own gravitational pull, and a star could thus collapse inward on itself. If a star is massive enough, when it collapses inward, it condenses so much mass into such a small space that the gravity generated is strong enough that an object traveling at the speed of light cannot escape the gravitational pull. In other words, the gravity is so strong not even light can escape, making the condensed star appear black. This condensed star is what we call a black hole.

If black holes have such an incredibly strong gravitational pull, why doesn’t everything in the universe get pulled into them? The answer to this question has to do with Newton’s theory of gravitation and a term called the “event horizon.” Although Newton’s theory of gravitation states that the gravity a body exerts increases with mass, it also states that gravity decreases proportionally with the distance between the body exerting the gravitational force and the body experiencing the gravitational force.

In practice, this means the farther away a body moves from the center of a black hole, the weaker the black hole’s gravity is for that body. The event horizon is the threshold outside of which a black hole’s gravity is too weak to pull something in. Anything inside of the event horizon cannot escape the force of the black hole’s gravitational pull, and anything outside will not fall into the black hole.

One of the many amazing properties of black holes is the time dilation that occurs between a body falling through a black hole and an observing body outside of the black hole. For example, say you and a friend decide to determine how much time it takes to fall through a black hole. Feeling brave, you tell your friend you will travel through the black hole while she remains a safe distance from the event horizon. How do you and your friend’s recorded time differ? For you, it takes a finite amount of time to travel to the event horizon, and a finite amount of time to reach the center of the black hole. For your friend, however, the situation appears differently.

As you get closer to the event horizon and thus farther from your friend, the light beams your body emits take a longer time to travel to your friend. The instant you cross the horizon, however, the light from your body cannot escape the black hole, and thus cannot reach your friend until an infinite amount of time has passed; thus, you appear to your friends to be frozen at the event horizon, despite the fact that you have crossed the horizon.

The above scenario ignores the crushing forces that are exerted on a body as it travels through a black hole. In fact, the gravitational forces in a black hole are so sensitive to distance that the forces on the bottom of your body would be significantly greater than the forces on the top of your body, and the black hole would rip you apart. Fortunately, we don’t have any stars in our solar system that have nearly enough mass to collapse into a black hole and turn us into Silly Putty. If we did, though, maybe it wouldn’t be a bad idea to have McConaughey on speed dial.