Black Holes: What are they and how do we know they're there?

Black holes are one of the most popular scientific topics, and many of their properties are straightforward to understand with an intro-level understanding of physics. Starts With a Bang has an excellent article describing the basics of what a black hole is and how we look for them--even though, by definition, we can't see them directly.

If you add enough matter to a star, Siegel writes, the gravity would be so strong that "not even light would be able to escape. As Hawking (and others before him, going all the way back to John Michell in the 18th Century) have noted, this would create a black hole in space, where matter (and other forms of energy) could fall in, but nothing — no matter, no light, no nothing — could get out."

But what does this concept of "escaping gravity" mean? If you wanted to "escape" the earth's gravity, for example, how would you know you had accomplished it?

The answer lies in thinking about energy.

You probably learned at some point in school that energy primarily comes in two forms: kinetic energy (energy associated with movement) and potential energy (energy associated with where you are). These concepts help you determine, for example, how hard you would need to roll a ball if you wanted the ball to make it over a hill. The higher the hill, the more kinetic energy you'd have to give it at the beginning.

This relationship is determined by a law called the conservation of energy: The total amount of energy in the universe has to remain the same. In the case of "escaping gravity," that means you need enough kinetic energy when you launch from the earth to overcome to amount of potential energy you have at launch (the size of the hill). "Having enough kinetic energy" means having a fast enough speed, and it's actually pretty straightforward to calculate this escape speed.

So, when we say that "light can't escape a black hole," what we means is that the escape speed from a black hole is higher than the speed of light!