Ask a Scientist: How do black holes make ripples in space?

Patrick Treuthardt is the assistant head of the astronomy and astrophysics laboratory at the Nature Research Center at the N.C. Museum of Natural Sciences. Here, he talks about black holes and the recent discovery that has rocked the science world. Questions and answers have been edited.

Q. Why are black holes black?

A. When we talk about black holes, we talk about escape velocity. This is the speed at which you have to travel to break away from an object’s gravity and leave it behind forever. When you are in orbit around Earth, you need less speed to break away than if you are closer to the surface. The closer you get to the surface, the larger your escape velocity becomes.

Black holes take this situation to extremes. As you get closer and closer to a black hole, the escape velocity gets larger and larger. All the mass is compressed to a single point so you can never actually get to the surface of the mass. Eventually, the escape velocity reaches the speed of light – the fastest anything in the universe can travel.

This distance from the black hole is known as the “event horizon” and is used to describe the size of a black hole. Within the event horizon, the escape velocity is larger than the speed of light. Outside the event horizon, the escape velocity is less than the speed of light. So, a black hole is black because when something passes within the event horizon it will never escape. Even light cannot escape.

Q. On Feb. 11, scientists announced that they had detected gravitational waves – ripples in the cosmic fabric of space and time caused by the cataclysmic collision of two black holes over a billion years ago. How can space ripple?

A. Space ripples because mass bends space (as described by Einstein’s theory of relativity). If we imagine three-dimensional space as a two-dimensional rubber sheet, placing a Ping Pong ball onto the sheet will create a small dimple. If we place a bowling ball on the sheet, a larger, deeper dimple will be created. Basically, if we imagine two bowling balls rapidly spirally into each other and orbiting each other faster and faster as this happens, they will create waves in the rubber sheet to get rid of energy.

Q. Why do we know that these waves were produced by the merging of two black holes, and not some other cosmic disturbance, like the collision of some other celestial bodies, or death of a massive star? Does this finding finally put to rest any doubt that black holes exist?

A. We know that this signal was due to the spiraling of two black holes because of the size and shape of the waveform signal that was detected. The spiraling in of other celestial bodies would have generated a different signature. A supernova could also produce gravitational waves if the explosion isn’t symmetric, but again the signal would be different. The detection of these gravitational waves is the strongest evidence for the existence of black holes that we have ever found.

This story was originally published February 27, 2016 at 9:18 PM with the headline "Ask a Scientist: How do black holes make ripples in space?."