Kate: Hey Jeremy, do you know what time it is?

Jeremy: I sure do, Kate. It’s time for us to answer another question on *The Sound of Science*, presented by NIU STEM Outreach and WNIJ.

Kate: Today’s question comes from Josh in Elmhurst, who asks:

Josh: I would like to know how small a human has to get to create a black hole.

Jeremy: What a great question Josh! To answer, let’s start by talking a bit about black holes in general.

K: Black holes are created when very large stars reach the end of their lives. As the star runs out of fuel, the outward force caused by nuclear fusion subsides and can no longer balance the force of gravity trying to collapse the star.

J: If the star is large enough, gravity can collapse it all the way down to a singularity. That means all the matter the star contained gets compressed into a space so small it has no physical dimensions.

K: That means a singularity can be unimaginably dense - so dense that the gravity around it becomes strong enough that not even light can escape its pull.

J: When we talk about the surface of a black hole, what we usually mean is something called “The Event Horizon.” This is the area around the singularity where gravity is strong enough to capture passing light. Sort of like the point of no return.

K: Light that passes near the singularity but stays outside the horizon is refracted, or bent, as it passes by, but it isn’t captured. That means scientists can observe black holes by looking at how light bends as it passes near them.

J: Scientists sometimes refer to the event horizon of a black hole as the Schwarzschild Radius. In 1916, Karl Schwarzschild came up with an equation used to predict the size of the event horizon, based on the mass contained within the singularity.

K: So to answer your question, Josh, we would just take the average mass of a person and plug it into Schwarzschild’s equation.

J: If we assume the average person has a mass of around 70kg, that would give us a Schwarzschild radius of 0.1 yoctometers. Or in other words, a decimal point, followed by 25 zeros and a 1.

K: In fact, that’s so small you could line up about 2 billion of them side by side and they still wouldn’t be as wide as a single electron.

J: What a great question Josh. If you have a question you’d like us to answer, be sure and send it to us at STEMoutreach@niu.edu.

K: Thanks for listening. This has been the Sound of Science on WNIJ, where you learn something new every day.