If a black hole came wandering through the solar system, would we notice? We're joined by the Bad Astronomer, Dr. Phil Plait, to chat about the possibility of Earth meeting its end via gravitational catastrophe. This Pulsar podcast is brought to you by #MOSatHome. We ask questions submitted by listeners, so if you have a question you'd like us to ask an expert, send it to us at sciencequestions@mos.org.

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ERIC: From the Museum of Science in Boston, this is Pulsar, a podcast where we search for answers to the coolest questions we've ever gotten from our visitors. I'm your host, Eric. And today we're going to put our planet in peril by answering a question we get with surprising frequency. Could the earth ever be destroyed by a black hole? Joining me is the person I usually think of when I'm contemplating planetary annihilation: the Bad Astronomer, Dr. Phil Plait, author of Death From the Skies: These Are the Ways the World Will End. Dr. Plait, thanks so much for being here on Pulsar.

PHIL: Oh my pleasure, I'm really happy to support the museum.

ERIC: So it turns out a lot of people ask us this question only having heard of black holes as awesome, powerful things in space, and not really knowing what they are. So can we start with an overview of what makes a black hole a black hole?

PHIL: Sure! What makes a black hole a black hole is its intense gravity. The gravity that you feel from an object depends on two things: how much stuff is in that object - how much mass it has, and how close you are to it. And if you take a lot of mass and squeeze it into a smaller and smaller volume, you can get closer and closer to it, because it's smaller. And when that happens, the gravity gets stronger and stronger. And at some point, if you have enough mass squeezed into a tiny enough volume, you get a black hole. Black holes have really, really, really strong gravity.

ERIC: Yeah that Hollywood, maybe like, general perception of a black hole being a hole that opens in space: not really true, it's just really powerful gravity. It still can do a lot of annihilation, but it's not something that's going to just appear somewhere, like, right next to the earth.

PHIL: And the power of the black hole comes from the fact that its gravity gets more intense the closer you are to it. And if you're heading towards the sun, there's only so close you can get to the sun before you kind of smack into its surface. And that's the most gravity you can feel. But if you take the sun and then compress it down into a ball that's just a few miles across. Yeah, it becomes a black hole, you can keep getting closer and closer to it, instead of being basically 400,000 miles from the center or something like that, you can get much, much, much closer, you can just be a few miles from the black hole. And so the gravity you feel is really, really strong. And that's where their destructive power comes from.

ERIC: Now we get that question a lot, too. What would happen if the sun magically turned into a black hole?

PHIL: Now gravity fades with distance. Replace the sun with a black hole, we wouldn't even really notice. I mean, we'd freeze, right? We wouldn't be getting light and heat. But the gravity we'd feel from the Sun is the same because the mass of the Sun stays the same. And we're the same distance from it. That's the critical thing. So if you take a black hole and put it really far away, you don't feel that much from it.

ERIC: Okay, so distance is a factor. But if one got closer, could a black hole destroy the Earth?

PHIL: And I want to be careful here, because I get this question a lot, too. And let me say, can a black hole destroy the Earth? Yes. Will a black hole destroy the Earth? Probably not. You know, you never say no. Because you have to kind of qualify it and say, look, there are a lot of black holes in the Milky Way galaxy, millions and millions of them really, and that sounds terrifying. But space is big. And that's why we call it space. And they're really, really, really far away. And if you do the math, it's actually fairly complicated. How often would a black hole get close enough to the sun into the earth to do damage? And the answer is essentially never, over the entire age of the universe. This probably has not happened. If it had happened, we wouldn't be here. So you know, right away that tells you over 4 billion years it hasn't happened, even once.

ERIC: So nothing to lose sleep over, which is great. But say one did wander into the solar system. What kind of things would we notice first?

PHIL: So if we pretend that this will happen, and again, probably not, but if it were to happen, your typical black hole, which is 10 or 20 times the mass of the Sun, is very small. It's only a few dozen miles across, and it's black, so it doesn't emit light. And if it's just out there in space, we won't see it. Now it may have some effect on starlight. Starlight passes it, it gets bent and warped. And we might notice that, that would be that would be pretty cool, although terrifying. But the effect it would have gravitationally on us would first probably be with the outer planets or even these chunks of ice and rock that orbit the Sun even out past Neptune. These are like gigantic comets and the gravity of the black hole would probably disturb those. And we see these things falling in towards the sun and think, Wow, we're getting a lot of comets lately, what's going on with that? Oh, maybe it's a black hole. And then as it gets closer, the orbits of the planets would change. And we'd see that because we know how the planets go around the sun. And then if it got close enough to Earth, then it would be bad. The moon might get pulled out of its orbit. And then there are the tides. Gravity changes with distance. The closer you are to something, the stronger the gravity. And if one side of the Earth is closer to the black hole than the other side, that means that there's a different force on the two sides of the earth. And that will stretch us like an egg. And if the black hole gets close enough, it will tear the earth apart.

ERIC: Sounds like all kinds of bad things could happen if a black hole were to wander by, but as you said, space is really big. What is the closest one that we know of to the earth?

PHIL: That's a hard question to answer. In fact, for a long time, it was this one called Cygnus X one that's over 1,000 light years away. There is one recently discovered, which I'm going to put in air quotes, which is closer, but it's not convincing that it's a black hole just yet. The problem is because they're black, we can't see them directly. But if they're orbiting a star, we can see their effects on the star. And it's not clear that what we're seeing going on with the star is actually because of a black hole.

ERIC: But still not really in our neighborhood. 1,000 light years away, that's really far. And that's not gonna affect us anytime soon, or ever.

PHIL: That's correct. For something to have an actual effect on us, it would have to be certainly no farther than a light year away. If we're talking about sort of a normal black hole that we we talk about when a star explodes and the core collapses, and it forms what we call a stellar mass black hole, it would have to be pretty close. And the odds of that happening are extremely rare. Think of it this way: there are hundreds of billions of stars in the galaxy, thousands of times more stars than there are black holes. And yet a star has never really passed close enough to the Earth to damage us over the four and a half billion years we've been around. So the odds of a black hole getting closer are a thousand times worse. So I'm not too concerned about it.

ERIC: Now, black holes don't show up too often in science fiction, but you got your nickname by pointing out the bad astronomy in movies, like nonsense with moon phases not changing, or magical orbits on space stations that have unexplained gravity. So I have to ask, are there any particularly bad or good representations of black holes from popular films?

PHIL: Yeah, if you watch Disney's The Black Hole from the 1970s, it's not great. But the movie Interstellar, which came out a few years ago, and I'll preface this with saying, I didn't care for it, not my cup of tea, but the representation of black holes, it was done very well. They talk about the intense gravity, they talk about how black holes can spin, they talk about the tides from a black hole. And they also talked about, and this was an important part of the movie, time dilation. The closer you get to a black hole, and the stronger the gravity is, your clock ticks slower. Time flows more slowly for you compared to somebody really far away from the black hole. And that's a real effect. It's not like a mechanical thing where your watch is just slowing down. Time literally moves more slowly. Not only was that depicted fairly well in the movie, it was a plot point. And it was kind of kind of an important thing that happened there. So that was pretty neat. And also just the visual representation of the black hole. The way light bends around it. That was all done really really well.

ERIC: Alright, so it sounds like we shouldn't lose any sleep over black holes. But that doesn't mean we shouldn't keep studying them because they they do sound pretty awesome. And they do sound like some of those powerful forces in the universe.

PHIL: That's correct.

ERIC: All right, Dr. Plait. Thanks so much for joining us here on Pulsar.

PHIL: Thank you.

ERIC: For more awesome and terrifying things that could destroy entire planets, come talk to our staff at the Charles Hayden Planetarium at the Museum of Science. And be sure to check out Death From the Skies: These Are the Ways the World Will End. wherever science-y books are sold. Until next time, keep asking questions.

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