365daysDate: August 3, 2009

Title: Black Hole Street Smarts


Podcaster: Lia Corrales and David Tam

Organization: Columbia University Astronomy

Description: We will explore the history and legacy of the most publicly known yet misunderstood astronomical object. We’ll be interviewing people on the streets of New York City to highlight common questions and misconceptions about Black Holes.

Bio: Lia Corrales was inspired into a life of astrophysical research through space documentaries and Stephen Hawking. She is now a second year Astronomy grad student at Columbia University. She also enjoys dancing, rock climbing, and reminiscing about Astrocamp.

David Tam is the Sr. Technician with the Columbia University Department of Physics. He received a B.A. in physics from Columbia in 2006, where he conducted research in experimental cosmology. His interests include machining and welding, the science of recording and radio broadcasting, and playing bluegrass cello.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Palomar Observatory.



LC – Lia Corrales
DT – David Tam


LC: Hello everyone, and welcome to Columbia Mondays. My name is Lia Corrales and I’m a second year astronomy graduate student at Columbia University in the City of New York.

DT: And I’m David Tam, I’m a technician with Columbia’s physics department.

LC: Today’s podcast will review one of my favorite topics in astronomy, black holes, from the public’s perspective.

DT: or, Black Hole Street Smarts!

LC: We went to Union Square here in New York City to find out what exactly the average person on the street knew about black holes. I wanted to know what people wondered about black holes. What we found was both exciting, and surprising! Several of the people we approached had a little bit of a black hole enthusiast in them.

LC: Are you a black hole enthusiast?

Woman: I am a black hole enthusiast!

DT: We started off trying to get a general sense of the public’s perspective on black holes.

LC: The first question we asked people was, Well, what do you think a black hole is?

Woman 1: About black holes?

LC: Yeah, do you know what they are?

Woman 1: I think I learned something about black holes. Whether I remember what it was right now… I know there’s a whole section in the Museum of Natural History. I remember spending like an hour there.

Woman 2: I think everybody’s heard of them, but just don’t know exactly what they are.

DT: We had a range of answers to these questions, but it turns out some of the people were mostly right.

DT: So, we’re wondering if we could ask you, what are your thoughts on black holes?

Man: Black holes.

DT: I mean, do you know what they are?

Man: Yeah.

DT: What do you know about them?

Man: Well, I know that it’s the final stage of a massive star.

LC: Oh wow!

Man: When it dies, all the gravitational pull, it’s greater than the forces that keep the star alive, so it concentrates into a singularity.

LC: A black hole is an object from which light cannot escape due to the force of gravity. It’s an object that has become so dense that no other known force can resist the effects of gravity. This is how it can become, theoretically, infinitely dense with zero size.

DT: I’ve heard a lot of people talk about black holes as rips in the fabric of space-time.

Man: So it kind of rips the time and space, it makes a hole, literally. Well, that’s what theory says.

DT: Is this true?

LC: Yeah, he’s doing a really good job.

Man: I don’t know if it’s literally like you take a paper or something and you make a hole, like literally a hole, that concept is, like, vague on me, but I think it’s a very interesting phenomenon.

DT: What do you think about this picture?

LC: Well, very often scientists like to invoke the image of a rubber sheet to talk about space and time, and massive objects can bend that rubber sheet. In other words, they bend space and time, causing light and other things to curve around them. So when people portray a black hole as a little rip in space-time, I don’t think it’s necessarily fair, and here’s why. Black holes tend to have lots of spooky properties, for example, they have, you know, infinite density, zero size. But, it’s really just the fact that light can’t escape from it that makes it difficult to probe the internal workings of a black hole. So, you can’t really send a probe or an astronaut in there in order to figure out what’s going on inside a black hole. So whatever happens beyond the point of no return, which we call the “event horizon,” is still very theoretical. Before I go on I should mention that there are really two types of black holes that an astronomer might talk about. There are super-massive black holes, which are the giant black holes found at the center of most galaxies. They can be millions to billions of times as massive as the sun. The way these monsters form is still a very active area of research in astronomy, so I’m not really going to talk about them too much. The other types of black holes we’ll talk about are stellar-mass black holes. These ones are usually ten to twenty times the mass of the sun. Those are the ones we’ll talk about most.

DT: And those were the ones that people tended to know the most about. So Lia, what would you say people knew the most about black holes?

LC: Great question. So a lot of people we interviewed had this vague idea that black holes come about because of some form of collapse.

Man: It’s, uh, when gravity, is, something collapses into itself.

Man: Isn’t it like, collapsing of, like, a…

Woman: …when a star collapses? Yeah?

LC: That’s right. So stellar-mass black holes form when a star collapses, basically. Not all stars form black holes, just the most massive ones. They shed their outer layers in an explosion, or supernova, leaving behind a tiny but massive core. This tiny remnant of a supernova will have one of two fates. It could become a neutron star or it could become a black hole. The main determiner of its fate is its mass versus its size. If it shrinks to a special tipping point called the Schwarzschild Radius, then bam, you’ve got a black hole!

DT: Once it crosses that special radius, it begins to suck things in from the Universe. Is that true?

LC: David, you should know better.

Man: …a mass that creates, that sucks in all the other, all forms of matter and light, into itself.

Woman 1: I know that what I saw was a little scary, kind of like, you know, it just basically all goes in there, and even it could suck up the whole, everything.

DT: Would you say most people kind of know that about black holes, that they suck stuff in?

Woman 1: They should!

Woman 2: Probably, I think that’s a lot of people’s perception of what they are.

LC: This is one of my favorite misconceptions about black holes. Black holes don’t suck!

DT: They rock!

LC: I know, totally! I think the mental analogy is often one of a space vacuum cleaner that kind of eats everything in its path or sucks things in. But the fact of the matter is that black holes are just as passive as anything else. If you want to get eaten by a black hole, you have to fall in. It doesn’t just drag everything in space towards itself – not anymore than a regular star does, in fact. Even though black holes are “passive” in a sense, they still produce some of the most spectacular displays when gas does fall into them. In fact, quasars, the brightest and most distant objects seen today in space, are powered when a large amount of gas happens to fall into a super-massive black hole.

DT: So if you’re orbiting a black hole, you shouldn’t expect to feel anything special. But what happens when you actually enter the black hole?

LC: Wow, a lot of things can happen.

DT: Are you astronomers?

Woman: No, but I did just watch something on Stephen Hawking the other day.

DT: That’s cool! What did you learn?

Woman: I learned that basically, it takes you about, let’s say two weeks to fall through a black hole from the event horizon, and you’re eventually stretched into spaghetti.

DT: Is that true?

LC: Yeah.

DT: Wow, who knew gravity could be so dangerous?

LC: I know, David. I think “spaghettification” is a cute term for what would be a painful experience if you ever found yourself falling into a black hole. Because a black hole is massive yet small, the amount of acceleration you feel on your feet will be very different from the amount you feel on your head. This is what physicists mean when they say that space-time is highly “curved” around a black hole.

DT: Oh, so its not really a rip in the fabric of space-time around a black hole.

LC: Its just a highly curved portion of space-time. So, if you fall into the black hole feet first, your feet would zoom away before your head could catch up, causing you to be stretched super thin, like a strand of spaghetti, as Stephen Hawking once put it. This is just one of the many reasons why probing a black hole with an instrument would be difficult.

DT: You mean an instrument like a space probe or an astronaut.

LC: Exactly.

Woman: If you’re on the event horizon and you’re watching, and you have a watch on, it doesn’t change ever. But the person on the outside observing the person who would be at the event horizon, they never seem to move, and it always doesn’t ever hit midnight, it always stays, like, there, so time never changes for the observer outside but the person inside… [unintelligible]

LC: The effect she’s talking about here is known as time dilation, which comes from Einstein’s theory of general relativity. Put simply, gravity changes your perception of time with respect to another observer. So, if you’re falling into a black hole, from an outside observer, it’ll look like you’re not moving at all. Your time will slow down to them. But as you’re falling in, and looking at the person outside, that person will appear to speed up in time.

DT: Huh, that’s pretty weird. So things approaching the black hole never actually appear to enter the black hole from our perspective.

LC: Yeah, that’s technically true. However, as you’re falling into the black hole, you will also appear dimmer, and redder, due to redshift and things like that. So, you’ll eventually see the person disappear, but it might take many years.

DT: So this is why it would be so difficult if we were ever to send a probe like an astronaut or a spaceship into a black hole, or for that matter, a wormhole that some theorists postulate to exist behind them. First, it would get ripped apart, and second, it would take a very long time for us to get any information back out of it. So with all these difficulties in studying black holes, why is it important to focus on them?

LC: That’s exactly what we asked two people that we met at Union Square.

DT: Why do you think it’s important that we have people working on studying black holes?

Woman 1: Because if we did not have people that study different things that ordinarily we wouldn’t study about, you know, we wouldn’t know exactly anything about it, we wouldn’t know how to handle it, we would worry about stuff.

LC: So, I can say that black holes are interesting to astronomers for a lot of reasons, and it isn’t just the amount of strange physics that occurs around them.

DT: Tell us about it.

LC: Super-massive black holes have been found at the centers of of nearly all galaxies, and as we continue to study them we find that the mass of the black holes are closely linked to the galaxies’ mass as well. It’s also closely linked to the the motions of stars and gas within the galaxies. Even though super-massive black holes are super massive, as their name suggests, they still only account for one in a thousand or less of the host galaxy’s mass. Yet, the link suggests that the evolution of galaxies and black holes is intimately linked. Astronomers are still trying to figure out how.

DT: And there are a number of ways astronomers can study black holes without having to send anything into them.

LC: Yeah. So far all of astronomy has relied on the broad spectrum of light signals, such as radio, optical, or X-ray. But black holes also provide an opportunity to observe the universe in gravitational waves. These waves can be emitted when two black holes merge. If gravitational wave detectors, like the LIGO project currently operating in Louisiana, or the proposed space observatory LISA, succeed in detecting these waves, we can observe the history of the universe in a whole new way. So black holes, not only do they include really cool physics and really strange extreme environments, but they provide a lot of insight.

DT: We also hope that the public, in New York City and elsewhere, will continue to appreciate black hole research as it’s expanding into this new frontier. Thanks for listening!

LC: This has been a podcast of Columbia University here in the City of New York. For more information about public events of Columbia Astronomy, visit Our next Columbia Monday podcast will be by Adam Fuller on August 10th, entitled “What Happened to Saturn’s Rings?” Have a great day, and keep listening.

End of podcast:

365 Days of Astronomy
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