Science Friday - SciFri Extra: Picturing A Black Hole

Episode Date: April 6, 2019

The Event Horizon Telescope is tackling one of the largest cosmological challenges ever undertaken: Take an image of the supermassive black hole at the center of our galaxy, using a telescope the size of the Earth. Now, the Event Horizon team has announced they have big news to share about those efforts. On Wednesday April 10th, it’s anticipated they will show a photo of the event horizon. Before they do, we wanted to share this 2016 conversation with Event Horizon project director Shep Doeleman and black hole expert Priya Natarajan, in which they discuss how you image an object as dark and elusive as a black hole. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

Hey there, Ira here. The Event Horizon Telescope is tackling one of the largest cosmological challenges ever undertaken. Take a picture of the supermassive black hole at the center of our galaxy using a telescope the size of the Earth. And now the Event Horizon team has announced they will have big news to share about those efforts on Wednesday, April 10th, where it's anticipated they will show a photo of the Event Horizon. But before they do, you can brush up on how exactly you photograph a black hole and what this project is all about. Here's a little primer from our archives, a selection from a conversation we had with Shep Dahleman and Priya Natarajan back in 2016 about that very question. We're talking this hour about mapping the universe's web of dark matter with Priya Natarajan. She's a theoretical astrophysicist and professor in the Department of Physics and Astrogyzstan.

at Yale, and author of Mapping the Heavens. Really good book to read. I'd like to bring on another astronomer's also studying something notoriously hard to see, and that is a black hole, and he has a plan for snapping a close-up of it. He wants to use a telescope as big as the Earth. No big deal. Shep Dolomyn is director of the Event Horizon Telescope Project. He's also an astrophysicist at the Harvard Smithsonian Center for,

astrophysics in Massachusetts. Welcome back to Science Friday. Thanks, A. Thanks, Aura. So what's so hard about looking at a black hole that you need a telescope as big as the Earth to take a picture of? Right. So first of all, black holes are by definition something you can't see.

I like to imagine it as trying to take a picture of a dinosaur. We know dinosaurs exist. We see their footprints in clay. We see their bones, but no one's ever seen one. And it's the same with black holes. You can see light, around them, but you can't see them themselves. So you have to find a way to build a telescope that has about 2,000 times the magnifying

power of the Hubble Space Telescope, because these are the smallest things in the heavens that are predicted by Einstein's theory of general relativity. You know, it's kind of weird because we tend to think of black holes as these massive objects swallowing up everything around them, when in reality you're saying that they're relatively small. They are the tiniest things you can imagine. They are the end result of gravity going haywire and collapsing a bunch of matter into a point source. But around that point is this wonderful membrane called the event horizon.

And that's the point where the gravity is so intense that even light can't escape. So go ahead. No, go ahead. I'm sorry. So all this gas and dust around the black hole is madly trying to get into a very small volume. and in a cosmic traffic jam it heats up to billions of degrees. So black holes can be some of the brightest things that we see in the sky. You know, in the film Interstellar, they use some equations by physicists to create what they thought was a real picture of a black hole. How close, you know, do you think they got to it?

So they got really close. I mean, no one's ever seen one, of course, but that is as close, I think, as we're going to get. It turns out there was someone in 1979 that made a wonderful illustration of what a black hole might look like using primitive computers. And it looks very much like what the computer graphics designers did for the movie Interstellar. And that's the kind of thing that we humbly hope to do with the Event Horizon Telescope. Can you just see black hole if you take a picture of it? Is that what it's going to look like? A hole in space?

So a black hole is surrounded by this, if you think of it as a three-dimensional flashlight of hot gas, that's this radiating light all around the black hole. So what you wind up seeing is what's called the shadow of the black hole. Light that normally would leave the black hole on the other side of the black hole gets bent around in a U-turn towards you. So you wind up seeing a ring of light around a relatively dim interior, and that's called the shadow. And that's what we are trying to measure an image with the event horizon telescope. Priya, what would you want to know from this very first picture of a black hole? What would you most interest you to see? Well, I think the shape of the shadow is a very important test of the predictions of general relativity

and really of physics in the strong gravitational regime. So the effects that we expect theoretically on light and light bending by black holes, this would be sort of one very important test at very sort of high resolution, small scale test, sort of a slightly more compelling test than I would say than the bending that we see by larger, more massive objects, because black holes are so compact. The clusters of galaxies that I was talking to you about are huge things on the sky. and they have a huge amount of matter. So the deflections that they generate, the matter generates,

they are in consonants with Einstein's theory. The question is when you have something that's really that compact, how well does the theory still work, right? So I think there's no reason to believe that there's any problem with the theory at the moment. But, you know, who knows what the Event Horizon Telescope could tell us? You know, what's interesting, you say, who knows? One of the hottest topics about black holes in these last few decades has been discussion, started by Stephen Hawking and other people

about what happens to this stuff that falls into the black hole? What happens to the information that's contained in all of that? Can you give us an idea what that debate is all about? So I think actually Stephen Hawking has a beautiful analogy. So one of the problems is that, as Shep mentioned, the event horizon is really seen as this point of no return. Because once you pass the event horizon,

which encases the singularity, and the singularity is the place where all the known physical laws that we know about and understand breakdown. So what really happens to information to matter when it crosses the event horizon is not very well understood. And we don't even have a framework. But Hawking had this interesting suggestion, and they're working on it with Andrew Strominger and Malcolm Perry at Harvard in Cambridge, which is, suppose you had an encyclopedia that was encased in a glass case, tight case, right? And you want to look up the capital of Rwanda. So you just go there, you look it up, you know, look at the page.

Now you burn the encyclopedia. And all the ashes of the encyclopedia are still in that box. Nothing's left that box, right? It's right in there. So theoretically speaking, the information on the capital of Rwanda is still in there. It's just that you don't know how to access it and how to extract it anymore. And I think that these are sort of the ways in which they're starting to think about sort of analogies for the event horizon to develop a quantum mechanical understanding because that's

sort of what seems to be lacking. And so string theory seems to be providing new insights. And so that's what the buzz is about at the moment. But I love this analogy because it really drives home the point that, you know, the information is possibly there. We just don't know how to extract it. We don't have the language mathematically to describe it. Yeah. Shep, you agree with this, this analogy is a good one? Well, it is an interesting one because the idea is, you know, where does the information go when it falls into the black hole? And that's why philosophers get very interested about black holes. If you talk to them about stars, they say they're beautiful, even neutron stars,

they say they're wacky. But when you talk to them about black holes, they get dreamy and they get very interested because this is an area of space time that is unaccessible to us. And that really is very, very startling and spooky. Our whole worldview is based on Newtonian determinism, that if you know what is happening around you, you can propagate it forward in time and know where you're going to be later. But what if you're falling into a black hole

and you can't tell somebody what happened to you? Is that part of, is that determinism or not? Or does determinism break down? So black holes, in addition to the information theory problem, they strike at the core of some of the, of Western thought. Jeff, this is an anniversary of sorts, isn't it, for the so-called Schwartzschild radius? Describe what that is.

Right. So the event horizon occurs for a non-spinning black hole at the Schwartzschild radius. And in 1915, Einstein came up with his field equations, his geometric interpretation of gravity. And it was communicated to Carl Schwarzschild, who was in the army in the trenches of World War I. And, you know, unlike how I like to do my work with a cup of coffee and maybe some music, he was solving Einstein's theoretical equations in the trenches. And he came up with this solution called the point source solution where he said, what if all the mass is concentrated into a point?

He didn't think it was realistic, but he said, let's focus everything into a point. And he found that at a certain radius called the Schwartzschild radius, even light couldn't escape because the gravitational field would be. to grade. And he wrote this down on a postcard, mailed it to Einstein, who very famously then presented it to the Prussian Academy of Sciences in 1916. So that's the celebration and anniversary that we're looking at today. The 100th anniversary of that. Right. And actually, you know, Einstein actually didn't expect that there would be any exact

solutions for his equations, field equations. So he was quite startled, actually. He didn't like the solution, but he was startled. And he also did not accept black holes, as Priya said before, for many, many years. Yeah, I mean, Einstein is this really intriguing character, right? He comes up with all these incredible radical ideas, and he is really unhappy and resists the implications of his own ideas. So this is something that I talk about in the book. And Einstein is a particularly interesting case. Yeah, he's the father, so to speak, of, you know, all kinds of science that he didn't agree with that was handled once he discovered it, especially quantum mechanics.

Stay with us. We'll be right back after this break. We have a couple of phone calls. Let's go to Philip in Portland, Oregon. Hi, Philip. Hi, I, how are you? Hey there. I've taken my call. You're welcome. Go ahead. My question is this. It seems like the science of the community has basically concluded that dark matter must exist because the equations that we have that predict gravitational lensing don't seem quite fit with the amount of matter that we're able to directly observe. And I'm just wondering what, you know, what, your guests think about the alternative, which might be that the equations aren't quite right. Hmm. Hmm. Can I take that?

Please, Maria, go right ahead. Yeah. I'm not touching that one. Okay. So actually, there's, Philip, there are many independent lines of evidence that point to the existence of dark matter, not just light bending. There's the motions of stars in galaxies and galaxies and clusters, which is not commensurate with the matter that is seen, right? And so Einstein's equations actually predict that the contents of fate and the geometry of the universe are interlinked. And so now we have an inventory of all the matter.

We also have an understanding of the geometry and the fate independently, right? And so they have to be sort of commensurate. So there's room for dark matter, although we haven't found the particle. I understand that it might seem that, you know, scientists have evidence, but they don't have a direct detection yet. So we're awaiting the detection of a particle, right? But I think what is, and this is not like ether, probably you're wondering whether this is going to go away. The only alternative that has been suggested and that's been worked on quite a lot is sort of a modification of the equations of Newtonian dynamics. And it's called Mond, this theory.

And the interesting thing is that versions of this theory can explain the motions of stars, the evidence for dark matter from galaxy scales. But this theory cannot really match up and give predictions for the light bending that is seen. So there is no real viable alternative theory at the moment, and there are independent lines of evidence that are very compelling for the existence of dark matter, although we are yet to detect the particle. But you know, the LIGO detection, remember, it took 40 years? Yeah. So there are many dark matter experiments that are ongoing at the moment, and I'm actually sort of optimistic that we just might. In particular, there's one experiment called Dharma that claimed a detection more than 15 years ago. but the community was not persuaded

and only recently a replication of that experiment in five different locations on the South Pole in Australia, South Korea, Spain and so with a particular crystal, sodium iodide crystal as a detector. So within a couple of years, we'll know maybe that was a signal and that maybe we should take that seriously. So I'm quite excited at the possibilities. You touched on this a little bit before.

Martha Hussein says, Is all dark matter the same? Is there anything to be inferred from the portion of dark matter to non-dark matter? Yes. I mean, at the moment, the simplest assumption that we're making is there's only one kind of dark matter, but there's no real reason to believe

that there can't be different kinds of matter. But the dominant component appears to be cold that is moving very slowly and practically collisionless. You know, for all, you know, one of my favorite candidates is all my unmet socks. Every time I do laundry, I miss socks. So, you know, that could be a component of dark matter as well. The greatest unsolved mystery of science.

Where do the socks go in the laundry? Shep, how will we know if we see something truly shocking and you take this image of the black hole? There's something really shocking that the image itself is correct, right? You're expecting something? Are you ready to upset 100 years of black hole theory for what you see? Well, as I like to say, it's never a good idea to bet against Einstein. but even he, I think, would be really marvelously excited by what we're about to do. So the idea is that you should see this ring of light around the black hole,

and the size and shape of that was predicted by Einstein. If we see some deviations from that, if it doesn't look round, if it's not as large or it's smaller than we think it should be, that would be an indication that either we're not looking at a black hole. It could be something weird and exotic, like a boson star or something that would be very difficult to think about how we could even construct it, but it's potentially possible. Or it would be, as Priya was discussing, a change in general relativity, a change in Einstein's equations.

So what we're looking for is this ring of light, and if it's the right size, let's say around the 4 million solar mass black hole at the center of our galaxy, the Milky Way, then that would tighten the noose incredibly. So in the center of our galaxy, there's this large black hole and some wonderful groups in Germany and UCLA have seen stars orbiting around this unseen mass. And that is very powerful evidence that it's a black hole. But they only come within, let's say, one or two thousand Schwarzschild radii of this black holes that are very far away and their motions are perfectly predicted by Newtonian dynamics. But we want to tighten the news to within one short shot radii. How so nobody will we have a photo? Oh, that's the million dollar question.

We are, our global team, and this really is a global team, Ira, are getting ready to take our first potentially imaging data set in the spring of 2017. That's when we'll add sites at the South Pole, in Chile, Hawaii, Arizona, Mexico. France and Spain. Truly an earth-sized array to look at this exotic object. Can't wait until it gets back from the drugstore. Thank you very, very much for taking time to be with us today. Sounds very exciting.

That was Shep Dolomyn, a member of the Event Horizons team, and physicist Priya Natarajan, speaking with us back in 2016 about the science of supermassive black holes and the Event Horizons team's effort to photograph one for the first time ever. And now Shep and his team are ready to share the results of their work this Wednesday, April 10th. So go to ScienceFriday.com slash Event Horizon all this week to check out our black hole coverage

and tune in this Friday for a special hour with the Event Horizon team.