Date: July 9, 2009
Title: Measuring the Black Hole
Podcaster: Carolyn Collins Petersen
Organization: Loch Ness Productions (http://www.lochnessproductions.com/index2.html)
Music by GEODESIUM (http://www.geodesium.com)
Special thanks to Dr. Shep Doeleman for his advice about the script for this segment. Also Special thanks to Nicole Gugliucci for her script review.
Description: A team of astronomers has used a special radio astronomy technique called VLBI to make some pioneering measurements of the event horizon around the black hole in the heart of the Milky Way Galaxy. TheSpacewriter Carolyn Collins Petersen talks with the head of that team, Dr. Shep Doeleman of MIT’s Haystack Observatory. The team’s work has been making big news recently, with articles in Nature and featured in upcoming BBC interviews. You can hear the story now from the team leader’s perspective.
Bio: Carolyn Collins Petersen is a science writer and show producer for Loch Ness Productions, a company that creates astronomy documentaries and other materials. She works with planetariums, science centers, and observatories on products that explain astronomy and space science to the public. Her most recent projects were the Griffith Observatory astronomy exhibits in Los Angeles and the California’s Altered State climate change exhibits for San Francisco’s California Academy of Sciences. She has co-authored several astronomy books, written many astronomy articles, and is currently working on a new documentary show for fulldome theaters, a vodcast series for MIT’s Haystack Observatory, and a podcast series for the Astronomical Society of the Pacific.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the Physics Department at Eastern Illinois University: “Caring faculty guiding students through teaching and research at www.eiu.edu/~physics/
Transcript:
Hi, I’m Carolyn Collins Petersen — the SpaceWriter.
Astronomers have known for years that the Milky Way Galaxy has a black hole in its central region. It’s called Sagittarius A*, and it has four million times the mass of the Sun. It’s invisible to our eyes, but very obvious in radio, infrared and x-ray emissions.
The black hole itself is like any other black hole — an immensely dense collection of matter with a gravitational pull SO strong that no form of light can escape from it. So how can astronomers detect the radiation coming from Sagittarius A*?
The emitting region is not the black hole itself, but a disk of material surrounding the black hole — called an accretion disk. As material in the disk swirls around, it gets compressed by the strong gravitational pull of the black hole. It heats up and emits radiation. The disk also has tremendously strong magnetic fields running through it, which also help to heat things up.
As it gets closer to the black hole, the material gets hotter, and gives off ever more energetic radio and x-ray emissions.
Eventually the disk material gets pulled across the point of no return — called the event horizon. After that, it’s completely beyond our detection.
The strong gravity of the black hole also acts as a gravitational lens, which warps the appearance of the radio and x-ray emissions that are being given off as material swirls down into the black hole.
As you can imagine, that distortion makes it very difficult for astronomers to observe and measure structure in the accretion disk.
In 2008, a group of astronomers led by Dr. Shep Doeleman at Haystack Observatory in Massachusetts observed Sagittarius A* using radio telescopes. The team used a technique called Very Long Baseline Interferometry – VLBI. It was a unique opportunity to study the region around a black hole. With this technique and using even more radio telescopes, they someday hope to answer some fundamental questions about black holes and the universe, such as “Was Einstein right about general relativity when it comes to strong fields?” And, is there really an event horizon? Can scientists estimate the spin of a black hole by resolving orbits of material near the event horizon? How do black holes accrete matter, and how do they create those powerful jets we see in places like the central region of the elliptical galaxy M87?
I asked Shep Doeleman about some of this. Shep, tell us how VLBI works.
SHEP: Okay, it’s probably best to think about how a normal telescope works. A normal telescope takes radiation from the sky bounces it off a very carefully shaped mirror, which then focuses the light onto a single point. That’s how we can make images of the sky. Think about now VLBI, in which we take different radio telescopes telescopes very far apart and we record data at each of these sites. We then bring the data back to a central facility, where we use a supercomputer that essentially acts as the lens – just as the mirror acted for the optical telescope, the supercomputer acts for the radio telescope as a lens. And, we’re able to produce data as though we had a telescope that as large as the distance between the two radio telescopes we used. That allows us to get very, very sharp images of the sky – about a thousand or two thousand times better than the Hubble Space Telescope.
A way to think about how we make images on the sky is in the following way: imagine that you had a friend across a pond, who was dipping their finger in the water. You get these beautiful circular rings out from their finger, and if you had some other friends on the distant shore with notepads and synchronized clocks, you could record the crests and the troughs that you found at each of those spots on the distant shore, bring your friends together, and they could – using their notepads and their synchronized clocks – recreate that beautiful ring that was created by your friend dipping their finger in the water. That’s what we do with VLBI. But, instead of friends with pads, we have radio telescopes that can tell us what the electromagnetic radiation is doing – from (on) different points of the Earth.
So, to do the VLBI, we needed atomic clocks, which keep extremely precise time, and we need high-speed recorders. When we get all that data, then we can use that to image of the sky with really unprecedented resolution; and that’s how we were able to make observations of the black hole in the center of our galaxy.
In April 2007, we used this VLBI technique to combine antennas in California, in Arizona, and also in Hawaii. We had each antenna look at the galactic center, we recorded data, and brought all the data back to Haystack Observatory, and we made a data set as though we had a telescope as large as the distance between California and Hawaii. We were able to measure for the first time the size of the emission around the black hole. We weren’t able to make an image, but we were able to get a very robust size. It was smaller than we thought it should have been. The reason for that is that the gravity around a black hole is so intense, that if you have a region of emission around the event horizon, it actually gets lensed to look larger than the event horizon when we look at it from the Earth. The gravity actually bends the light like a magnifying glass. And the size we found was less than that size that we should have seen. And the way we understand that is that we’re probably looking at a hot spot in the accretion disk – the flow of matter around the black hole that’s undergoing this death spiral into the event horizon. We’re seeing a bright spot off to one side.
We see the bright spot off to one side because of the Doppler Effect. The portion of the disk that’s accelerating toward us seems bright, and the one going away from us seems dim, so we’re probably a small spot on the approaching edge of the disk of accreting matter.
Carolyn: Is this something of a clumpy accretion disk, do you think?
SHEP: It could be clumpy. A lot of simulations that take into account the magnetic field around the black hole and hot gases that are swirling around it seem to show a fairly smooth flow, but every once in a while, Sag A* does something very odd – it flares. So, from the radio, submillimeter, near-infrared, even up to the x-rays, we’ll see a huge burst of activity. What people think is going on there is that a little clump of dense matter is falling into the black hole, and we think that with VLBI, we can only see the light curve that is evident when this happens, so just seeing the total intensity change as a function of time, but we can actually watch it spiral in as it orbits the black hole.
There are ways of doing that with VLBI that don’t require that we make an image – we can just see some of the VLBI data that would show something orbiting. That’s one of the things that we’re really excited about in this project – being able to watch in real time as something moves around the black hole event horizon.
Carolyn: What’s the next step for your team?
SHEP: We really want to refine these measurements, and there’s a clear path to making all the data we have much better. The first is to get more telescopes. Right now, we only have enough telescopes in our array so that we can measure the size of Sag A*. What we really want to do is get enough telescopes, maybe we need about ten of them, so that we can make true images. And then, we’d be able to answer questions like “Was Einstein right?” Newtonian gravity has been replaced by Einstein’s theory of gravity.
We haven’t really tested Einstein’s theory of gravity in the most extreme environments in the universe, and that’s where we’re going to see gravity break down, if it does break down. So, towards the black hole, we expect to see a very interesting signature. The light coming towards us directly from the front side of the black hole is somewhat dim, because it has to fight through this strong gravitational well, and it gets red shifted. Whereas, all the light that leaves from behind the black hole gets bent around by the extreme gravity of the black hole, into a ring around that internal shadow. So, you wind up seeing not a blob of emission, but we should wind up seeing a shadow directly towards the black hole, surrounded by a ring of light. And that’s what Einstein predicts. What we want to do is measure the size of that shadow, and if it’s the right size, then Einstein will be correct. If it deviates significantly from what Einstein predicts, then we have to start scratching our heads and saying “What is going on here? Was Einstein wrong? Do we not understand something fundamental about black holes?”
That’s why the measurements we hope to make in the future with more telescopes are going to really tell us something fundamentally new about space-time.
Carolyn: You can learn more about Sagittarius A* and the VLBI interferometry measurements by pointing your browser to www.thespacewriter.com/wp and click on the 365 Days of Astronomy tab.
Thanks for listening!
End of podcast:
365 Days of Astronomy
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