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Testing relativity with a telescope as big as the Earth
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Testing relativity with a telescope as big as the Earth

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How’s this for a lofty goal:

“We’re trying to take a picture of a black hole with a telescope the size of the entire Earth,” University of Arizona astronomer Daniel Marrone said.

Marrone and a dozen other U astronomers, theorists, engineers and graduate students are working on the Event Horizon Telescope, a global scientific quest to create the first image of a black hole.

His description of what he is doing was tailored to a class of first-graders. Marrone visited his son Peter’s class earlier this year, in part to atone for being away so long installing equipment for the attempt at the South Pole Telescope.

“I think the thing that fascinated the kids the most was, ‘Did I see penguins in Antarctica?’”

He did, and he had pictures to prove it.

They don’t yet have a picture of a black hole, though, but the telescope Marrone is helping assemble by linking an array of telescopes around the globe may already have seen one. That image, if it exists, is hiding in a mound of data.

“We have enough information to try and create an image. It won’t be the world’s best image, and we hope to do a lot better,” Marrone said.

The first big attempt, tentatively set for spring 2017, will train an array of the world’s largest-millimeter telescopes on Sagittarius A* (Sagittarius A Star), the black hole at the center of our home galaxy, the Milky Way.

The shape of that image will either bolster theories of relativity that have stood since Albert Einstein first postulated them — or they won’t, which would be a more exciting result.

No escape

A black hole, by definition, is unseeable. Thought to begin with a collapsed star, it is a phenomenon so dense that nothing — no energy, matter or light — can escape it. Stars orbit it. The fabric of space/time warps in its vicinity. It devours anything that comes close.

Gravity, usually called a “weak force” in physics, dominates at the event horizon. It overwhelms all other forces. Pity the poor photon trying to escape.

But while you can’t see a black hole, the theory of general relativity, proposed by Einstein 100 years ago, predicts that the rim of a black hole — its event horizon — will glow with the light of the intense radiation caused by matter being compressed into near-nothingness and be circumscribed by a ring of photons circling it, but not captured by it.

In 1973, physicist James Bardeen calculated that a black hole should cast a shadow more than twice its size.

Bardeen was still teaching at the University of Illinois when theoretical astrophysicist Dimitrios Psaltis started his graduate studies there. Psaltis, now a professor at the UA Department of Astronomy and Steward Observatory, has seen Bardeen’s described phenomenon — virtually.

He and his team — which includes his wife, Feryal Ozel, also a professor of theoretical astrophysics at the UA — have fed all of the data about black holes and all of Einstein’s theories into “El Gato,” a UA supercomputer amped with an array of 140 graphics-processing units, or GPUs.

The GPUs have been “hacked” for the purposes of the astrophysicists, Psaltis said.

Made for video-gaming devices, they assumed that all light travels in a straight line. Now the photons travel according to Einstein’s special laws.

Just like a video game

In his office, Chi-Kwan Chan, a post-doctoral researcher at Steward, offers a visitor a set of virtual-reality goggles marked Oculus Rift. They are a development prototype of a video-gaming device bought by Facebook last year and not yet on the market.

The room disappears and two images appear — 3-D simulations of the black hole. The images produced by the supercomputer move so quickly that they can’t be controlled with keyboard strokes. Chan can make them move and spin by hand, employing a Leap Motion controller.

Chan said he was never a gamer. As a child, his parents refused to buy him a video game console. Now he gets to program them and use them for scientific purposes.

Psaltis said the computer simulations can be tweaked to show how the images would appear under various alterations of the laws of physics, and those images can then be compared to the image obtained by the Event Horizon Telescope. The sharper the image acquired, the better the scientific return will be.

One unexpected result of the simulations, he said, is that no matter where you place the observer or how fast you allow the black holes to spin, the shadows appear nearly circular.

That makes it easy to test Einstein, he said. If the actual image deviates from circular, that would violate the theory of general relativity.

Getting that image is not easy. Sagittarius A* is massive — with a measured density of about 4 million suns — but it occupies a tiny space in the sky as seen from Earth.

Its circumference is about the size of Mercury’s orbit around the sun, and it is 26,000 light years away. Its place in the sky is small and it is hidden behind the dusty veil of the Milky Way.

There are bigger black holes, but they are even farther away. One, the secondary target for the Event Horizon Telescope, is 53.5 million light years away, but it is so big that its space on the sky is equivalent to Sagittarius A*. The researchers will try to image that one, too.

Linking the telescopes

Seeing a black hole requires mega-magnification and just the right wavelength of light. It also requires immense coordination and cooperation from the weather globally.

That’s where the Event Horizon Telescope comes in. The telescopes being linked to create it are being outfitted with receivers in the 0.86-mm and 1.3-mm range.

“Turns out that is exactly the right wavelength to see through the dust and to image the event horizon,” Psaltis said.

Shorter wavelengths of light, such as visible or infrared, scatter before reaching Earth. Radio waves do not.

In order to create enough resolution, the Event Horizon team, led by Sheperd Doeleman at MIT’s Haystack Observatory, set out to link telescopes in France, Hawaii, Mexico, Arizona, Argentina and the South Pole.

He and Psaltis co-wrote an article explaining the principle behind the project for last month’s special edition of Scientific American celebrating the 100th anniversary of Einstein’s Theory of Relativity.

The Event Horizon Telescope, they wrote, “exploits a technique known as very long baseline interferometry, in which astronomers at radio dishes across the globe observe the same target simultaneously, record the data they collect on hard drives, and then later combine all those data using a supercomputer to form a single image.”

That combination will give the telescope resolving power 2,000 times better than the Hubble Space Telescope and will allow it to make an image akin to seeing something the size of a DVD disk on the moon.

Sounds simple enough, but there are, of course, a lot of details to work out before that happens. Which is what brought UA astronomer Marrone to the Amundsen–Scott South Pole Station last winter — summer there — where he sat on the ice with a crate containing a sophisticated maser-powered — yes, maser, not laser — atomic clock that needed to be plugged in to stay warm.

“We unloaded it from the C-130 (cargo plane) and it just sat there sitting on the ice forever. I’m looking at my watch. Eventually I got the fork lift driver and said, “Hey, could you move this to a warm place?’”

The instrument was unharmed, despite the cold and the “jerkiest ride you could imagine” to its resting place.

The clocks need to be installed at each of the telescopes and the telescopes also need to be fitted with receivers that work in the 0.86- and 1.3-millimeter range.

a grapefruit on the moon

Marrone built the receiver for the South Pole Telescope in his basement laboratory in the Steward Observatory building on the UA campus.

It was supposed to go with him to the South Pole this December but that plan was sabotaged by a couple of “oops” moments.

Both copies of a super-conducting junction that Marrone calls “the heart of the receiver” were ruined in fabrication — “amazingly, as though cursed.”

That means Marrone gets to stay home this winter instead of working at the South Pole Telescope.

Next winter, it’s back to the South Pole, where he will install the receiver just in time for the big experiment that will bring in the Event Horizon Telescope’s largest partner, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.

ALMA is the giant of the astronomy world, built by an international coalition that includes the European Southern Observatory and the U.S. National Science Foundation. It is capable, all by itself, of the best resolution ever achieved by a radio telescope array.

Link the collecting power of its 66 antennas with the resolution of a telescope that spans the globe, and you can see a grapefruit on the moon or a black hole 26,000 light years away.

Arizona brain power

By the time the Event Horizon Telescope conducts its experiment in spring 2017, the South Pole Telescope should be hooked up with receivers and clocks ready at multiple sites in Chile, France, Hawaii and Arizona.

Here, the Submillimeter Telescope on Mount Graham is already part of the network and the new 12-Meter Alma Prototype Telescope on Kitt Peak will join as well.

Lucy Ziurys, director of the Arizona Radio Observatory, said the UA has been part of the very long-baseline experiment for 15 years, and participated in a multi-telescope experiment with Doeleman and MIT that produced “preliminary images” and measurements of the mass and size of Sagittarius A* for a paper in Nature in 2008.

The Arizona Radio Observatory recently received a $1.4 million grant from the National Science Foundation to equip the 12-meter telescope on Kitt Peak for very long-baseline experiments, she said.

Marrone said Arizona has become “one of the most exciting sites for EHT work. This place is putting a lot of brain power and people power into doing this.”

Theory team members Psaltis, Marrone and Chan all said they expect the image of the event horizon to match the computer simulations.

That, Psaltis said, would be the first demonstration of relativity outside the confines of the solar system — the first confirmation of relativity in extreme physical circumstances.

It would be the first confirmation of a black hole, said UA astrophysicist Ozel. “We think it’s there. We have indirect evidence that we have black holes throughout our galaxy, distorting the fabric of space/time, but we’ve never seen one.”

“Taking a picture of this thing that we know, through pure reason, should be there but have never been able to see, that’s pretty good already,” Marrone said. Finding something unexpected would be an even bigger deal.

What happens then?

“You check your data,” Marrone said.

Contact reporter Tom Beal at or 573-4158.

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