Signals From the Void

Scientists are trying to get the first direct look at the black hole at the center of our galaxy. How close will they come to seeing the unseeable?

By Seth Fletcher Posted 07.23.2012 at 11:00 am 36 Comments

Leading Light: The 66-dish Atacama Large Millimeter Array in Chile could soon become the centerpiece of a global telescope array designed to see the black hole at the center of the Milky Way. ESO/Jose Francisco Salgado

Even on the clearest nights under the clearest skies on Earth, the galactic center and the dense cluster of stars around it are all but invisible to the human eye. Visible light can’t make it through the clouds of dust and plasma that clog the Milky Way’s glowing central globe. But radio waves can. That much became known in 1932, when a Bell Telephone Laboratories physicist named Karl Jansky noticed that whenever the galactic plane rose above the horizon, the sky exploded with radio transmissions. Since then, radio astronomers have discovered several means by which to bring the galactic center into increasingly sharp focus.

The first and most important method was the same one the Event Horizon Telescope uses today—connecting multiple geographically distant radio telescopes to form an interferometer, which adds together waves collected at different telescopes to produce a new, stronger wave. In the early 1960s, almost as soon as the National Radio Astronomy Observatory in Green Bank, West Virginia, was completed, astronomers started pointing its two-station interferometer at the galactic center. Then in 1966, observing relatively low-frequency radio waves, astronomers there detected the first signs of what we now know as Sagittarius A*. The resolution was far too low to yield a definitive observation, but eight years later, Green Bank astronomers using an upgraded interferometer capable of capturing higher-frequency waves proved that something extremely dense and bright existed at the center of the galaxy. Something was sitting at the core like a gyroscope, hovering in place while the rest of the Milky Way churned around it. Eight years later, one of the astronomers named the object—which, when viewed from Earth, appears to lie in the constellation Sagittarius—Sagittarius A*.

Since then, increasingly sensitive detectors and more powerful computers have enabled radio astronomers to observe at ever-higher frequencies and peer deeper into the center of the galaxy with greater clarity. Higher frequency radiation, which consists of shorter wavelengths, provides finer resolution. More important, the radiation that comes from the most extreme environment in the galactic center—the very edge of the event horizon—tends to be very high-frequency. At wavelengths longer than two millimeters, observing the galactic center is “like looking through frosted glass in the bathroom,” Doeleman says. At wavelengths of one millimeter and below, the frosted glass “magically clarifies.”

To capture those one-millimeter waves, astronomers have to travel. Atmospheric water vapor blocks waves in the one-millimeter range, which is why high-frequency radio telescopes are built in places where the atmosphere is thin and arid enough to let the one-millimeter light in. High, dry places like Mauna Kea. Places like the 17,000-foot plateau in Chile’s Atacama Desert (the world’s driest) where the Atacama Large Millimeter Array is under construction.

The telescope should see a disc of darkness surrounded by a glowing halo.ALMA, soon to become the world’s most powerful radio-telescope array, is expected to join the Event Horizon Telescope array in 2015. Once it does, it will become the critical station in Doeleman’s planet-spanning array. With ALMA on board, the EHT needs to add two, maybe three more key telescopes to approach the data-gathering capacity it will take to see Sagittarius A*’s event horizon. The EHT crew will also need to install their most advanced equipment—including the new recorders currently under development at Haystack, which should record data eight times as fast as the ones they use today—at every station. But once that’s done, their virtual telescope should be able to gather enough data to make an image.

Like all radio-telescope images, the picture will be an encoded map of a small patch of sky—a map of the immediate vicinity of Sagittarius A* in which the brightness of each pixel represents the intensity of radiation coming from that speck of space. It could take one excellent night to get it; it could take several very good nights of combined data. But at the end of some observing run, there will be a picture.

Theorists have used supercomputers to predict what the picture should look like. If the black hole is calm, the telescope should see a disc of darkness surrounded by a glowing halo, like an eclipse. One side of the disc may contain a fat blob of light. That’s a hotspot, a clumped-up cloud of accreting matter orbiting the event horizon. If Sagittarius A* is caught consuming some giant cloud of matter, the black hole may look like a ball of fire.

Doeleman is quick to emphasize that the EHT will be gathering data years before and after Sagittarius A*’s shadow first comes into focus. The more telescopes Doeleman can add, the finer the detail they’ll be able to resolve. Yet some theorists argue that, scientifically, the picture is almost beside the point. “I don’t think the end-all and be-all of this is producing an image,” Broderick says. “Eventually there will be an image, but it won’t tell us much more.” Viewed this way, the image is candy. Viewed this way, the Event Horizon Telescope is a science project designed to generate, almost by accident, a work of art.

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