Looking into the galactic center is hard. So much dust and gas lies between us and the center of the Milky Way that very little of the visible light emitted there makes it to us. We can peek through that dust and gas by collecting x-rays, infrared radiation, and radio waves. Even then, however, resolving the tiny speck of sky that contains the Milky Way’s central black hole, with enough clarity to see the black hole’s shadow, is extremely difficult.
You need a telescope roughly the size of the Earth to do it. This might sound impractical. Fortunately, it’s possible to mimic the performance of an Earth-size telescope by coordinating existing radio telescopes scattered around the world.
That’s the idea behind the Event Horizon Telescope (EHT). If all goes well, by the end of next year the EHT will be a coordinated array of radio telescopes stretching from the South Pole to Hawaii to Chile to Mexico, plus many points in between. The astronomers behind the EHT have been already been observing for years using a smaller telescope array. In 2007, a three-station version of the EHT resolved Sagittarius A*, the black hole at the center of the Milky Way, with unprecedented clarity, detecting something (“structure” is the proper term) on the scale that we would expect from the black hole’s event horizon. It was a big deal, the farthest into the inner sanctum of a black hole that anyone had ever seen. The goal now is to make the EHT powerful enough to take the black hole’s picture.
Shep Doeleman, an astronomer with joint appointments at the Massachusetts Institute of Technology and the Harvard-Smithsonian Center for Astrophysics, leads the international group of researchers who are working to make this happen. Scientists from the U.S., Japan, Taiwan, Chile, Mexico, and several European countries are involved in the project.
The EHT will combine many of the world’s most advanced radio telescopes: the Submillimeter Array (SMA) and the James Clerk Maxwell Telescope (JCMT) in Hawaii; the Combined Array for Research in Millimeter-wave Astronomy (CARMA) in California; the Submillimeter Telescope (SMT) in Arizona; the Large Millimeter Telescope (LMT) near Puebla, Mexico; the Atacama Large Millimeter Array (ALMA) and the Atacama Pathfinder Experiment (APEX) in northern Chile; the South Pole Telescope (SPT); the Greenland Telescope (GLT); the Plateau de Bure interferometer in France; and the 30-meter dish at Pico Veleta in Spain. Once properly outfitted, these telescopes will set out on a few key nights each year to observe the same black hole simultaneously. Together, they will function as one giant telescope.
The reason it’s possible to combine the observations of different telescopes into a single picture is interference, a convenient property of all waves. If you multiply waves and they are in phase–crest meets crest, trough meets trough–they produce a bigger wave. Combine waves that are out of phase, and they reduce one other. The EHT is an extreme example of an interferometer; it operates using a technique known Very Long Baseline Interferometry (VLBI). The “Very Long Baseline” part is key. The reason the EHT can resolve the tiny patch of sky where Sagittarius A* lives is its size. As with any telescope, its diffraction limit is expressed as the wavelength of light it collects divided by the diameter of its collecting surface–in this case, the distance between telescopes. The distance between the EHT’s telescopes is huge. As a result, when the EHT collects light with a 1.3 mm wavelength, it will have an angular resolution of 25 microarcseonds. To put that into perspective, a microarcsecond is a millionth of an arcsecond. The diffraction limit of the human eye–the smallest patch of sky we can resolve with the naked eye–is about 16 arcseconds. That means the EHT is nearly a million times sharper than the human eye–acute enough to spot a grapefruit on surface of the moon, or to read the writing on a nickel in Los Angeles from New York.
Actually doing VLBI requires a nerve-fraying amount of coordination among the astronomers at each station. On observation night, researchers at each telescope point their dishes at Sagittarius A*. They track the black hole through the night, using the Earth’s rotation to view it from different angles. They store the data they collect on off-the-shelf hard drives. Someone then hauls the hard drives to the nearest town and FedExes them back to MIT’s Haystack Observatory, where the results are correlated on a supercomputer.
Here’s an analogy: Imagine you and a dozen friends standing around the edge of a pond. Each person has a stopwatch and a notepad. Each person knows his or her position on the pond bank to great accuracy. Now someone throws a rock in the middle of the pond. Each person records the shape of the resulting waves at the precise moment they reach their portion of the bank. Then everyone gets together and combines their notes. “You say to each friend, I know exactly where you were when you recorded peaks and troughs,” Doeleman says. With the information gathered by the group, “You can basically show what the wavefront looked like.”
How, exactly, is it possible to combine the data collected at the different sites? What can that data tell us? What do we know about the black hole itself? The only way to handle this subject properly is to spread it over several blog posts. Soon, we’ll talk about what has to happen at each telescope for it to participate in the EHT; how much needs to be done before the EHT can get a picture of Sagittarius A*; and what we know about the black hole at the center of the galaxy.