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What’s on the (Event) Horizon?
Everyone knows that it’s impossible to see black holes, right? We’ve observed the effects they have on nearby stars and gases. We’ve detected the gravitational waves that come crashing through the universe when black holes collide. We’ve seen jets of energy that belch from ravenously feeding black holes. Although scientists have seen blurry images that suggest the presence of a black hole, we’ve never really, truly seen one.
A group of scientists from around the world is hoping to capture an image of the black hole Sagittarius A*—or more precisely, its event horizon—at the center of the Milky Way galaxy. They’ve also got their sights set on a monster black hole, estimated at six billion times the mass of the sun, in a galaxy known as Messier 87 (M87). This galaxy is at the center of the cluster of galaxies in the Virgo constellation of stars, about 50 million light-years away. The M87 black hole is much farther away from us than Sagittarius A*, and it’s also a whole lot larger. An image of the event horizon of either black hole would be the most direct evidence to date of the existence of black holes.
This is an artist’s illustration of the corona (represented in purplish colors) surrounding a black hole. Coronas surrounding supermassive black holes are sources of highly energetic particles.
Scientists are starting with Sagittarius A*. They have calculated that it would appear as a dark circle in the middle of a gas cloud in the constellation Sagittarius at the center of the Milky Way. Its tremendous gravity will distort and magnify any image scientists take, sort of like the distortions of a funhouse mirror. So scientists would expect to see a shadow of the black hole about 50 million miles (80.5 million km) wide.
This is a combined image from two Chandra observations of the giant elliptical galaxy Messier 87—50 million light-years from Earth. Bright arcs and dark cavities in the multimillion-degree-Celsius atmosphere of M87 surround a central jet. Much farther out, two plumes extend beyond the rings. These features, plus radio observations, show that repetitive outbursts from the central supermassive black hole have been affecting the entire galaxy for at least one hundred million years.
Sagittarius A* is so compact and so very far away that getting a good look at it would be like someone on Earth trying to take a picture of a grapefruit sitting on the moon. You would need a very large telescope to get enough focus and clarity to see the grapefruit.
A radio telescope is the ideal instrument to look more closely at Sagittarius A*. (Radio telescopes, like other telescopes, detect electromagnetic waves from sources in space—in this case, radio waves.) Because radio waves are very long, they can pierce through Earth’s atmosphere and through the galactic dust between Earth and the center of the Milky Way galaxy. Visible wavelengths can’t do that. Scientists also believe that the hot gas at the Sagittarius A* event horizon will shine brightly when the high-frequency (0.05 inches, or 1.3 millimeter) radio wavelengths hit the gas.
However, to get the kind of resolution needed to see the event horizon of Sagittarius A*, scientists would need an Earth-sized radio telescope. That’s impossible. So the next best thing is a network of nine radio telescopes situated around the world: the Event Horizon Telescope (EHT). The trick—and it’s not a small one—is getting all the telescopes to work together, looking at the same object at the exact same time, and coordinating all the data to create a single high-resolution image.
In 2005 a team of astronomers led by Zhiqiang Shen of the Shanghai Astronomical Observatory in China caught a fuzzy glimpse of Sagittarius A*. The team used an array of ten radio telescopes stretching from Hawaii to the Caribbean island of Saint Croix (one of the US Virgin Islands). Known as the Very Long Baseline Array (VLBA), the networked telescopes provided data to create an image showing that the diameter of Sagittarius A* was less than 90 million miles (145 million km) across—less than Earth’s distance from the sun. Ionized electrons and protons in space blurred the image, however, and the black hole’s event horizon appeared larger than scientists expected. This is not unlike the way frosted glass blurs and enlarges the image of something on the other side of the pane of glass.
EHT scientists hope to sharpen that blurry image. The EHT telescopes are all over the world, which gives EHT images much better resolution than VLBA images. And the additional EHT radio telescopes will be tuned to slightly higher radio frequencies so that the waves can more effectively make their way through the cosmic haze.
Scientists check for ice on the dish of the Large Millimeter Telescope (LMT) Alfonso Serrano. The LMT is part of a network of radio telescopes known as the Event Horizon Telescope. The LMT sits atop a dormant volcano in Mexico, at an altitude of 15,092 feet (4,600 m).
Radio astronomy is not always easy. Water vapor in Earth’s atmosphere absorbs radio waves, so radio observatories tend to be built at high, dry places. Still, powerful storms do sometimes roll in at these high elevations, making observing difficult to impossible. And because radio signals sent far into space are very weak, it’s important to filter out as much radio frequency noise from Earth as possible. It’s like tuning a radio to get rid of the static between stations. The more remote and the quieter the location, the better for radio astronomy!
One of the most powerful radio telescopes on Earth is the Atacama Large Millimeter/submillimeter Array (ALMA). It is in the Atacama Desert of Chile on a 16,400-foot (5,000 m) plateau, one of the driest places on Earth. Filmmakers use it as a location to shoot scenes that are meant to take place on Mars. In fact, scientists have found that the soil of the Atacama Desert is just as lifeless as that on the surface of Mars. And because the plateau is at such a high altitude, oxygen levels are low. Scientists and engineers working at the ALMA site wear portable oxygen tanks to avoid passing out. They are allowed to spend just six hours a day at this high altitude.
ALMA is actually made of many small antennae that are 40 feet (12 m) in diameter. The antennae combine data to work together as a single large telescope. The ambitious goal of the EHT team is to network ALMA with other independent radio observatories in similarly remote and forbidding places across the world. This requires a mind-boggling degree of coordination. For example, on an agreed-upon observation night, scientists at each telescope point their dishes at Sagittarius A*. Each telescope has an atomic clock. The clocks are synchronized to an extreme level of accuracy—only a one-second shift every one hundred million years. The telescopes track the black hole throughout the night, using Earth’s rotation to view it at different angles. The telescopes store data on hard drives, shipping them off to MIT. There, a supercomputer puts all the data together for analysis.
CHIRP
Coordinating the information from each observatory is tricky. Katie Bouman, a graduate student in electrical engineering and computer science at MIT, explained how the telescopes work together. Imagine, she said, “that you have a big pond. You and your friend are sitting very far apart on the shore at the edges of the pond. There are a bunch of frogs jumping up and down in the center of the pond, which causes ripples. You can’t see the frogs; you can only see the waves as they approach you at the shore.”
Bouman points out that you can measure the wave patterns that reach you and compare them to the patterns that reach your friend a little farther away. The ripples, or waves, will interfere with one another. Some will amplify the waves to form crests (peaks). Some will cancel them out, forming troughs (low points or valleys). By merging your information about the waves in the water, you can figure out where the frogs are.
Similarly, the radio telescopes are like you and your friend at the edge of the pond. Scientists can study the data they receive from the radio waves emitted by a black hole. From the waves, they can then create an image of the black hole at the center of our galaxy. It’s a technique called interferometry. It’s a powerful tool for merging data from different telescopes.
The EHT scientists face some additional challenges in creating an image of Sagittarius A*. Radio waves from cosmic sources usually reach any two telescopes on Earth at slightly different times. Earth’s atmosphere can slow them down, exaggerating differences in arrival time and throwing off the calculations needed to merge the information from the two telescopes.
Bouman developed a new algorithm (a process or set of rules to be followed in calculations) that she calls Continuous High-resolution Image Reconstruction using Patch priors (CHIRP) to address that problem.
It multiplies the data from three telescopes to cancel out the delays caused by Earth’s atmosphere.
CHIRP solves another problem for the EHT. With just a few telescopes scattered across Earth, the EHT data is incomplete. Scientists can use the data to make any number of images of the black hole. The images could all match the data but not necessarily be a true reflection of the black hole.
“This [situation of incomplete data creating an image] poses a real conundrum, because we’ve never seen a black hole,” Bouman said. “We have some idea of what a black hole might look like, but we don’t want to impose too much prior information on that. We don’t want to get into a circular argument where we’re reconstructing something that we expected to see.”
CHIRP helps create an image that both fits the data and agrees with what we already know about black holes. To do this, Bouman fed a huge database of real-world images, from pictures of galaxies to cats and houses, into the computer’s algorithm. “Even though there’s a huge variation in these kinds of images, if you break them up into 64-pixel patches [small visual units], there’s a lot in common,” she explained. Over time, the computer learned which patches come together in a way that produces an image that best fits the data. “When we’re trying to reconstruct an image of the black hole, we’re treating those little patches [of galaxy images] as puzzle pieces, fitting them together in a collage to make an image that’s likely, based on what we know about images in general.”
If everything goes well, EHT scientists hope to see a clear picture of the Sagittarius A* black hole emerge sometime in 2017. So far, they have already taken terabytes (one terabyte is one million million bytes) of radio frequency data, and they will continue to take many more. You can follow the project at http://www.eventhorizontelescope.org/.
A Theory (Possibly) Confirmed
What’s the scientific payoff for the EHT? It’s not just that scientists want to create pretty pictures of some of the most mysterious objects in the universe, although that would be fantastic. EHT will provide a laboratory of sorts to test the predictions and limits of Einstein’s general theory of relativity. For example, his theory predicts that the strong curvature of space-time near a black hole will produce a dark shadow surrounded by a bright ring of photons. If scientists do capture an image of the event horizon shadow, it will be a major confirmation of the theory.
Just as exciting is the prospect of watching Sagittarius A* dine, perhaps on a blob of gas. We would expect to see the gas orbit the event horizon at nearly the speed of light and watch its last moments before it is swallowed into the black hole. The fast-moving gas should emit light from a particular place in the black hole, giving scientists a great tool for measuring how light and matter are affected by extreme gravity.
Sagittarius A* is a picky eater, as black holes go. It’s on a near-starvation diet. The gigantic M87 black hole in Virgo, on the other hand, is a much less picky and messier eater. It has swallowed an entire medium-sized galaxy over the past billion years and regularly burps out huge jets of energy into the universe. One day EHT scientists hope to capture an image of this black hole too!