The landscape appeared more Martian than terrestrial—barren for the most part, with a reddish hue, and dotted with salt-splashed mountain ridges. We passed a few wild donkeys among the desert shrubs and a field of cacti along the way. Ahead of the final ascent by bus, we heard dire warnings about the risks associated with altitude sickness, ultraviolet exposure, and dehydration. A paramedic checked my heart rate, blood pressure, and oxygen-saturation level, and clipped a pulse oximeter on my finger—all part of the requisite ritual before visiting the Atacama Large Millimeter Array (ALMA), an elaborate grid of radio dishes erected on a high plateau in northern Chile.
At 16,500 feet, there is half as much oxygen in the air as at sea level. Walking, let alone working long hours, at such altitude often results in nosebleed, vomiting, and fatigue. In rare cases, fluid accumulation in the lungs or the brain could be fatal. Some people in our group needed to use oxygen canisters, even though our visit only lasted a couple of hours. The extreme conditions on the Chajnantor plateau that strain the human body are the very reasons that astronomers have built the world’s most expensive ground-based telescope there, at a cost of nearly $1.5 billion. At the high and dry site, where the annual rainfall averages about four inches, the air contains very little moisture, which would otherwise absorb the millimeter-wave emissions that come beaming down from the heavens above. Living up to astronomers’ lofty expectations, ALMA has spotted planets forming around nearby young stars and dusty galaxies in the far reaches of the universe, and a whole lot else in between.
Soon ALMA will take on perhaps the most ambitious endeavor of its brief existence. Over 10 days in early April, it will join radio telescopes at five other sites spanning the globe, from Hawaii to the South Pole, in an attempt to capture the shadow of a supermassive black hole that sits at the heart of our Milky Way galaxy, and an even bigger one in the neighboring galaxy M87. Together, the far-flung network of observatories will function as a single Earth-sized telescope to try to image, for the very first time, the so-called event horizon, the black holes’ boundary of no return.
“ALMA makes a huge difference,” Heino Falcke of Radboud University in the Netherlands, who chairs the science council of the Event Horizon Telescope (EHT), told me. For one, with 66 dishes, “it provides an enormous win in sensitivity,” he added. For another, the center of our galaxy passes almost directly above northern Chile.
Earlier tests, with fewer telescopes in North America and Hawaii, probed the magnetic field near the Milky Way’s black hole and the jet launching mechanism of M87’s. Now, with dishes in Chile and Antarctica greatly expanding its footprint, the network is said to be sharp enough to see a DVD on the moon. That is just about sufficient resolution, researchers expect, to see the shadows cast by the black holes’ event horizons in silhouette against the glowing gas in their vicinity.
Even with a virtual telescope nearly the size of the Earth, the challenge remains formidable because, despite their enormous masses, black holes are rather compact—not to mention dark. The one at the Milky Way’s core, dubbed Sagittarius A*, is four-million times as massive as the Sun, but only about 18 times bigger. At a distance of 26,000 light-years, that barely amounts to a pinprick in the sky.
At optical wavelengths, Sagittarius A* is obscured by dust in our sight line. Even at most radio wavelengths, ionized gas clouds in the galaxy’s heart wash out our view. Sub-millimeter waves are best for looking through the veil for a glimpse of the black hole’s shadow, as Falcke and colleagues first suggested nearly two decades ago. Still, water vapor in the Earth’s own atmosphere poses a challenge, which explains why telescopes like ALMA are built at arid, high-altitude sites. The EHT team, led by Shep Doeleman of the Harvard-Smithsonian Center for Astrophysics, has settled on a wavelength of 1.3 millimeters as the best compromise for their April observations.
“Don’t expect a quick result,” Falcke cautioned. The team will take about a year to analyze the combined dataset from the different sites. In fact, they will not even have access to the South Pole Telescope’s observations until October, when planes can retrieve a data drive following the Antarctic winter.
So what do the researchers expect to see? “We are at the hairy edge of getting a good image,” according to Falcke. “If we are lucky, we may see something that looks like an ugly peanut.” One side of the image would be brighter than the other because relativistic effects boost the apparent brightness of material moving toward us. “The first images probably won’t look quite like the beautiful simulations,” Falcke added. “Still, confirming the event horizon’s existence would be a big deal. Seeing is believing after all.”
If the April run turns out to be a success, the EHT team will redouble their efforts. “Eventually, the image should look like a blurry new moon, where the dark portion would represent the event horizon’s shadow,” said Falcke. Adding radio dishes in Namibia and Greenland—or perhaps even in space—to the network would sharpen the EHT’s vision further. By measuring the size and shape of the shadow, the researchers hope to test whether Einstein’s theory of general relativity holds up in the intense gravity next to monster black holes, which are, after all, one of the most extreme and mysterious physical environments in the observable universe.
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