Black holes are, by definition, difficult to photograph. Their gravity is so immense that they can effectively cut anything that falls through the event horizon off from the rest of the universe. This includes light waves. That means there are no telescopes that can directly photograph anything that’s on the other side of a black hole’s event horizon, but the new Event Horizon Telescope comes close with its ability to observe anything that’s right on the edge of the event horizon.
The Event Horizon Telescope is actually a collection of networked radio receivers around the world, including the South Pole, Chile, the French Alps and the U.S. From April 5 through April 14, the network will be switched on to observe radio frequencies being emitted by a very specific object in a very specific location. It will make these observations using a technique known as Very-Long-Baseline Interferometry, which is the same technique used to observe radio waves emitted by objects like quasar. For this study, the radio receivers will focus on a frequency of 230 GHz, which should grant the ability to “see through” anything that may be blocking a line-of-sight view of the event horizon, such as concentrations of dust and gas.
The Event Horizon Telescope will have a resolution of 50 microarcseconds, which would theoretically give it the ability to spot a grapefruit on the Moon if that grapefruit had the ability to emit a strong radio signal. For the purpose of this study, though, the telescope’s first target will be the black hole at the center of the Milky Way Galaxy, known in astronomical circles as Sagittarius A*. This black hole has never been directly observed before and astronomers have had to be satisfied with observing its effect on nearby stars. The black hole has an event horizon that is 20 million kilometers across and is 26,000 light years away from Earth, which makes it a challenging target to study. With the Event Horizon Telescope, though, astronomers believe they could get the equivalent of a close-up view of activity in the black hole’s immediate environment and possibly make some direct observations of the event horizon itself.
But Will It Look Like That Black Hole in Interstellar?
The team behind the movie Interstellar actually deserve credit for attempting to depict a realistic black hole. If Einstein was right, the appearance of the event horizon would resemble a crescent of bright light surrounding a dark blob. The ring of light would still be there and emitting radiation, but the crescent would be easiest to detect because the Doppler effect would make material moving in our direction appear brighter. The Event Horizon Telescope team are hopeful that they will also be able to measure the shadow cast by the black hole.
Team member Feryal Özel said of their predictions, based on Einstein’s theories, “We know exactly what general relativity predicts for that size. … [I]t’s never a good idea to bet against Einstein, but if we did see something that was very different from what we expect we would have to reassess the theory of gravity.”
If the weather cooperates during the viewing window in April, scientists expect to gather enough data to have a clear picture of what the black hole actually looks like by late 2017 or early 2018. They expect that public interest in the first direct images of a black hole’s event horizon will be high. As Özel told the 227th meeting of the American Astronomical Society, “One thing that could excite the public almost as much as a Pluto flyby would be a picture of a black hole, up close and personal.”