The scientific definition of a black hole is a geometrically defined region of spacetime that has an extremely strong gravitational pull—a pull so strong that nothing can escape from it, not even light. According to the theory of general relativity (the geometric theory of gravitation published by Albert Einstein in 1915), a sufficiently compact mass can deform spacetime to create a black hole. While black holes were first posited as early as the 18th century, the event horizon (the point of no escape from the pull of a black hole) was defined in 1958 by David Finkelstein as “a perfect unidirectional membrane: causal influences can cross it in only one direction.”
The Event Horizon Telescope (EHT) project will photograph the event horizon of Sagittarius A*, the black hole at the center of the Milky Way, in 2017. It’s currently in the process of completing its technical preparations and theoretical calculations. Nine radio telescopes around the world located in Antarctica, Chile, Hawaii, Spain, Mexico and Arizona were chosen to photograph the event horizon.
“We’re almost there. The phasing in of the instruments has been done, the receivers are in place and the theoretical work has been done,” said Feryal Ozel, one of the members of the EHT team and professor at University of Arizona. “There are quite a few challenges that need to be overcome to take a picture of a black hole—it’s something that’s extremely small in the sky. But what we’re hoping for is a full array observation in early 2017.”
Although many renditions of the Milky Way Galaxy show the black hole at the center as huge, it is relatively small compared to the distances measured in space. Sagittarius A* is only 14,912,908.6 miles across (24 million kilometers), or about 17 times bigger than the Sun. It is also 25,000 light years away, which is a distance of 146 quadrillion miles. However, taking photos of the event horizon of Sagittarius A* may be troublesome. Clouds of gas and dust surround the black hole, which may make it difficult to get a closer look. Researcher of the EHT agree to a specific wavelength of 1.3mm by deriving theoretical calculations from simulations.
“We’ve run upwards of a million simulations, for many different configurations of what that gas might look like. And in all cases, we think that the 1.3mm wavelength is the right choice to see down to the event horizon,” she said. Ozel further commented that it was also an “incredibly lucky coincidence” that any wavelength was feasible, given the nature of black holes. It is still unclear what the photos will look like, but Ozel has an idea: “Hopefully it will look like a crescent—it won’t look like a ring.” The glowing gas spinning around the black hole and the Doppler effect (the change in frequency of a wave or other periodic event for an observer moving relative to its source) should make the event horizon photos look much brighter. “The rest of the ring will also emit, but what you will brightly pick up is a crescent.”
Einstein’s theory of relativity, which has been accepted by most (if not all) scientists over the past 100 years, will be thrown into question when the photos from the event horizon come in 2017. The theory states that a mass as large as a black hole can bend spacetime, and the curve can be mathematically calculated. This means the size of the shadow cast by Sagittarius A* will match the theory of general relativity, or it won’t.
“We know exactly what general relativity predicts for that size,” she said. “Now we’re taking that, and we’re multiplying it literally by millions and millions and millions in terms of the space-time curvature.” The EHT team isn’t stopping at just Sagittarius A*. It also plans to look at galaxy Messier 87, which is 53.49 million light years away. It has been reported that an immense jet of plasma was blasted into space by that black hole, so photographing the event horizon of a black hole may also give us more insight into how black holes work.