Black holes are among the most amazing objects in the known Universe. But despite the fact that they are suspected to lurk at the center of most galaxies, the reality is that no one has ever been able to actually photograph one. That is because black holes, as their name indicates are very very dark. They are so gigantic that they irreversibly consume everything that crosses their event horizon, including light, making them impossible tophotograph. But that could be about to change, when a new telescope network switches on in April this year.
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Called the Event Horizon Telescope, the new device is made up of a network of radio receivers located across the planet, including at the South Pole, in the US, Chile, and the French Alps. The network will be switched on between 5 and 14 April, and the results will put Einstein's theory of general relativity through its paces like never before. The Event Horizon Telescope works using a technique known as VLBI (very long baselineinterferometry), which means the network of receivers will focus in on radio waves emitted by a particular object in space at one time.
For the black hole, they will be focusing on radio waves with a wavelength of 1.3 mm (230 GHz), which gives them the best chance of piercing through dust and clouds of gas, blocking the black hole. And because there are so many of these antennae all tuned in on a single spot, the resolution of the telescope should be 50 microarcseconds. To put that into perspective, it is the equivalent of being able to see a grapefruit on the surface of the Moon.
That is important, because the first target will be the massive black hole at the center of our galaxy, called Sagittarius A*, which is actually only the size of a pinhole in our night sky. We have never directly detected Sagittarius A*, but scientists know it exists because of the way it effects the orbit of nearby stars. Based on the behavior of these stars, scientists forecast that the black hole is likely about four million times more gigantic than our Sun, but with an event horizon diameter of just 12.4 million miles (20 millionkm). At a distance of around 26,000 light-years away from Earth, that makes it a pretty small aim.
But the Event Horizon Telescope will target to observe the immediate environment around the black hole, and it should be able to get enough resolution to see the black hole itself. Sheperd Doeleman from the Harvard-Smithsonian Centre for Astrophysics told Jonathan Amos at the BBC thisweek that, "There is great excitement, in April we are going to make the observations that we think have the first real chance of bringing a black hole's event horizon into focus." So what can we expect to see if the project is successful?
The scientists forecast the black hole will look like bright ring of light around a dark blob. The light is being produced by dust and gas particles that are accelerated to high speeds just before they are ripped apart and consumed by the black hole. The dark blob would be the shadow cast over that chaos. But if Einstein was right, we should see more of a crescent of light than a ring because an affected Doppler effect should make the material moving towards Earth appear much brighter.
Team member Feryal Özel said in a press conference "Hopefully, it will look like a crescent - it won't look like a ring, the rest of the ring will also emit, but what you will brightly pick up is a crescent." If the team is able to measure the dark shadow cast by the black hole that will be gigantic, because general relativity makes some pretty specific forecasts about what size it should be, based on how much the black hole should bend space-time. Özel said "We know exactly what general relativity predicts for that size. Get to the edge of a black hole, and the general relativity tests you can perform are qualitatively and quantitatively different."
What happens if we see something else? Doeleman told Amos that it is absolutely a possibility, and it would shake up the world of physics as we know it. He said "As I have said before, it is 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. I don't expect that is going to happen, but anything could happen and that's the beauty of it."
Given all the data researchers will need to process, we should not expect the first images of a black hole until the end of the year, or even the start of 2018. And that is assuming there's good enough weather to get a clear picture in the April viewing window. But when those first pictures come in, it is going to be a pretty thrilling moment for humanity.
"One thing that could thrill the public almost as much as a Pluto flyby would be a picture of a black hole, up close and personal," Ӧzel said at the 227th meeting of the American Astronomical Society.
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