The Event Horizon Telescope project
The event horizon at the mouth of a black hole is the point of no return: once light crosses this threshold, there is no going back. This means it's proved impossible for astronomers to physically see what's going on beyond this point. But there might be a way to do it by looking at the shadow cast by the event horizon and using this to infer what made that shadow. Shep Doelman is an astronomer at the MIT Haystack Observatory and Harvard Smithsonian Centre for Astrophysics, and he's part of an initiative called The Event Horizon Telescope project, which aims to get the first image of the shadow cast by the black hole at the centre of our own galaxy, as he explains to Georgia Mills...
Shep - There's a part of the Universe that is forever separate from our experience, and that's inside inside the event horizon, and the size and the shape of that event horizon is predicted by Einstein's equations which have withstood all the tests that we've subjected them to in the solar system and the larger universe, and now we'd like to go to the one place where they might break down, at the event horizon itself.
Georgia - Past the event horizon, by its very nature, you can't really see into it. How would you be able to study something like that?
Shep - Well, in a paradox of their own immense gravity, black holes, which by definition are dark are some of the brightest objects in the Universe. You can think of it this way: the black hole is insanely powerful and it's trying to attract all of this gas, dust, and ionised plasma into a very small volume and you get a cosmic traffic jam. Everything is rubbing up against each each and, just as your hands get warm when you rub them together, all this gas and dust heats up to billions of degrees. So it's a little bit like trying to suck an elephant through a straw; it's very hard to do and, when you ultimately do it, it's a big mess. So the black hole and the event horizon are illuminated by this three dimensional flashlight that lights up the space time, and one of the characteristics of the event horizon is that the light gets bent by gravity and so you wind up with a shadow feature. The way you can think about the shadow is that the light that is moving away from you, from the back side of the black hole, gets bent around in these curve trajectories back toward you so it illuminates a ring of light around the event horizon, and it's the size and shape of that ring that we're after with the Event Horizon Telescope Project.
Georgia - So how are you planning to find this shadow?
Shep - Black holes are the smallest objects that we know of and to see something that small you've got to make an entirely new kind of telescope. So we need something, to put it in perspective, that has a magnifying power that's at least 2,000 times better than the Hubble Space Telescope. The best candidate for us to observe one of these black hole shadows is in the centre of our own Milky Way galaxy and radio waves are the perfect medium for that. They can pierce the gas, and the dust, and the ionised plasma that lies between us and the centre of our galaxy. So we need to make a telescope that has 2,000 times the magnifying power of the Hubble that sees radio waves.
Georgia - So the problem here is that you need a radio telescope that's 2,000 times stronger and that, to me, would seem like it would need to be bigger. How are you going to go about this?
Shep - The magic of the Event Horizon Telescope is that we don't make one single telescope, but we use radio dishes that are spread around the globe and we link them together using GPS to synchronise them perfectly, and then we install atomic clocks at each of the sites. And all of the telescopes swivel to look at the black hole at the centre of our galaxy at the same exact moment and, when that happens, you get an earth sized virtual telescope and the radio waves are recorded perfectly at each of the sites. Then hard discs are shipped back on a 747, or the airliner of your choice, to a central facility and when you do that we wind up getting a data set that's equivalent to having a telescope the size of the Earth. And I would hasten to add that when you're making and Earth sized telescope you need and Earth sized group and I'm just pleased as punch to work with some of the most talented astronomers on the face of the Earth from: Taiwan, Japan, Chile, the United States, Europe. It's a really big enterprise.
Georgia - This kind of massive collaboration thing; it seems to me that's what science is all about. How did you get all these different telescopes on board with you?
Shep - Part of the secret sauce of the Event Horizon Telescope is that we're not building any new telescopes. We are developing all the instrumentation that allows us to turn telescopes that already exist into this global linked array. So, what we have done is we've gone to the directors and boards of telescopes around the world; we've explained this project to them and the science payoff is so interesting and exciting to the community that they've allowed us to come in with specialised equipment, install it at the telescopes and make these observations, and we've had some very interesting results so far. We wouldn't really be having this conversation if we hadn't already seen very small shadow sized features towards the black hole at the centre of the galaxy, and now we're going to take those size measurements one step further and see if we can make an actual image.
Georgia - And then at some point in the near future - well when is this going to happen, when are we going to get the telescopes to join up and assemble and all point at the same direction at once?
Shep - Well we're on, as they say, an aggressive timeline. We have already made these precursor observations which show us that we're on the right track and we are, over the next year, finishing the build out across the entire global array. The first possibility for us to really make an image would be in the spring of 2017, that's when of the larger telescopes (The Alma Array, in Chile), will join the Event Horizon Telescope and that will increase our sensitivity by a factor of ten, and also increase our resolution by a factor of two, and that would be the first time when we would have a shot at making a credible image. Knowing that all those 4 millions suns are within that ring would be the strongest evidence that we have, as least as humans (aliens might have better evidence), that black holes actually exist.