(Image: Dick van Aalst/漏2011 Radboud University)
Why photograph a black hole?
Black holes were predicted a century ago, but I have the feeling that we understand them even less these days. We still don鈥檛 have conclusive evidence for the presence of an event horizon 鈥� their point-of-no-return surface. Also, event horizons and quantum theory just don鈥檛 go together. Something needs to change, and it鈥檚 not entirely clear what that is.
Advertisement
How do we even know there鈥檚 a black hole in the Milky Way鈥檚 core?
Stars in the galactic centre orbit at some 10,000 kilometres per second, meaning there must be a central mass that is more than 4 million times our sun鈥檚 mass. The only thing that we 鈥渟ee鈥� in the very centre is a radio source called Sagittarius A*. Its very short, sub-millimetre radio waves probably arise from jets of hot gas emitted by material plunging into a supermassive black hole.
How will your planned giant network of radio telescopes help?
The black hole鈥檚 event horizon is probably 25 million kilometres across, but it鈥檚 27,000 light-years away. To image it at sub-millimetre wavelengths you need a telescope as big as Earth. A worldwide network of radio telescopes can obtain the same resolution.
Aren鈥檛 US astronomers working on a similar idea?
I first discussed these ideas 10 years ago with Shep Doeleman of the Massachusetts Institute of Technology, who now heads the US-led Event Horizon Telescope project. It makes no sense for us or them to work with a subset of the available telescopes, so we are now trying to set up a global project. We need each other.
What exactly are you looking for?
We hope to see how radio waves from the black hole鈥檚 surroundings are bent and absorbed, just as in Christopher Nolan鈥檚 movie Interstellar. The result should be a sort of central 鈥渟hadow鈥�. By comparing the size, shape and sharpness of this shadow with theoretical predictions, we can test general relativity. If the shadow is half as big 鈥� or twice as large as predicted, say, general relativity can鈥檛 be correct.
What are the biggest challenges?
The technology is daunting, but now under control. For each telescope you have to record hours of data at a rate of 64 gigabits per second and ship hard discs with petabytes of data between continents. Budget issues have eased a little bit with grants from the European Research Council and the US National Science Foundation.
When will we have our first black hole portrait?
In 2000, I said a result might be in within a decade, so I鈥檇 better temper expectations a little bit. Will it be another 10 years? I hope not, but in the end it takes the time it takes.
Read more: 鈥�General relativity at 100: Einstein鈥檚 unfinished masterpiece鈥�
Profile
is a radio astronomer and astroparticle physicist at Radboud University in Nijmegen, the Netherlands. In 2013, the team he co-founded received a 鈧�14 million grant from the European Research Council
This article appeared in print under the headline 鈥淗ow I鈥檓 going to photograph a black hole鈥�
