THE QUEST TO PHOTOGRAPH A BLACK HOLE
Stellar-mass black holes are difficult to see in detail because, one, they are small, and, two, they are black. The supermassive black holes in the hearts of galaxies are hugely bigger but unfortunately hugely farther away, making them appear small too. However, one supermassive black hole is both big and near.
Sagittarius A*, 26,000 light-years away in the centre of our Milky Way, weighs in at 4.3 million solar masses. It is the target of the Event Horizon Telescope (EHT), an array of cooperating radio telescopes scattered across the globe. The radio signals recorded at each site combined on a computer in Haystack, Massachusetts, to simulate a giant dish the size of the Earth. The bigger a dish is and the shorter the observing wavelength – EHT is using 1.3mm – the more it can zoom in on details in the sky.
The challenge is to image Sagittarius A*’s event horizon, which only appears as big in the sky as a grapefruit would appear on the Moon when viewed from Earth. What the astronomers want to know is whether the event horizon behaves as predicted by Einstein or even whether it exists. Recently, Stephen Hawking suggested that it might not. “An image would symbolise a turning point in our understanding of black holes and gravity,” says EHT leader Shep Doeleman of the Massachusetts Institute of Technology.
The first image of a black hole event horizon may be obtained in 2018. Almost certainly, it will be an iconic image to rival the double helix of a DNA spiral or the Apollo 8 image of the Earth rising above the Moon.