For the first time ever, scientists will have detailed images of the supermassive black hole which lies at the heart of our galaxy. But you can’t see them just yet, for several reasons.
First of all, the images are stored in hard drives that won’t be accessible until later this year, and it’s not because somebody forgot the password: they’re stored at the South Pole, and it won’t be possible to fly them out until October.
Storing hard drives at the South Pole might seem like poor planning, but there’s a good reason for that seemingly weird location, and it has to do with how the images were obtained in the first place.
In order to zoom in on their target, astronomers used telescopes all over the world — Europe, the Americas, and the South Pole — with each telescope contributing to the images, as part of an overall project called the Event Horizon Telescope.
But these aren’t conventional photographs. Instead of using light in the visible spectrum like the kind we’re used to, astronomers instead looked for radio waves emanating from their target, using a technique known as Very-long-baseline interferometry (and yes, radio waves are a kind of light, but with wavelengths much longer than those which make up visible light).
By using telescopes all over Earth simultaneously, they were able to capture images much more effectively than any single telescope would be capable of, because the group of telescopes can act collectively like a much larger telescope, one with a diameter as big as the distance between them.
So now we’re waiting for the hard drives from the South Pole. And these aren’t images you can just send as an email attachment — we’re talking about more than a thousand hard drives full of data.
Even once the logistics are all sorted out and the data has been put together, there’s another reason why you won’t be able to see images of the black hole. And that’s because the images won’t really show the black hole at all.
Does that mean the project didn’t work? Not at all. The fact is, it’s impossible to capture an image of the black hole itself. Wrap your head around this: black holes have such a strong gravitational pull that any incident light will never escape it. And as you may know, that’s how we’re able to see anything at all — by absorbing the incident light that bounces off the object we’re seeing. So, no incident light escaping an object means it can’t be seen (hence the name: “black” holes).
Okay, okay. So then what are these images even going to show? Well, as particles fall into the black hole, they move at such high velocities that they emit lots of light before disappearing forever into the black hole. And it’s that light, emitted outside the so-called “event horizon” of the black hole, which was picked up on the telescopes. So in the end, what we’ll see won’t be the black hole itself, but the halo of light that surrounds it, emitted by particles spiralling into oblivion.
Once emitted, that light then had to travel 26,000 years from the centre of the galaxy to Earth. So the image we’ll be seeing is actually quite old: we’ll be seeing a snapshot of what the black hole looked like during the last ice age.
As for the black hole itself, it’s in a region at the centre of our galaxy called Sagittarius A*, and it’s called a “supermassive” black hole for a reason — it’s over 20 million km across. Another set of images captures a similar black hole at the centre of another nearby galaxy, known as M87.
Aside from being a cool thing to have, these images will also help us to understand important scientific questions. For one thing, they’ll help scientists to check certain predictions of Einstein’s Theory of General Relativity, which governs the behaviour of supermassive black holes.
Not that anyone expects Einstein to be disproven — as one of the scientists in the project said, “it’s never a good idea to bet against Einstein.”
And on the off chance that Einstein is proven wrong by this, he’ll have been upstaged by light that was already well on its way here thousands of years before he was born.