A telescope the size of the Earth
A combination of nine radio telescopes around the globe promises to produce the best ever image of the black hole at the heart of the Milky Way. Cathal O’Connell reports.
The most ferocious storm in our galaxy rages right at its centre. It is a maelstrom hotter than any star: a swirling disk of superheated gas firing two jets, one above and one below, that whirl and twist like tornadoes. In the middle sits an eye of perfect stillness – a sphere of total black, millions of kilometres across.
This is the supermassive black hole at the centre of the Milky Way. Now, using a telescope the size of the Earth, we are about to catch our first picture of it.
Black holes are perhaps the most mysterious objects in the universe. They are regions of space so warped by gravity that not even light can escape. They surround objects of apparently infinite density called gravitational singularities – where the laws of physics, as we know them, break down.
To ‘see’ a black hole might seem an impossibility, but the region just outside the edge, or event horizon, of the black hole is actually incredibly bright.
Matter falling into a black hole gets heated, through some poorly understood mechanism, to millions of degrees Celsius. This makes the biggest ones – such as the supermassive black holes at the centre of galaxies – among the brightest objects in the universe.
Many of these have been imaged before, but only as bright spots, never with any detail of their inner workings.
To see one directly, our best bet is to point our telescopes at the constellation Sagittarius and a bright spot known as Sagittarius A*, where the Milky Way’s very own supermassive black hole is located about 25,000 light-years away.
There is a problem, though. We cannot just take a snap of Sagittarius A* with the Hubble Space Telescope because our view is obscured by gas and dust. We have to turn to radio waves, which can pass unimpeded through the galaxy.
We also need the largest radio telescope ever built, because typical radio telescopes can only detect objects millions of times larger than Sagittarius A*. Because it is so far away, Sagittarius A* is a tiny speck in the sky, just 37 microarcseconds across – about the equivalent size to a grape sitting on the surface of the moon.
The Event Horizon Telescope (EHT) comprises an array of nine radio telescopes around the world: in Chile, the United States, Mexico, France, Spain and Antarctica. By triangulating the data from each, the EHT works like one humongous radio dish, thousands of kilometres across. The signal will not be perfect, but should be enough to capture the bright spot of Sagittarius A* and the black silhouette at its centre.
Such an image could allow us to test our under-standing of physics and cosmology in new ways, especially Einstein’s theory of general relativity.
One of the first things physicists will look at is the shape of the black hole itself. The theory of general relativity predicts black holes to be perfectly spherical, meaning the EHT’s image of the silhouette should appear circular. Any kind of squashed shape could be the first observational disagreement with the accepted orthodoxy – setting up a potential revolution in physics.
Another mystery relates to the accretion disc, the swirling cloud of material in motion around the hole. How does it heat up? Physicists often describe the process as a kind of ‘friction’ – as if the gas particles rub against one another as they churn around the disc. Yet we know the gas would be too diffuse for direct physical contact. Something else must be going on, perhaps related to strong magnetic fields driving turbulence. Again, direct images could give us the answer.
The evolution of supermassive black holes is tied to the growth of galaxies themselves. To understand these processes, we will need to look beyond the Milky Way. The EHT should be powerful enough to image the supermassive black hole in the centre of the Messier 87 galaxy, in the constellation Virgo, more than 50 million light-years away. Although Messier 87 is more than 2,000 times more distant from us than Sagittarius A*, its black hole is 1,500 times more massive, so it should appear only slightly smaller than Sagittarius A*.
The nine telescopes trained all of their ‘eyes’ on Sagittarius A* in April 2017. Since then, scientists have been compiling the data, rendering the image and comparing it to models of what we expect the black hole to look like. Astronomers and astrophysicists are now anticipating having the first images from the EHT soon.
The result could be the major astrophysical event of 2018, heralding a new age in black hole astronomy – all by looking into the eye of the storm raging at the centre of our galaxy.