When Stephen Hawking died in March, accolades poured in from fellow scientists about his work on those most enigmatic objects in the universe, black holes.
Now astronomers are racing to achieve a feat the late professor would doubtless have been thrilled by.
They’re planning to unveil the first-ever image of a black hole.
On the face of it, this sounds like a contradiction in terms.
Black holes are notorious for having gravitational fields so strong nothing can escape from them – not even light.
But an international team of researchers believes it has the means to reveal them: a telescope as big as the Earth.
Made by linking together instruments scattered across the globe, they have turned this leviathan towards the centre of our galaxy, where a colossal black hole is thought to lurk.
And they think they may have captured its hulking presence amid the destruction it wreaks on its surroundings.
Known as Sagittarius A-star (Sgr A*), no-one knows exactly how it got there, nor how old it is. But it’s thought to have a mass equivalent to several million suns, its titanic gravity tearing apart entire stars which wander too close by.
Over millions of years, this has created a disc of searingly hot debris around the black hole, which is expected to show itself as a pitch-black shadow on the disc.
It’s this unique signature that the astronomers hope to show the world within months.
The key challenge facing the astronomers is that Sgr A* owes its incredibly strong gravity to cramming the mass of millions of stars into a region of space far smaller than our solar system.
Being located at the heart of our galaxy, it’s a long way away. Even travelling at the speed of light it would take 26,000 years to get there.
The upshot is that being able to image the tell-tale shadow is equivalent of being able to see the width of one human hair at a distance of over 1,000 km.
No ordinary telescope is capable of such a feat, which is why the astronomers are using a trick known as Very Long Baseline Interferometry (VLBI) to link together eight radio telescopes in locations ranging from Chile and the US to Mexico, Spain and Antarctica.
The result is effectively a single instrument as big as the Earth.
It’s called the Event Horizon Telescope (EHT), after the region of a black hole within which nothing can escape.
The individual telescopes have now all been trained on Sgr A* for a week, the precise timing of their observations being recorded using ultra-accurate atomic clocks.
By matching up the timing of the observations of each telescope, the EHT team are now stitching them together to re-create the effect of one telescope thousands of kilometres across.
It’s an intricate process, and the astronomers – led by Professor Shep Doeleman of Harvard-Smithsonian Centre for Astrophysics – are taking their time.
That’s because this is about more than taking cosmic happy snaps. Studies of the precise shape of the shadow will give current theories of space and time a real work-out.
And it’s entirely possible they will find the first cracks in Einstein’s most celebrated achievement: his theory of gravity, known as General Relativity (GR).
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It has survived every challenge thrown at it since its publication in 1915, and explained a host of cosmic mysteries, from the behaviour of planets orbiting the sun to the expansion of the universe.
In 2015, its centenary year, astronomers finally confirmed GR’s prediction that the very fabric of space and time can ripple, creating gravitational waves – whose discovery was marked in last year’s Nobel prizes.
Even so, many theorists believe GR cannot be the final word in understanding gravity.
That’s because it lacks any obvious links to that other cornerstone of fundamental physics: quantum theory, the laws of the sub-atomic world.
Most theorists believe that GR and quantum theory are just part of a single, unified description of all the forces in the universe, called the Theory of Everything (ToE).
So far, however, evidence for the ToE has been hard to come by. Hopes that tell-tale signs would turn up in experiments at the Large Hadron Collider, near Geneva, have come to nought.
By looking for effects at the cosmic scale, the EHT offers a fresh opportunity to glimpse what even Einstein never imagined possible.
And that’s most likely to reveal itself in the shape of Sgr A*’s shadow.
According to Einstein’s theory of gravity, by swallowing the light pouring from the hot debris around it, this giant black hole should create a circular shadow.
However, the EHT has the power to spot distortions in the shadow that hint at other possibilities.
Already theorists are speculating that the EHT will reveal that Sgr A* isn’t just a giant star-eating monster, but a gateway to another part of the cosmos.
Known as a space-time wormhole, this astounding idea was investigated by Einstein himself in the 1930s. He showed it’s possible for a black hole to contain a tunnel providing a short-cut to another galaxy. And it might just be possible to venture down it.
Theorists have long believed this tunnel would collapse the instant it forms, trapping any travellers within it.
However, new research suggests that quantum effects might prevent this collapse. If so, space-time wormholes would no longer be science fiction.
If Sgr A* is indeed a portal to another galaxy, it would affect the shape of shadow it casts – and the EHT could detect it.
Such a discovery would rank as one of the most mind-blowing of all time. And it would certainly have thrilled the late Professor Hawking.
Robert Matthews is Visiting Professor of Science at Aston University, Birmingham, UK
