Quest to uncover a black hole

Using Einstein’s theory of gravity, Interstellar, Chris Nolan’s latest film, is giving scientists new insights into the appearance of these phenomena. An international team is now working to reveal the presence of one such object, writes Robert Matthews.

A scene from the film Interstellar, which explains correctly that black holes are not invisible. Courtesy Warner Bros

With box-office receipts already topping half a billion US dollars (Dh1.83bn), director Christopher Nolan's new movie Interstellar has been hoovering up money like a black hole.

Not bad for a film whose plot centres on some of the most esoteric concepts in physics, from relativity to space-time wormholes.

Still, you don’t have to know anything about Albert Einstein’s theory of gravity to appreciate the mind-bending special effects.

Even experts who have been sniffy about some of the science concede that the movie’s computer-generated imagery (CGI) breaks new ground.

And not just for sheer spectacle. Based on Einstein’s theory of how gravity affects light, the CGI is so accurate that it has given scientists new insights into the appearance of black holes.

There is, however, just one problem with such insights: they can only be theoretical until we actually observe one of these cosmic conundrums.

The idea of seeing a black hole may sound like a contradiction in terms. But if there is one bit of science that Interstellar gets dead right, it is that black holes are not invisible. They reveal their presence via the destruction they wreak on their surroundings.

And that is exactly what an international team of astronomers is now searching for, using the world’s biggest scientific instrument. Known as the Event Horizon Telescope (EHT), once completed it will be trained on what astronomers regard as the best candidate for a giant black hole.

No one knows how the object code-named Sagittarius A-star (“Sgr A*”) came to position itself 26,000 light years away at the centre of our galaxy. Its age is also a complete mystery. What is not in doubt is that it has an incredibly strong gravitational field.

Studies of nearby stars show that they are hurtling around Sgr A* at speeds of up to 6,000 kilometres a second – a sizeable fraction of the speed of light. That implies that they are in orbit around something with the same gravitational pull as several million suns.

Something that hefty should be pretty hard to miss, yet conventional telescopes scouring the location of Sgr A* are unable to see any obvious source of the gravity. It seems all that mass has been crammed into a region smaller than our solar system.

And only one object is thought to be capable of possessing such incredible density – a black hole.

The supposed invisibility of black holes has been their defining characteristic since they were first mooted more than 200 years ago.

Using nothing more complex than Newton’s law of gravity, English scientist John Michell predicted as long ago as 1783 the existence of objects with gravity so strong that not even light could escape them.

But not until 1967 were the objects given their now-familiar name by American theoretical physicist John Wheeler.

Since then, it has become increasingly clear that black holes might make themselves visible indirectly, through their effects on their surroundings.

Gas and dust can end up trapped in a so-called accretion disc around a black hole. The friction within the disc heats it up until it is brilliantly radiant at every wavelength. Then Einstein’s theory of gravity begins to work its magic.

By warping the very fabric of space and time across which light travels, the black hole’s gravity distorts the appearance of the accretion disc.

Common sense suggests that the disc should look the same all the way around the central black hole. But calculations of the sort used by Interstellar’s CGI team predict a host of distortions, brightening and shifting the light on one side relative to the other.

It is these asymmetries that the international team of astronomers led by Dr Shep Doeleman of the Massachusetts Institute of Technology are hoping to witness for the first time using the Event Horizon Telescope.

The challenge is truly astronomical. If Sgr A* really is a black hole, then its size – as dictated by the “event horizon” after which the telescope is named – will be barely 20 times the diameter of the sun.

Detecting the predicted effects from an object so far away is equivalent to studying a single hair from a distance of 1,000km.

Even so, Dr Doeleman and his colleagues think that the EHT is up to the task. Unsurprisingly, it’s not like a normal telescope.

Using a technique pioneered by British astronomers in the 1950s, the team is linking up a global network of dozens of telescopes electronically, thus mimicking the power of a single instrument thousands of kilometres across.

In theory, that should be enough to discern the effects of Sgr A* on its surrounding accretion disk.

But the sheer distance to the object creates another problem: any light travelling from it has to penetrate tens of thousands of light-years of dust-strewn space.

To get around this problem, Dr Doeleman and his colleagues have included radio telescopes in the EHT network, sensitive to radiation able to cut through this cosmic fog. Even though the global network is still years from completion, the EHT team has made some major discoveries.

By linking observatories in Arizona, California and Hawaii, the team has created an instrument 2,000 times more sensitive than the Hubble Space Telescope.

Trained on Sgr A*, it has been able to reveal the fate of hot gas as it spiralled in towards the object. If Sgr A* were a conventional astronomical object, some of this gas would smash into its surface, triggering violent outbursts.

But observations of Sgr A* have seen no such outbursts – as if the material has simply vanished.

And that, of course, is exactly what is expected if it had descended into the black hole. Once over the threshold of the event horizon, nothing – not even light – can escape to tell the tale.

The team hopes to see more details of Sgr A* as more telescopes are added. If they succeed in demonstrating the reality of black holes, it will be another triumph for Einstein’s theory, although not for the man himself.

Ironically, the scientist most often associated with black holes went to his grave believing that they were nothing but pure science fiction.

Robert Matthews is visiting reader in science at Aston University, Birmingham, England.