“ The RadioAstron telescope’s resolution is so good, it could pick out a snooker ball on the moon”
If you thought black holes were strange, try a white one.
Katia Moskvitch joins the hunt
Thar she blows
PHYSICS is full of opposites. For every action, there’s a reaction; every positive charge has a negative; every magnetic north pole has a south pole. Matter’s opposite number is antimatter. And for black holes, meet white holes. Black holes are notorious objects that suck in everything around them. Famously, not even light can escape their awesome gravity.
White holes, in contrast, blow out a constant stream of matter and light – so much so that nothing can enter them. So why have so few people heard of them?
One reason is that white holes are exotic creatures whose existence is speculated by theorists, but believed by few because no one has ever seen one. Now Nikolai Kardashev and his colleagues at the Astro Space Centre in Moscow are hoping to change that using a vast radio telescope with a view equivalent to that of a dish 30 times wider than Earth. They are aiming to identify what lies at the heart of many galaxies. If they confirm the existence of white holes, they will cast into doubt our current notion of what lies at the centre of galaxies – including our own. It would also be vindication at last for physicist Igor Novikov, who was the first to theorise their existence in 1964. Back then, black holes were called frozen stars, and were even less well understood than they are today. Novikov did what theoretical physicists do when confronted with situations that are impossible to test in the laboratory: he used pure reasoning to ask what would happen to a black hole if time were to flow backwards. His thought experiment yielded a new kind of object that spewed matter and light continually: a white hole. Others ran with the idea. What if a black hole was attached to a wormhole, a shortcut through space-time that connects two regions of our universe, or maybe even two different universes? The black hole would draw in matter, while at the other end of the wormhole there would be a white hole emitting it. Many physicists, though, have found the notion of a white hole hard to swallow. After all, black holes are thought to form when a massive star collapses under its own gravity; the collapsing matter results in a singularity at its core. This is the heart of a black hole, where all physical quantities diverge to infinity and all the known laws of physics break down.
But in the time-reversed version, “a white hole existed in the past, and somehow exploded outward”, says Novikov. Even he concedes the fundamental problem: “Researchers accepted that, from a mathematical and theoretical standpoint, white holes could exist. But there were questions about how such an object could actually form.”
Wormholes offered a way, but there were theoretical problems with them, too. They seemed to collapse as soon as they formed, shutting down the white hole too. Novikov himself outlined this instability problem in the 1970s. A decade later, however, theoretical physicist Kip Thorne of the California Institute of Technology showed that wormholes could indeed be stable, which gave the white hole theory a boost. Even this month, Carlo Rovelli and Hal Haggard at Aix-Marseille University in France showed that quantum theory can transform a collapsing black hole into an expanding white hole.
Perhaps the fact that we have found no signs of a white hole, despite peering ever deeper into space, is a more fundamental problem.
Enter a space telescope called RadioAstron whose wildly elongated orbit takes it out to a distance of 350,000 kilometres – nearly as far as the moon and 30 times wider than Earth’s diameter. Launched from Kazakhstan’s Baikonur Cosmodrome in 2011, its dish is only 10 metres across. But when its signals are combined with those from radio telescopes on Earth, the resulting images are as sharp as those from a dish 350,000 kilometres wide.
Right now, RadioAstron’s resolution is 20 times better than that of any telescope on the ground. It is so good that it can pick out objects covering an angle of 27 microarcseconds – the size a snooker ball
would appear on the moon as viewed from Earth. Kardashev and his colleagues have used RadioAstron to survey 100 active galactic nuclei, the compact regions at the centre of galaxies that are much brighter than expected.
Many astronomers think that these owe their brilliance to supermassive black holes. As the black hole sucks in gas, the unlucky matter is sent swirling around and gets hot enough to sparkle before plunging into oblivion.
But could some of these dazzling displays instead be due to matter and light streaming out of a supermassive white hole? Novikov thinks so: “Certain active galactic nuclei are not black holes, as most researchers suggest, but exist in the form of white holes, linking our universe to another universe.”
If RadioAstron can make a detailed enough image, then it should be easy to tell black holes and white holes apart, says Kardashev. “If it’s a black hole, then in the middle there should be a dark spot on the image,” he says.
“And if it’s a white hole, then there should be a bright spot in the centre.”
But perhaps the reason we haven’t seen a white hole is that we’ve been looking in the wrong place at the wrong time. Alon Retter, an astrophysicist who now works for Israel Aerospace Industries in Tel Aviv, thinks so.
What’s more he believes that we may already have caught one flickering into existence. In 2006, NASA’s Swift satellite detected a gamma-ray burst called GRB 060614. Such bursts are usually associated with supernovae or regions of high star formation, but GRB 060614 was neither. Retter believes that it may instead have been a white hole. He argues that white holes will appear as temporary flashes, rather than shining continuously, because all that matter and light coming out will collapse under its own gravity into a black hole. Kardashev and Novikov agree with Retter’s ideas. “The nature of these flashes in the sky is still unclear,” says Kardashev. “So once we spot a gigantic powerful gamma-ray burst with a lot of radio radiation, we will take a close look with RadioAstron and try to determine its shape and size for the first time.” That could provide important clues about its source. “It may be a white hole or a wormhole. Maybe the flashes are coming from another universe.” Retter calls his idea a “small bang” – a
spontaneous emergence of a white hole. If we extrapolate this thought, he says, we could assume that our entire universe is the result of a white hole that emerged as the big bang. Hardly anyone is hunting for white holes these days, but hopefully that will change. With each passing day, RadioAstron is beaming back more observations of fine structures in active galactic nuclei. “It is not theoretically excluded that the central engine in active galactic nuclei is something more interesting than a supermassive black hole,” says Konstantin Postnov, an astrophysicist at Moscow State University. “So let’s keep our eyes open and not discard even very exotic possibilities.”