The hunt for white holes
The search for these eruptions in space-time has begun. Will they leave the pages of science fiction and enter reality?
The search for these eruptions in space-time has begun – but will they ever leave the pages of science fiction and enter reality?
The cosmos is full of odd and extreme phenomena. Take everyone’s favourite behemoth, the black hole, for example. These dark, mysterious objects hungrily devour anything and everything within their reach. However, there is a light side to accompany the dark – and the dark black hole has its light equivalent in the intriguing white hole.
In simple terms, white holes are time-reversed black holes. If you were somehow able to film a black hole in action and play the tape in reverse, you would see a white hole. Rather than furiously dragging in and trapping matter, as a glutinous black hole does, a white hole spews material out into space, allowing nothing to enter it.
White holes exist within the general theory of relativity. In mathematical terms, white and black holes are possible solutions that fit within the set of equations compiled by Einstein in the 1910s. These equations describe how mass interacts with and warps the fabric of space. Any object with mass has a gravitational effect on its surroundings; the greater the mass, the greater the gravitational effect.
The laws of general relativity, as with most of the laws of physics, can technically run both forwards and backwards. “If places where matter and energy can enter but cannot leave fit the equations, like black holes, then places where matter and energy can leave but cannot enter, like white holes, must as well,” explains Jeff Filippini of the University of Illinois at Urbana-Champaign. “There’s nothing wrong with the idea mathematically – but that doesn’t necessarily mean that white holes are real.”
Unlike black holes, which we know exist, white holes are currently thought to be hypothetical objects that are unlikely to exist in reality. “We don’t have any evidence for the existence of white holes in the visible universe,” adds Steven Giddings of the University of California, Santa Barbara. “They’re a theoretical prediction based on the time-reversal symmetry of the laws of nature. But time reversal also predicts that if we see a coffee cup fall and shatter on the ground, in principle there can be situations where all the pieces come together and assemble a coffee cup that flies up in the air, and we don’t see things like that, which are in some respects similar to white holes.”
While time-reversed processes – snooker balls coming back together to form a unbroken triangle, broken eggs reforming their shells, leaping coffee cups and white holes – violate no laws of physics, “we don’t expect to see [them] happen naturally,” explains Filippini.
Black holes form when massive stars use up their nuclear fuel, can no longer support themselves and collapse to create an infinitely dense point known as a ‘singularity’. This point has such a strong gravitational field that it is surrounded by a region from which nothing can move fast enough to escape – not even light, rendering it invisible.
One of the major stumbling blocks for white holes is that, unlike with black holes, there is no known process that could create one. “Black holes are known to exist, and we know ways they can form, so they satisfy the equations and exist in the real universe,” says Filippini. “But there is no process that I’m aware of that could actually make a white hole.”
Some scientists have hypothesised that white holes could form via exotic processes. One theory involves quantum tunnelling, an odd phenomenon that allows matter to behave in bizarre ways on very tiny ‘quantum’ scales. This process could potentially enable black holes to transform into white holes.
Crudely put, if you fell into a valley without sufficient energy to climb out, you would be stuck there until a kindly passer-by offered to help. In quantum physics this is not true. Instead it is possible to ‘tunnel’ your way through an intervening hill into another valley. “Our thought was that maybe you could actually tunnel between different solutions of the equations of general relativity using a quantum theory of gravity,” says Hal Haggard of Bard College, New York, one of the scientists behind the idea. “Quantum mechanically, you might be able to start with a black hole and have it tunnel into a white hole. However, these tunnelling processes are very rare, and nobody currently has a good grasp on how improbable they are. It could be that this is just so rare that it would never happen.”
White holes might instead form as a result of a black hole forming in some other part of
“WHITE HOLES ARE A THEORETICAL PREDICTION BASED ON TIME-REVERSAL SYMMETRY THAT FORMS THE LAWS OF NATURE”
Steven Giddings
“The Big Bang, the singularity point at the beginning of the Universe, is clearly a gigantic white hole”
Alon Retter
the universe – or another universe within the multiverse. In such a scenario, the white and black holes would be connected by a passage stretching through space and time, dubbed a wormhole. Matter would fall into the black hole, travel through the wormhole and pop out of the white hole somewhere else.
Although wormholes are a valid solution to Einstein’s equations, “they’re very tricky to describe in a way that doesn’t sound like craziness,” says Haggard. Wormholes are similar to white holes in that while they are mathematically possible, we know of no process that could form one. “There’s more evidence against the possibility of wormholes than there is against white holes. To create one you’d need a very unusual, strange type of matter that we’ve never seen – so wormholes would be an extremely surprising phenomenon.”
While we can’t observe black holes directly, we can detect them by hunting for bursts of X-rays from superheated material and spotting stars and gas performing quirky dances around seemingly empty areas of space. From this scientists can infer the existence of a massive, invisible object lurking nearby that is disrupting its surroundings. This proved true for the centre of our own galaxy, which is believed to host a supermassive black hole called Sagittarius A*. White holes, however, would be eminently observable. They would spring to life, pour huge amounts of energy out into space and then disappear. In 2006 scientists observed something like this – a sudden burst of energy a couple of billion light years from Earth. This lasted for 102 seconds before stopping and disappearing.
Sudden, energetic explosions are not unusual in the universe. The most energetic events in the cosmos are flurries of high-energy particles known as gamma-ray bursts (GRBs); these are produced by extreme processes such as star collisions, stellar deaths known as supernovae and even star-black hole or black hole-black hole mergers. GRBs come in two classes dependent on their duration and other properties, and each is thought to form differently. Scientists assumed the 2006 blast to be an example of a GRB, and named it accordingly as GRB 060614. However, its properties were peculiar and placed it in neither known class. More confusing was the fact that there appeared to be no suitable progenitor in the patch of sky that hosted GRB 060614; it seemed as if the explosion had come from nowhere.
In 2011 a duo of scientists suggested that the event may have been the first observed occurrence of a white hole. “Recent observations suggest that there is a third class of GRBs, the prototype of [which] is GRB 060614,” wrote Alon Retter, one of the scientists behind the theory. “[These] GRBs are long, close, lack any supernova emission and are naturally explained by white hole blasts.” Retter and his colleagues have even suggested that the Big Bang itself might have been a white hole springing to life. “Most astrophysicists believe that there are no white holes, but we and a few other researchers are convinced that they must exist,” says Retter. “The Big Bang, the singularity point at the beginning of the universe, is clearly a gigantic white hole that ejected all or most of the mass in the universe.”
A Big Bang-like explosion is indeed superficially similar to what we might expect to observe if we saw a white hole, and aspects of GRB 060614 also ring true, but both ideas are highly speculative.
“Was the Big Bang a white hole? I wouldn’t go there,” says Haggard. “The bounce of a Big Crunch – collapse of the universe – to a Big Bang – expansion of the universe – is analogous to the bounce of a black hole to a white hole, but I’m not sure that there’s a correspondence in the physics of the two situations.”
Another phenomenon that could possibly be linked to white holes is that of fast radio bursts (FRBs). These are incredibly short-lived, intense bursts of energy of unknown origin from outside our galaxy. However, there are indications that these FRBs may pulse periodically, which would make them less likely to be connected to white holes.
Despite their status as hypothetical objects, there are observatories that would be able to detect them. The Laser Interferometer Gravitational-Wave Observatory (LIGO) is the largest gravitational-wave hunter in existence. Gravitational waves are linked to the most extreme and high-energy phenomena in the universe, like black holes, neutron stars and supernovae. LIGO has characterised unusual types of black hole, discovered collisions between them
and more. The observatory could also spot white holes, although there appear to be no current plans to search for them directly.
Other potential white hole observatories include the Cherenkov Telescope Array (CTA), which is currently under construction and will hunt for highenergy gamma-ray sources, and the Fermi Gammaray Space Telescope (FGST), a space telescope currently in orbit around Earth that aims to characterise GRBs. “We must look for the structure of magnetic fields near the centres of galaxies,” says Russian theoretical astrophysicist Igor Novikov, who points out that general relativity allows for the existence of white holes. “If the structures of the magnetic fields appear to be magnetic monopoles [magnetic phenomena that have only one pole, which have so far proven elusive] that are macroscopic in size then this is a wormhole.” It turns out that wormholes – specifically their white holes – will emit their own radiation, in contrast to black holes that spew intensive radiation from the swirling gas that surrounds them.
While white hole research remains highly speculative and the concept remains hypothetical, the same was once true for black hole research, Haggard points out. “When people first discovered black hole solutions in general relativity they said, ‘this could never really happen; it’s such a strange idea’,” he says. “That was the textbook explanation of them for many years: ‘this is just a weird solution’. We now know that’s wrong! Black holes turned out to be ubiquitous, and there’s a lot of evidence for them now. My stance is to be openminded about white holes; mathematics doesn’t say they can’t exist, so we should look into them.
“We’d love for astrophysicists to go out and look for experimental signatures and either constrain them more tightly so that it’s less likely that they’re there, or find evidence, which would very much increase their odds of existing in the universe.”