ON 2 NOVEMBER 2012, an intense burst of radio waves flashed across the skies above the Caribbean island of Puerto Rico. There was no spectacular fireworks display visible to human eyes, but the signal was captured by the 305-metre-wide radio dish of the Arecibo Observatory, nestled among the island’s forest-covered peaks.
Radio astronomers had been waiting for one of these for a while: a fast radio burst. Lasting only a few thousandths of a second, these super-bright pulses are thought to come from deep space, and are extremely rare. The Arecibo burst was only the eleventh ever detected. There was a suspicion that all previous readings, from a single radio telescope in Australia, were down to a technical glitch.
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They weren’t. But what these strange pulses are remains a mystery. Some think they come from super-dense stars or black holes dancing with those stars. Some have even floated the idea – one few people are willing to buy – that they could be “hailing signals” from aliens.
“At stake are not only black holes and space-time, but the start of everything”
The latest idea is that they could encode something equally astonishing: a signal from black holes behaving in an entirely new way. If so, it would transform our understanding of these most enigmatic cosmic objects, and mark the beginning of the end of a quest to reconcile two fundamentally irreconcilable descriptions of the physical universe. It might even explain the beginning of it all.
“That’s our big excitement for all this,” says of Bard College in Red Hook, New York, who has worked on the theory. “But I’m being a little cautious.”
The idea grows out of the rift between general relativity, which explains how space and time curve to create gravity, and quantum theory. Quantum theory describes the behaviour of natural force fields, such as those associated with electricity and magnetism, and the subatomic particles, or quanta, that make up those fields. To do this, it assumes that space and time are a fixed and rigid framework through which these fields pass – while general relativity, formulated by Einstein almost exactly 100 years ago, insists that space and time are malleable fields in their own right. But if space-time is a field, presumably quantum theory must apply, meaning it can be subdivided into little bits.
It’s a fundamentally confusing picture, and nowhere does that become more dramatically apparent than in our attempts to understand black holes. These regions of space-time in which matter is so dense and gravity so overwhelming that nothing can escape are, in essence, the ultimate cosmic trash compactors. Once something falls in, whether it’s a photon or an idealistic NASA pilot thrown off course by a Hollywood scriptwriter, it never comes out.
At least this is what general relativity tells us. According to Einstein’s theory, all matter ends up at the centre of the black hole where it forms a singularity: a pinprick of infinite density at which the laws of physics break down. This is where today’s cosmologists disagree with Einstein. “No physicist really believes that is what happens,” says Haggard.
Hence the search for a successful theory of quantum gravity, which would explain how gravity behaves over extremely small distances, such as those in the centre of a black hole. The problem is, we have no clue what such a theory would look like. General relativity has never flunked a test; no one has ever measured a signal that it can’t neatly explain. Until we do, theorists are working in the dark.
“We are in dire need of a guiding light to show us the correct path to quantum gravity,” says at the University of Nottingham, UK. “It would be one of the most important discoveries you could make.” Hence the interest in the fast radio bursts. “If you could observe a black hole doing something that does not come from general relativity, that would be a revolution,” says Sotiriou.
Loopy bits
The new idea comes from work Haggard did with Carlo Rovelli, a theoretical physicist at the University of Aix-Marseille, France. Rovelli is one of the founders of a model for a unified theory known as loop quantum gravity. It proposes that space-time is made of interlocking loops that form a fabric akin to chain mail. When seen from afar, this fabric looks smooth and continuous, but viewed close up it is woven from tiny, indivisible pieces. These loops would be the fundamental quanta of space-time; nothing could be smaller.
When Rovelli and Haggard considered what loop quantum gravity would mean for black holes, they came up with a startling conclusion: a black hole would eventually reach a density at which loopy bits of space-time could shrink no more. According to their calculations, published in 2014, there would be no singularity. Instead the loops would generate an outward pressure, resulting in a “quantum bounce” – an explosion that would destroy the black hole.
This is not the first time physicists have toyed with black holes doing unexpected disappearing tricks. Stephen Hawking suggested a mechanism based on the laws of thermodynamics, which govern heat and energy, that would make a black hole evaporate, albeit over a stupendous length of time. More recently, Abraham Loeb of Harvard University suggested that black holes could appear to explode if they were surrounded by a veil of matter that suddenly dissipated.
Rovelli and Haggard’s bouncing black holes are different. Most importantly, their quantum bounce would create a white hole, a massive object that spews out particles and radiation. “It’s like running a movie of a black hole in reverse,” he says. “The white hole emits particles but never absorbs them.”
Haggard has calculated that white holes are possible under the mathematical rules laid out by the equations of general relativity for the behaviour of space-time. But that does not mean a black hole actually does turn into its belching alter ego. “Stitching together different space-time solutions does not prove it is possible for a black hole to evolve into a white hole,” says Sotiriou.
Haggard and Rovelli think the transition could be the result of a quantum phenomenon called tunnelling, which allows subatomic particles to spontaneously change from one state to another. Quantum tunnelling underlies nuclear fusion, among other things. In our sun, it allows protons to overcome an otherwise insurmountable energy barrier in order to fuse and release energy.
It is not so crazy to think that quantum gravity could be subject to the same peculiar ways. Haggard and Rovelli’s idea is that all the matter collapsing to form a black hole singularity can never actually reach that point. As it approaches the size of an individual space-time loop, the probability that the entire black hole undergoes a quantum tunnelling event becomes greater and greater, until boing! It suddenly becomes a white hole (see diagram).
Not everyone is convinced: Sotiriou for one thinks the process is too much of a conceptual leap.”That definitely goes against conventional wisdom,” he says. “I think that needs clarification before this idea can be taken seriously.”
The best way to persuade the doubters would be to spot a black hole in the throes of such an explosive reversal. In 2014, Rovelli and two colleagues – at the University of Grenoble in France and of Radboud University in Nijmegen, the Netherlands – set out to determine how such an event would make itself known.
at a conference in Trieste, Italy, and calculated that an exploding black hole would generate signals at a wavelength equal to its diameter. In that sense, black holes are rather like loudspeakers: larger ones transmit longer wavelengths, or lower pitches, than smaller versions. Assuming that the exploding specimens were primordial black holes – a class of small black hole thought to have formed in the gravitationally violent aftermath of the big bang – they came up with a wavelength of a few millimetres. In other words, the expected signal was on the boundary between infrared and radio waves.
This rang a bell for Vidotto. She thought back to the mysterious ping from the Arecibo Observatory, and realised that its wavelength was in the same ballpark as their prediction for a signal from bouncing black holes.
That was especially exciting given how rare these events seem to be. Fast radio bursts were discovered in 2006, when radio astronomer Duncan Lorimer, then newly arrived at the University of West Virginia in Morgantown, was sifting through an old set of data from the Parkes Observatory in New South Wales, Australia. One signal stood out. “We just didn’t know what to make of it, it was so bright,” says Lorimer. After a further hard look, he concluded that the signal was not a technical snafu and almost certainly came from the far reaches of the universe – although he could not figure out what had produced it.
Only nine more of these fast radio bursts have been detected since, all from Parkes data, leading to questions about whether it was a quirk of the telescope – that is, until 2012 and the Arecibo signal.
It is easy to see why Rovelli and his colleagues are energised by that burst, which was announced to the world in 2014. But though their predictions for an exploding black hole signal and the fast radio bursts so far are a close match, they are not a perfect one. Then again, the calculations are based on an estimate for the mass of a black hole, which is in turn based on an estimate of how long a bounce would take.
Narrowing things down means tweaking the theory to take into account effects such as time dilation. Under the laws of relativity, time slows down in a gravitational field. The stronger the force field produced by a massive object, the slower time will run. A clock on a spacecraft orbiting 10,000 kilometres above Earth runs faster than a clock on the surface because the planet’s gravitational field weakens with distance. So in a black hole, where the field is about as strong as it can be, time pretty much comes to a standstill – for an outside observer at least.
The twist is that we experience time passing at the same rate wherever we are. So let’s imagine you could witness a quantum bounce from inside a black hole. For you the explosion would take milliseconds, but for anyone outside it would appear to take billions of years because the black hole generates such a strong gravitational field.
Now consider that the more massive the black hole, the stronger its gravity and so the greater the extent of the time dilation, meaning bigger black holes would appear to take longer to bounce. This gives Rovelli and his colleagues a potential way to test their hypothesis. Primordial black holes are generally tiny, isolated objects, their size fixed at their birth shortly after the big bang. If the theory is right, they are like ticking time bombs whose clock is set by their mass. The smaller ones will experience less time dilation, and so from our perspective will explode earlier in cosmic history. These explosions should also generate fast radio bursts at shorter wavelengths.
In other words, distant fast radio bursts should have shorter wavelengths than nearby ones – a pattern we would not expect to observe if the bursts had any other origin. “If we saw this, it would be extremely strong confirmation for our idea,” says Rovelli.
At the moment, all known fast radio bursts have been relatively close, so the change of wavelength with distance cannot yet be tested. Also, with only 11 of them, there are too few for any serious statistical analysis. So now it is a waiting game. Over time, researchers hope to detect more fast radio bursts and build up a proper data set.
Another type of signal thought to be produced by exploding black holes might offer an alternative route to validation: blasts of super-high-energy photons known as gamma rays. Much like radio bursts, the precise gamma ray signal emitted by an exploding black hole emits is determined by the total amount of matter and energy contained within. The signals would be distinct from the gamma ray bursts that astronomers regularly see, and right now we wouldn’t be able to detect them.
There is, however, a new observatory currently under consideration: the Cherenkov Telescope Array, which would boast more than a hundred telescopes spread over two sites in Spain and Chile. It should be capable of spotting these signature bursts but will not be ready until 2023, assuming it gets the go-ahead.
Big boing?
There is a lot at stake here – our understanding of not just black holes and space-time, but also the origin of the universe. There is only one other place where general relativity predicts a singularity arising: at the very moment of the big bang. But if the granularity of space-time prevents singularities happening in the first place, how did our cosmos come about? Perhaps not with a big bang, but with a big boing.
The big bounce hypothesis has been around for a while, and says that our universe was not born of an explosion that came out of nothing, but from a previous universe that collapsed. The new idea fits with that picture: as all the matter in that doomed cosmos came together in a catastrophic crunch, an enormous quantum bounce would have taken place once it reached the scale of the individual loops of space-time. This would have set our big bang in motion.
Lest anyone gets too carried away, however, there are more prosaic ideas to consider. Lorimer thinks fast radio bursts are most probably produced by young versions of rapidly-spinning neutron stars known as pulsars. This idea is “perfectly consistent with what we see”, he says, and is “grounded in things that we really understand”.
If so, it would resign us to yet another false start for quantum gravity. But then perhaps the mysterious signals really are the call sign of a distant alien civilisation, and they can tell us where we are going wrong.
(Images: Bose Collins, Yury Prokopenko/Getty)
This article appeared in print under the headline “Quantum bounce”
