When black holes turn white
Can bouncing black holes help physicists find the ultimate theory of everything?
Has a new detection finally revealed the ultimate theory of life, the universe and everything?
Somewhere out there in the vastness of space lurks a black hole smaller than the full stop at the end of this sentence. Minuscule but mighty, it could hold the key to unlocking some of the greatest mysteries in the universe.
Black holes are the ultimate cosmic laboratory, a way for physicists to test out their theories in an environment so extreme that space and time are curved and warped. Even light cannot resist their eternal grasp, so we see no light reflected from them at all. We can only spot them when their gravity affects something visible or they merge to create gravitational waves. Few places have such a high amount of energy in such a small space.
But what happens if you fall into one? The bad news is you’re unlikely to survive the ordeal. The difference in gravity between your feet and your head would eventually get so extreme that it would overcome the forces holding your atoms together. You’d be torn apart into thin strips of human spaghetti, which is where the process gets its whimsical name: spaghettification. Where do your spaghettified atoms ultimately end up? What’s at the bottom of a black hole?
“With a black hole you get sucked in, but with a white hole things can only come out”
Our best answer currently comes from our leading theory of gravity: Einstein’s General Theory of Relativity. It tells us that a singularity awaits – an infinitely small, infinitely dense point where space and time cease to be. Hit it and you’re immediately erased from existence. Yet if you crush something down much smaller than an atom you enter the arena of quantum physics. At the moment we’re yet to take its weird and wonderful rules into account at the bottom of black holes because we have no way of combining it with general relativity. The search for such a theory of ‘quantum gravity’ is the ultimate goal for many physicists. A Nobel Prize would surely be in the offing for anyone who finds one that accurately describes our universe. It might also help us explain where our cosmos came from because, according to general relativity, the other place you find a singularity is at the moment of creation – the Big Bang – where time and space sprang into existence.
Carlo Rovelli, director of the quantum gravity group at Aix-Marseille University in France, doesn’t believe in singularities. “You cannot compress things too much,” he says. “It is a universal thing in nature.” He argues we need quantum gravity to help explain what happens instead. Rovelli is a founder of one approach to this thorny problem of getting the two theories to play nicely together: loop quantum gravity (LQG). According to Einstein, the fabric of space-time is smooth. However, proponents of LQG suggest that it isn’t. “That’s not surprising,” says Rovelli. “Other things in the universe like light and the energy of electrons come in chunks.” He suggests space is not smooth, but grainy – it’s also made of tiny little chunks or loops. Think of it like a piece of cloth; at first glance it may seem smooth, but look at it under a microscope and you’ll see that it’s really made of a series of stitches.
If you apply this logic to the depths of a black hole you get a remarkable result. Occasionally a black hole might ’bounce’ into its polar opposite: a white hole. “With a black hole you get sucked in, but with a white hole things can only come out,” says Francesca Vidotto from Radboud University in The Netherlands.
What exactly triggers the change? According to Vidotto it is simple chance. Quantum physics is defined by probability. You can never say exactly where an object is or what state it is in, only where it is more likely to be when you make a measurement. But the smaller an object, the more likely it is for unusual things to happen. Vidotto says an object has a timescale over which it can display these weird quantum properties. “For large objects, like a person or a cat, this time is much larger than the age of the universe,” she says. “For a planet-sized black hole it is about the age of the universe.” But for a black hole just half a millimetre across you’d expect it to have happened fairly often already across the cosmos. We normally think of black holes as much bigger than that – formed by the deaths of the most massive stars. However, astronomers also imagine there may be primordial black holes out there. Tiny ones formed in the early universe shortly after the Big Bang. Some of those could now be making this odd transition into a white hole.
If that’s true we should be able to see evidence of it happening with our telescopes. “You would expect an explosion,” says Vidotto. Such a detonation would trigger the rapid release of huge
amounts of energy. How energetic this radiation is depends on the size of the black hole. For black holes the size of your hand or smaller you’d expect it to be the radio part of the spectrum. And over the last decade astronomers have found a handful of unexplained events that might just fit the bill: fast radio bursts (FRBs).
The first was spotted in 2007 and, while there are still many mysteries surrounding them, it is clear they are coming from beyond our galaxy. The nearest emanated from over a billion light years away. Some astronomers have even suggested they might be attempts by aliens to get in contact. Far more likely is that they have some astronomical origin, but what exactly? Perhaps they are generated by colliding black holes or neutron stars. However, there is a way we might be able to prove once and for all that they really are coming from black holes bouncing into white holes.
According to calculations by Rovelli and Vidotto, more distant bursts should have more energy than those nearby. That’s because black holes are thought to evaporate over time by releasing Hawking radiation, named after the late physicist Stephen Hawking. Younger black holes in the distant universe should therefore be bigger and release more energy than older black holes closer to us that have had more time to evaporate.
This is in direct contrast to the way things normally work in astronomy. As the universe expands it dilutes the amount of energy in a given amount of space. There’s more space between us and a distant object to stretch, so far-away objects have their energy watered down more than those close to us. With bouncing black holes you’d expect the two effects to cancel each other out, meaning these explosive events would have a similar energy across a wide range of cosmic distances. According to Vidotto, observing this behaviour “would be a smoking gun for our theory”.
There are some potential snags, however. The FRBs discovered so far are not of the exact energy you would expect from a black hole to white hole bounce. That may not be the end of the world according to Hal Haggard from Bard College in
New York. “Given how imprecise the calculations are it’s not surprising,” he says. “It’s in the right ball park.” More concerning is that astronomers have identified a repeating fast radio burst called FRB 121102. First discovered in 2012, more than 15 distinct pulses are associated with the same source. “There’s nothing in the white hole theory that calls for that,” says Haggard. “If more and more of these repeating bursts are found then that goes against this proposal.”
He believes the white hole interpretation is extremely speculative, but the pay-off is potentially huge. “It’s exciting because there are so few ways
“It’s exciting because there are so few ways to test quantum gravity currently”
to test quantum gravity currently on the table.” But confirming a black to white hole transition wouldn’t immediately crown loop quantum gravity the victor. Haggard says the approach taken so far is “a generic model that doesn’t leverage anything specific about the theory of quantum gravity you’re using”. However, further detailed observations of how the explosions played out could do the trick. “Detailed analysis of the signals would be able to distinguish between theories, and that’s why this is so exciting,” says Haggard.
Given the high stakes, fortunately there are other ways a black hole to white hole bounce could show itself. According to Vidotto the explosive event should also generate gamma rays – the highest energy part of the electromagnetic spectrum. Although we do already have gammaray telescopes in space peering into the universe, Vidotto says “they are not yet optimised to see in
“What is remarkable is that no new physics is needed. No strings, no new forces and no new particles”
such high-energy gamma rays”. Future gamma-ray observatories may well be up to the task, however. In the meantime there’s a third way in: synchrotron emission. Particles like electrons would be accelerated through strong magnetic fields during the high-energy explosion, emitting radiation as they do so. “The challenge is how can we distinguish these cosmic rays from all the other sources in the sky,” says Vidotto.
If any one of these endeavours is ultimately successful, confirming a black hole to white hole transition won’t just help with the mystery of quantum gravity. It could also tackle an equally perplexing puzzle currently frustrating astronomers: dark matter. When we look at galaxies and clusters of galaxies there appears to be far more gravity than can be accounted for using visible material like stars and gas alone. Instead astronomers have suggested there is some hidden material skulking in the shadows which acts like a galactic glue, helping bind galaxies together with its own gravitational pull. The most fashionable contender for this dark matter has been supersymmetry – the idea that alongside the familiar sub-atomic particles like electrons and protons there are bigger particles that are their mirror images. The lightest of these supersymmetric particles has been the go-to explanation for dark matter for well over a decade. Despite a lot of searching, no one has ever found a supersymmetric particle.
That’s causing some physicists to look elsewhere for an explanation. Rovelli believes the remnants left behind as a black hole transitions into a white hole could go some way to providing the missing gravity. Being so small, they would be hard to detect other than by their collective gravitational pull. “What is remarkable is that no new physics is needed. No strings, no new forces and no new particles,” Rovelli says, referring to string theory – an alternative way to attack the problem of quantum gravity. Haggard agrees it’s possible that “they could make up a substantial fraction of dark matter”. He also says that “dark matter may not be one thing – it may be a mixture of particles we haven’t discovered and something else”. That something else could be black holes turning white.
For now astronomers are left in a tantalising position. Through fast radio bursts we might not only have the first clues that black holes can morph into their polar opposites, but also a way to tackle the ultimate questions about the nature of space and time itself. Then again, we may not. Only more observations with more telescopes from one end of the electromagnetic spectrum to other will tell us whether to call the Nobel committee or return to the drawing board. The stakes couldn’t be higher.
What is a white hole?Black holes are places where you can go in and you can never escape, while a white hole is a place where you can leave but cannever go back.
Differences in the strength of gravity across an object stretches it as it approaches a black hole
Although invisible, a black hole can often be detected by its effect on its surroundings
Einstein said that space and time are woven together into a smooth, continuous fabric called space-time
Could dark matter be made up of black holes?