# DO BLACK HOLES LEAK INTO PARALLEL UNIVERSES?

## Information entering one of these high-gravity objects might not be destroyed but oozing into another cosmos entirely

Invisible, enigmatic and infuriating, black holes are astounding. Formed from the explosive deaths of the most massive stars, they push our very understanding of space and time to its limit. They are regions of such concentrated gravity that escaping from their clutches is impossible for those venturing too close. Once you've crossed the event horizon, you'd have to travel faster than the speed of light to escape but nothing can travel faster than the speed of light. Breach the event horizon and you're doomed to oblivion. What's more, you cannot hail anyone for help.

These monsters are so vexing because at various times they are both big and small. They start as the size of a star, where Einstein's general theory of relativity rules the roost. But, as the core of the dead star collapses to form the black hole, matter is concentrated down into an ever-smaller space. Eventually it moves into a realm dominated by the rules of the super-small – the weird and wonderful world of quantum physics.

Both of these theories have rightly been lauded for their individual explanatory power. Einstein published his revolutionary theory in 1915 and so far it has passed every test thrown at it with flying colours. The recent discovery of the gravitational waves it predicted was a real triumph. Equally, our modern technological age is built on a thorough understanding of quantum physics. Yet physicists cannot get the two theories to play together nicely. There is no currently accepted theory of “quantum gravity” that combines the two neatly on the same scale. Black holes in particular embarrass us by confronting us with the reality of this dilemma.

One of the most famous attempts to reconcile the two theories with the physics of black holes was provided by Stephen Hawking in 1974. In a well-studied quantum phenomenon, a pair of subatomic particles can simultaneously pop into existence as long as they disappear again very quickly. Hawking imagined this happening right on the event horizon of a black hole. One particle is doomed, the other is free to escape. They can never be reunited, meaning a black hole must slowly lose energy to its immediate environment. According to Hawking, black holes evaporate over time in this way through the emission of one half of these particle pairs – an effect known as Hawking radiation.

However, that idea immediately threw up a problem because his calculations showed that the nature of Hawking radiation depends solely on the mass of the black hole. Yasunori Nomura, a researcher at the Berkeley Center for Theoretical Physics, likes to imagine throwing two books into the void. “One is Shakespeare, the other is Penthouse,” he says. While both books contain different words, they both have exactly the same mass. As it only depends on the mass of the black hole, Nomura says the resulting Hawking radiation is identical in both cases. “It looks like the information about whether it was Shakespeare or Penthouse is completely lost,” he says. Quantum

Stephen Hawking was one of the first to successfully apply quantum physics to black holes

“A black hole can never completely evaporate away. Instead, a minuscule husk would always remain”

physics says that information cannot be created or destroyed. So where does the information go? This problem has become known as the ‘Black Hole Information Paradox’.

Many physicists have wrestled with how to solve this thorny issue. In 2015, Hawking himself detailed a new idea, re-exploring the notion he'd had 40 years earlier. His radical solution to the information paradox is that the information contained within the two books never actually makes it into the black hole. “I propose that the information is stored not in the interior of the black hole as one might expect, but on its boundary, the event horizon,” he said at a conference in Sweden on Hawking radiation held that year. According to Hawking, information about three-dimensional objects falling in ends up encoded as a two-dimensional hologram on the event horizon. Later, outgoing Hawking radiation re-delivers this information back into the universe. Given enough time, someone would, in principle, be able to recover the information contained within the books. Hawking would go on to tell the conference that black holes are not the eternal prisons they were once thought to be.

Nobel prize-winning physicist Gerard ’t Hooft has another idea. An object crossing the event horizon will begin to feel dramatic changes in its gravitational field. Hawking radiation will be affected by these gravitational changes and so carry out with it information about what the incoming object was. However, both Hawking and ’t Hooft's ideas have a significant snag: quantum physics not only forbids information from being destroyed, it also outlaws it being duplicated. The object falling in will carry one copy of its information, while another either sits as a hologram on the event horizon or is carried outwards by Hawking radiation. The mystery is far from solved.

Other researchers found a less drastic ray of hope when they discovered a way that Hawking radiation might preserve the information contained within objects added to the black hole without the need for holograms or duplicates. However, they could only get this to happen by dramatically severing the quantum link between the two particles that initially created the Hawking radiation. Cutting the cord would lead to a sudden burst of energy. With this process happening all along the event horizon, crossing over it would be like entering hell. You'd soon be incinerated by what physicists have a dubbed a ‘firewall’. This creates a new paradox. Einstein's general theory of relativity forbids anything special happening when you cross over the event horizon. Like the Earth's equator, it is a purely mathematical line. Why should you be set alight just because you pass from the equivalent of

“The information is stored not in the interior of the black hole, but on its boundary, the event horizon”

stephen hawking

one hemisphere into another? Physicists call this ‘The Firewall Paradox’. Applying quantum physics to black holes suggests the existence of Hawking radiation. At first that implied information can be destroyed

– The Information Paradox – unless crossing the event horizon singes you into a ball of smoke – The Firewall Paradox.

“I'm just not comfortable with this idea,” says Ana Alonso-Serrano at the Max Planck Institute for Gravitational Physics in Germany. She's been looking for an alternative way out and now believes she may have found one. “You don't need a firewall,” she says. To come to this conclusion, AlonsoSerrano looked at some of the current models for how quantum gravity might work. She specifically investigated something called the Generalised Uncertainty Principle (GUP), which says the more you know about a black hole's size the less you know about its energy. Her work shows that more and more Hawking radiation would be given off as the black hole evaporates, changing the amount of information it carries away. “Information isn't lost

– it is hidden in the Hawking radiation,” she says. Alonso-Serrano admits that her solution “is not a complete resolution” to the problem, but it has the potential to eliminate the pesky firewall. Her work also shows that a black hole can never completely evaporate away. Instead, a minuscule husk would always remain.

According to Hawking, a black hole should gently glow in Hawking radiation

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Gerard 't Hooft thinks the gravity of infalling objects imprints a black hole's Hawking

radiation The discovery of gravitational waves in 2015 finally confirmed a major prediction of Einstein's general theory of relativity

It's possible that every quantum event fractures the universe into copies