So there you are, about to leap into a black hole. What may possibly await you if, against all chances, you survive? Where would you wind up, and what enticing tales would you be able to tell if you could wiggle your way back?
As Professor Richard Massey says, the basic answer to all of these queries is, “Who knows?” As a Royal Society research fellow at Durham University’s Institute for Computational Cosmology, Massey is well aware of the mysteries surrounding black holes. “Falling through an event horizon is literally passing beyond the veil — once someone falls past it, nobody could ever send a message back,” he said. “They’d be ripped to pieces by the enormous gravity, so I doubt anyone falling through would get anywhere.”
That sounds like a disappointing — and painful — response, and it is. Since Albert Einstein’s general theory of relativity was thought to have predicted black holes by linking space-time with gravity’s action, it has been known that black holes form when a massive star dies, leaving behind a small, dense remnant core. If this core has more than three times the mass of the sun, gravity would overwhelm it to the point in which it collapses in on itself into a single point, or singularity, which is thought to be the black hole’s infinitely dense core.
The resulting uninhabitable black hole would have such a strong gravitational pull that even light would be unable to leave it. So, if you find yourself at the event horizon — the point where light and matter can only pass inward, as proposed by German astronomer Karl Schwarzschild – there is no way out. Tidal forces, according to Massey, would reduce your body to strands of atoms (or “spaghettification”), and the thing would eventually end up crushed at the singularity. The idea that you could pop out somewhere — perhaps at the other side — seems utterly fantastical.
What about a wormhole?
Or is it? Scientists have investigated the possibility that black holes could be wormholes to other galaxies over the years. They may even be, as some have suggested, a path to another universe.
Such an idea has been around for a long time: in 1935, Einstein worked with Nathan Rosen to propose bridges that connected two different places in space-time. But it gained some fresh ground in the 1980s when physicist Kip Thorne — one of the world’s leading experts on the astrophysical implications of Einstein’s general theory of relativity — raised a discussion about whether objects could physically travel through them.
“Reading Kip Thorne’s popular book about wormholes is what first got me excited about physics as a child,” Massey said. Wormholes, on the other hand, do not appear to exist.
Indeed, Thorne, who lent his expert advice to the production team for the Hollywood movie Interstellar, wrote: “We see no objects in our universe that could become wormholes as they age,” in his book “The Science of Interstellar” (W.W. Norton and Company, 2014). Traveling through these theoretical tunnels, according to Thorne, would most likely remain science fiction, since there is no strong evidence that a black hole could allow for such a passage.
But, the problem is that we can’t get up close to see for ourselves. We can’t even picture anything that happens within a black hole – since light can’t escape their massive gravity, then nothing can be captured by the camera. As it stands, theory suggests that anything which goes beyond the event horizon is simply added to the black hole and, what’s more, because time distorts close to this boundary, this will appear to take place incredibly slowly, so answers won’t be quickly forthcoming.
“I think the standard story is that they lead to the end of time,” said Douglas Finkbeiner, professor of astronomy and physics at Harvard University. “An observer far away will not see their astronaut friend fall into the black hole. They’ll merely get redder and fainter as they go closer to the event horizon [because to gravitational red shift]. But the companion falls right in, to a place beyond ‘forever.’”
Maybe a black hole leads to a white hole
Certainly, if black holes do lead to another galaxy or universe, there must be something opposing them on the other side. Is this a white hole, as suggested by Russian cosmologist Igor Novikov in 1964? Novikov proposed that a black hole is linked to a white hole in the past. Unlike a black hole, a white hole allows light and matter to escape but not to enter.
Scientists have continued to explore the potential connection between black and white holes. Carlo Rovelli and Hal M. Haggard claimed in a 2014 paper published in the journal Physical Review D that “there is a classic metric satisfying the Einstein equations outside a finite space-time area where matter falls into a black hole and then emerges from a while hole.” In other words, when black holes die, all of the material they have eaten may be spewed out, and black holes may become white holes.
The collapse of a black hole would be halted rather than destroy the information it absorbs. Instead, it would experience a quantum bounce, allowing information to escape. If this is the case, it would shed some light on a notion made in the 1970s by former Cambridge University cosmologist and theoretical physicist Stephen Hawking, who speculated that black holes release particles and radiation — thermal heat — as a result of quantum fluctuations.
Finkbeiner said, “Hawking said a black hole doesn’t last forever.” As reported in his 1976 paper published in Physical Review D, Hawking calculated that radiation would cause a black hole to lose energy, shrink, and disappear. Given his claims that the radiation emitted would be random and would have no information about what had fallen into it, the black hole’s explosion would erase enormous amounts of information.
This meant Hawking’s theory violated quantum theory, which states that information cannot be destroyed. Physics states information just becomes more difficult to find because, should it become lost, it becomes impossible to know the past or the future. Hawking’s theory gave rise to the ‘black hole information paradox,’ which has long perplexed physicists. Some have said that Hawking was just mistaken, and the man himself admitted to making a mistake during a scientific meeting in Dublin in 2004.
So, should we return to the idea of black holes emitting preserved information and releasing it via a white hole? In their 2013 study published in Physical Review Letters, Jorge Pullin at Louisiana State University and Rodolfo Gambini at the University of the Republic in Montevideo, Uruguay, applied loop quantum gravity to a black hole and found that gravity increased towards the core but reduced and plonked whatever was entering into another region of the universe. The findings supported the theory of black holes functioning as portals. Singularity does not exist in this study, thus it does not build an impenetrable barrier that crushes whatever it encounters. It also means that information does not disappear.
Maybe black holes go nowhere
Physicists Ahmed Almheiri, Donald Marolf, Joseph Polchinski, and James Sully, however, believed Hawking was on to something. They developed on a notion known as the AMPS firewall, sometimes known as the black hole firewall hypothesis. According to their calculations, quantum mechanics could turn the event horizon into a massive wall of fire, and anything that came into contact with it would burn in an instant. In that sense, black holes lead nowhere because nothing could ever get inside.
However, this contradicts Einstein’s general theory of relativity. Someone crossing the event horizon should not experience great hardship since an object would be in free fall and, according to the equivalence principle, that object — or person — would not feel the extreme impact of gravity. It could obey physics laws found elsewhere in the cosmos, but even if it did not violate Einstein’s principle, it would undercut quantum field theory or imply that information can be lost.
A black hole of uncertainty
Step forward Hawking once more. In 2014, he published a paper in which he rejected the idea of an event horizon, claiming that gravitational collapse would instead produce a ‘apparent horizon.’
This horizon would suspend light rays attempting to escape the black hole’s core and would last for a “period of time.” According to his rethinking, seeming horizons temporarily hold matter and energy before dissolving and releasing them later. This explanation fits well with quantum theory, which states that information cannot be destroyed, and, if proven, argues that anything may escape from a black hole.
Hawking went as far as saying black holes may not even exist. “Black holes should be redefined as metastable bound states of the gravitational field,” he wrote. There would be no singularity, and while gravity would cause the visible field to travel inwards, it would never reach the centre and become consolidated within a dense mass.
Even so, whatever is emitted will not be in the form of the information swallowed. It would be impossible to figure out what went in by looking at what is coming out, which causes problems of its own — not least for, say, a human who found themselves in such an alarming position. They would never feel the same way again!
One thing’s for sure, this particular mystery is going to swallow up many more scientific hours for a long time to come. Rovelli and Francesca Vidotto recently proposed that a component of dark matter could be formed by remnants of evaporated black holes, and Hawking’s paper on black holes and ‘soft hair’ was published in 2018 and describes how zero-energy particles are left around the event horizon — an idea that suggests information is not lost but captured.
This contradicted physicist John Archibald Wheeler’s no-hair theorem, which said that two black holes would be indistinguishable to an observer because none of the special particle physics pseudo-charges would be conserved. It’s an intriguing concept, but it’s a long way from being accepted as the solution to where black holes lead. If only we could figure out how to jump into one.