Black hole physics provides a test for any potential quantum gravity theory. Whatever your theory is, it must explain what happens to the information recording a black hole’s history.
It took two decades for scientists to come up with a solution. They suggested that the information stored in a black hole is proportional to its surface area (in two dimensions) rather than its volume (in three dimensions). This could be explained by quantum gravity, where the three dimensions of space could be reconstructed from a two-dimensional world without gravity – much like a hologram. Shortly afterwards, string theory, the most studied theory of quantum gravity, was shown to be holographic in this way.
The laws of quantum mechanics insist that information about the past is never lost, including the record of whatever fell into a black hole. But Hawking’s calculation contradicted this. He applied both quantum mechanics and Albert Einstein’s theory of gravity to the space around a black hole and found that quantum jitters cause the black hole to emit radiation that’s perfectly random, carrying no information. As this happens the black hole shrinks and eventually disappears.
But does its information disappear with it, meaning quantum mechanics is wrong? Or does the problem lie with Einstein’s theory? When forced to choose, many physicists back the quantum rule and suspect that information somehow escapes in the black hole’s radiation, which in that case isn’t random after all. Figuring out how the information gets out should point the way past Einstein’s theory to a more complete quantum theory of gravity. Yet after 45 years of grappling with this “black hole information paradox,” no one has pinpointed the alleged misstep in Hawking’s calculation.
Now, though, several noted black hole physicists think they may be closing in on a solution. Not even the researchers themselves fully grasp the physical implications of the math they explore in a recent paper. But in the abstract mathematical threads, they and others see the outlines of a bridge to the black hole’s interior, an escape route for trapped data. They located this hidden path using an imperfectly understood technique for spying on a black hole from a higher dimension.
“It’s magic,” said Ahmed Almheiri, a physicist at the Institute for Advanced Study (IAS) in Princeton, New Jersey, who co-authored the recent paper. “We know how to prove” that the shift in perspective works, he said, “but we don’t have a complete understanding of why this is happening.”
Raphael Bousso, a physicist at the University of California, Berkeley and an expert on the information paradox, said of the new work, “I find it interesting enough that I’m really working hard” to understand it.
Right now, the Earth is pulling a little bit harder on whichever part of your body is closer to the ground. You can’t feel or see the effects of that pull, called tidal force, because, relative to the size of the universe, the Earth isn’t all that big. So, the force of gravity in return isn’t all that strong.
But since black holes are much more massive, the tidal force there is much stronger than on Earth. “If you are an astronaut falling towards a black hole feet first”, explains Ojha, “ the pull on your feet is so much stronger than the pull in your head, so in the direction that you are falling, you get stretched [by the tidal forces].” The very real astrophysics term for this process is pretty straightforward: spaghettification.
Past the event horizon exists what’s known as the singularity: an infinitely dense point where space and time warp and cease to exist as we know them. They are very small regions in space-time where an almost infinite amount of mass concentrates. Under those conditions, the equations Einstein created for gravity break down, says Avi Loeb, director of the Black Hole Initiative at Harvard University.
Objects like the singularity are explained by quantum mechanics. The problem is that the way particles behave in quantum mechanics is extremely different from the way objects behave in Einstein’s theory of general relativity. So, the inside of a black hole is a contradiction: a place where gravity is extremely strong, but at the same can be understood only through quantum mechanics. Scientists have been looking for the “theory of everything” for a century. In Ojha’s words, “black holes are a representation of the biggest questions in Physics right now.
Most of the current hypotheses—countless, according to Daniel Jafferis, a quantum gravity professor at Harvard—are mathematically possible. But that doesn’t mean that all of those solutions are actually happening. So remember: Anything you read beyond this point is possible but not necessarily true.
The general relativity description of black holes suggests that once you go past the event horizon, the surface of a black hole, you can go deeper and deeper. As you do, space and time become warped until they reach a point called the “singularity” at which point the laws of physics cease to exist. (Although in reality, you would be die pretty early on on this journey as you are pulled apart by intense tidal forces).
In Mathur’s universe, however, there is nothing beyond the fuzzy event horizon. Currently, a rival theory in quantum gravity is that anybody falling into a black hole hits a “firewall” and is immediately destroyed. The firewall proposal has been criticized since (like fuzzballs) firewalls have drastically different behavior at the horizon than general relativity black holes.
But Mathur argues that to an outside observer, somebody falling into a fuzzball looks almost the same as somebody falling into an Einstein black hole, even though those falling in have very different experiences. Others working on firewalls and fuzzballs may well feel that these arguments rely on properties of the example he used. Mathur used an explicit description of a very special kind of fuzzball to make his arguments. Such special fuzzballs are probably very different to the fuzzballs needed to describe realistic astrophysical black holes.
The debate about what actually happens when one falls into a black hole will probably continue for some time to come. The key question to understand is not whether the horizon region is reconstructed as a hologram — but exactly how this happens.