In March 1974, Stephen Hawking published . It contained the revelation that black holes 鈥 gravitational giants from which nothing, not even light, can escape 鈥 don鈥檛 grow and grow until the end of time, but instead slowly shrink as they release particles in a phenomenon now called Hawking radiation.
The implications were mystifying. Hawking鈥檚 calculations showed that the radiation should be random, offering no way to predict what types of particles will emerge. The problem was that anything that falls into a black hole contains information 鈥 what sorts of particles it is made of, their configurations, their quantum states 鈥 and if what comes back out is random, that information is lost forever as soon as the object is sucked in. But physics operates on the idea that, if we know all the information about a system, we can reconstruct its past and predict its future.
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Can black holes really do the impossible, destroying anything and everything they pull in? That prospect is called the black hole information paradox. It has occupied physicists for decades, not only because it highlights the profound disconnect between general relativity, Albert Einstein鈥檚 theory of gravity, and quantum theory 鈥 but also because it offers the hope of a reconciliation.
Now, 50 years after its inception, the paradox is all but solved. And yet physicists aren鈥檛 celebrating as you might expect because their solution hasn鈥檛 resulted in a long-sought quantum theory of gravity. In many ways, it has only deepened the mystery of what happens inside black holes 鈥 and what that tells us about how the universe works at its most fundamental level.
The black hole information paradox
The black hole information paradox is a conflict between two apparently incontrovertible facts. The first stems from quantum mechanics, which describes the workings of atoms and subatomic particles. It says that all processes are what physicists call unitary, meaning that if you have all the available information about the system now, you should be able to calculate all of its past properties. 鈥淚f you have a glass of water and you drop some ink in it, after a while, all the water looks uniform,鈥 says at the Institute for Advanced Study in Princeton, New Jersey. 鈥淏ut if you kept track of all the molecules in the system, you could, in principle, figure out where you dropped the ink into the water, so that information is retained.鈥
鈥淭hat鈥檚 one of the big tenets of quantum physics, that you should be able to rewind the movie,鈥 says . 鈥淚f one object violates that principle, then the theory breaks down and you have to throw away the theory.鈥
Binning quantum mechanics isn鈥檛 an option, however, because it has stood up to every test physicists have thrown at it. Nor can we throw out thermodynamics, which deals with work and energy and follows a similar rule about unitarity. If black holes destroyed information, as Hawking calculated, two of the most thoroughly tested areas of physics would have to be incorrect. But for decades, physicists couldn鈥檛 prove his calculations wrong.
Something had to give, which brings us to the second apparently incontrovertible fact: that semiclassical physics, a well-established combination of general relativity and quantum mechanics in which the two theories are treated roughly separately, describes what goes on at the edge of a black hole.
Hawking did his original calculations based on semiclassical assumptions, and it took a while to find fault. Slowly but surely, though, his workings began to crack. In 2004, Hawking himself realised that the amount of information left behind in space-time when a black hole completely evaporates should be equal to the amount that was present when it formed, regardless of how much had fallen into the black hole in the meantime. 鈥淚f you jump into a black hole, your mass energy will be returned to our universe, but in a mangled form, which contains information about what you were like, but in an unrecognisable state,鈥 Hawking said in a 2004 press conference. The information was coming out, he conceded, suggesting that semiclassical physics alone isn鈥檛 enough to make sense of black holes. But his research said nothing about how.
A further path out of the paradox appeared on the horizon as researchers began to make inroads into what a theory of quantum gravity might look like, and physicists began to consider how quantum effects might modify Hawking radiation. 鈥淭o solve the paradox, you have to accept that everything is quantum, not just tiny particles 鈥 [but also] planets, black holes, the universe,鈥 says Calmet. 鈥淚f you think of the black hole as classical, you are lost.鈥
One way in which a quantum black hole is different from a classical one is in how it affects the surrounding space-time. Classical black hole space-time is perfectly spherical because black holes themselves should be perfectly spherical, thanks to their extremely powerful gravity. But quantum fluctuations mean that a quantum black hole should warp space-time in a very slightly uneven way 鈥 and that asymmetry can store information, says Calmet.
The information that comes out would be thinly spread across a huge volume of space-time
In 2023, he and his colleagues calculated that this slight asymmetry in space-time should affect the Hawking radiation, so that it isn鈥檛 completely random. 鈥淓ven when the black hole evaporates completely, the information remains in the radiation and in space-time,鈥 says Calmet. 鈥淭hey鈥檙e very tiny effects, but they鈥檙e enough.鈥 The information that comes out would be thinly spread across a huge volume of space-time, which is why it was so hard to mathematically find.
and at the University of California, Berkeley, made a similar finding in late 2023, although with a slightly different mechanism. Hawking radiation is the result of pairs of particles being produced at the very edge of a black hole, one of which falls in while the other radiates outwards. These pairs are connected through quantum entanglement, a phenomenon in which the measured properties of each particle depend on those of the other.
If we could measure the particles that escape, that might give us some information about the properties of their doomed partners. This could create 鈥渆ntanglement islands鈥 of information about the inside of a black hole, stretching out from its edge. Researchers thought that these islands 鈥 perhaps more accurately described as peninsulas 鈥 would protrude from a black hole by a length smaller than anything we could measure, making them effectively non-existent. But Bousso and Penington found that they could stick out by as much as an atom鈥檚 width, again warping space-time and allowing information out.
That isn鈥檛 very far, of course, but in principle it is measurable. And what matters for the black hole paradox is that the information is theoretically accessible, not that accessing it would be remotely practical.
In any case, at this point, most physicists agree that information is preserved, not destroyed. Hence the claims that the black hole paradox has finally been resolved. 鈥淭his has been a big change in the last four years or so,鈥 says Calmet, though not everyone believes that it amounts to a conclusive resolution.
鈥淭here鈥檚 been progress for sure, but I definitely wouldn鈥檛 say that all the i鈥檚 are dotted and all the t鈥檚 are crossed,鈥 says at King鈥檚 College London. 鈥淭he evidence is now very overwhelming that the information is preserved, but I鈥檓 not convinced that we know how to do the calculations to recover the information.鈥
The biggest issue is that the resolution hasn鈥檛 led to a new formulation of quantum gravity 鈥 something that goes beyond the semiclassical physics Hawking used in his original calculations. 鈥淲e couldn鈥檛 understand it with known physics, so it seemed like it could point the way to what fundamental physics really underlies everything,鈥 says at the University of California, Santa Barbara. 鈥淏ut I think it鈥檚 been the opposite.鈥 Allowing information to escape a black hole doesn鈥檛 require string theory or any other particular formulation of quantum gravity. That leaves open the question of which, if any, of the many formulations is correct.
What happens inside a black hole?
Part of the problem is that even if we figure out exactly how quantum physics works at the edge of a black hole 鈥 the general shape of which we seem to understand, although the details are still a bit fuzzy 鈥 what happens further in remains a mystery.
鈥淭he calculation part appears to be done, but there does remain the interesting question of what does it mean for physics inside the black hole?鈥 says Marolf. 鈥淚f you fall into an evaporating black hole, what does that feel like? Could you live in there if you made it in? Can you just go about your normal life or is the physics so weird that you would be immediately destroyed? Those are the things that we argue about.鈥
For a while, physicists skirted the issue by positing what they called a firewall 鈥 the idea that physics on the inside of a black hole was so unlike that in the rest of the universe that it was impossible to access. Instead, there would be a moat of extraordinarily high-energy particles right at the edge that would incinerate anything that got too close. Some even argued that space-time would cease to exist beyond the firewall, so black holes would have no interior at all.
That idea has fallen from grace in recent years, replaced by insights gleaned from arcane calculations about the relationship between a black hole鈥檚 interior and exterior. Many of these hypotheticals deal with the entropy, or disorder, present inside a black hole. This is related to how many quantum states exist there, similar to how the information content of a book is related to how many letters fill its pages.
Directly observing the interior of a black hole is out of the question
In classical physics, a black hole can contain an infinite number of quantum states, but recent calculations indicate that there are fewer than expected, because many configurations of states that appear distinct are actually equivalent. A black hole that might seem from the outside to have a volume of 20,000 cubic kilometres, say, could actually be only one-tenth that big when observed from the inside. 鈥淚t seems like, indeed, physics is very strange inside a black hole, that things you would have thought are apples and oranges are actually the same, but we don鈥檛 quite [fully] know what that means physically yet,鈥 says Marolf. It does seem to imply, however, that an astronaut outside a black hole would measure very different properties from what an astronaut inside the void would see.
It isn鈥檛 clear how to figure out the details of those differences theoretically, and directly observing the interior of a black hole is out of the question, so physicists are at an impasse for now. 鈥淚t is sort of sad in that sense, but until we build a black hole big enough to jump in, I think it鈥檚 hard to tell,鈥 says Marolf.
In the meantime, the best we can do is to work with experimental systems that have shown promise at mimicking some black hole properties. Most of these are made of perfect fluids that flow with no friction or viscosity. These strange substances, which include Bose-Einstein condensates and superfluid helium, move similarly to fields in space-time, particularly gravity. As a result, they can be used to create peculiar flows that behave in some ways like black holes. How these flows interact with sound is similar to how black holes interact with light, allowing researchers to use them as artificial black holes.
Such models have even displayed behaviours similar to Hawking radiation, but many physicists are sceptical that they can capture the full complexity of black hole behaviour. 鈥淧eople talk about analogue black holes a lot, but they just aren鈥檛 the same. They don鈥檛 evaporate in the same way,鈥 says Marolf.
Inside a black hole, the roles of space and time are radically recast, which is difficult or even impossible to reproduce in a laboratory. 鈥淭hese analogues, there鈥檚 an analogy, but it鈥檚 not really getting at the big issue, which is that space and time are changing roles inside that black hole,鈥 says Lambert. 鈥淲hen you鈥檙e outside a black hole, it鈥檚 a point in space and it stays that way. But when you fall into a black hole, it ceases to be a point in space for all time, it becomes a point in time for all space. It鈥檚 an object everywhere in your future.鈥 This inescapability, this altering of space and time themselves, is what is challenging to duplicate or even explain.
Another potential route to analogue black holes comes through quantum computing. So far, it has proven impossible to simulate a black hole in detail on any existing computer 鈥 the interactions between gravity and quantum mechanics at the edge of these objects are simply too complicated. But quantum computers are better suited than conventional machines to deal with that sort of complexity. They run on quantum effects, so it is far easier for them to simulate the quantum world.
The question with quantum computing, then, isn鈥檛 whether it can simulate the quantum effects in a black hole, but whether it can square those effects with gravity. 鈥淚t might be that with sufficiently big quantum computers we could realise these analogue systems that display some version of gravity,鈥 says Maldacena. Tech companies are aiming to build quantum computers in the size range that he says would be necessary within the next decade or so, so that may be a path forward.
Not all black hole researchers are quite as hopeful, though. 鈥淥n the one hand, I don鈥檛 personally have, nor do I know of, anything that seems like a useful idea for sorting [this mystery] out. On the other hand, it took us almost 50 years to figure out that information comes out at all,鈥 says Marolf. 鈥淢aybe future scientists will have new, better ideas.鈥
If they do, it could pave the way towards a better grasp of not just black holes themselves, but of all the physics underlying them too. That physics is crucial for understanding the origins of space-time itself.
鈥淏lack holes are wild 鈥 it鈥檚 crazy what they do 鈥 and that might let us get a grip on the role of information in both gravity and quantum mechanics,鈥 says Lambert. 鈥淚t promises to be quite revolutionary. It鈥檚 an area where, if we ever understood it, we could be in for quite some revelations.鈥
