Black holes are the most mysterious objects in our Universe. Black holes are regions of spacetime where the gravity is so strong that nothing escapes it, not even light. Black holes form when a very massive star collapses inwards due to its gravity. Black holes have a horizon known as the event horizon. It is the point beyond which nothing can return. Beyond the event-horizon, time breaks down and has no meaning. The event horizon is also called the Schwarzschild radius after Karl Schwarzschild. At the centre of the black hole lies a spacetime singularity where the curvature of spacetime becomes infinite.
A Little about Quantum Mechanics
The black hole information paradox arises from a conflict between the quantum nature of the Universe and general relativity. Quantum mechanics and general relativity are theories of the extreme. While general relativity applies to large masses, quantum mechanics applies to systems at the subatomic level. This creates a problem since a black hole is very small but has a large mass.
The basic idea of quantum mechanics is that the Universe is probabilistic. It means that a phenomenon observed is only one of the many probable outcomes. Let's imagine a scenario. We have a treasure chest(a classical system) in which a coin is present. It is either heads or tails. We open the chest(known as the "measurement" of a system) and find that the coin shows heads. If we consider 1000 chests, prepared in the same way, then we will get 1000 heads. Now let's consider the 1000 coins to be a quantum system where the outcome is probabilistic. If the probability of getting heads or tails is 50% then on measuring we will get 500 heads and 500 tails. Thus the state of the coin is 50% heads and 50% tails.
There is a concept in quantum mechanics that a system exists in all its probable states till we measure it. In other words, a quantum coin is both heads and tails at the same time till we open the chest. Now imagine that each chest contains two coins. There are four probable states of the two coins. It's given that the probability of finding two heads is 50% and two tails is 50%. If one of the coins turns out to be heads then it rules out the possibility of the second coin being tails. Thus, there is entanglement between the two coins.
Entanglement
Quantum entanglement is the relation between two particles of a quantum system. It means that the two particles depend on each other in a certain way. The entanglement between particles is a crucial piece of information about the whole system.
The coins mentioned before are entangled. If we measure one of them without knowing that they are entangled, we exclude crucial information about the entire system.
The Black Hole Information Paradox
It is a known fact that pairs of entangled particles pop in and out of existence all the time. A pair may come into existence such that one of them is beyond the event horizon of a black hole. That particle gets sucked in by the black hole and passes into oblivion. The other particle is emitted into space and carries radiation with it, known as Hawking radiation. Black holes evaporate over time due to the emission of Hawking radiation. We have information about the particle which flew out but not the one which was sucked in. Stephen Hawking said, in 1976, that because of quantum entanglement the disappearance of the second particle is a net loss of information.
Quantum mechanics does not allow information about the Universe to be lost. On the other hand, general relativity says that information about the two-particle system is irrevocably lost.
A possible solution?
A paper established in May 2019 by Ahmed Almheiri, Netta Engelhardt, Donald Marolf, and Henry Maxfield suggests that gravity contributes to entanglement in a specific way which is enough to counterbalance the loss of information due to evaporation. The mechanism supposedly employs the use of wormholes of an unusual type. Wormholes are "shortcuts" between two points in spacetime. This mechanism is like the proverbial "black box". Physicists would love to know what is inside this black box, why do these wormholes appear and the true nature of quantum gravity.
This paradox has been challenging physicists for 45 years. Physicists are still several light-years away from a proper answer. Resolving the black hole information paradox would be a stepping stone towards understanding the most fundamental building blocks of our Universe, the big bang and black holes. It could also answer the big question: WHERE DID IT ALL COME FROM?