In the last few months a lot of interesting papers on the black hole theory have been published. For those, who haven’t been following the news, here’s a short summary of what’s been going on.
Almost fifty years ago, through the work of such scientists as Stephen Hawking and Jacob Bekenstein, a new theoretical understanding of black holes was formed. In particular, Hawking’s work showed that black holes could be treated thermodynamically, which led to the discovery of Hawking radiation. Interestingly, Hawking in his work showed that the results based on general relativity and quantum field theory violated information conservation. Long story short, Hawking claimed that Hawking radiation does not preserve information, which is one of the most important principles in physics.
In a famous though experiment Bob barely escapes a black hole while Alice falls in. But what will then happen to Allice?
What followed is now often referred as Susskind-Hawking black hole war, as described in Susskind’s book of the same title. Susskind and his colleagues argued that information is in fact preserved in some way on the boundary of the system, in this case, the event horizon. This is known as the holographic principle and currently many physicists believe that it solved the information paradox of black holes.
So, it seems that black holes are still strange, but not strange enough to destroy information. However, it didn’t take long for scientists to come up with a new problem with black holes just to cause more headaches to theoretical physicists. The “firewall” phenomenon was proposed in 2012 by Ahmed Almheiri, Donald Marolf, Joseph Polchinski, and James Sully as a possible solution to an apparent inconsistency in black hole complementarity. In particular, Hawking radiation involves two mutually entangled particles and the outgoing particle must be entangled with all the Hawking radiation the black hole has previously emitted. This causes a paradox, which as it appears, breaks the monogamy of entanglement — a principle that forbids entanglement of more than two independent systems at the same time. This paradox is very important, as it could potentially lead scientists to give up one of the three well-tested theories: Einstein’s equivalence principle, unitarity, or existing quantum field theory.
The idea of “firewalls” offers a possible way to resolve this. In order to break the entanglement between the in-falling and outgoing particles huge amount of energy is required, which, according to Almheir, Marolf, Polchinski and Sully, would create a “firewall” at the event horizon. This would break the equivalence principle.
Other possible solutions of the paradox, which do not employ “firewalls”, include a suggested modification of quantum field theory by Steve Giddings such that entanglement would be gradually lost as the outgoing and in-falling particles separate, resulting in a more gradual release of energy inside the black hole. Juan Maldacena and Leonard Susskind , on the other hand, suggested that the two entangled particles are connected by wormholes. Finally, another recent idea, which got much attention from the press, was Hawking’s “apparent horizon” black hole. This, however, was received with great criticism, since most scientists were confused about how this would solve the paradox.
So, since most of these ideas are very recent and can easily be accessed online , I decided to share some of these important papers. The first one is the infamous preprint by Hawking titled “Information Preservation and Weather Forecasting for Black Holes“, in which Hawking argues that black holes do not exist, at least in a way we are familiar with — an event horizon must be replaced by an apparent horizon.
The other two papers describe the “firewal” paradox in more detail. In “Black Holes: Complementarity or Firewalls?” Almheiri, Marolf, Polchinski and Sully present their idea of firewalls. Whereas in “Cool Horizons for Entangled Black Holes” Susskind and Maldacena offer their solution to the firewall paradoxes.
For more papers on black holes check out this link. Also, for more articles and videos on the whole “firewall” madness use these links:
- Stephen Hawkings New Black Hole Theory
- Fiery Black Hole Debate Creates Cosmological Wild West
- Leonard Susskind on Black Holes