Black Holes: Getting Around The Firewall Paradox
March 7, 2013

New Insights Into The Life And Death Of Black Holes

John P. Millis, Ph.D. for — Your Universe Online

Black holes are incredibly difficult objects to understand, partially because their very existence seemingly challenges the physical laws of the Universe. Because of their extreme nature, various peculiarities arise that give scientists pause.

One such case was proposed by Stephen Hawking, the famed cosmologist, back in the 1970s. He noted that because of the extreme gravity that exists within the event horizon of a black hole that any information that passes that boundary is irretrievably lost.

As a result any two black holes of the same size and spin would be indistinguishable, and the history of how each black hole arrived at its current state would be impossible to determine. Even though the two black holes could have arisen by completely different means, there would be no features that would clue the observer in to this fact.

The trouble with this theory was that other physics theories are based on the idea that no information is ever truly lost. It was decades before Hawking himself realized that this was not the case. And not until Leonard Susskind proposed the holographic principle, based on string theory, and satiated the astrophysics community.

But, in 2012, a theory known as the firewall paradox was proposed. On the most basic level, the paradox posits the existence of two quantum-entangled particles. Such particle pairs must maintain opposite quantum states until observed — so, for instance, if one is spin up, the other must be spin down. This entanglement must be preserved until one of the particles is “observed”, which usually means that it interacts with a photon or some other particle.

It had previously been shown that quantum entanglement could survive the fall across the event horizon. However, the firewall paradox observes that the only way that the information of the consumed particle could be preserved is if the particle outside the event horizon becomes entangled with another particle — in violation of quantum mechanics.

So, either the information from the first particle becomes lost as soon as it crosses the event horizon, or the entanglement becomes broken. If the latter, then a large amount of energy will be released in the form of highly energetic particles — the so-called firewall.

Now, a team led by Dr. Sam Braunstein and Dr. Stefano Pirandola, at the University of York, believe that they have found a way around the Firewall Paradox. According to Dr. Braunstein, “We are the first to show the necessity of entanglement across all black hole event horizons and to consider what happens as black holes age. The greater the entanglement, the later the curtain descends. But if the entanglement is maximal, the firewall never occurs. Indeed, entanglement has long been believed to exist for some types of black holes, taking on exactly this maximum value. Our work confirms and generalizes this claim.”

In effect, they argue that under certain conditions — namely that the extent at which entanglement can exist across the event horizon — that the firewall never arises, and the information is preserved. Furthermore, their work also shows that, in general, the entanglement threshold will be met.

The paper "Better late than never: Information retrieval from black holes" is published in Physical Review Letters.