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"Black Holes have Hair!" Scientists Say

A new groundbreaking work in theoretical physics claims to change our understanding about black holes, and in particular about the information loss paradox. The work consists of 2 recently published papers which can be found at the end of this article.

In the first paper, appearing in the journal Physical Review Letters, Professor Xavier Calmet from the University of Sussex School of Mathematical and Physical Sciences, Professor Roberto Casadio (INFN, University of Bologna), Professor Stephen Hsu (Michigan State University), along with PhD student Folkert Kuipers (University of Sussex), show that black holes have a gravitational field that, at the quantum level, encodes information about how they were formed. This goes against the "ho hair theorem" of Wheeler.


In 1967 Werner Israel presented at King's College in London his "Uniqueness Theorem". This is also known as "No Hair Theorem" a phrase popularised by John Archibald Wheeler, and regarded offensive by Richard Feynman. According to which in the absence of angular momentum, gravitational collapse with sufficient mass will lead to a Schwarzschild black hole, irrespective of the shape of the collapsing star. A classical black hole can be described be only three parameters: Mass, Angular Momentum, Charge. The no hair theorem, appears to be wrong according to the recent theoretical work.


Calmet and Hsu mention: "Recently it was shown [8] that the quantum state of the graviton field outside the horizon depends on the state of the interior. No-hair theorems in general relativity severely limit the information that can be encoded in the classical gravitational field of a black hole, but the situation is quite different at the quantum level. This result is directly connected to recent demonstrations [9], [10], [11] that the interior information is recoverable at the boundary: they originate, roughly speaking, from the Gauss Law constraint in quantization of gravity. It provides a mechanism (“quantum hair”) through which the information inside the hole is encoded in the quantum state of the exterior gravitational field."


Calmet and Hsu conclude:

"Hawking's information paradox has been the focus of intense interest for almost 50 years.


In his 1992 lecture on the subject, John Preskill [5] wrote:

I conclude that the information loss paradox may well presage a revolution in fundamental physics. The resolution described here is conservative: the quantum state of the exterior gravity field is determined by the interior black hole state, allowing the latter to influence Hawking radiation produced at the horizon. Two distinct quantum states of the black hole may produce the same semiclassical external geometry, but the graviton states differ at the quantum level. The relationship between interior and exterior quantum states is not governed by classical no-hair theorems. Indeed, it has gradually been appreciated that gravity itself prevents the localization of quantum information [4], [9], [10], [11], [21], [22], [23], even behind a horizon. We stress that all formulations of the paradox require a degree of factorization between the black hole internal state and the radiation (see, e.g., (6)), which is clearly not true of our equation (4).


Certain aspects of our expressions (2)-(4) are very clear: the black hole information is spread over many branches of the final radiation state, and macroscopic superpositions of different spacetime geometries play a role in the evaporation. Some of the difficulty in resolving the paradox may originate from a reluctance to accept these aspects of quantum dynamics."


Prof. Calmet also added: "Black holes have long been considered the perfect laboratory to study how to merge Einstein’s theory of general relativity with quantum mechanics. It was generally assumed within the scientific community that resolving this paradox would require a huge paradigm shift in physics, forcing the potential reformulation of either quantum mechanics or general relativity. What we found, and I think is particularly exciting, is that this isn’t necessary. Our solution doesn’t require any speculative idea, instead our research demonstrates that the two theories can be used to make consistent calculations for black holes and explain how information is stored without the need for radical new physics. It turns out that black holes are in fact good children, holding onto the memory of the stars that gave birth to them.”


Roberto Casadio, Professor of Theoretical Physics from the University of Bologna, also adds: “A crucial aspect is that black holes are formed by the collapse of compact objects and then, according to the quantum theory, there is no absolute separation between the interior and the exterior of the black hole. In the classical theory, the horizon acts as a perfect one-way membrane which does not let anything out and the exterior is therefore the same for all black holes of a given mass. This is the classical no-hair theorem. However, in the quantum theory, the state of the matter that collapses and forms the black hole continues to affect the state of the exterior, albeit in a way that is compatible with present experimental bounds. This is what is known as quantum hair.”


Stephen Hsu, Professor of Theoretical Physics and Professor of Computational Mathematics, Science, and Engineering from Michigan State University, concludes: “The concept of a causal horizon is central to the notion of a Black Hole. What is behind the horizon cannot, in classical physics, influence the exterior. We showed that there are intricate entanglements between the quantum state of the matter behind the horizon (inside the hole) and the state of gravitons outside. This entanglement makes it possible to encode quantum information about the black hole interior in Hawking radiation that escapes to infinity.”



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