February 07, 2016

Stephen Hawking’s latest black-hole paper could be a fresh way to solve a black-hole conundrum

Almost a month after Stephen Hawking and his colleagues posted a paper about black holes online, physicists still cannot agree on what it means.

Some support the preprint’s claim — that it provides a promising way to tackle a conundrum known as the black hole information paradox, which Hawking identified more than 40 years ago. “I think there is a general sense of excitement that we have a new way of looking at things that may get us out of the logjam,” says Andrew Strominger, a physicist at Harvard University in Cambridge, Massachusetts, and a co-author of the latest paper.

Others are not so sure that the approach can solve the paradox, although some say that the work illuminates various problems in physics. In the mid-1970s, Hawking discovered that black holes are not truly black, and in fact emit some radiation. According to quantum physics, pairs of particles must appear out of quantum fluctuations just outside the event horizon — the black hole’s point of no return. Some of these particles escape the pull of the black hole but take a portion of its mass with them, causing the black hole to slowly shrink and eventually disappear.

In a paper published in 1976, Hawking pointed out that the outflowing particles — now known as Hawking radiation — would have completely random properties. As a result, once the black hole was gone, the information carried by anything that had previously fallen into the hole would be lost to the Universe. But this result clashes with laws of physics that say that information, like energy, is conserved, creating the paradox. “That paper was responsible for more sleepless nights among theoretical physicists than any paper in history,” Strominger said during his talk.

The mistake, Strominger explained, was to ignore the potential for the empty space to carry information. In their paper, he and Hawking, along with their third co-author Malcolm Perry, also at the University of Cambridge, turn to soft particles. These are low-energy versions of photons, hypothetical particles known as gravitons and other particles. Until recently, these were mainly used to make calculations in particle physics. But the authors note that the vacuum in which a black hole sits need not be devoid of particles — only energy — and therefore that soft particles are present there in a zero-energy state.


Arxiv - Soft Hair on Black Holes



Abstract

It has recently been shown that BMS supertranslation symmetries imply an infinite number of conservation laws for all gravitational theories in asymptotically Minkowskian spacetimes. These laws require black holes to carry a large amount of soft (i.e. zero-energy) supertranslation hair. The presence of a Maxwell field similarly implies soft electric hair. This paper gives an explicit description of soft hair in terms of soft gravitons or photons on the black hole horizon, and shows that complete information about their quantum state is stored on a holographic plate at the future boundary of the horizon. Charge conservation is used to give an infinite number of exact relations between the evaporation products of black holes which have different soft hair but are otherwise identical. It is further argued that soft hair which is spatially localized to much less than a Planck length cannot be excited in a physically realizable process, giving an effective number of soft degrees of freedom proportional to the horizon area in Planck units.

Conclusion

They have reconsidered the black hole information paradox in light of recent insights into the infrared structure of quantum gravity. An explicit description has been given of a few of the pixels in the holographic plate at the future boundary of the horizon. Some information is accessibly stored on these pixels in the form of soft photons and gravitons. A complete description of the holographic plate and resolution of the information paradox remains an open challenge, which we have presented new and concrete tools to address.

SOURCES - Arxiv Soft Hair on Black Holes, Nature

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