International School for Advanced Studies (SISSA) | 2016 May 26
A Look Beyond the Event Horizon
[img3="Predicted appearance of a non-rotating black hole.Black holes are still very mysterious celestial bodies which, according to the majority of physicists, do not, however, escape the laws of thermodynamics. As a result, these physical systems possess an entropy though no real agreement has been reached about the microscopic origin of this propriety and how it should be calculated. A SISSA/Max Planck Institute (Potsdam) group has achieved important results in this calculation by applying a new formalism (Group Field Theory) of Loop Quantum Gravity (LQG), a very popular approach in the area of quantum gravity. The result is consistent with the famous Bekenstein/Hawking law, whereby the entropy of a black hole is proportional to a quarter of its surface area, while it avoids many of the assumptions and simplifications of previous LQG theory attempts. Additionally, it lends support to the holography hypothesis, whereby the black hole that appears three-dimensional can be mathematically reduced to a two-dimensional projection.
Credit: Brandon Defrise Carter / Wikimedia Commons"]https://upload.wikimedia.org/wikipedia/ ... khole.jpeg[/img3][hr][/hr]
In principle, nothing that enters a black hole can leave the black hole. This has considerably complicated the study of these mysterious bodies on which generations of physicists have debated ever since 1916, the year their existence was hypothesized as a direct consequence of Einstein’s general theory of relativity. There is, however, some consensus in the scientific community on the fact that black holes possess an entropy, because their existence would otherwise violate the second law of thermodynamics. In particular, Jacob Bekenstein and Stephen Hawking have suggested that the entropy -- which we can basically consider a measure of the inner disorder of a physical system -- of a black hole is proportional to its area and not to its volume, as would be more intuitive. This assumption also gives rise to the “holography” hypothesis of black holes, which (very roughly) suggests that what appears to be three-dimensional might in fact be an image projected onto a distant two-dimensional cosmic horizon just like a hologram which, despite being a two-dimensional image, appears to us as three-dimensional.
As we cannot see beyond the event horizon (the outer boundary of the back hole), the internal microstates that define its entropy are inaccessible: so how is it possible to calculate this measure? The theoretical approach adopted by Hawking and Bekenstein is semiclassical (a sort of hybrid between classical physics and quantum mechanics) and introduces the possibility (or necessity) of adopting a quantum gravity approach in these studies, in order to obtain a more fundamental comprehension of the physics of black holes. ...
Horizon Entropy from Quantum Gravity Condensates - Daniele Oriti, Daniele Pranzetti, Lorenzo Sindoni
- Physical Review Letters 116(21):1301 (27 May 2016) DOI: 10.1103/PhysRevLett.116.211301
arXiv.org > gr-qc > arXiv:1510.06991 > 23 Oct 2015 (v1), 24 Nov 2015 (v2)
