## Recent study lends support to the holography hypothesis.

Black holes are perplexing celestial beasts. In practice, nothing that enters a black hole can leave so studying them is extremely difficult. Given this difficulty, there is debate about what black holes are made of and how they form.

Although there is some consensus in the scientific community that black holes must have entropy or their existence would violate the second law of thermodynamics, no agreement has been reached about the origin of this entropy, or how to calculate its value. Now, a SISSA/Max Planck Institute (Potsdam) group has achieved results in the calculation of entropy that lends support to the holography hypothesis.

Theoretical physicists Jacob Bekenstein and Stephen Hawking suggested that the entropy — a measure of disorder or randomness — of a black hole is proportional to its area and not its volume, an idea that stemmed the formation of the “holography” hypothesis of black holes.

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This hypothesis suggests that a three-dimensional image can be projected as two-dimensional — just like how a hologram is two-dimensional, but appears three-dimensional to us.

Since we can’t see beyond a black hole’s event horizon — the outer boundary of the black hole that once passed nothing can escape, not even light — the research team used a theoretical approach to quantum gravity called Loop Quantum Gravity (LQG).

To understand quantum gravity, we need to discuss Planck’s length. Planck’s length is the smallest measurement of length with any meaning and is also where space-time stops being continuous (as we see it), and becomes grainy. The universe at this dimension is described by quantum mechanics. Quantum gravity looks at gravity in the terms of quantum mechanics and ventures to understand how gravity behaves at the Planck scale.

In a new study published in *Physical Review Letters*, Daniele Pranzetti of the Max Planck Institute for Quantum Optics, along with colleagues, applied a new formulation of LQG to black holes. “We obtained a description of black hole quantum states, suitable to describe also ‘continuum’ physics, that is, the physics of space-time as we know it,” explained Pranzetti in a SISSA press release.

Considering a black hole as a continuous spherical object, as Pranzetti suggests, the black hole can be described as a condensate, which is an assemblage of “atoms” with the same properties. Even if there are a lot of atoms, their collective behavior could be determined by studying the properties of just one.

The results also provide support for the holographic hypothesis. All the black hole’s information could be found on a two-dimensional surface, making studying black holes a little bit simpler.

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