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The Strongest Evidence That Black Holes Are Not Completely Black

Black holes have an escape velocity larger than the speed of light. Since nothing can move faster than that, nothing can escape. This is the simplest mechanical explanation of a black hole. But once you add thermodynamics and quantum mechanisms into the mix, things get messier.

With all this in mind, physicist Stephen Hawking put forward the hypothesis in 1974 that black holes are actually not black; instead, they emit radiation, they lose energy, and over time they shrink. However, the amount of radiation is too small to be observed in astrophysical black holes, so how can we test this idea?

Professor Jeff Steinhauer from the Israel Institute of Technology has not only found a way to test it, but in a new paper, published in Nature Physics, has revealed the strongest evidence yet that this black hole emission, now known as Hawking radiation, is very real. Steinhauer constructed an acoustic black hole – a trap that has a specific frequency much greater than the energy of the sound “particles” (the phonons), which can only move at the speed of sounds.

"If there's a phonon inside the black hole, it can't go against the flow because the flow it's faster than the speed of sound. It's like a person trying to swim against the current. If the current is faster than they can swim, they go backward instead of forward," Prof Steinhauer told IFLScience.

This might seem simplistic, but it’s a fairly accurate model of the real thing. And more importantly, this acoustic black hole was observed emitting the long sought Hawking radiation.   

Hawking’s idea was necessary because relativity and quantum mechanics don’t work well together. Black holes require both theories, so there is a continuous and extensive investigation of their properties by approximating some of the equations we have.

"The point of studying black holes is to learn about the new laws of physics, not just about black holes themselves," added Steinhauer.

The concept of Hawking radiation comes from one of these approximations. Every bit of spacetime has energy and sometimes that energy can suddenly turn into a particle-antiparticle pair before interacting and turning back into energy. If this particle production happened on the event horizon of a black hole, one particle could be captured by the object’s gravity and fall in, while the other escapes. The escaped particle would be linked to its companion lost in the black hole, by a property called entanglement. The Hawking radiation from Steinhauer’s black hole sported this entanglement, providing the strongest experimental evidence that Hawking radiation is very real.

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