The Strongest Evidence That Black Holes Are Not Completely Black


Black holes have an escape velocity greater than the speed of light. Since nothing can move faster than that, nothing can escape. This is the simplest mechanical justification 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 theory in 1974 that black holes are actually not black; instead, they emit radiation, they lose energy, and with time they shrink. Still, the amount of radiation is too small to be detected in astronomical 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 study, published in Nature Physics, has discovered the strongest evidence yet that this black hole emission, now known as Hawking radiation, is very real. 


Steinhauer created an acoustic black hole – a trap that has a precise frequency much greater than the energy of the sound “particles” (the phonons), which can only move at the speed of sound.


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


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


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


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


The concept of Hawking radiation comes from one of these estimates. Every bit of space time has energy and sometimes that energy can rapidly turn into a particle-antiparticle pair before interacting and turning back into energy. If this particle production occurred on the event horizon of a black hole, one particle could be captured by the object’s gravity and fall in, while the other one escapes.



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