Black holes are massive collections of mass – with gravity so strong that nothing can escape, not even light. Stellar-mass black holes appear when massive stars explode. Supermassive black holes exist in the hearts of galaxies and usually contain the mass equivalent of millions of suns. Is a black hole the birth of a universe or the end of it? What happens when matter disappears at the event horizon? What is gravity? What is Hawking radiation? What can black holes tell us about the nature of quantum entanglement?
In 1974, the Cambridge physicist Stephen Hawking theorized that black holes should create and emit sub-atomic particles, known today as Hawking radiation. Observation of this proposed phenomenon remained a “holy grail” for the fields of atomic physics, nonlinear optics, solid state physics, condensed matter superfluids, astrophysics, cosmology, and particle physics. Until last year, it remained just theoretical. But publishing in Nature Physics, Prof. Jeff Steinhauer presented first proofs that such radiation could exist.
In his lab at the Faculty of Physics, Steinhauer has constructed a sonic black hole – an analogue of the real thing. “We observe a thermal distribution of Hawking radiation, stimulated by quantum vacuum fluctuations, emanating from an analogue black hole,” says Steinhauer. “This confirms Hawking’s prediction regarding black hole thermodynamics.” Pairs of entangled phonons (particles of sound) appear spontaneously in the void at the event horizon of the analogue black hole. One of the phonons travels away from the black hole as Hawking radiation, and the other partner phonon falls into the black hole. The pairs have a broad spectrum of energies. It is the correlation between these pairswhich allowed Steinhauer to detect the Hawking radiation.
“We saw that such high energy pairs were entangled, while the low energy pairs were not. This entanglement verifies an important element in thediscussion of the information paradox as well as the firewall controversy,” explains Steinhauer.
This observation of Hawking radiation, performed in a Bose-Einstein condensate, verifies Hawking’s semi-classical calculation, which is viewed as a milestone in the quest for the graviton – a fundamental particle of matter which should exist, but which hasn’t yet been found.
Steinhauer has been working exclusively on the proof since 2009 in his hand-assembled lab at Technion, replete with lasers and dozens of mirrors, lenses, and magnetic coils to simulate a black hole. Motivated by an overriding curiosity regarding the laws of physics since he was a child, he says that evidence for the existence of quantum Hawking radiation brings us one step closer to uncovering the laws of our universe.