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Stephen Hawking famously predicted that black holes shrink because they emit radiation.
But since he shared his theory 42 years ago, the phenomenon has been difficult to prove, with the radiation too weak to observe using current techniques, leading proof of its existence to be dubbed a ‘holy grail’ in the field of atomic physics.
Now a scientist in Israel has made the first observation of so-called Hawking radiation.
In 1974, Hawking theorised that quantum effects mean black holes cannot be completely black, but must emit radiation, resulting in them losing mass.
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Because the amount of radiation emitted is very small, it has not been observed at an actual astrophysical black hole.
Instead, scientists have created acoustic black holes in which to study the phenomenon in the lab.
Professor Jeff Steinhauer, a physicist at the Israel Institute of Technology in Haifa created an acoustic model or a black hole – from which sound rather than light cannot escape – to observe Hawking-like radiation.
He has been working in his hand-assembled lab, complete with lasers and dozens of mirrors, lenses, and magnetic coils to simulate a black hole, since 2009.
The observation was performed in a Bose-Einstein condensate – a quantum state of matter where a clump of super-cold atoms behaves like a single atom.
In the experiment, Steinhauer studied how particles behave on the edge of his “black hole” – the equivalent of an event horizon, which is essentially the “point of no return” in spacetime, beyond which events cannot affect an outside observer of an analogue black hole.
He found that pairs of phonons – particles of sound – appear spontaneously in the void at the event horizon.
One of the phonons travels away from the black hole as Hawking radiation, and the other partner phonon falls into the black hole.
It is the correlations between these pairs, which have a broad spectrum of energies, which allow for the detection of the Hawking radiation.
Steinhaurer said the Hawking radiation is characterised by a Hawking temperature of 1.2 nano-Kelvin.
The results, published in the journal Nature Physics, provide the strongest claims to date that an analogue of Hawking radiation can be seen in lab-made “black holes”, as Steinhaurer has described the first observation of thermal, quantum Hawking radiation in any system.
“We observe a thermal distribution of Hawking radiation, stimulated by quantum vacuum fluctuations, emanating from an analogue black hole,” he said.
“This confirms Hawking’s prediction regarding black hole thermodynamics."
The experiment shows the Hawking and partner particles within a pair can have a quantum connection called “entanglement.”
Steinhauer explained: "We saw that high energy pairs were entangled, while low energy pairs were not."
Read more: What are black holes? WIRED explains
This entanglement verifies an important element in the discussion of the information paradox – the idea that information that falls into a black hole is destroyed or lost.
It also supports the firewall controversy – the theory that a wall of fire – resulting from the breaking of the entanglement between the Hawking particles and their partners – exists at the event horizon of a black hole.
Steinhaurer said evidence for the existence of quantum Hawking radiation brings us one step further in our endless journey of discovering the laws of the universe.
The experiment may also help Stephen Hawking win a Nobel Prize for this landmark theory.
This is because theorists can only win the coveted award once their ideas have been proven correct.
This article was originally published by WIRED UK
