Beyond the Point of No Return

By Amanda Jaffe-Katz

Forty years ago, British theoretician Stephen Hawking, a physicist at Cambridge University, hypothesized that rather than being completely dark as their name implies, black holes should emit a faint glow of particles – known today as Hawking radiation.

Physicists wanting to test this theory were, however, literally left in the dark: Black holes in space are notoriously inaccessible. William Unruh, of the University of British Columbia, suggested that it might be possible to create a terrestrial analogy to the cosmic puzzle, and to investigate the laws of quantum physics via sound – in what he dubbed “dumb holes” – analogous to light in a gravitational black hole. Unruh argued that a sonic event horizon could be created by making part of a body of fluid flow faster than the speed at which sound travels in that fluid.

In 2009, experimental physicist Prof. Jeff Steinhauer at Technion’s Faculty of Physics, indeed reported the world’s first successful observation of a sonic black hole, by means of a very low temperature atomic Bose-Einstein condensate. Now, Steinhauer has detected Hawking radiation for the first time in the lab, with this dumb hole.

Steinhauer explains that his lab work involves an organized set of steps to create the conditions for and subsequently observe Hawking radiation. Each stage, when accomplished, has been published separately over the last five years.

First, he created an analogue black hole; next, he was able to measure a small quantity of phonons (units of sound energy); then, he applied the methodology to the black hole and looked for the distribution of phonons that occur naturally. Now, with his latest publication in Nature Physics in October 2014, Steinhauer says that, “In general, an experimental measurement of something heretofore predicted is very important for pushing physics forward. I showed that the Hawking mechanism really works.”

Such findings could one day help resolve the ‘black hole information paradox’ – a major enigma in astrophysics that questions whether information that falls into a black hole really disappears forever.

Significantly, the lab-created mechanism is an analogue of a charged black hole, which manifests two event horizons – an inner and an outer one. The horizons create paired particles of sound, or phonons. One phonon – having positive energy – escapes the horizon, while the negative energy partner remains trapped inside the black hole. While a single phonon is too weak to observe, the phonons inside the black hole bounce back and forth between the inner and outer horizons, giving rise to self-amplifying Hawking radiation. “Because the amplitude has grown exponentially, this allows me to observe Hawking radiation, which is the output of the black-hole laser formed between the horizons,” Steinhauer says.

Steinhauer takes pictures of what happens at the dual event horizons and calculates the atoms’ flow speed from these images.

“Phonons move faster at very short wavelengths,” Steinhauer explains his findings, “but it is not yet clear whether light (photons) can also do this.”

In the recently published article, Steinhauer concludes: “This work suggests a method for probing the inside of a black hole… The experimental techniques presented here could be used for further analogue gravity experiments, such as simulating the expansion of the early universe.”

Steinhauer is already discussing the next steps with international theoreticians with whom he collaborates. One would be to enhance the detectors to sense radiation from a single horizon, which could help determine whether the pairs of phonons are entangled – another predicted quantum feature of real black holes.