How can a black hole emit sound?

Still hole emits Hawking radiation

The attraction of a black hole is so great that not even light can escape its gravitational influence. But in 1974 Stephen Hawking found out that black holes could still emit radiation. So far, however, this so-called Hawking radiation has not been proven experimentally because its signal is extremely weak. Jeff Steinhauer from the Israeli Institute of Technology in Haifa has now made progress in this regard: With the help of a Bose-Einstein condensate, he created an analogue of a black hole in the laboratory, which instead of matter or light allows no sound to escape. Beyond this silent hole, Steinhauer was actually able to detect a kind of Hawking radiation from sound, as he reports in the journal "Nature Physics".

Hawking radiation is supposed to emanate from the event horizon of a black hole if a virtual - i.e. not real - particle-antiparticle pair is created there. According to quantum electrodynamics, such pairs of particles constantly emerge from the vacuum, but matter and antimatter usually immediately annihilate each other again. In the vicinity of the event horizon, however, it is possible that the antiparticle crashes into the black hole, while the matter particle escapes and the black hole actually emits radiation. If astronomers could detect this radiation, it would be a direct observation of a black hole. But even if it existed, the Hawking radiation would be far too weak to be picked up by telescopes at the moment. In order to be able to research black holes experimentally at all, physicists make do with analogies: They exploit the fact that certain systems obey laws similar to those of gravity.

For his experiment, Steinhauer used ultra-cold rubidium atoms, which form a Bose-Einstein condensate. In this state of matter, the atoms behave like a liquid without any viscosity, i.e. internal friction. If these atoms are now moving at supersonic speed, sound waves that are inside the condensate can no longer escape it and are thus trapped like in a black hole. In order to realize such a model system, Steinhauer irradiated the Bose-Einstein condensate with laser light and accelerated the rubidium atoms in a limited area to supersonic speed. As a result, an analogue to the event horizon developed at the inner and outer borders of this shaft-shaped region. If sound waves were now trapped in this silent hole, they moved back and forth between the two borders and were thus amplified. This amplification mechanism finally made it possible to detect the mimicked Hawking radiation in the form of phonons - quasiparticles of sound - outside the silent hole.

In addition to the detection of Hawking radiation in the form of sound, the experiment suggests a possibility of detecting the radiation of real astrophysical black holes. Because under certain circumstances - for example when they are electrically charged instead of neutral - there could also be an inner event horizon inside black holes. In this case, electromagnetic radiation could also be amplified analogously to the sound waves between the two horizons and thus amplify the Hawking radiation signal sufficiently to enable a discovery on earth.