Black holes in the Universe are mysterious phenomena. Cosmologists hardly know anything about them, and even if something is known about them, they are all just hypotheses. These hypotheses are very hard to verify, as black holes cannot be controlled or modified in a lab. Parameters and influence of conditions cannot be tested on black holes. However, many of the behaviours of black holes can be studied using a model: sonic black holes.
Sonic Black Holes: The Principle Behind It
A sonic black hole is similar to a black hole, except it’s for sound. It is a localised point in space that sound cannot escape, hence the adjective sonic. This usually happens in a fluid that is flowing at a speed higher than the speed of sound in that medium. Fluids where black holes can be created are often called gravity analogs because the behaviour of the system resembles gravity.
It has many properties which are similar to the black holes found in the Universe. It was William Unruh who first proposed in 1981 that sonic black holes may be constructed to study black holes. First of all, sonic black holes have an event horizon. The event horizon is the “border” of the black hole i.e. the edge after which light can no longer escape the black hole. Once anything has crossed the event horizon, it can no longer be retrieved. For a sonic black hole, this is where the flow speed of the fluid becomes supersonic, i.e. faster than the speed of sound.
Hawking radiation is thermal radiation that may be emitted by a black hole, hypothesised by Stephen Hawking. This means that the black hole would be losing information, and therefore slowly shrink as it emits the radiation and finally ends up evaporating. Hawking radiation is very difficult to detect and has posed many problems. However, the construction of a sonic black hole may help with that, as sonic black holes may also have this “radiation”, and since it can be created in a lab, it could be detected. You can read about another trial to simulate black hole radiation optically here.
Sonic Black Holes: In Practice
Creating sonic black holes isn’t so easy though. The fluid used needs to be a perfect fluid ideally (so with no viscosity). This is because if the fluid is viscous, detection of many phenomena and characteristics of black holes is hindered by the turbulent and mostly unpredictable flow of the fluid at the event horizon.
The first sonic black hole was successfully created in 2009 by a team from the Technion-Israel Institute of Technology, three decades after Unruh’s proposal. It was created using a cloud of cooled rubidium atoms. Many measurements were made on it and Hawking radiation was detected later on. You can read more about the ground-breaking construction here.
We don’t know much about black holes, but scientists are finding new ways of creating analogs of black holes on Earth. These systems behave in a similar way to black holes, and could, therefore, make it easier to investigate certain aspects and predict the evolution of a black hole over time. It is interesting to observe similar behaviour in different systems where different parts of physics come to play. This goes to show the underlying principles of the Universe and everything around us, making us wonder about how nature likes to follow simple principles and structures.
Editor: Maria ‘Stefi’ Ticsa