Black hole in the bath

A groundbreaking experiment has allowed researchers to simulate the dissipation of energy around a black hole using waves generated in a bath tub, says Aswin Sekhar

An artist’s interpretation of a supermassive black hole, a billion times the mass of the sun, surrounded by matter flowing onto the black hole in what is termed an accretion disk. Also shown is an outflowing jet of energetic particles, believed to be powered by the black hole’s spin

Black holes are possibly the most mysterious objects in the universe. The physics that explains these massive cosmic bodies is equally esoteric and beyond the comprehension of all but a select few. The most complex physics can, however, be demonstrated by a deceptively simple experiment. Recently, physicists at the University of Nottingham, UK, showed how black holes behave, using a bathtub and coloured water.

The team at the School of Mathematical Sciences, led by Silke Weinfurtner, simulated a natural phenomenon called superradiance - the process of extracting energy from black holes - in a filled bathtub where the black hole corresponded to the drainage hole in the centre. A specially designed sensor recorded the superradiant scattering effect. The study was published in the journal Nature Physics.

"Black hole phenomena are hard, if not impossible, to study directly. So there are very limited experimental possibilities," said Weinfurtner in a statement.

This study is significant because the behaviour of black holes has been studied in a lab for the first time. Astrophysicists can only indirectly observe black holes by using hints from the motion of other celestial bodies around them. Says Quanzhi Ye, a solar system astronomer at California Institute of Technology, US, "The physics around black holes is totally different from what rules daily life. When you have something that can produce a number of exotic things and yet can't be seen, you can't resist the temptation to find out more."

Albert Einstein is considered the reluctant father of black holes because it is his theory of general relativity that provides the basis for them but he himself was loathe to accept their existence. Indian astrophysicist and Nobel winner S. Chandrasekhar found that stars bigger than 1.4 times the sun - called the Chandrasekhar limit - will evolve into a black hole.

"Black holes are amongst the most remarkable members of the modern astrophysical zoo - ranging from supermassive objects to objects of stellar mass (tens of times heavier than the sun), down to much smaller, so-called micro black holes, with masses of the order of just a trillion tonnes, which might have formed during the very earliest stages of the Big Bang," says Mark Bailey, a noted astrophysicist and senior member of the International Astronomical Union.

Kiwi mathematician Roy Kerr gave us an image of the black hole - like water swirling around a drain - but the name came later, in 1967, from physicist John A. Wheeler of the US.

The traditional concepts of black holes evolved due to research in quantum mechanics. "Stephen Hawking showed that all black holes 'evaporate' over time, through a quantum-mechanical process now termed Hawking radiation. For black holes a few times heavier than the sun, the effects of evaporation are negligible. However, if black holes with much smaller masses were to form in the early universe, the Hawking radiation effect would be significant for those," comments Amol Patwardhan, a neutrino astrophysicist at the University of Wisconsin-Madison in the US. This has led experts in quantum mechanics as well as string theory to look at the possibility of very small or micro black holes.

Reputed string theorist and director of IISER Pune, Sunil Mukhi, says, "Micro black holes are a very good example of solitonic particles - concentrated lumps of energy which are expected to behave just like fundamental particles. Since large black holes are abundant in the universe, it is natural that small (even tiny) black holes should exist as well."

The remarkable thing is that solitonic particles can be produced in pairs from "nothing" if sufficient energy is provided. This means that, theoretically, micro black holes could be produced in labs. The only problem is that they would evaporate too fast.

"A black hole with a mass of 400 tonnes would evaporate in about five seconds," says Suvrat Raju, a quantum mechanics expert at ICTS-TIFR, Bangalore. A captive black hole would enable scientists to study it closely. There are, however, no indications that existing particle accelerators produce micro black holes in noticeable numbers.

"Some have been tempted to speculate that they [micro black holes] will instead [of evaporating] grow in size, become big black holes and swallow us all up. This delicious fantasy is not supported by any reliable calculation," chuckles Mukhi.

Since people do not anyway understand the physics underlying black holes, the absence of any support for this paranoid thought - fit to be turned into a disaster movie - is not that reassuring. It is, however, important to note that all particle accelerators - whether it be CERN in Geneva or VECC at Calcutta - are extremely safe. CERN scientists have been talking to the media and general public to convince people that the apprehensions about micro black holes are baseless. The recent study must reassure Luddites as it has shown that we don't need a black hole to study its properties.

The author is an Indian astrophysicist working in Norway

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