Scientists at Low Temperature Laboratory planning to model a black hole

Academy Professor Matti Krusius and Antti Finne, M.Sc. (Eng.), were invited to a recent science breakfast, hosted by the Academy of Finland, to talk about their ongoing work to produce a first-ever laboratory simulation of a black hole. A black hole is created as a result of the most extreme concentration of matter.

Scientists have been arguing about the possible existence of black holes for an entire century. Today the existence of black holes is supported by various astrophysical phenomena. Experimental observations of their existence are necessarily indirect because not even light can escape from a black hole. However, indirect observations cannot explain the structure of a black hole and its surrounding event horizon.

Over the past couple of decades scientists have designed several experiments for the purpose of modelling the physics of the event horizon of a black hole, as based upon the theory of relativity, but all proposals have so far proven impracticable. However, work is now under way at the Helsinki University of Technology Low Temperature Laboratory, Finland, one of the Academy’s Centres of Excellence in Research, to set up the experiment and conduct the necessary measurements.

Simulation can help to unravel the mysteries of the universe

According to Academy Professor Matti Krusius, the advances that have been made over the past years in cosmological observations have revolutionised scientists’ understanding of the Universe. However, there remain many intriguing questions about the birth, structure, and future fate of the Universe that have so far not been answered by means of direct measurement. One of the ways to tackle these enigmas is by means of analogue models, i.e. by simulating the event in some other seemingly remote area of physics. This is exactly what scientists at the Low Temperature Laboratory are doing while attempting to simulate a black hole.

One of the world’s first successful cosmological analogue measurements was completed at the Low Temperature Laboratory in 1996. This experiment involved measuring the defects nucleated in the macroscopic structure of matter by a very rapid change of state, specifically the change of liquid helium from the normal to the superfluid phase. Scientists believe that the Early Universe, during its expansion and cooling in the aftermath of the Big Bang, went through a whole series of changes in state. It is presumed that the defects created in these processes are responsible for the inhomogeneous density of visible matter that is seen in the current Universe, clear proof of which is the uneven distribution of galaxies into long chains interspersed by vast empty spaces.

The 1996 analogue measurement did not provide a direct answer to the origin of the cosmic inhomogeneity, but the experiment did prove that the proposed mechanism via defect formation in rapid changes of state is physically sound. During the past two years measurements of the cosmic microwave background radiation have suggested that the so-called inflation model can provide a better explanation for the observed inhomogeneity in the distribution of matter.

Answers lie in superfluid interface oscillations

Professor William Unruh originally drew attention to the similarities between fluid dynamics and the theory of relativity as early as 1981. In his model the speed of fluid flow is increased beyond the speed of sound, thus creating an event horizon for the sound carried forward in the fluid. In practice, however, this is not physically possible for ordinary fluids.

A superfluid, on the other hand, can within certain limits flow without loss. The plans that scientists at the Low Temperature Laboratory have for modelling a black hole are based upon interface oscillations between two superfluids. The purpose is to create a situation where two fluids are moving at different speeds relative to each other. At some critical speed the interface will no longer remain stable, but begins to oscillate and forms surface waves. When the thickness of the superfluid layers is reduced to a sufficient extent, the equations describing the interface oscillations become similar to those giving rise to the event horizon of a black hole. Thus by varying the thickness of the fluid layers in the course of their measurements, the scientists will be able to observe whether it is indeed possible to create an analogue model of the black-hole event horizon.

Scientists at the Low Temperature Laboratory are now in the position that they can control the superfluid interface and produce interface oscillations. The next challenge is to study other intervening phenomena that need to be understood before it is possible to interpret the black-hole-like measurements and compare with the relativistic models. Although the experimental setup as well as the necessary measurement technology are already in place, it will still be another 2-3 years before the scientists will know the final answers from these simulation studies.