Mini-detectors for the gigantic

The gravitational waves created by black holes or neutron stars in the depths of space indeed reach Earth. Their effects, however, are so small that they could only be observed so far using kilometer-long measurement facilities. Physicists therefore are discussing whether ultracold and miniscule Bose-Einstein condensates with their ordered quantum properties could also detect these waves. Prof. Ralf Schutzhold from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the TU Dresden has now carefully looked at the basis of these suggestions and has soberly determined in the journal Physical Review D (DOI: 10.1103/PhysRevD.98.105019) that such evidence is far beyond the reach of current methods.

As early as 1916 Albert Einstein submitted an article to the Prussian Academy of Sciences, in which he demonstrated that moving masses such as giant stars orbiting each other leave behind a dent in space and time, which spreads at the speed of light. These dents are known as gravitational waves and should move precisely like radio waves, light and other electromagnetic waves. The effects of gravitational waves, however, are normally so weak that the world-famous physicist was convinced that they presumably could never be measured.

The reason for this skepticism is that the power of these gravitational waves is rather weak. Even, for example, the quite large mass of the Earth, which covers almost thirty kilometers every second on its way around the much larger sun, produces gravitational waves with a power of merely three hundred watts. That wouldn't even be enough to power a commercial vacuum cleaner with an Energy Star label. The influence of these gravitational waves on the Earth's orbit can therefore hardly be measured.

When Black Holes Merge
The situation looks a bit better when, in contrast, considerably larger masses are involved. When two huge black holes merged at a distance of 1.3 billion light years from Earth, of which one possessed the mass of approximately thirty-six suns and the other a mass of twenty-nine suns, space and time trembled.

During this merging, a mass that measured three times that of our sun transformed into a gigantic gravitational wave, whose remnants reached Earth 1.3 billion years later on September 14th, 2015, at 11:51 AM Central European Time. Because the waves, however, propagate in all directions over such enormous distances and spread to an unimaginably large space, their power was hugely diminished.

On Earth, therefore, only an extremely weak signal was received, which was registered using two four-kilometer-long perpendicular vacuum tubes in the United States. Two special laser beams shoot back and forth between the end points of these facilities. From the time required for one light beam to reach the other end, the researchers can very precisely calculate the distance between the two points.

"As the gravitational waves reached Earth, they shortened one of the two measurement distances by a tiny fraction of a trillionth of a millimeter at both facilities, while the other perpendicular stretch was extended by a similar amount," says HZDR researcher Ralf Schutzhold, outlining his colleagues' results.

Therefore, on February 11th, 2016, following a detailed analysis of the data, the researchers had for the first time directly detected the gravitational waves predicted by Albert Einstein. Three of the contributing researchers were promptly awarded the Nobel Prize in physics in 2017.

Atoms in Synchronization
Astrophysicists can now use these waves to observe massive events in space, in which two black holes merge or huge stars explode. Physicists are simply asking themselves whether this won't also work with facilities that are much easier to deal with than the four-kilometer-long perpendicular vacuum tubes.

One possibility could be what is known as Bose-Einstein condensates, which Satyendranath Bose and Albert Einstein had already predicted back in 1924. "Such condensates can be thought of as heavily diluted vapor from individual atoms that are cooled to the extreme and therefore condense," explains Schutzhold. Researchers in the United States only succeeded in doing so in 1995.

At extremely low temperatures, which are only very slightly above the absolute zero of minus 273.15 degrees Celsius, most atoms of metals such as rubidium are in the same quantum state, while they form a chaotic hodgepodge as vapor at higher temperatures.

"Similar to laser light particles, the atoms of these Bose-Einstein condensates move, so to speak, in synchronization," says Schutzhold. Gravitational waves, however, can change sound-particles or sound-quanta, which physicists call phonons, in these synchronized atom-condensates. "This is a bit similar to a big vat of water in which waves generated by an earthquake change the existing water waves," says Ralf Schutzhold, describing the process.

Little Evidence is too Little
When the head of HZDR's Theoretical Physics Department, however, took a closer look at the fundamentals of this phenomenon, he ascertained that such Bose-Einstein condensates had to be several orders of magnitude larger than is currently possible in order to detect gravitational waves emanating from merging black holes.

"Today, Bose-Einstein condensates with, for example, one million rubidium atoms are obtained with great effort, but it would take far more than a million times that number of atoms to detect gravitational waves," says Schutzhold. There is in fact an alternative where a kind of vortex is formed in the Bose-Einstein condensate, in which gravitational waves directly generate phonons that are more easily observable.

"But even with such inhomogeneous Bose-Einstein condensates, we are still orders of magnitude from detecting gravitational waves," regrets the physicist.

The HZDR researcher nevertheless provides a hint as to possible proof: if the noble gas helium is cooled down to less than two degrees above absolute zero, a superfluid liquid is formed that is in fact not a pure Bose-Einstein condensate, but contains just under ten percent of such synchronized helium atoms.

Because much larger quantities of this superfluid helium can be produced, many orders of magnitude more Bose-Einstein condensate atoms can be created this way than with direct production. "Whether superfluid helium is, however, really a way to detect gravitational waves can only be shown with extremely complex calculations," says Schutzhold. The mini-detectors for gravitational waves still therefore lie some time in the future.

Research Report: "Interaction of a Bose-Einstein condensate with a gravitational wave"

Related Links
Helmholtz-Zentrum Dresden-Rossendorf
The Physics of Time and Space

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