Physicists Struggling to Observe Gravitational Waves Using Bose-Einstein Condensates

In 1916, Einstein theorized that moving masses like giant stars leave dents in space and time, which then spreads at the speed of light. Those "dents" became known as gravitational waves -- moving similarly to other waves like radio, light, electromagnetic. 

The problem with original studies of gravitational waves came from their weaknesses.

Most gravitational waves that reach Earth don't have enough power to power a standard commercial vacuum cleaner. This has made them an extensive challenge for physicists to hunt down. 

One instance in 2015, however, fascinated physicists and astronomers alike. Two huge black holes merged some 1.3 billion light years from Earth. One of those black holes measured the mass of 36 suns and the other measured 29 suns.

When the effects of that merger reached Earth in September 2015, the weak signal was enough to register movement in two four-kilometer-long vacuum tubes in the United States. 

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That instance fascinated researchers at the Helmholtz-Zentrum Dresden-Rossendorf laboratories. 

"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," said HZDR researcher Ralf Schützhold, outlining his colleagues' results.

After a year of analyzing data, researchers had successfully detected gravitational waves over 100 years after Einstein first predicted them. Those researchers won the 2017 Nobel Prize in Physics. 

How Bose-Einstein condensates could spot gravitational waves

While astrophysicists celebrated the 2016 research, others wanted to know if gravitational waves could be spotted using much smaller equipment and at smaller facilities. They looked to Bose-Einstein condensates for a potential answer. 

Originally predicted by Satyendranath Bose and Albert Einstein in 1924, the condensates exist at extremely low temperatures. Most atoms of metals like rubidium exist in the same quantum state, despite being chaotic as vapors at higher temperatures. 

"Such condensates can be thought of as heavily diluted vapor from individual atoms that are cooled to the extreme and therefore condense," explained Schützhold. 

"Similar to laser light particles, the atoms of these Bose-Einstein condensates move, so to speak, in synchronization," Schützhold said.

Physicists hoped that the gravitational waves could change the phonons (sound-particles) in synchronized atom-condensates. Researchers would then be able to spot and measure those changes. 

"This is a bit similar to a big vat of water in which waves generated by an earthquake change the existing water waves," Schützhold explained. 

Why Bose-Einstein condensates don't work

The short answer as to why the condensates can't be used? They don't exist at a magnitude large enough to register the gravitational waves. 

"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," Schützhold said.

That's not to say it couldn't one day be possible, the HZDR researchers noted. The team wants to look more closely at super-cooled helium. While helium doesn't qualify as a true Bose-Einstein condensate, it would have 10 percent of synchronized atoms. The researchers want to explore if that's a high enough percentage to register gravitational waves. 

"Whether superfluid helium is, however, really a way to detect gravitational waves can only be shown with extremely complex calculations," Schützhold concluded.