Dropping ultra-cold quantum gas down an elevator shaft could help prove Einstein wrong. Scientists have shown that it’s possible to keep sufficiently close tabs on quantum mechanical objects in free fall to tell whether two such objects experience gravity the same way.

In 1907, Einstein suggested that if you were in a windowless elevator that was plunging towards Earth in free fall, you would feel the same weightlessness as if you were floating in outer space.

This notion, known as the equivalence principle, laid the foundation for general relativity. It explains why a pebble and a piano fall at the same speed if dropped from the same roof, despite their different masses. It’s also a necessary first step toward describing the effects of gravity as curvature in spacetime.

“It’s a very important cornerstone,” said physicist Ernst Rasel of the Leibniz University of Hannover in Germany. But, he added, the equivalence principle “is just a postulate — it’s not coming out of a law.”

So of course, physicists have spent the past century trying to break it. Earlier tests used man-made, macroscopic objects like rotating pendulums to make sure two elements of different masses fall toward Earth at the same speed. Physicists also have bounced laser light off mirrors left on the moon in the 1960s to make sure the Earth and the moon feel the same acceleration from the sun’s gravity. So far, to the limits of the experiments’ accuracy, Einstein’s idea has held up.

To get more precise measurements, physicists are turning to smaller and smaller test masses, all the way down to the atomic scale where conventional laws of motion give way to quantum mechanics. In the June 18 Science, Rasel and his colleagues describe a technically tricky but conceptually simple test of how quantum objects experience gravity in a free-fall: Drop one down an elevator shaft.

Though the elevator trial hasn’t directly tested general relativity yet, it proves that tests with two different kinds of atoms can be done with unprecedented sensitivity.

Rasel’s team used a quantum object called a Bose-Einstein condensate, a gas so cold its atoms all act as one particle. The team used lasers to cool rubidium atoms to a few billionths of a degree above absolute zero. Because rubidium is a type of particle called a boson, several of its atoms can pack together in the same quantum state and act as one particle, or one matter wave.

Normally, preparing a Bose-Einstein condensate takes a room full of carefully aligned lasers, vacuum chambers and delicate electronics. But Rasel and colleagues crammed all the necessary equipment into a capsule 24 inches wide and 85 inches tall.

The team then dropped the capsule down the 480-foot-tall drop tower at the Center for Applied Space Technology and Microgravity in Bremen, Germany, which was designed for similar experiments in low gravity.

“It’s just so impressive as an experiment,” comments Paulo Nussenzveig of the University of Sao Paulo in Brazil. “It’s inspiring. If these people did it, then it can be done.”

The researchers managed to watch the Bose-Einstein condensate carefully enough that at the end of the fall, they were confident that nothing but gravity acted on the quantum gas. Then, just to be sure, they repeated the drop 180 times.

“We’ve demonstrated that you can do this reliably,” Rasel said. This reliability will be important for doing similar experiments in space, which Rasel said is the ultimate goal.

“One of the things that is important for all these relativity tests, you will measure very small effects, so you need to accumulate data over a long period of time,” Nussenzveig noted. “That means running the experiment again and again and again and again.” In space, physicists will be able to watch a quantum gas fall continuously for years, as opposed to the four seconds it spends in the drop tower.

Next, Rasel and his colleagues plan to drop quantum gases of two different elements, rubidium and potassium, to see if they behave the same way. If their matter waves are out of phase, it’s a sign that something’s wrong with the equivalence principle.

Watching Bose-Einstein condensates in free fall could also help sketch a map of the intersection between quantum mechanics and general relativity. These twin pillars of modern physics don’t play well together — scientists have yet to come up with a description of the universe that describes both at the same time.

“Performing this experiment with an intrinsically quantum mechanical object inside it can in principle lead to new insight, bringing together these two very important developments from 20th century physics,” Nussenzveig said. “This is opening up a new testing ground.”

Image and Video: ZARM – University of Bremen

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