Ultracold atoms in space will let us stress test Einstein's relativity

Physicists are close to performing the most accurate tests of Einstein’s ideas about gravity ever undertaken. Their first-of-its-kind experiment involves using two kinds of extremely cold atoms aboard the International Space Station (ISS).

A key principle of Einstein’s theory, and one that researchers have been testing for decades, is the equivalence principle. This states that all objects fall with the same acceleration when gravity is the only force acting on them.

One of the most sensitive tests of the principle so far relied on putting very cold rubidium atoms into freefall at a special facility in California; another test involved exploring the effects of gravity on materials of precisely measured mass that were launched into space on a satellite.

Naceur Gaaloul at Leibniz University Hannover in Germany and his colleagues have now built an experiment that combines elements of both of these earlier tests by using ultracold atoms in space.

They used the Cold Atoms Laboratory (CAL) on the ISS, which was launched in 2018 and built to examine quantum effects noticeable in atoms only when they are extremely cold and when gravity is extremely low. Within the CAL, atoms are confined to a chip and made very cold by being pushed, pulled and hit by magnetic forces and lasers.

At temperatures only billionths of a degree above absolute zero, quantum effects make these atoms behave like a collection of overlapping “matter waves” rather than distinct particles. In the new experiment, the researchers cooled potassium and rubidium atoms on the same chip and then manipulated them in a way that effectively turned the chip into two separate devices called interferometers.

Interferometers measure acceleration based on patterns made inside them by clashing matter waves. The ISS is always in freefall – it is always accelerating due to gravity – so if the two interferometers record different acceleration values, the equivalence principle would be broken.

While the researchers have now successfully made the two interferometers in the CAL, they need to optimise the two devices further before they can use them to fully test the equivalence principle.

“The equivalence principle is the bedrock of our understanding of gravity, but these experiments could go beyond just testing general relativity. There could be new particles which are not included in the Standard Model that manifest as breaking this principle,” says Timothy Kovachy at Northwestern University in Illinois. He says that the accuracy of atom-based interferometers increases the longer the atoms are in freefall, and since there are time constraints on how long such freefall can be maintained on Earth, reaching extreme precision requires going to space.

And doing so is a growing and competitive field, says Gaaloul. In 2017, he was part of a team sponsored by the German Space Agency, or DLR, that achieved atom interferometry using ultracold rubidium atoms – but not potassium atoms – aboard a research rocket. The DLR team will launch another rocket in the coming months, this time with both potassium and rubidium atoms on board.

The CAL experiments are expected to yield results that are hundreds of times more accurate than those obtained with satellite-based tests, and hundreds of thousands of times more accurate than the results of Earth-based experiments, but Gaaloul says that ultimately it will be necessary to go beyond the ISS as well. “Because of the vibrations from astronauts biking and other things that are going on, the ISS is not perfect for precision experiments,” he says. “But here we will make sure of techniques for equivalence principle tests which will ultimately happen on a dedicated satellite.”