Even for scientists who have dedicated their lives to understanding gravity, the force’s relentless downward pull is sometimes a drag. Consider, for instance, the researchers who study Bose-Einstein condensates (BECs) as precise probes of fundamental physics. BECs emerge when a dilute gas of atoms is cooled close to absolute zero and begins behaving as a single, strange chunk of quantum matter—similar to how wriggling water molecules transform into a block of ice once they are chilled. These odd assemblages magnify otherwise hidden quantum-mechanical effects such as the wavelike nature of matter, making them visible at macroscales. Yet sometimes gravity’s pernicious influence can get in the way.
Earthbound escapes from gravity’s hold involve subjecting BECs to free fall, usually for short spates inside tall drop towers or airplanes flying in parabolic arcs. But the best approach is arguably to leave Earth behind, placing BECs in rockets to experience longer periods of weightless free fall in outer space. Recently, a team of physicists supported by Germany’s space agency reported on doing just that. In Nature Communications this past February, they published the results of a 2017 experiment that manufactured BECs on a millimeter-sized chip in a suborbital sounding rocket almost 300 kilometers above the planet’s surface. The BECs then crashed together in the microgravity conditions, allowing the physicists to study the collisions in exquisite detail. Their mission, MAIUS-1, was the first to successfully collide BECs in space, and it points the way toward new space-based tests of fundamental physics.
Clash of the Condensates
When two BECs collide, instead of bouncing off one another like atoms usually do, they interact as waves. When their peaks line up, they form an even taller wave. If the peak of one matter wave overlaps with the trough of another, they cancel each other out, leaving behind empty space. An encounter between two misaligned condensates results in a wave-interference pattern: alternating bright stripes where the two waves enhanced each other and dark stripes where they annihilated each other. Creating and studying these patterns in matter is called atom interferometry.
Onboard the MAIUS-1 rocket, a carefully choreographed system of lasers split the ultracold atoms into multiple matter waves before letting them collide. Images captured inside the rocket, and analyzed once the spacecraft returned to Earth, showed a detailed striped interference pattern that emerged from slight differences in the shapes and positions of each BEC’s peaks and troughs. By studying such details, the researchers could tell whether, prior to crashing, the matter waves had been changed by interacting with light or any other forces in their surroundings.
“Atoms are sensitive to all of it,” says Naceur Gaaloul, a physicist at Leibniz University Hannover in Germany and co-author on the study. The stripe pattern produced by colliding BECs, Gaaloul says, is a bit like an archeological dig: it helps scientists determine the precise precrash history of the matter waves and pinpoint anything that could have moved their peaks and troughs.
Gravity’s pull complicates all of this because it makes BECs fall while they move toward each other, resulting in vanishingly brief clashes and blurred interference patterns. The microgravity conditions of space remove these limitations.
According to Maike D. Lachmann, a physicist at Leibniz University Hannover and the study’s lead author, escaping gravity has always been her team’s motivation. “The whole thing started in a collaboration, which was aiming to do experiments in a drop-tower facility,” she recalls. “But the long-term goal was always going to space.” Dropping ultracold atoms from a nearly 150-meter-high tower bought scientists several seconds of microgravity. The MAIUS-1 rocket bumped that up to nearly six minutes.
“Microgravity is just really where you want to be,” says Cass Sackett, a physicist at the University of Virginia, who was not involved with the study. “I expect that as time goes on, we will see atom interferometers in space that are better than anything that’s been on the ground.” In fact, in 2018 NASA launched an ultracold atom experiment into space. The space agency’s Cold Atom Laboratory (CAL) has been cooling atoms onboard the International Space Station (ISS) ever since.
CAL’s ability to create quantum states in microgravity for scientists to play with captivated many physicists, including Sackett. Anita Sengupta, an aerospace engineer who served as CAL’s project manager during the first five years of its development and was not part of the new study, echoes this sentiment. “My personal motivation behind the mission was to engineer a facility to explore the fundamental physics of the BEC, to open a new doorway into the quantum world,” she says. Researchers using CAL have recently performed atom interferometry experiments similar to the work of the MAIUS-1 team as well, Sengupta adds.
Cool Apps for Cold Atoms
Regardless of the specific space-based platform being used, one common research goal for atom interferometry is to test the fundamental principle that bodies of all compositions fall at the same rate under the influence of gravity. According to Lachmann, conducting the MAIUS-1 matter wave interference experiment multiple times using batches of elementally different atoms would test this idea to unprecedented levels of precision. In the unlikely event gravity moved one set of atoms more than the other, their two stripe patterns would be visibly different.
The extreme precision offered by atom interferometry also ushers in the small possibility that signatures of exotic forces, perhaps those associated with some models of dark energy, could be spotted through the technique.
A more immediate and practical application for devices such as the MAIUS-1 chip could emerge in celestial navigation. Because BEC interference patterns are so sensitive to even the smallest fluctuations in gravity, they can be used to map out details of gravitational fields. Similar to how maps of underwater currents help ships navigate, these gravitational-field maps could be useful for fine-tuning a spacecraft’s deep-space maneuvers.
During its mission, the MAIUS-1 team already achieved several technological advances. The scientists’ experiment fit on a single ruggedized chip rather than being laid out on a large table like the arrangement in most terrestrial laboratories—because it had to survive the rocket’s bumpy flight through Earth’s atmosphere. Also, the researchers could not communicate with the rocket after it launched, so autonomous systems cooled, manipulated and imaged the atoms. In the future, they want to equip the rocket with commonly used navigation sensors and compare those sensors’ performance to that of their chip.
For now, NASA and MAIUS-1 scientists are collaborating on developing upgrades for future installation on CAL onboard the ISS that will offer more options for microgravity experiments, including using atoms that have magnetic spins or that interact with one another strongly. Combining their experiences of trying to wrestle atoms away from gravity’s pull, researchers hope to put fundamental physics under an even more powerful magnifying glass in outer space.