Conduct atom-drop gravity tests

Science News pointed readers on May 31 to a class of physics experiments that test gravity by dropping atoms and tracking their motion with atom interferometers. The experiments are designed to probe the equivalence principle — the idea, central to general relativity, that different objects fall the same way in a gravitational field. Researchers are pursuing the work in tall vacuum tubes, drop towers and orbiting laboratories, using ultracold atoms whose quantum behavior can be measured with high precision. The immediate goal is better gravimetry; the longer-term target is tighter tests of gravity theory. ### What are scientists actually dropping? Atoms are the falling objects, but the measurement is not a simple stopwatch test. In atom interferometry, laser pulses split and recombine matter waves, letting physicists read out tiny differences in phase as atoms move through free fall. NASA said in a May 6, 2025 explainer that atom interferometers can precisely measure gravity, magnetic fields and other forces, and that the agency’s Cold Atom Lab on the International Space Station has demonstrated atom interferometry in orbit. Science News reported in 2020 that researchers launched clouds of two rubidium isotopes about 8.6 meters high in a vacuum tube and compared how they accelerated as they rose and fell. The result, published in Physical Review Letters, found that the two atom types matched in acceleration within about one part per trillion, according to the report. (science.nasa.gov) ### Why does comparing two kinds of atoms matter? The equivalence principle is the target. Science News said the rubidium-isotope experiment reaffirmed that an object’s acceleration under gravity does not depend on its mass or composition, a foundation of Einstein’s general relativity. Physicist Mark Kasevich of Stanford University told the outlet that tests with atoms “stress” the principle in new ways because atoms are governed by quantum mechanics. (sciencenews.org) NASA used similar language for its space-based program. The agency said the dual-species atom interferometer on the ISS served as a pathfinder for future experiments aimed at testing the universality of free fall, which it described as a key tenet of general relativity. (sciencenews.org) ### Why are physicists building taller and longer systems? Longer free fall increases sensitivity. ZARM in Bremen says the phase measured by a matter-wave interferometer often scales quadratically with free-fall time, which is why its drop-tower work focuses on quantum gases in extended free fall. The group says the Bremen facilities let researchers study Bose-Einstein condensates and other ultracold systems for gravity tests and related quantum measurements. (science.nasa.gov) Leibniz University Hannover is building a larger ground-based setup. Its Very Long Baseline Atom Interferometry project describes a 10-meter-baseline instrument for absolute gravimetry, gravity gradiometry and tests of fundamental physics. The Hannover group says extending baselines from tens of centimeters to several meters could push universality-of-free-fall tests to levels comparable with classical lunar laser ranging and torsion-balance experiments. (zarm.uni-bremen.de) ### Are these experiments only about Einstein tests? Gravity testing is one use, not the only one. NASA said atom interferometers are also being developed for aircraft and ship navigation and could support future measurements in planetary science and Earth observation. The agency’s Cold Atom Lab is using persistent microgravity in orbit to extend free-fall conditions beyond what is practical on the ground. (iqo.uni-hannover.de) The Hannover project lists geodesy and high-accuracy gravimetry alongside equivalence-principle tests. Its published goals include absolute gravimetry with inaccuracy below 1×10^-9 meters per second squared and gravity-gradient resolution below 5×10^-10 per second squared. ### What comes next for these atom-drop programs? (science.nasa.gov) NASA said upgraded hardware is being prepared at Jet Propulsion Laboratory for launch and installation into the Cold Atom Lab on the ISS, where researchers have already reported dual-species interferometry in space. (iqo.uni-hannover.de) In Hannover, the next concrete milestone is the 10-meter VLBAI test stand, which the university says will operate as a dual-species gravimeter using ytterbium and rubidium atoms. In Bremen, drop-tower teams are continuing extended-free-fall experiments with ultracold gases and Bose-Einstein condensates. (iqo.uni-hannover.de) (science.nasa.gov)