'There is no conflict between quantum physics and gravity': Physicists prove Einstein's 'happiest thought' holds true at quantum scales

For the first time, physicists have measured the minute shift an object undergoes by falling through Earth's gravity, an effect predicted by Einstein's relativity but never observed. Researchers used ultracold rubidium atoms in a superposition state, where a particle exists in two paths simultaneously. One path involved the atom falling, while the other kept it motionless.

By recombining the atoms, scientists detected an almost imperceptible difference between the two paths. The findings, published in Science Advances, demonstrate that Einstein's equivalence principle, a cornerstone of general relativity, still holds true in the quantum realm. This principle posits that acceleration and gravity cannot be distinguished locally.

The classic illustration of this principle is Einstein's elevator thought experiment, in which a person in a windowless elevator cannot determine whether they are in a stationary elevator on Earth or an accelerating elevator in space. Despite being tested with great precision for heavy objects, the equivalence principle has never been observed for quantum objects.

The experiment required an instrument with one arm at rest and another in free fall, which no previous experiment could achieve. The researchers chilled about 20,000 rubidium atoms into a Bose-Einstein condensate, released them, and ran an interferometer 113 micrometers beneath an atom chip patterned with thin gold wires. By manipulating magnetic fields, one half of the atom was launched upward, while the other remained motionless under gravity's influence.

The separation between the two halves was about 7.5 micrometers, and the researchers measured the interference signal as they extended the free-fall time. The phase between the two parts of the atom followed the cube of the falling time, matching the theoretical prediction made in 1927. The result suggests that there is no conflict between quantum physics and gravity, as both theories coexist at this level of accuracy.