Almost a century ago, physicists grappled with a mind-bending question: what happens when a quantum wave experiences gravity? Now, a team led by Ron Folman at Ben-Gurion University has built a revolutionary interferometer, demonstrating that an atom can both fall and stay still simultaneously. This experiment could finally test if quantum mechanics and Einstein's theory of gravity are at odds.

Long before this experiment, Galileo taught us that objects in free fall are governed by clear rules of motion. But here, in the quantum realm, every object, from atoms to spaceships, behaves like a wave. ‘Everything that is a wave, like sea waves or sound waves, goes up and down, and this is measured by something called a phase,’ explains Folman. ‘A phase just tells you if you are at the top or bottom of the wave.’

The challenge has always been in measuring this phase, which requires splitting a particle into two paths for comparison. Only in the late 1990s did scientists cool atoms to near absolute zero, allowing their wave nature to become observable. This breakthrough paves the way for a new era of quantum gravity experiments.

Their interferometer, a sophisticated apparatus, gives an atom two paths: one that falls freely, and another where the atom is held still. Both paths end at the same place and time, enabling the team to observe the effects of free fall on the atom’s wave-like properties. This could be a pivotal moment in reconciling quantum mechanics with general relativity.

The implications are profound. If the experiment reveals a contradiction, it could suggest a new theory of quantum gravity. Conversely, if it aligns, we might finally glimpse a unified theory of nature. For now, this experiment offers a tantalising glimpse into the fabric of reality, where quantum mechanics and relativity meet.