An international team of physicists measured a subtle effect predicted for a quantum object in free fall. The result is a new test of how Einstein's description of gravity works alongside quantum physics.
The researchers cooled atoms and used a new device called a quantum Galileo interferometer to split a wave associated with an atom into two paths. One part was held still in the laboratory while the other was allowed to fall under Earth's gravity.
When the two paths were brought together again, the team measured a difference in their quantum phase. The result matched the phase predicted by the equivalence principle, one of the foundations of Einstein's theory of gravity.
The experiment does not solve the larger mystery of how gravity and quantum physics might fit together. It does show that, in the conditions tested, the two ideas can agree, opening a new way to look for answers.
