Researchers report in Science Advances that they have measured a long-predicted gravity effect on a quantum object as it fell. Quantum mechanics and general relativity (Einstein’s theory of gravity) work extremely well on their own, but it is still not clear how they fit together.
The measurement tests the equivalence principle, a central idea in Einstein’s gravity theory. It is hard to test it with quantum objects, because they can act like waves and follow more than one path at once.
The researchers used a new apparatus is called a Quantum Galileo Interferometer, clouds of rubidium atoms cooled to just above absolute zero, and devices that make controlled magnetic fields.
How the two paths were compared
Microwave pulses first put the atoms into a superposition, a quantum state in which each atom is associated with two paths at the same time. Magnetic fields from the chip then treated the two parts of the wave differently. One part felt an upward force that cancelled Earth’s pull, so it stayed still relative to the laboratory. The other part was given a brief upward magnetic kick and then switched into a state that barely felt the field, so it fell freely. A later magnetic pulse brought the two parts together. Their overlap produced interference.
The measured phase matched the value expected if Einstein’s equivalence principle is applied to such a falling quantum wave. Earlier work has used quantum particles to measure gravity, but the authors say this is the first direct measurement of that predicted free-fall phase.
The result does not merge quantum theory with gravity, and it does not show that gravity itself is quantum. It only shows that the equivalence principle still held in the case tested. It also does not test an argument by co-author Roger Penrose that quantum mechanics might fail for much heavier objects held in superposition for longer times. The masses and durations here were too small for that. Related work with heavier objects, including nanodiamonds, is planned at Ben-Gurion University. Vlatko Vedral of Oxford said the experiment pushes quantum mechanics toward gravity and that the usual quantum predictions still held.