Scientists observe Einstein's gravity in the quantum world

An international team including Nobel Prize-winning physicist Professor Sir Roger Penrose has observed a long-predicted effect of gravity on a falling quantum object for the first time. The result shows that a fundamental ...

For more than a century, physicists have relied on two extraordinarily successful descriptions of nature. Quantum mechanics explains the strange behavior of atoms and other tiny objects. Einstein's theory of gravity explains how objects fall and how gravity shapes the universe. Yet physicists still do not fully understand how the two fit together. Now, an international team has performed an experiment that probes the point where they meet. In the study, the researchers observed a distinctive change in the quantum properties of atoms as they fell under gravity. Crucially, the effect they measured is the same one predicted when Einstein's equivalence principle, a cornerstone of his theory of gravity, is applied to a quantum object.

The equivalence principle states that for an observer in free fall, gravity should locally disappear. Someone falling freely in an elevator, for example, would experience weightlessness. While this theory has survived extraordinarily precise tests involving ordinary matter, it was unclear how it could be experimentally tested with quantum objects, which can behave as waves and effectively travel along more than one path. At the heart of the experiment is a new apparatus the researchers call the Quantum Galileo Interferometer. It allowed them to do something unusual: effectively split the quantum wave associated with an atom into two paths, hold one in place while allowing the other to fall freely, and then reunite them to see how gravity had changed the falling wave.

The atom chip used in the experiment (fabricated at Ben-Gurion University of the Negev). In the experiment the chip was upside-down and the atoms manipulated just under it. Credit: Ben-Gurion University of the Negev Read more

The 2D MOT apparatus which feeds the science chamber with cold atoms. At the center is a glass cell held under vacuum between coils producing magnetic fields. The atoms go into the science chamber through the tiny hole that may be seen in the center of the yellow (reflecting) surface. Credit: Or Dobkowski Read more

A general picture of the experimental setup. At the heart is a vacuum chamber in which conditions such as those in space mean that atoms can be kept undisturbed. On the left is a 2D MOT, a device that feeds atoms into the science chamber where the atom chip is positioned. Around the chamber are antennas, coils and optical fibers, enabling atoms to be trapped and cooled, then manipulated into two distinct paths. Finally, the relative phase between the two paths is detected. Credit: Or Dobkowski Read more