Is gravity truly a quantum force?

Exploring quantum gravity—for whom the pendulum swings.

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Einstein’s theory of general relativity offers the framework for understanding gravity, about a century ago. The theory outlined gravity as a curvature of space-time.

But the theory falls near the center of a black hole or in the first moments of the universe. Physicists need a truer picture of gravity to describe these extremes accurately.

Here’s when quantum mechanics dominates, yet physicists have been unable to unite quantum theory with gravity for decades.

atomic interferometer
(1) In an atomic interferometer, the atom’s wave function is split into left and right arms. The left and right arms are then recombined, producing an interference pattern. Credit: S. Kelley/NIST

According to physicists, like the other forces of nature- gravity must have a quantum form.

A new experiment could potentially settle the persistent question: Is gravity indeed a quantum force? The experiment, performed by the scientists at Berkeley Lab and the National Institute of Standards and Technology (NIST), takes advantage of two of the weirdest properties of quantum theory: the superposition principle and entanglement.

In theories of quantum gravity, the graviton is the hypothetical quantum of gravity. It mediates the force of gravitational interaction between two massive objects. So, if a graviton truly exists, it should connect or entangle the properties of two massive bodies.

The experiment offers a clever way to test if two massive bodies can indeed become entangled by gravity.

atom's wave function
When the experiment begins, the atom’s wave function is unaffected by the pendulum. This means the two arms of the single atom interfere fully with each other. Credit: S. Kelley/NIST

The experiment would use a cold cloud of atoms trapped inside a nuclear interferometer. The interferometer has two arms—a left arm and a right. The superposition principle suggests that, if each atom in the cloud is in a pure, undisturbed quantum state, it tends to be portrayed as a wave occupying both arms simultaneously. When both portions of the wave recombine one from each arm, they will produce an interference pattern that uncovers any progressions to their ways because of forces like gravity.

A small, initially stationary mass suspended as a pendulum is introduced just outside the interferometer. The suspended mass and the atom are gravitationally attracted. If that gravitational tug also produces entanglement, what would that look like?

The suspended mass will become correlated with a specific location for the atom—either the right arm of the interferometer or the left. As a result, the mass will start swinging to the left or the right. If the atom is located on the left, the pendulum will start swinging to the left; if the atom is located on the right, the pendulum will start swinging to the right. Gravity has entangled the atom’s position in the interferometer with the direction in which the pendulum begins swinging.

gravitational attraction
(3) If gravitational attraction indeed causes an entanglement between the pendulum and the atom, the pendulum will partially measure the position of the atom, concentrating it into one arm or the other. Credit: S. Kelley/NIST

The position entanglement means that the pendulum has effectively measured the atom’s location, pinpointing it to a particular site within the interferometer. Because the atom is no longer in a superposition of being in both arms simultaneously, the interference pattern vanishes or diminishes.

Almost half period later, the swinging mass loses all memory of the gravitational entanglement once it returns to its starting point. And when it returns to the starting position, it’s equally likely that the pendulum will pick out a location for the atom in the left or right arm. At that moment, entanglement between the mass and the atom has been erased, and the atomic interference pattern reappears.

Half a period later, the pendulum swings to one side or the other. This reestablishes the entanglement and diminishes the interference pattern again. As the pendulum swings back and forth, the pattern repeats—interference, reduced interference, interference.

half oscillation period
After each half oscillation period, the pendulum will return to where it started, losing all memory of the gravitational entanglement it had created and restoring full interference. Credit: S. Kelley/NIST

Daniel Carney, now at the Lawrence Berkeley National Laboratory, said, “This collapse and revival of interference would be a “smoking gun” for entanglement. It is difficult for any phenomenon other than gravitational entanglement to produce such a cycle.”

Jake Taylor of NIST’s Joint Quantum Institute at the University of Maryland said, “Although the ideal experiment maybe a decade or more from being built, a preliminary version could be ready in just a few years. A variety of shortcuts could be exploited to make things easier to observe. The biggest shortcut is to embrace the assumption, similar to Einstein’s theory of general relativity that it doesn’t matter when you start the experiment—you should always get the same result.”

“Non-gravitational sources of quantum entanglement must be considered, which will require careful design and measurements to preclude.”

Journal Reference:
  1. Daniel Carney et al., Using an Atom Interferometer to Infer Gravitational Entanglement Generation, PRX Quantum (2021). DOI: 10.1103/PRXQuantum.2.030330