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Starts With A Bang —

Has a new experiment just proven the quantum nature of gravity?

At a fundamental level, nobody knows whether gravity is truly quantum in nature. A novel experiment strongly hints that it is.
This artist’s illustration depicts how the foamy structure of space-time may appear, showing tiny bubbles quadrillions of times smaller than the nucleus of an atom that are constantly fluctuating and last for only infinitesimal fractions of a second. Rather than being smooth, continuous, and uniform, at the quantum scale, spacetime has fluctuations inherent to it. Although we strongly suspect that gravity is quantum in nature, we can only be sure via experiment. (Credit: NASA/CXC/M. Weiss)
Key Takeaways
  • Three of our fundamental forces of nature — the electromagnetic and strong and weak nuclear forces — are known to be quantum in nature.
  • However, the oldest known fundamental force, gravity, has only been shown to exhibit behavior described by Einstein's general relativity: a classical and continuous theory.
  • By demonstrating that particles display the Aharonov-Bohm effect for gravitational forces, previously only seen with electromagnetic ones, we might have our first clue to gravity's quantum nature.
Ethan Siegel
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If you were to break down the matter in our Universe to its smallest and most fundamental subatomic constituents, you’d find that everything was made up of individual quanta, each of which possesses both wave and particle properties simultaneously. If you pass one of these quantum particles through a double-slit and don’t observe which slit it passes through, the quantum will behave as a wave, interfering with itself on its journey and leaving us with only a probabilistic set of outcomes to describe its ultimate trajectory. Only by observing it can we determine precisely where it is at any moment in time.

This bizarre, indeterminate behavior has been thoroughly observed, studied, and characterized for three of our fundamental forces: the electromagnetic force and the strong and weak nuclear forces. However, it’s never been tested for gravitation, which remains the one remaining force that only has a classical description in the form of Einstein’s general relativity. Although many clever experiments have attempted to reveal whether a quantum description of gravity is required to account for the behavior of these fundamental particles, none has ever been performed decisively.

However, a long-studied quantum phenomenon, the Aharonov-Bohm effect, has just been discovered to occur for gravity as well as electromagnetism. A greatly underappreciated result, it could be our first clue that gravity is truly quantum in nature.

quantum gravity
In general relativity, the presence of matter and energy determine the curvature of space. In quantum gravity, there will be quantum field theoretic contributions that lead to the same net effect. So far, no experiment has been able to establish whether gravity is quantum in nature or not, but we’re getting closer. (Credit: SLAC National Accelerator Laboratory)

The quantum question

In the world of quantum physics, few experiments are more demonstrative of the bizarre nature of reality than the double-slit experiment. Originally performed with photons more than 200 years ago, shining light through two thin, closely-spaced slits resulted not in two illuminated images on the screen behind the slits, but rather in an interference pattern. The light that went through each of the two slits must be interacting before they reach the screen, creating a pattern that displays light’s inherent wave-like behavior.

Later on, this same interference pattern was shown to be generated with electrons as well as photons; for single photons, even as you passed them through the slits one at a time; and for single electrons, again even as you passed them through the slits one at a time. As long as you don’t measure which slit the quantum particles go through, the wave-like behavior is easily observable. It is evidence of the counterintuitive, but very real, quantum mechanical nature of the system: Somehow, an individual quantum is capable of going through “two slits at once” in a sense, where it must interfere with itself.