Laboratory tests of whether gravity is quantum

6 problems

• Close the decoherence-diffusion window for classical gravity

  Open

  If gravity is classical while matter is quantum, consistency requires gravity to both blur superpositions and make the gravitational pull jitter randomly. Measuring both effects together can exclude such theories.

• Does gravity-mediated entanglement prove the gravitational field is quantum?

  Contested

  The standard argument says a classical go-between cannot create entanglement, so seeing it would prove gravity quantum. A 2025 paper argues that classical gravity acting on quantum matter fields can still create some entanglement through virtual matter particles (short-lived intermediate states), so the inference is disputed.

• Observe entanglement created only by gravity between two masses

  Open

  Two tiny crystals, each in a superposition of two places, are let fall side by side; if gravity is quantum, their attraction should entangle them within seconds. This needs masses of roughly a hundred-billionth of a gram kept coherent for seconds with almost no other force between them.

• Must the gravitational field obey quantum superposition?

  Open

  If a mass is in two places at once, is its gravitational field also in two configurations at once, or is gravity a classical field that only responds to the average? Experiments at low energy can in principle decide.

• Detect single-graviton absorption in a massive acoustic resonator

  Contested

  A heavy resonator cooled to its lowest vibration state could absorb a single quantum of a passing gravitational wave, called a graviton, and jump up by one vibration quantum. Detecting such jumps is argued to be within reach, but whether they would prove that gravity is quantum is disputed.

• Measure the gravity of a mass that is in superposition

  Open

  Gravity has been measured from masses of about 90 milligrams, but never from an object that is itself in a superposition of two places. Doing so would show whether a quantum source pulls on a probe through its average position or through each branch separately.