When a tennis ball bounces off the ground, you could be forgiven for thinking its movement is smooth. But this is just an illusion. The ball is constantly switching between closely packed quantum states of gravitational energy — so quickly that the transition seems smooth. Now, physicists have measured these transitions by bouncing not tennis balls, but ultracold neutrons to test the laws of gravity over scales no one had studied before.
For everyday objects and over short distances, the gravitational force — whether as described in Newtonian physics or in Einstein's more complicated general theory of relativity — reduces to a simple rule: gravitational energy is directly proportional to height, and tennis balls go up and down tracing parabolas. For confined particle that has quantum behaviour, such as the neutron, the energy is still proportional to height, but can only exist at particular levels, rather than over a continuum1 (see 'Gravity leaps into quantum world'). Some potential explanations for the mysteries of dark energy and dark matter — which together make up 95% of the Universe — predict subtle deviations from the levels that we would expect if gravity were acting alone.