Cobra Effect · Quantum mechanics
Gravity reaches a quantum wave
A tilted crystal, a beam of neutrons, and gravity shifting a quantum wave.
7 cards, read aloud in 4:17, with a test and sources.
Spring 1974. At a social gathering, two physicists start talking about gravity and waves.
Albert Overhauser and Roberto Colella, both at Purdue University, knew that a neutron travels as a wave. Gravity was known from falling apples and orbiting planets. Nobody had seen it act on the wave side of a particle. A new kind of instrument, first made to work with neutrons in Vienna that same year, might let them look. They published the idea that November.
The instrument was carved from a single crystal of silicon.
Three thin slabs were left standing on a shared base, all part of one flawless crystal a few centimetres across. A neutron’s wave meets the first slab and divides to take two paths. The middle slab turns both paths back toward each other, and at the third slab they meet. Where they meet, the two waves add or cancel, and that decides which of two counters the neutron lands in. Each neutron is counted whole. It is the wave that takes both paths. The two paths ran centimetres apart.
In 1975, at a reactor in Michigan, they tilted the crystal.
Samuel Werner of the Ford Motor Company’s research staff joined them, and they set up at the University of Michigan’s research reactor in Ann Arbor. With the crystal lying level, both paths ran at the same height. Then they rolled the crystal about the incoming beam, so that one path ran a little higher than the other. On the higher path the neutron climbs against gravity and slows very slightly, so its wave stretches a little. By the time the paths meet, the two waves have slipped out of step.
As the crystal tilted, the counts in the two counters rose and fell against each other.
More tilt meant more height between the paths, and more slip between the waves. In step, then out of step, then in step again, several times over. The Earth’s gravity was shifting a quantum wave, and the counters showed it. The shift came out about a tenth smaller than predicted. Later runs traced most of that gap to the crystal bending under its own weight, and brought the match to about one percent.
It was the first result that needed both gravity and Planck’s constant to explain.
A falling apple needs only gravity. The bands in a quantum experiment need only Planck’s constant, the number that sets the scale of quantum effects. The size of this shift depends on both at once. But notice what it does not show. Gravity enters the sum as an ordinary, steady pull, just as an electric force would. The neutron is quantum. Nothing in the experiment asks whether gravity is.
Since then, the wave side has been tested again and again.
In 1991 Mark Kasevich and Steven Chu at Stanford split the waves of cooled sodium atoms, and used them to measure the pull of gravity. By 1999 the same method had measured it to about one part in ten billion. In 2002 a team in Grenoble found that very slow neutrons resting above a flat mirror can only have certain energies, like the rungs of a ladder, set by gravity. Every one of these treats gravity as an ordinary pull on a quantum thing. Every one agrees with the theory.
So when you hear that gravity and quantum theory never meet in the lab, ask which way round.
A quantum wave feeling gravity has been measured since 1975. A quantum wave making gravity never has. In 2021 a team in Vienna measured the pull of a gold ball two millimetres across, the smallest source of gravity anyone had measured. Anything yet shown to travel as a wave is vastly lighter than that. Closing the gap is how physicists hope to learn whether gravity itself is quantum.
Sources
- Observation of Gravitationally Induced Quantum Interference, R. Colella, A. W. Overhauser and S. A. Werner, Physical Review Letters, 1975. The abstract of the original paper, a single sentence. The paper itself is three pages, behind the journal’s paywall.
- 1975: The year that quantum mechanics met gravity, Greg Gbur, Skulls in the Stars. A physics professor walks through the original paper in plain language, including what it does and does not show.
- Gravitation and quantum interference experiments with neutrons, Hartmut Abele and Helmut Leeb, New Journal of Physics, 2012. A technical review of the 1975 experiment, its more precise successors, and the neutrons that rest in steps above a mirror.
- Measurement of gravitational coupling between millimetre-sized masses, Westphal, Hepach, Pfaff and Aspelmeyer, Nature, 2021. The free version of the Vienna paper on the pull of a two millimetre gold ball.
Nearby ideas
- Superposition and the double slit. One electron at a time, and still a pattern only waves should make.
- Is gravity quantum?. What Feynman asked in 1957, the rival answers, and the test that could decide.
- Falling feels like floating. Why falling feels like floating, and how that led Einstein to gravity.
- The photoelectric effect. The experiment that showed light arrives in packets, not only as waves.
- Atomic spectra and energy levels. Why each element glows in its own few colours, like a fingerprint.
- The uncertainty principle. Why pinning down where a particle is spreads out where it is going.
- Spin, and the magnet that split a beam. The magnet experiment that showed atoms answer in just two ways.
- Quantum tunnelling. How particles slip through walls they lack the energy to climb.