GoKawiilAlmost a hundred years ago, physicists theorized out what free fall should do to a quantum wave. If the solution they came up with is wrong, quantum mechanics and Einstein’s theory of gravity flatly contradict each other. But testing it has been impossible because nobody has managed to build an interferometer that could perform the necessary measurement.
Now, a team led by Ron Folman, a physicist at Ben-Gurion University of the Negev, with collaborators in Germany, the UK, and the US, including Nobel laureate Roger Penrose, has done it. They built a new interferometer that gives a single atom two possible paths at once: one that involves a free fall, and another where it is held perfectly still. Both paths end at the same place at the same moment, allowing the team to measure what the fall does to a wave-like property of the atom.
Long time coming
Ever since Galileo, physicists have known how to describe a falling object—where it is, how fast it goes, or how quickly it accelerates. Quantum mechanics, though, insists that every object is also a wave. “Every particle, doesn’t matter if it’s a car or a spaceship or an atom, is a wave,” Folman says. “Everything that is a wave, like sea waves or sound waves, goes up and down. And if you’re up or down, this is measured by something called a phase. A phase just tells you if you are at the top of the wave or at the bottom.”
The wave nature of an atom shows itself only when the atom is barely moving, which only happens when it is cooled down to nearly absolute zero. For many years after theorists had first looked at this problem, this sort of temperature wasn’t an option—cooling atoms down to such temperatures only became possible in the late 1990s. “But this was just the start of the journey of this experiment,” Folman says.
The second obstacle is that a phase cannot be measured on its own—it appears only when you perform a comparison. “It can only be a relative measure done by splitting a single particle into two trajectories, and then bringing the two trajectories together,” Folman says. He told Ars that this is the same logic behind the double-slit experiment, where particles fired through two narrow slits toward a background screen form an interference pattern that reveals their phase.