Scientists Just Found Where the Quantum World and Our World Meet: Free Fall

A group of scientists has shown that an object in “the quantum phase of free fall” is affected by gravity. If this sounds obvious, that’s not at all true—it’s actually really hard to link quantum phenomena in general to classical physics in concrete ways. In fact, this new observation comes many decades after it was first theorized by Albert Einstein. The finding is therefore an important step toward understanding how quantum and classical physics, two very different branches of the field, might one day be unified.The scientists’ research appears now in an article in the journal Science Advances, from a team that includes 95-year-old Nobel Prize winner Roger Penrose. In the intro, the authors explain how gravity and the idea of the free fall are so vital to the handshake zone between quantum mechanics and general relativity: “Einstein used the geometry of space-time to describe gravity and free fall. [O]ne should be able to describe nature from different frames of reference. [T]he principle of equivalence has guided our understanding of gravitation for over a century.”In other words, an object falling in one place, in one context, should be scientifically equivalent to an object falling in a very different place and context. That doesn’t just apply to closer contexts, like on Earth’s surface and throughout our solar system. It means the tiniest things and the most enormous things must be explainable using the same mechanisms, in the emptiness of space and in the ignited plasma inside our stars.The revolutionary experiment involves “a novel cold-atom interferometer,” meaning a newly designed version of a tool that disturbs something in order to measure what happens. A cold atom is one that has been chilled to just above absolute zero, which slows it down enough for its quantum behavior to show. Many quantum technologies and effects, from quantum computers to superconductivity, depend on temperatures close to absolute zero.The interferometer is not a piece of equipment, but rather a careful arrangement of individual molecules held in place by optical (laser) tweezers and other micro forces. At its heart is a unique Bose Einstein condensate, a cloud similar to a gas but defined by its extreme cold. The researchers diluted a cloud of rubidium atoms until the atoms were far enough apart to look at individually. Then they applied different magnets until one atom held still while another was able to “free fall” on the nano scale.They used math and definitions to confirm that their observations of this setup qualified as free fall. In turn, they measured “g,” the gravitational acceleration in respect to Earth, calculated in a way that conformed to the quantum experimental setup. “Although measuring g is clearly not the goal of our experiment, the phase of a freely falling object is proportional to g so the measurement of the phase may be considered a measurement of gravity,” the team concluded.This single experiment is the start of a long process, not a definitive finding. “As an outlook, let us introduce the term quantum equivalence principle (QEP). As there seems to be no agreement on how to exactly define the QEP, we leave this for future theoretical and experimental work,” the team explained.Caroline Delbert is a writer, avid reader, and contributing editor at Pop Mech. She's also an enthusiast of just about everything. Her favorite topics include nuclear energy, cosmology, math of everyday things, and the philosophy of it all.