Quantum particles can feel the effects of gravitational fields that you never touch

Scientists have discovered that the Aharonov-Bohm effect also applies to gravity.

Gravity is caused by massive objects warping spacetime, according to general theory of relativity (illustrated). A quantum effect reveals that even if subatomic particles are not subject to gravitational forces, they can feel the effects of warping.  PLUS VCHAL/ISTOCK/GETTY IMAGES
Gravity is caused by massive objects warping spacetime, according to the general theory of relativity (illustrated). A quantum effect reveals that even if subatomic particles are not subject to gravitational forces, they can feel the effects of warping. 

Even if you keep your distance, a black cat in your path is bad luck if you're superstitious. Similarly, in quantum physics, particles can sense the influence of magnetic fields with which they have no direct contact. Scientists have now demonstrated that this strange quantum effect applies not only to magnetic fields but also to gravity — and it's not a myth.

Normally, a particle must pass through a magnetic field to feel its influence. However, physicists Yakir Aharonov and David Bohm predicted in 1959 that the conventional wisdom would fail in a specific scenario. A magnetic field contained within a cylindrical region can affect particles that never enter the cylinder, such as electrons. 

The electrons in this scenario do not have well-defined locations but are in "superpositions," quantum states defined by the probability of a particle materializing in two different places. Each fractured particle travels around the magnetic cylinder in two different directions at the same time. The magnetic field, despite never touching the electrons and thus exerting no force on them, shifts the pattern of where particles are found at the end of this journey, as various experiments have confirmed (SN: 3/1/86).

The same strange physics is at work for gravitational fields in the new experiment, physicists report in the journal Science on January 14. "Every time I look at this experiment, I think to myself, 'It's amazing that nature is that way,'" says Stanford University physicist Mark Kasevich.

Kasevich and colleagues fired rubidium atoms into a 10-meter-high vacuum chamber, hit them with lasers to put them in quantum superpositions tracing two different paths, and then watched the atoms fall. The particles were not, however, in a gravitational field-free zone. Instead, the experiment was designed so that the researchers could filter out the effects of gravitational forces, revealing the strange Aharonov-Bohm effect.

The study not only reveals a well-known physics effect in a novel context but also demonstrates the possibility of studying subtle effects in gravitational systems. Researchers, for example, hope to use this technique to better measure Newton's gravitational constant, G, which reveals the strength of gravity but is currently known less precisely than other fundamental constants of nature (SN: 8/29/18).

This experiment relies on a phenomenon known as interference. Atoms and other particles in quantum physics behave like waves that can add and subtract, similar to how two swells in the ocean combine to form a larger wave. The scientists recombined the atoms' two paths at the end of their flight so their waves would interfere, then measured where the atoms arrived. The arrival locations are extremely sensitive to phase shifts, which change where the peaks and troughs of the waves land.

The researchers placed a 1.25-kilogram hunk of tungsten at the top of the vacuum chamber. To isolate the Aharonov-Bohm effect, the scientists repeated the experiment with and without the mass, as well as with two different sets of launched atoms, one close to the mass and the other lower in altitude. Each of those two groups of atoms was divided into superpositions, with one path being closer to the mass than the other, separated by approximately 25 centimeters Other groups of atoms with superpositions spread over shorter distances completed the crew. 

Comparing how different sets of atoms interacted, both with and without the tungsten mass revealed a phase shift that was not caused by gravity. That change was caused by time dilation, a feature of Einstein's theory of gravity, general relativity, that causes time to move more slowly when it is close to a massive object.

The two theories underlying this experiment, general relativity and quantum mechanics, do not coexist well. Scientists are at a loss as to how to combine them to describe reality. So, according to Guglielmo Tino of the University of Florence, who was not involved in the new study, "probing gravity with a quantum sensor is really one of... the most important challenges at the moment" for physicists.

Chris Overstreet et al. Observation of a gravitational Aharonov-Bohm effect. Science. Vol. 375, January 14, 2022, p. 226. doi: 10.1126/science.abl7152.


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