![]() These comparisons illustrate the daunting challenges the USTC team faced. That value is more than 10 billion times smaller than the typical nuclear Larmor frequency in Earth’s magnetic field and is roughly a thousandth of our planet’s rotation rate of once per day. If gravity coupled directly to spin with the same strength that it couples to mass, spins would precess at a frequency of about 10 nHz in Earth’s gravitational field. Similarly, if a spin–gravity interaction existed, spins would precess in a gravitational field. ![]() It causes spins tilted from the magnetic-field axis to precess-wobble like a spinning top-at a characteristic frequency called the Larmor frequency. This phenomenon, known as the Zeeman effect, is the basis for nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). Consider the analogous case of a nuclear spin in a magnetic field: the spin’s energy depends on its orientation relative to the field because of its magnetic moment. The USTC team developed an exquisitely accurate experiment to test whether the energy associated with the spin of an atomic nucleus depends on the spin’s orientation relative to Earth’s gravitational field. So how does a quantum spin interact with a gravitational field? That question remains open. By contrast, gravitational fields are understood through general relativity: a classical theory that describes angular momentum arising only from the rotation of large, massive bodies. ![]() Intrinsic spin is a purely quantum form of angular momentum whose essence does not involve the physical rotation of a particle its explanation emerges from Dirac’s unification of quantum mechanics and special relativity. Although no evidence for this interaction was found, the search yielded strong constraints that have implications for the existence of hypothetical forces of nature and for the origin of the matter–antimatter asymmetry of the Universe. Now a team of researchers from the University of Science and Technology of China (USTC), led by Dong Sheng and Zheng-Tian Lu, has stepped into this breach by searching for an interaction between a particle’s intrinsic quantum spin and Earth’s gravitational field with unprecedented sensitivity (Fig. The path toward reconciling these theories is obscured by the dearth of experiments probing phenomena at the intersection of quantum physics and gravity. ![]() Yet they are seemingly irreconcilable, hinting at a deeper truth. Both pillars have withstood numerous stringent tests and have had myriad predictions spectacularly confirmed. And the second is Einstein’s theory of general relativity, which describes the nature of gravity. ![]() The first is quantum field theory, which underpins the standard model of particle physics. Our understanding of physics is supported by two theoretical pillars. Researchers from the University of Science and Technology of China have used this difference in precession frequency to look for signs of an interaction between the spins and Earth’s gravitational field (illustrated by the light blue shading). APS/Carin Cain Figure 1: In an applied magnetic field (green arrow), nuclear spins of xenon-131 (blue) and xenon-129 (purple) wobble, or precess, at different frequencies (indicated by the single- and double-headed black arrows). ![]()
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