Physics in the region where quantum theory meets gravity is being explored by new research into the link between a particle's intrinsic spin and Earth's gravitational field.

Two theoretical pillars form the basis of our understanding of physics. The first of these is quantum field theory, which forms the basis of the standard model of particle physics. The second is general relativity, which Einstein determined how gravity works. Both pillars have passed a variety of rigorous tests and a wide variety of predictions have been brilliantly confirmed. However, their apparent mismatch points to a more fundamental truth. The lack of studies examining events at the nexus between quantum physics and gravity makes it difficult to reconcile these theories.

Now, a group of scientists led by Dong Sheng and Zheng-Tian Lu from the University of Science and Technology of China (USTC) has filled this gap by conducting a highly sensitive search for interactions between a particle's intrinsic quantum spin and the Earth's gravitational field. Although no evidence for this interaction has been discovered, the research has produced important limitations that have implications for the origin of the universe's matter-antimatter asymmetry as well as the existence of speculative natural forces.

Thanks to Dirac's integration of quantum physics with special relativity, intrinsic spin is essentially a purely quantum form of angular momentum that does not involve the physical spin of a particle. In contrast, general relativity, a classical theory that describes only the angular momentum resulting from the rotation of large, heavy objects, is used to explain gravitational fields. What is the relationship between quantum spin and gravitational field? This question is still unanswered.

**The USTC team set up an incredibly sensitive experiment to determine whether the energy associated with the spin of an atomic nucleus depends on the spin's orientation with respect to Earth's gravitational field.**

Consider the equivalent case of a nuclear spin in a magnetic field: the magnetic moment of the spin causes its energy to depend on the direction of the spin relative to the field. The Zeeman effect, which forms the basis of both magnetic resonance imaging (MRI) and nuclear magnetic resonance, is a phenomenon. It results in precession, a distinct frequency known as the Larmor frequency that causes spins bent from the axis of the magnetic field to wobble like a top. Similarly, if there is a spin-gravity coupling, the spins will also be precessed in a gravitational environment.

In the Earth's gravitational field, the spins would spin at a frequency of about 10 nHz if gravity directly applied the force it exerts on the mass to the spin.

This figure is almost one-thousandth of our planet's daily rotation rate, and is 10 billion times smaller than the usual nuclear Larmor frequency in Earth's magnetic field. These contrasts highlight the enormous challenges faced by the USTC team. In order to identify a potential spin-gravity interaction, it was particularly important to understand and tightly regulate the systematic inaccuracies caused by magnetic fields and gyroscopic events associated with the Earth's rotation.

The method used by the USTC team used a spin-polarized gas composed of xenon-129 and xenon-131 isotopes. In an applied magnetic field, the researchers simultaneously measured the nuclear spin precession frequencies of the two isotopes.

To reduce systematic errors caused by gyroscopic effects, the direction of this field was carefully oriented parallel to the Earth's axis of rotation. By dividing the two precession frequencies, the team definitively canceled out the effects due to the magnetic field. This frequency ratio was monitored as the direction of the magnetic field was repeatedly changed, and the differences between the ratios for the two different field directions were calculated. This difference is first-order proportional to the amount of precession caused by non-magnetic factors such as the torque exerted by gravity on the spins. After careful examination of the data, the researchers found no indication of a spin-gravity interaction.

The USTC experiment is particularly sensitive to the force of gravity to bind to neutron spins due to the structure of xenon-129 and xenon-131 nuclei. The measurements placed the tightest constraint on any intrinsic spin coupling with gravity. The determined neutron limit reduces the previous restrictions by 17 times, surpassing the electron and proton restrictions by 400 and 6000 times, respectively. The experiment is sensitive to spin precession frequencies that are a hundred times lower than Earth's rotational speed.

It is impossible to tell the difference between the spin-gravity interaction that the USTC experiment is looking for and a long-range force mediated by a strange boson like an axion.

A reasonable option for explaining dark matter is action, a hypothetical particle predicted by numerous theoretical extensions of the standard model. Earlier limitations on the strength of certain axion-mediated forces, as well as severe constraints identified from astrophysical data, have been significantly exceeded by the USTC results.

It is of particular interest that the USTC experiment investigates a spin-gravity interaction that deviates from the symmetry-corresponding parity (P) and time reversal (T) fundamental symmetries upon reflection of the coordinate axes over the origin [6]. According to quantum field theory, interactions that violate T symmetry will also break the combined CP symmetry, where C means charge conjugation or conversion of a particle into an antiparticle.

The genesis of the universe's matter-antimatter imbalance is a longstanding physics puzzle, and the missing piece is a currently unidentified source of CP violation. Investigations for CP-violating effects in neutrino physics as well as permanent electric dipole moments of electrons and other fundamental particles violating CP have been fueled by this enigma. The investigation of spin-gravity interactions has become much more challenging with the possibility that gravity may violate CP symmetry.

**Important theoretical work that began shortly after Einstein developed general relativity revealed that the theory could be fundamentally changed by incorporating internal spin into its framework.**

It can be assumed by analogy that gravitational effects on orbital angular momentum will have equal effects on spin, since intrinsic spin is ultimately a form of angular momentum.

This idea marks a challenging experiment. According to general relativity, large rotating masses will move space-time with them as they spin. Gyroscopes are pressed as a result of this frame drift observed, for example, by the Gravity Probe B expedition. The sensitivity of the USTC experiment is still many orders of magnitude away from detecting spin precession caused by frame drift. However, there are empirical suggestions that such a test may be possible in the future.

Source: physics.aps.org/articles/v16/80

📩 17/05/2023 11:35