For me, as a geophysicist with a fondness for geology, “particle” means the diameter of individual grains in a sedimentary rock. Particle physicists, however, ponder even smaller particles. And they think there may be an additional force that explains how such particles interact.
Subatomic particles have a property called spin, which is a quantized and directional form of angular momentum intrinsic to the particle. The standard model of particle physics predicts the exchange of special particles between ordinary fermions (particles with half-integer spin, like electrons and neutrons) to result in a force called long-range spin–spin interaction. Spin-spin interactions behave differently from interactions predicted by electromagnetism.
Investigations of spin-spin interactions have involved a laboratory source of polarizing neutrons and electrons, such as by shining polarized light through a cell containing helium and rubidium atoms, and seeking a response in a detection apparatus.
The detection apparatus, based around a co-magnetometer, compares the spin precession frequency of mercury and cesium atoms about a magnetic field; that is, the angular frequency at which their spin axes change orientation. The Cs precession is used to fix the magnetic field at a constant magnitude. The Hg frequency is observed when the orientation of the magnetic field is changed relative to that of Earth.
The frequency is measured when the apparatus is positioned such that the magnetic field has a large northerly component. Then a 180° rotation gives the magnetic field a southerly component. If long-range interactions exist, the Hg frequency would change as the source is flipped, whereas no such change would occur due to the magnetic field alone.
Bringing things down to Earth
Particle physicist Larry Hunter of Amherst College and geophysicist Jung-Fu Lin of the University of Texas at Austin had the idea to use Earth as a source of spin-polarized electrons instead of the laboratory source. Their results are published in Science.
Our planet’s magnetic field causes electrons in the iron-rich mantle to become spin-polarized: the direction of their spin is no longer completely random. That’s an advantage because the number of polarized “geoelectrons” exceeds the number in a laboratory source by a factor of at least 1017. And while the laboratory source is located less than a meter from the detection apparatus, a geoelectron is located several thousand kilometers away.
Of course, the mantle as an electron source cannot be flipped around: rather, the detector apparatus is rotated to see whether a change in Hg precession occurs. Hunter and team created a map of polarized electron-spin density and direction within the earth based on measurements of electron spins in minerals at high temperatures and pressures. They combined this with information regarding the magnitude and direction of Earth’s magnetic field between core and surface to reach an indication of interactions that might be expected at the laboratory in Amherst, Massachusetts.
Certain spin-spin interactions fall off in inverse proportion to the separation r between particles. Normal dipole-dipole interactions due to magnetic interaction fall off as the inverse cube of r, much faster than spin-spin interaction. This means that electrons deep inside the earth are so far away that magnetic interactions are difficult to detect, while detection of spin-spin interactions is most sensitive to potential that scales as 1/r.
Results and bounds
Alas, no Hg signal was detected in Hunter’s study, but it did establish limits on possible interactions lower than those established in the laboratory. For the 1/r potential, there is another potential to which to compare: gravity scales as 1/r as well. That places an upper bound on the strength of the spin-spin force, which is a million times smaller than gravitational attraction between a neutron and an electron.
“What’s exciting is that some theorists have suggested . . . that gravity may not just be the interaction between masses but also depends on spins,” says Hunter. Placing bounds on possible spin interaction provides the sensitivity to investigate things smaller than ordinary gravity.
Further, if increasingly sensitive instrumentation does reveal a signal, particle physicists would be thrilled to learn about a new force of nature. Geophysicists would be excited to determine iron concentrations in Earth’s lower mantle based on the orientation of a detector on its surface.
“There is a low probability that we’ll actually discover the [long-range spin-spin] force. But if we do, the payoff would be enormous,” concludes Hunter.
*Assuming both physicists are vectors.
