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The Hunt for the Fifth Force of Nature Deep Within Earth's Guts

Meet the long-range spin-spin interaction.

Quick refresher: there are four known fundamental forces of nature: the strong force, the weak force, electromagnetism, and gravity (though electromagnetism is sort of the same thing as the weak force). Most of the stuff we deal in day to day experience has to do with electromagnetism and gravity: light, heat, electricity, not flying off into space, etc. The others have to do with stuff going down at the atomic and subatomic levels, like radioactive decay and holding atomic nuclei together.


We're still learning about all of these things, of course; we've yet to even observe the particle that gives us gravity, the graviton. But in a universe that is still super weird and full of unknown stuff, another force would hardly be unwelcome. Naturally, this is a thing being considered by some particle physicists. One of these fifth forces is called the long-range spin-spin interaction and, as Einstein might put it, it's spooky. That is, it acts at a distance, linking or coupling subatomic particles with other subatomic particles hundreds or thousands of miles away.

In a paper out today in Science, researchers describe how this theorized force could help us figure out more of what's happening in the guts of planet Earth, if nothing much else. The thing is that a lot of what happens in those guts is still a mystery--such as how concentrations of iron and other minerals vary with depth--revealed indirectly and imperfectly through Earthquake monitoring and lab-based modeling. There is no Hubble telescope for the inner-space of Earth, which is kind of a weird thing to consider.

The general idea of the new research, led by Amherst College's Larry Hunter, is that some electrons making up minerals in the Earth's mantle should align themselves according to the planet's magnetic field. These are called geoelectrons, and the team created a computer model describing theorized densities and orientations of these electrons. Hunter et al's next task was putting the model to experiment, with the help of a new device designed to look for interactions between these geoelectrons and particles on Earth's surface.

The long-range spin-spin interaction in which the spin-sensitive detector on Earth’s surface interacts with geoelectrons (red dots) deep in Earth’s mantle. Credit: Marc Airhart and Steve Jacobsen.

Said task was basically to examine how the energy of surface particles changed as their orientation to Earth's interior change. "We know, for example, that a magnet has a lower energy when it is oriented parallel to the geomagnetic field and it lines up with this particular direction — that is how a compass works," says Hunter. "Our experiments removed this magnetic interaction and looked to see if there might be some other interaction with our experimental spins. One interpretation of this 'other' interaction is that it could be a long-range interaction between the spins in our apparatus and the electron spins within the Earth, that have been aligned by the geomagnetic field. This is the long-range spin-spin interaction we were looking for."

Sadly, they didn't find it. But the researchers were able to put a lower boundary on the amount of force the spin-spin interaction should exert: less than one millionth the strength that gravity exerts on the same particles. Considering that gravity is already the weakest force by a huge degree, that makes the spin-spin interaction, like, nothing. But it is a rather spooky nothing, which is always welcome.

Top image: University of Chicago

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