Theorists had already known the force they were seeking would be weak and could only be detected over very long distances. So the scientists needed a creative way to look for it. They needed to find a place where tons of electrons were crowded together to produce a stronger signal.
"Electrons have a big magnetic moment," Hunter said. "They align better with the Earth's magnetic field, so they are the obvious choice." Anything that nudges the spins of electrons that line up with the Earth's magnetic field will change the energy of those spins by a small amount. [50 Amazing Facts About Planet Earth]
So the Amherst and University of Texas team decided to use the electrons that are in the mantle of the Earth, because there are a lot of them — some 10^49. "People before prepared samples of spin-polarized neutrons and such," Hunter said. "Their source was close, and controllable. But I realized that with a bigger source you could get better sensitivity."
The reason is that even though only one in about 10 million mantle electrons will align their spin to the Earth's magnetic field, that leaves 10^42 of them. Even though it's not possible to control them the way one would in a lab, there are plenty to work with.
The scientists first mapped out the spin directions and densities of electrons inside the Earth. The map was based on the work of Jung-Fu Lin, associate professor of geoscience at the University of Texas and a co-author of the new paper.
To make the map they used the known strength and direction of the Earth's magnetic field everywhere within the planet's mantle and crust. They used the map to calculate how much influence these electrons in the Earth would have had on spin-sensitive experiments that were done in Seattle and Amherst.
The Amherst team then applied a magnetic field to a group of subatomic particles — neutrons in this case — and looked closely at their spins. The Seattle group looked at electrons.
The change in the energy of the spins in these experiments depended on the direction they were pointing. Spins rotate around the applied magnetic fields with a distinct frequency. If the electrons in the mantle are transmitting some force that affects them, it should show up as a change in that frequency of the particles in the lab.
Besides narrowing the search for new forces, the experiment also pointed to another way to study Earth's interior. Right now, models of Earth's interior sometimes give inconsistent answers as to why, for example, seismic waves propagate through the mantle the way they do. The fifth force would be a way to "read" the subatomic particles there — and might help scientists understand the discrepancy. It would also help geoscientists see what type of iron is down there and the actual structure it has. "It would give us information that we mostly don't have access to," Lin said.
Editor's Note: This article has been updated to correct the last name of physicist Larry Hunter.
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