Earth's mantle helps hunt for fifth force of nature

Hey CERN, think you've got a big particle detector? Try using the entire Earth to hunt for a new fundamental force of nature.

So say Larry Hunter of Amherst College in Massachusetts, and colleagues. They have created a map of the spins of electrons deep within the Earth's mantle, which could be used to reveal the as-yet-unseen force as well as the strange particles ? known as "unparticles" ? that might carry it.

Such an experiment could also yield new insights into the planet's lower mantle, hundreds of kilometres below the surface.

We currently know of four fundamental forces: gravity, electromagnetism and the strong and weak nuclear forces. The hypothetical fifth force can be thought of as a version of magnetism that does not weaken as quickly with distance.

Unparticle exchange

Electromagnetic fields are produced when two particles exchange virtual photons that pop in and out of existence. But some physicists think another kind of virtual particle could also be exchanged, giving rise to a fifth fundamental force.

One candidate is the unparticle, a mysterious entity dreamed up several years ago. It has an unusual trait: its mass varies depending on the way we measure it, due to a property called scale invariance. As a result, unparticle exchange would not drop off as quickly as electromagnetism with distance ? potentially giving rise to measureable long-range effects.

Theory says this fifth force should tweak the amount of energy needed to flip the spin of an electron or neutron, due to interaction with another particle far away. Detectors have been set up to measure the effect of particles a few metres away on the spins of neutrons in the lab, but those have so far come up empty.

Different tack

So Hunter's team suggest taking a different tack. "We realised you can get a much bigger source if you use the whole Earth," he says.

Extreme conditions in the planet's mantle can affect the spins of electrons in various minerals, and these in turn affect the Earth's magnetic field. So by looking at geomagnetic field data, the researchers were able to infer the electron spins and how these spins would interact with particles in the lab via the fifth force.

They combed the results of three previous laboratory spin experiments to look for signs that electrons in Earth's mantle had influenced what was measured ? and found nothing.

But that doesn't rule out the fifth force, says Hunter. Rather, it tells us that this force must be very weak. Indeed, it suggests that any interaction between an electron in the mantle and a neutron in the lab via this weird new force must be a million times weaker than their gravitational attraction.

That's a useful new constraint on the theory, which originally said the two forces should have similar strengths, says Hunter, and means newer hunts for the force will have to be much more sensitive.

Incredible gift

"We often spend a decade struggling to get a factor of 10 improvement in a measurement," says Hunter. "This was like an incredible gift, as we didn't even have to do a new experiment."

It might sound like a long shot, but Hunter reckons it's worth it ? since a positive result would be evidence of a fifth force. "It's one of those low probability but high pay-off activities," he says. "Boy, if you see something, it really is exciting."

The experiment also raises another possibility: if future, more sensitive experiments reveal the fifth force, it could allow geophysicists to map out the electrons in the lower mantle in more detail by looking at how they influence particles on the surface. "You could set up detectors at various places on the Earth and probe down," says Hunter.

Journal reference: Science, DOI: 10.1126/science.1227460

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