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Slight Twitches in Atomic Timekeeping May Reveal Dark Matter

How the GPS network might become a giant detector for cosmological defects.

​It may be possible to detect dark matter fields using networks of atomic clocks. As described in a n​ew paper published in Nature Physics, a highly subtle and still ambiguous variety of dark matter could cause time glitches of sorts in the quantum oscillations that ultra-precise atomic clocks depend on. As the GPS satellite network d​epends on such precision, we happen to already have a de facto detector orbiting the Earth.

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There's really no other scientific instrument in existence with the precision of an atomic clock, which can keep time with a fractional inaccuracy of around 0.0000000000000​001 percent. Time is kept by observing the frequency of microwave radiation emitted as electrons change energy levels in an atom of usually caesium or rubidium. A second ​is defined as 9192631770 cycles.

Accuracy in an atomic clock is maintained by protecting these quantum oscillations from environmental noise and other background perturbations. Time itself does not get more real or objective, nor measurement more precise.

"It is natural to ask if such accuracy can be harnessed for dark-matter searches," Andrei Derevianko and Maxim Pospelov, physicists at the University of Nevada and University of Victoria respectively, write in the current study. Indeed, atomic clocks are ​a​lready used to hunt for the sorts of irregularities in the fundamental constants of nature—such as the mass and charge of an electron, or the speed of light—that might introduce some unanticipated disruption.

The simulated effects of a dark matter field on synchronized atomic clocks. Image: Andrei Derevianko and Maxim Pospelov

So-called light dark matter fields might also introduce a disruption. This is a very different variety of dark matter than the usually assumed WIMPs (weakly interacting massive particles), which are actually very heavy particles that just don't experience the same basic forces of physics, like electromagnetism, as the non-dark universe. That's what all of these unde​rground dark matter detectors are looking for, but the actual nature of the stuff is hardly as settled as it might sometimes seem.

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Light dark matter fields would appear "dark" because their constituent particles have so little mass. This mass is so small that even the normal rest mass-energy of a particle—its "internal" energy—is smaller than the quantum fluctuations that fill up empty space (vacuum energy). It's a strange way of existing without really existing, like a signal buried in background noise.

These fields would be considered topologica​l defects. As the early universe cooled from its hyper-energetic, hyper-dense post-Big Bang state, all of the forces of nature sort of crystallized or condensed, like water vapor meeting the side of an icy cold glass of water. Our whole existence is just those beads of condensate; heat things back up and all of the forces and all of the mass our universe is built upon turn back into a featureless blob of pure energy.

This blob would have been what's known as symmetric—the same for everyone and everything everywhere—and when all of the physics we know and love condensed out of it, the symmetry was broken. All we are is broken symmetry, provisional organization.

This form of dark matter is a bit different, however. As a to​pological defect, it's a relic or artifact of the symmetric universe that used to be. Imagine that glass condensate full-on freezing, turning into a chunk of (even more asymmetric) ice. A defect would then be like a trapped air pocket, a little bubble of dark-light matter. The so-far imagine forms of cosmological defects include strings, domain walls, and monopoles. They're all pretty weird.

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These defects are theorized to interact with our asymmetric world via "portals" (really). The exact nature of these portals might be found in some so-far undiscovered or undescribed physics, but that doesn't mean we can't observe their effects.

"The main consequence of the interaction is a temporary shift of all masses and frequencies inside the [topological defect]," Pospelov and Derevianko write. "Thus, the signature we are proposing to search for is a transient variation of fundamental constants." When the dark-light matter field is passing through some massive object, it acts as a new force. This force, according to the physicists, would change the frequency of oscillations in the clock.

The GPS system can register your whereabouts down to distances of less​ than a meter because of the intense synchronization of its satellites. This is maintained with help from atomic clocks. It's probably clear where this is going: Intensely synchronized clocks should be able to register when some mystery force is acting on one by, well, desynchronizing.

"Several networks of atomic clocks are already operational," the study explains. "Perhaps the most well known are the Rb and Cs microwave atomic clocks on-board satellites of the Global Positioning System (GPS) and other satellite navigation systems. We envisage using the GPS constellation as a 50,000-km-aperture dark-matter detector, with added capabilities due to the extensive terrestrial network of atomic clocks on the GPS tracking stations."

The proposal isn't unreasonable. As the authors note, the International Space Station is due to be fitted with an atomic clock setup in the near future, and optical clock synchronization networks already exist between laboratories in Europe. Who'd of thought that the same technology logging your location for Facebook could be used to hunt dark matter?