The well-worn phrase describes the quintessential quiet place, so silent that a listener can hear the tiny "toc" of a pin dropping. It's a good metaphor for the conditions required for certain physics experiments that are searching for the subtle indicators that would signal the discovery of radical new phenomena at work in the universe.
Physicists and astrophysicists are listening for the equivalent of a pin drop in the universe, the faint "toc" of a dark-matter particle hitting a detector. Dark matter is ubiquitous, streaming around–and indeed through–us all the time. Taken together, the combined mass of dark-matter particles provides the gravitational glue to keep our solar system from wandering off into inter-galactic space. But trying to detect the feeble signals of dark-matter particles on the earth's surface is like listening for a pin drop after a home run in the bottom of the ninth with the bases loaded. The background noise caused by billions of cosmic rays coming in to earth from outer space drowns out the pin drop of a rare dark-matter signal. To detect dark matter, physicists need the equivalent of Proust's cork-lined room, an experimental environment so free of background noise that they can discern dark matter's whispered signal.
The solution for dark matter experiments–and for searches for other rare processes with key implications for shaping the universe–is to go deep underground, where thousands of feet of rock provide shielding from the cosmic ray background. Thus sheltered, experiments can detect the signals of dark matter or the evidence that fundamental forces may ultimately combine. Deep underground, the pin drops that will change the way we see the universe will come through loud and clear.
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