To make a perfect mirror, physicists confront the mystery of glass
Quanta Magazine said:The Laser Interferometer Gravitational-Wave Observatory can sense movements thousands of times tinier than the width of an atom partly because of the instrument’s near-perfect mirrors. The mirrors bounce laser beams back and forth down the arms of LIGO’s L-shaped detectors. Changes in the relative lengths of the arms reveal when a gravitational wave flutters past Earth, stretching and squeezing space-time.
They’re nothing like regular mirrors. In your bathroom mirror, light reflects off metal, which has glass in front of it merely for protection. But LIGO’s 100-kilowatt laser would fry any metal. Instead, its mirrors are made entirely of glass.
Normally, glass isn’t reflective. Metal reflects because light waves shake its freely moving electrons, which absorb and reemit photons in the process. Glass, by contrast, lets most light pass through because its electrons stay within their atoms and don’t interact much with light. But LIGO makes mirrors out of glass using a trick first invented in 1939. The mirrors consist of 70 layers of glass that alternate between silicon oxide glass (or “silica,” the material of most glass) and tantalum pentoxide (“tantala”). Each layer reflects a small fraction of the light. The thickness of each layer is chosen with exquisite precision so that, for the exact wavelength of LIGO’s laser, all the reflections constructively interfere, adding up to a mirror that’s 99.9999% reflective.
LIGO’s mirrors are imperfect, however, because of a strange form of noise that is baked into glass, a mysterious substance in general. Glass consists of atoms or molecules that are haphazardly arranged like those in a liquid yet somehow stuck, unable to flow. Physicists believe that the noise inherent in glass comes from small clusters of atoms switching back and forth between two different configurations. These “two-level systems” ever so slightly change the distance laser light travels between LIGO’s mirrors, since the surface of each glassy layer shifts by as much as an atom’s width.
“LIGO at this point is literally limited by that,” said Frances Hellman, a glass specialist at the University of California, Berkeley and a member of the 1,000-person LIGO scientific team. Despite the detectors’ “astonishing vibration isolation, damping, all kinds of stuff that has led to the extraordinary sensitivity,” Hellman said, “the one thing they haven’t been able to get rid of are these funny little atomic motions in the mirror coatings.” Given the thousandth-of-an-atom amplitude of the gravitational waves LIGO is looking for, the atomic motions are a big problem.
There’s hope, though. Fueled by recent theoretical insights about the nature of glass, Hellman’s group and others are racing to find more perfect glass to use in LIGO’s mirrors. Advanced LIGO Plus, the next iteration of the experiment, slated to begin in 2024, will require mirrors that are less than half as noisy as the current ones. In conjunction with other upgrades, this improvement will translate into seven times more gravitational-wave detections — approximately one every few hours.
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