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.

Already, the researchers have found some candidate glasses that might satisfy the design requirements, but they’re still hoping to discover a clear winner. “For a long time it was a slightly random approach,” said Iain Martin, a glass physicist at the University of Glasgow who is also part of LIGO. “Now we are in a position of being much more guided in our search.”

Hellman’s group is looking for something approaching “ideal glass,” a hypothetical phase of matter predicted decades ago. The molecules in ideal glass are theoretically packed together in the densest possible random arrangement, a perfectly stable state that has no two-level systems at all. Ideal glass, if it exists, would explain what’s happening in all glass; it would be the state that molecules in regular glass are trying to reach.

In 2007, the quest for ideal glass led the physicist Mark Ediger to invent a new glassmaking technique that produces much more stable glass than before. Instead of cooling a liquid until it hardens, as glassblowers have done for 4,000 years, Ediger and his team dropped molecules one by one onto a surface as if they were Tetris pieces, allowing them to find snug fits. A 2014 experiment by Hellman and her Berkeley team indicated that “ultra-stable” silicon glass created in this way has far fewer two-level systems than normal glass.