In 2015, The LIGO Scientific Collaboration made the historic discovery of two coalescing black holes and the waves of curved space, gravitational waves, that erupted out of this cosmic explosion. And then, just two years later, LIGO’s Rai Weiss, Kip Thorne and Barry Barish went on to win the Nobel Prize in Physics.

The discovery made by LIGO's two giant detectors in Louisiana and Eastern Washington State was the culmination of a half-century-long search.

Kip Thorne knew as early as 1978 that according to theory, LIGO would have to reach a sensitivity of 10-to-the-minus 18 meters in its measurement of the movement of the mirrors. That is one with 17 zeros in front of it. (The gravitational waves sweep across the observatories, stretching and squeezing the arms — with their mirrors at each end — infinitesimally. The measurement of that change in the distance between the mirrors is the measurement of the event’s signal.)

Theory also said there might well be an ultimate limit to LIGO's sensitivity.

It was Vladimir Braginsky, a Soviet scientist, and Kip, who were concerned that once LIGO achieved a measurement in the quantum world there was a barrier to extremely sensitive measurement that had stood firm for nearly a hundred years.

It was based on Werner Heisenberg's famous uncertainty principle of 1927, one of the pillars of quantum physics, which said that in a quantum world the precise continuous measurement of subatomic particles, such as photons, the particles of light, was impossible. Because, once the position of a particle was measured, the uncertainty of its momentum or movement must accordingly increase, and vice versa. These gravity waves are the elusive messengers from a realm of the universe that is invisible to our satellite cameras and Earth-based telescopes: The violent warped side. Warped such that LIGO is now finding almost one collision of black holes every day somewhere in the vast depth of space and time in the visible universe.

The gravitational waves are so faint, it was like seeing one strand of hair on the surface of the nearest star, Alpha Centauri, 25 trillion miles away.

The detection happened 40 years after Rai Weiss of MIT and Kip Thorne of Caltech began talking about Rai's concept of using mirrors and lasers to make this discovery. Kip and Rai knew very well Albert Einstein's prediction of gravitational waves more than a half century earlier, but Kip at first doubted there would ever be a technology sensitive enough to detect them. Because, despite the giant detectors that a young LIGO collaboration finally began building in the 1990s, which they called LIGO, and which were upgraded to Advanced LIGO in 2015, despite their purest glass mirrors ultimately weighing 40 kilograms or almost 90 pounds, and their vacuum tube arms two and a half miles long for the state-of-the-art laser beams, the largest self-contained vacuum systems on the planet, LIGO's measurement of these waves was, in fact, a quantum measurement. A measurement in the subatomic world.

Bringing a measurement at the level of seeing one hair 25 trillion miles away to distances on Earth itself would be like measuring a subatomic particle 10,000 times smaller than a proton in the nucleus of an atom. A quantum measurement, indeed

So all continuous measurement here is uncertain. Everything is probability. A cloud of probabilities to only estimate these specific qualities of the laser light measuring the gravitational waves.

Kip, Braginsky and others knew early on this might be a severe limitation to how sensitive the LIGO observatories could ever become. This inherent quantum uncertainty might always demolish, that was the term they used, the faintest information hidden in their laser light.

That is another way of saying it was an ultimate barrier to how far they could see into the universe. Because, the farther away a collision of two black holes, say 10 billion light-years away, rather than 1.4 billion, the even fainter the signal would be. So the far more sensitive LIGO's measurement would have to become to detect it.

In 2024, more than five decades later, LIGO succeeded in breaking this theoretically insurmountable barrier, known as the Standard Quantum Limit. This is the story of that astonishing achievement.