On September 14, 2015, LIGO caught a fleeting signal from two black holes spiraling together.
The episode opens by marking the 10th anniversary of the detection of gravitational waves by LIGO, framing it as the culmination of a cosmic story stretching back over a hundred years. Cole Miller from the University of Maryland provides a richly detailed account of how the story actually began with a stumbling start: Einstein himself made significant mathematical errors in his first gravitational wave paper of 1916 and even his corrected 1918 version contained mistakes. For decades Einstein oscillated, publishing papers both supporting and denying the reality of gravitational waves, and the scientific community did not reach consensus that they were real until the 1950s. The mechanism is elegantly explained: when two objects orbit each other, they lose energy as gravitational waves, which causes their orbit to gradually tighten. For binary systems involving neutron stars or black holes, this inspiral can take hundreds of millions of years before the objects finally merge. The first indirect evidence came in 1974 with the discovery of a binary pulsar system, where the orbital decay was measured with extraordinary precision using pulsar timing over years and decades. This matched the predictions of general relativity exactly and was considered strong indirect evidence for gravitational waves. But a direct detection, actually measuring the distortion of spacetime as a wave passes through Earth, required building instruments of almost incomprehensible precision. LIGO was designed to detect changes smaller than a thousandth the diameter of a proton across its four-kilometer detector arms. The climactic event was the merger of two black holes, a cataclysmic event that released more energy than all the stars in the observable universe emit in a given moment, all in the form of gravitational waves. When LIGO detected the signal on September 14, 2015, it opened an entirely new observational window on the universe. Miller explains that this detection does not just confirm general relativity but inaugurates a new era of multi-messenger astronomy, allowing scientists to study cosmic events through both electromagnetic radiation and gravitational waves simultaneously. The anniversary rebroadcast allows a new generation of listeners to appreciate how science builds across generations toward breakthroughs that seemed impossible to earlier researchers.
