In 2017, NASA's Fermi Gamma-ray Space Telescope connected gamma rays, the highest-energy form of light, with new cosmic messengers -- gravitational waves and high-energy neutrinos. For the first time, the discoveries linked these new signals to the one sky watchers have known for millennia -- light. First, gravitational waves and gamma rays were emitted from merging neutron stars. Fermi saw the first-ever light detected from a gravitational wave event. Then, just weeks later, Fermi connected a high-energy neutrino seen by the IceCube experiment at the South Pole to a black-hole-powered galaxy, which fires a jet of matter that emits both neutrinos and gamma rays. This is no overnight success story. The origins of both these breakthroughs span more than a century. As the 19th century closed, scientists worked to understand many new phenomena including radioactivity, and new forms of light-- X-rays and gamma rays. Light was expected to need a medium, called the "aether," in order to move through space, which meant its speed should change when measured in different directions on the moving Earth. Yet no changes were seen. Solving this puzzle led to Einstein's special theory of relativity, which assumed light in a vacuum moves at a constant speed that nothing can exceed. His theory formed a theoretical basis for particle physics... ...which in 1912 incorporated an unexpected source -- a rain of particles from space called cosmic rays. Einstein's general theory of relativity, his theory of gravity, regarded space-time as the fabric of the cosmos. Space-time tells matter how to move, and matter tells space-time how to curve. As scientists probed the subatomic realm, one type of radioactive decay suggested the presence of a new lightweight particle, dubbed the "neutrino." Later, Einstein and Nathan Rosen showed that accelerating masses can create gravitational waves that ripple across space-time. Following World War II, technological advances permitted new kinds of observations. In the mid-Fifties, neutrinos were detected for the first time. Richard Feynman showed that gravitational waves must move matter, which means they're detectable. In a few years, the first efforts to do so began. The 1960s brought the first gamma rays seen in space, the first neutrinos detected from the Sun's interior, and something new -- later called gamma-ray bursts, or GRBs -- was caught by satellites looking for banned tests of nuclear weapons. In 1971, Rainer Weiss conceived of a way to detect gravitational waves using lasers, one of the roots of LIGO. 1987 delivered the brightest supernova in nearly 400 years. Three experiments caught neutrinos from the star's collapse. Instruments on balloons saw gamma rays from radioactive elements in the explosion's debris. The 1990s and 2000s brought: new satellites for exploring the gamma-ray universe; the construction and first operation of LIGO; and AMANDA, a neutrino detector built under the ice at the South Pole. In 2005, NASA's Swift satellite showed that short gamma-ray bursts likely come from merging neutron stars. Soon after, NASA launched Fermi, providing our best-ever view of the gamma-ray sky. AMANDA morphed into IceCube, which was completed in 2010. It monitors a cubic kilometer of ice under the South Pole for neutrinos. The same year, LIGO shut down for years of upgrades. IceCube reported more than two dozen high-energy neutrinos -- likely arrivals from beyond our galaxy. In 2015, the upgraded LIGO saw the first gravitational waves. The source: merging black holes over a billion light-years away. And in 2017, gamma-ray counterparts accompanied both: a gravitational wave event; and a cosmic neutrino source. Multimessenger astronomy -- and its promise of greater insight into the most powerful processes in the universe -- has arrived. [Music] [Music] [NASA Astrophysics] [Beeping] [Beeping] [Beeping]