Detecting gravitational waves

On September 14, 2015, LIGO directly detected gravitational waves from a pair of merging black holes, a century after Einstein predicted their existence. The signal, GW150914, confirmed a key prediction of general relativity and marked the birth of gravitational-wave astronomy. This episode traces the slow buildup—from indirect hints in binary pulsars to the engineering feat of kilometer-scale interferometers—then explains how LIGO's laser interferometry senses tiny spacetime distortions. It also highlights the role of luck and teamwork, and it looks ahead to future detectors like LISA that promise even deeper insights into the violent, gravitational universe.

Context: From Einstein to Gravitational Waves

In this episode the timeline of gravitational waves begins with Einstein's general theory of relativity in 1915 and continues with his early, mixed conclusions about the existence of gravitational waves. The podcast notes that acceptance of these waves as real did not solidify until the mid-20th century, with the concept finally being embraced in the 1950s. The indirect evidence came from the discovery of a binary pulsar in 1974, where precise timing showed orbital decay consistent with energy loss by gravitational waves. Pulsars function as incredibly precise clocks, enabling measurements of orbital shrinkage to the millimetre per day, a testament to the subtlety of gravitational radiation. The host and guest emphasize that gravitational waves probe gravity in its strongest regime, a regime Einstein's theory has largely withstood, though mysteries remain in strong gravity scenarios.

"General relativity, Einstein's theory of gravity, has passed every test that it's been put to" - Cole Miller

From Idea to Instrument: The LIGO Detectors

To detect the waves directly, researchers built the Laser Interferometer Gravitational Wave Observatory, or LIGO, a pair of giant L-shaped detectors with 4-kilometre arms. The principle is simple in concept: split a laser beam, bounce it along two perpendicular arms, and recombine it so the beams cancel. A passing gravitational wave stretches space and alters the relative travel time, producing a tiny signal. The podcast explains that LIGO opened in 2002, but its true potential was unlocked after upgrades to boost sensitivity. The field owes much to pioneers like Joe Weber and Rainer Weiss, whose ideas culminated in a detection method that made use of lasers and precision metrology.

"The chances of seeing something as strong as was seen were not good. The community got extremely lucky to see it as well" - Cole Miller

The GW150914 Event: A Moment That Changed Science

On September 14, 2015, the GW150914 signal appeared in the data from two detectors 3,000 kilometres apart in Hanford and Livingston, showing the characteristic chirp of two black holes spiraling together, merging and releasing a torrent of gravitational waves. The event was extraordinary not just for the astrophysics but for the rarity of a signal this strong during the start of LIGO's science run. The transcript notes that the detection was almost serendipitous: the detectors were not yet in full science mode, yet the data revealed a convincing, clear oscillation in spacetime. The detection earned its creators a share of the 2017 Nobel Prize in Physics and opened new avenues for understanding gravity, dense matter, and relativistic astrophysics.

"September 14th, 2015 will go down in history as the date that these waves were first directly detected" - Benjamin Thomson

Impact, Recognition, and the Road Ahead

The host notes that gravitational waves provide a new lens on the universe. General relativity has passed many tests, but gravity's weakness on everyday scales masks its strength in extreme conditions like black hole mergers and neutron stars. The GW150914 event catalyzed a new field, with subsequent detections of black hole and neutron star mergers enriching our understanding of gravity, matter at nuclear densities, and the behaviour of spacetime. The episode also discusses the Nobel Prize awarded to Weiss, Barish, and Thorne, and highlights the community effort behind such breakthroughs. Looking forward, plans for space-based detectors like LISA promise to extend this new window and to explore gravitational waves from other sources, unlocking a more complete, multi-messenger view of the cosmos.

"In the 2030s, researchers are looking to space with the laser Interferometer Space Antenna or LISA mission" - Benjamin Thomson