Listening to spacetime: how gravitational waves opened a new kind of astronomy

Gravitational waves gave scientists a second way, besides light, to observe the universe. They are tiny ripples in space and time, set off by violent events such as two black holes colliding. Three conversations trace how they went from Einstein's guess to working observatories and on toward the question of how the universe began.

Barry Barish, who shared the Nobel Prize in Physics for his contributions to LIGO, explains the history and the machine. Einstein saw by insight in 1916 that gravity should make waves, but the question stayed open until a conference in the late 1950s, when Richard Feynman's thought experiment showed the waves must carry energy and so must be real. Waves come from objects orbiting each other, like two black holes, and because space is extremely stiff the effect is tiny. LIGO splits a laser down two four-kilometer arms tuned so the beams cancel; a passing wave stretches one arm and squeezes the other. The change is about one part in 10 to the 21, which Barish puts at about a thousandth the size of a proton. Taming the Earth's shaking took four layers of shock absorbers and active noise cancellation. After about a decade of seeing nothing, a rebuilt detector with active seismic isolation caught its first black hole collision almost instantly in 2015.

Barish's central point is that the field is limited by technology, not by science. Before the pandemic shutdown LIGO caught about one event a week, and a detector ten times more sensitive would see ten times farther and a thousand times more volume, moving from astronomy to cosmology. He calls the result "a new kind of astronomy," and sees black holes, including one system of 140 solar masses too big to come from a single star, as an opportunity for unifying physics.

Michelle Thaller, an astrophysicist and retired NASA executive, describes the detector in nearly the same terms and recalls her joy at friends' success. She says the waves already show black holes and neutron stars colliding. Her biggest hope is the early universe: light could not travel freely until about 400,000 years after the Big Bang, and gravitational waves might reach further back. Yet she says physics cannot describe the first instant yet, and insists, "You have to be honest about what you don't know."

Brian Keating, an experimental physicist at UC San Diego, hunted the waves from cosmic inflation, which no LIGO could catch. His BICEP telescope looked for their imprint on the oldest light, the cosmic microwave background. In 2014 the BICEP2 team announced a detection before peer review, and the signal turned out to be galactic dust, a lesson in confirmation bias.

The three agree on how the detector works, that the waves carry information light cannot, and that the universe's first instant is still out of reach. They differ on the route there: Barish and Thaller picture better detectors gradually seeing further back, while Keating says inflation's waves can only be seen indirectly, through the cosmic microwave background. Their spoken numbers also vary, for example on the size of LIGO's measurement and the age of the first collision.

What to do with it: use Barish's fun-house mirrors or still pond to picture the effect, read The First Three Minutes, give a child a small telescope and a notebook, follow upcoming instruments such as LISA and the Simons Observatory, and treat big announcements with care, checking whether the team ruled out mimics.