LIGO · Gravitational waves
Can you hear space itself shaking?
That time, that place
Einstein had predicted it back in 1916: when massive things move violently, spacetime itself ripples, and the ripples spread outward as waves.
He also thought detection impossible. The signal was simply too small.
Why this question
Consider the scale.
You set two arms four kilometers long at right angles and measure how their lengths change. The change is about one ten-thousandth the diameter of a proton.
A passing truck, waves on a distant shore, faint tremors in the ground — all of these move things far more than that. The signal had to be pulled out from beneath overwhelming noise.
What was found
It took forty years and more than a billion dollars. At the outset many called it reckless.
One safeguard was to build not one detector but two. Only a signal appearing at both sites, three thousand kilometers apart and essentially simultaneously, would count — noise at one site would not show at the other.
On 14 September 2015, days after the upgraded instruments came online, a signal arrived. At both sites, seven milliseconds apart.
It was the ripple thrown off 1.3 billion years ago as two black holes spiraled together and merged. It had been traveling for 1.3 billion years. The observed signal lasted two tenths of a second.
Ripples in spacetime were a prediction and not measurable
The same waveform arriving at two detectors 3,000 km apart, seven milliseconds apart
Events that emit no light can now be observed. To seeing the universe we have added hearing it