According to general relativity, gravity is not a force that pulls on matter through empty space. Instead, curved space itself pushes objects to fall toward Earth and pushes on matter at the planet’s surface. The adage goes, “Mass tells space how to curve and curved space tells matter how to move.”

The closest analogy in three dimensions is that matter makes a dent in elastic space-time. Planets orbit around the dent at just the right speed to keep stable orbits. Stationary objects tend to be pushed down the slope of the dent.

Using this concept of gravity, Einstein predicted that objects accelerating in space-time would generate gravity waves in much the same way as accelerating electric charges generate electromagnetic waves.

Gravity is a very weak force compared with electromagnetism. A small magnet can pick up an iron object against the gravity of the massive Earth. For this reason, gravity waves would be extremely weak and nearly impossible to detect unless the objects that generate them are extremely massive.

Approximately 1.3 billion years ago two small black holes, each with a mass of about 30 times that of our sun, orbited around each other. The motions caused gravity waves to spread out from the orbiting pair like ripples on a pond. Like the pond ripples, the waves lose force as they travel away from the source.

The energy that the waves carried away from the two orbiting black holes slowly caused the two orbits to decay, drawing the two black holes ever closer together.

Eventually they collided and combined in a supermassive gravitational explosion. The sequence of events was invisible and would have caused no notice except for distortion of surrounding starlight that passed through their event horizons.

When the collision took place, it converted, in a fraction of a second, about three times the mass of the sun into gravitational waves with a peak power output about 50 times that of the whole visible universe.

In September 2015, 1.3 billion years after the massive event, the extremely weak waves arrived at LIGO detectors, one in Hanford, Wash., the other in Livingston, La.

LIGO stands for Laser Interferometer Gravitational-Wave Observatory. Each of the two detectors uses interference of a split laser beam to measure the stretching and shrinking of space as gravity waves pass through it.

The split laser beam travels the length of two perpendicular tunnels, each 1 mile long. Mirrors at the two ends reflect the beam back to the split mirror, where they recombine and project onto the detector.

If the tunnels are exactly the same length the light cancels out. If a gravity wave passes through, it alternately stretches one arm and compresses the other, causing the beams to interfere and produce a rhythmic signal at the detector.

The effect is unimaginably small, about .001 the width of a proton. That speaks to the precision of modern physics as well as the ingenuity of the experiment.

Despite the distance between the two detectors and their independence from one another, the signals they recorded were nearly identical. Because the waves arrived at Livingston seven milliseconds before they arrived at Hanford, scientists knew that the signal originated in the Southern Hemisphere.