If the moon were to disappear instantly, how long would it take to quit experiencing measurement of its gravity?

We actually know this answer, thanks to the Laser Interferometer Gravitational-Wave Observatory (LIGO)! Gravity travels at the speed of light, so we on Earth would see the Moon’s gravity go away at the same time it vanished (about 1 second after it happened on the Moon).

On August 17, 2017 8:41 Eastern Daylight Time, the two LIGO centers (in Louisiana and Washington) detected a gravitational wave passing through Earth. You might know from Einstein that gravity bends space. Certain motions, like two objects orbiting each other, bend space in a way that looks like a wave. Picture the way a an object bobbing up and down in water generates waves going outward.

There are two problems with measuring gravitational waves. The first is that this distortion is tiny. Thus, astronomers needed to look for very dense objects making very small orbits. The candidates were thus black holes and neutron stars orbiting another black hole or neutron star. When a massive star (at least 8–10 times the mass of our Sun) dies, its core becomes either a black hole or neutron star as the rest of the star explodes in a supernova. if the core retains less than about 3 times the mass of the Sun, gravity will pull the core so tightly that electrons will be forced into the atomic nuclei, creating neutrons, and a quantum mechanical effect called neutron degeneracy pressure will be what stops gravity. Above about 3 times the mass of the Sun, nothing the core has left[1] stops gravity from compacting the core so much it becomes a black hole.

Anyway, black holes and neutron stars (we can call them ‘compact objects’ for shot) are the densest objects in the universe, and because they are small (a stellar mass black hole ‘s event horizon is about the size of a city, and a neutron star is not that much larger), another object can orbit it very closely, such that a ‘year’ might be seconds long. Normal matter will be shredded by tidal forces this close to a compact object, but another black hole or neutron star has its own gravity to hold it tightly against tides.

So, we have a binary compact object. It generates gravitational waves as the two objects orbit each other, and because that is energy going out, the two objects spiral together until they collide. We first measured this slow spiral in a binary neutron star in the 1970s, but gravitational waves from a collision weren’t seen until 2016. And that gets us to the second obstacle — how do you measure space warping when a physical ruler warps with space?

The answer is light. The speed of light in a vacuum is a constant. That means if space gets stretched between point A and point B, the light takes longer to go A to B, and if space gets compressed, light takes less time. Each LIGO facility is essentially two kilometers-long tunnels at right angles to each other with the air removed, with mirrors at the far end. A laser beam is split in two, sent down each tunnel to reflect off the mirror, come back to the central facility and combined. If the distance along one arm changes, the two light waves from the laser will be slightly out of sync when they recombine, and that can be measured — this is the ‘laser interferometer’ part of the name. This is still very hard to do, and one of the reasons that two observatories had to be built — an earthquake or a truck driving over a nearby road can create vibrations or other changes that rock the mirrors at one place… but not in the same way that the other observatory is moving. Only a cosmic event would trigger both observatories in a distinctive way in less than a second.

Okay, so back to the story now that you have the background. It’s 2017, and LIGO made its first confirmed gravitational wave detection last year. Based on analysis, that was two black holes on their final inspiral and collision. The direction was hard to pin down, because LIGO isn’t a camera. But the Europeans had brought their own gravitational wave observatory online, Virgo. The more observations of the same event, the better a position can be found. And in August, all three facilities were observing when LIGO detected a compact object merger.

At the same time, a gamma-ray telescope in orbit, Fermi, detected what we call a short-gamma ray burst. Gamma rays are highly energetic light that generally mean something big happened. Satellites to detect them were first launched to monitor nuclear testing, but we discovered that there was also astronomy to be done. Gamma-ray burst were occasional pulses of gamma rays from one spot in the sky, and it wasn’t clear what they were for a long time. Some of them were linked to visible light ‘afteglows’ in galaxies enough that they seemed to be some kind of big stellar explosion, and Fermi already had the software that when it saw a gamma-ray burst, a mailing list got an email that send the coordinates and time to look for an afterglow.

Based on the timing of the LIGO and Fermi detections, astronomers assumed that this was unlikely to be coincidence. The LIGO astronomers both asked the Virgo team to look to see if they saw anything so they could narrow down a position, and started trying to quantify what they saw. The results were ’two neutron stars — they weren’t massive enough to be black holes — spiraled in and collided, about 130 million light years away’[2]. Also, Virgo didn’t see the collision… which was actually useful as each observatory had two ‘blind spots’ where gravitational waves coming from a blind spot would change both arms of the observatory by the same amount. The LIGO observatories could then pinpoint which blind spot the source was in. And, it turned out that, yes, it was the same part of the sky that the Fermi gamma-ray burst was in. It seemed likely that the gamma-ray burst, the visible light afterglow, and the gravitational waves were all caused by two neutron stars colliding.

Because we saw the gamma-ray burst and the gravitational waves at the same time, despite them having to travel 130 million light years, that tells us that gravity and light move at the same speed in vacuum or near-vacuum. This was what was predicted, but it’s nice when observations confirm what you expect to happen.