In the wake of recent news that astronomers finally detected the space-warping boom of colliding neutron stars, measuring the merging of black holes might seem kind of old hat.
You might have moved on, but researchers are still picking through the data gathered from those previous ground-breaking thunderclaps. Now two teams of physicists used figures from the variety of gravitational waves to narrow estimates on just how fast we think gravity moves, and while their results aren't shocking, they are strangely comforting.
A few centuries ago, Isaac Newton assumed gravity's tug was instantaneous; a claim later Albert Einstein refuted by reasoning the force travelled at the speed of light. Going by Einstein's reckoning, space isn't just an empty stage for matter to move on, but is a major actor itself. Mass pulls on the very fabric of space, curving time and distance in such a way that objects accelerate towards one another.
Just as the speed of a massless particle of light in a vacuum is restricted by the Universe's upper speed limit, the massless distortions of spacetime would also be energy zipping along at top speed. Or, to be more precise, gravity moves at 299,792,458 metres per second, a rate we can just call c.
Of course you'd be a fool to bet against Mr. General Relativity himself, but good science demands that even geniuses need to be checked against reality and in spite of being intimate with Earth's strong grip, the force of gravity is kind of hard to measure.
“Until the advent of gravitational wave astronomy, we had no way to directly measure the speed of gravity,” Neil Cornish, a physicist from Montana State University, told Phys.org.
The numbers are pretty insane.
As objects dozens of times more massive than our Sun orbit one another thousands of light years away, they lose energy by making space ripple. In that final moment before finally colliding, that effort equals something like 10 times the amount of energy pouring from every star in the Universe. Mind blown? By the time it reaches us, each wave is ten thousand times smaller than a proton, and passes in just one fifth of a second. We rely on a network of 4 kilometre (2.5 mile) long light beams arranged at right angles to spot those signature distortions.
It might all sound simple in practice, but the technology behind the detectors – worthy of a Noble Prize – is about as cutting edge as it gets. The growing pool of data collected by these detectors is opening the way for scientists everywhere to dig for evidence on everything from hidden dimensions to the basic properties of space.
“The speed of gravity, like the speed of light, is one of the fundamental constants in the Universe,” says Cornish.
By comparing the exact timing of the gravitational waves as they hit different observatories across the globe, researchers can get a good idea of the wave's overall speed. Cornish's team of researchers combined the timings of the first three detections to narrow down the speed of the waves to between 55 and 142 percent of c. If enough detectors stay in top working order, this method could be used to calculate the figure to within just 1 percent of c by measuring just another five gravitational waves.
Before you start marking off the days on your calendar, another team made up of a small army of physicists used the burst of gamma rays captured from last month's neutron star collision to come up with their own estimate. Their method was a little more precise. Ok, a whole lot more precise.
They found the difference between the lightning flash of the gamma ray burst and the thunderclap of the gravitational wave was extremely close - within -3 x 10^-15 and 7 x 10^-16 of c. Close enough to call it a tie, really. To be fair, the previous team couldn't have predicted the neutron star collision, so hats off to them for going old school. Having multiple methods come to similar conclusions also gives us confidence we're on the right track, and that's pretty damn cool.