The dead cores of two stars collided 130 million years ago in a galaxy somewhat far away.

Because of how violent the collision was, a gravitational wave was produced. Together with that gravitational wave, the star explosion’s light also travelled throughout space. On August 17, at 6:41 a.m. Eastern, they simultaneously touched down on Earth.

The incident sparked headlines across the globe as the beginning of “multimessenger astronomy.” For this time, astronomers had waited a generation. However, it was also the first time that gravity had been directly proven to move at the speed of light.

In order to answer that question, it is necessary to quickly review Albert Einstein’s general relativity, also known as the gravity theory, which was the first theory to predict gravitational waves.

“Absolute time,” as proposed by Isaac Newton, was disproved by Einstein. Newton believed that regardless of how we as mortals experience it, time advances everywhere at the same rate. It did not flinch. According to that logic, a second on Earth is equivalent to a second close to a black hole (which he was unaware of).

Newton also believed that gravity happened instantly. Distance was irrelevant.

It’s All Relative

But then Einstein showed that time is relative. It changes with speed and in the presence of gravity. One of the ramifications of that is that you can’t have simultaneous actions at a distance. So information of any kind has a finite speed, whether it’s a photon — the light-carrying particle — or a graviton, which carries the force of gravity.

General relativity specialist Jolien Creighton of the University of Wisconsin-Milwaukee and a member of the LIGO team that made the first detection of gravitational waves claims that there is a “speed of information” in relativity, or the fastest rate at which information can be sent from one point to another.

According to Creighton, an electron’s movement in electromagnetism alters the electric field, which propagates at the speed of light. The same principles govern gravity. When a mass is shaken, the gravitational field changes at the same rate as the gravitational wave.

Gravitational waves and electromagnetic waves both move at the speed of information, hence the reason why their speeds are equivalent is straightforward, according to Creighton.

There is also a simple way to visualize this. Imagine that the sun has just disappeared. Earth wouldn’t simply disappear into space. Earth would turn off and simultaneously take off in a straight path after eight minutes.

Soucre: blog.scientiststudy.com