It has been about 1.3 billion years since those two black holes collided. So that’s how long it took from the time the actual event occurred to the moment, September 14, 2015 at 5:51 a.m. EDT, when we humans received our first clear empirical evidence that gravity waves exist — a cool century after Albert Einstein first predicted them.
Some people have reacted to this discovery, to the incredibly massive scale of the event itself both in time and space, by pondering how insignificant humans are in this universe. Others have responded the opposite way: As far as we know, we are the first beings in the Universe to appreciate gravity waves.
But what about that qualifying phrase “As far as we know”? While that expanding wavefront was passing galaxy upon galaxy over the course of 130 million centuries, perhaps sentient creatures on other planets detected the same signal, and realized, just as we did, what it is. They might have thought, just as some of us have thought: “As far as we know, we are the first beings in the Universe to appreciate gravity waves.”
We can defined the Gravity Wave Index, or GWI, of any universe as the number of independent such discoveries per billion years for any given black hole collision. The GWI for our universe may very well be in the thousands, or even millions, depending on how many planets out there contain sentient species.
But we know for certain now that it is at least about 0.769.