Celebrating Day 1 of the Gravitational Wave Era

A long time ago in a galaxy far, far away two black holes with combined masses about 65 times that of the Sun violently merged. That event caused spacetime ripples—gravitational waves—which propagated outward for hundreds of millions of years, passing by Earth just a short while after people had built instruments sensitive enough to detect and measure the slight passing disturbance. 

We didn't have to invest billions of dollars, decades of time, and an unfathomable amount of people-years to build apparatuses sensitive enough to listen for gravitational waves. And there are plenty of alternative paths of humankind where a General Theory of Relativity would not have been developed ever, making the predictions it did. Yet, 100 years ago Einstein did develop this theory and we did invest the time and energy to test this theory. And today, as we have learned, nature rewarded this effort big time.

From ScienceMag.com

This detection is a triumph of human creativity, ingenuity, perseverance, and curiosity. It will be celebrated forever, I suspect, as one of the crowning achievements of modern science. Not just of theoretical physics but of worldwide scientific projects and all that those entail, from the physical to the bureaucratic. This is a technological feat beyond comprehension. The amount of motion detected due to that event is comparable to the change of distance to the nearest star by a length the thickness of a human hair. The engineers, computer scientists, technicians and statisticians have pushed the boundaries of precision and control of macro systems on a micro scale. 

_“This detection is a triumph of human creativity, ingenuity, perseverance, and curiosity. It will be celebrated forever, I suspect, as one of the crowning achievements of modern science.” _

The list of things we learned to today are staggering. Gravitational waves do exist (they were inferred from pulsar timing decades ago, but are now directly detected). Gravitational waves appear to travel at the speed of light. Black holes do exists and they spin (the astrophysical evidence has been incontrovertible for years but we now have a GR-only confirmation). Large black holes do exist in binaries and merge within a Hubble time.

Surprisingly such BH-BH mergers were not the most likely events expected in the LIGO/VIRGO frequency range. Instead neutron star-neutron star (NS-NS) mergers were thought to be the prime candidates: we know from radio observations that NS-NS systems are plentiful throughout our own Galaxy so it was natural to extend what we knew (from the electromagnetic view) to what we surmised (in the gravitational wave spectrum). Indeed NS-NS mergers may be more a more common event in the universe but big BH-BH mergers are “louder” and so the volume we probe is much larger.

Herein lies the only real “disappointment” in today's announcement: unlike NS-NS or NS-BH mergers, BH-BH mergers are unlikely to produce detectable electromagnetic signatures (there's just not that many baryons around). And so, while a great many of us astronomers had been contemplating joining this party, this event was VIP only. That is, the follow-on science lives almost entirely in the domain of physicists analyzing waveforms and performing supercomputing experiments rather than astrophysicists studying light.  The next gravitational wave event may be more abiding.

As VIRGO and LIGO have not yet reached their design sensitivities, the next few years may prove to be one of the wildest intellectual rides of our lives. Nature clearly has more surprises in store for us. But for now, let's celebrate this glorious beginning, day 1 of the gravitational wave era.