We Just Heard the Spacetime 'Chirp' of Black Hole Rebirth
The LIGO gravitational wave signal of two black holes merging is depicted in this plot -- both LIGO stations detected the same event and the physics is clear: we've just witnessed the rebirth of a black hole.
The massive galaxy cluster Abell 2218 is filled with some stunning examples of gravitationally-lensed galaxies -- observational evidence of the effects of Einstein's theory of general relativity.
Today will go down in the history books as the day we directly detected gravitational waves. The discovery, made by scientists of the LIGO Science Collaboration, is a Nobel Prize-worthy event that equals, and maybe even surpasses, the discovery of the Higgs boson in 2012 and the realization by Edwin Hubble in 1929 that the universe is expanding.
“We can now hear the universe,” said LIGO physicist and spokesperson Gabriela Gonzalez during Thursday’s National Science Foundation’s historic meeting in Washington D.C. “The detection is the beginning of a new era: The field of gravitational astronomy is now a reality.”
Although gravitational waves were first theorized by Einstein’s theory of general relativity a century ago, it’s only now that humanity has the technology to physically measure these ripples in spacetime. Using LIGO, which stands for Laser Interferometer Gravitational-Wave Observatory, exquisitely tiny fluctuations in spacetime can be measured and, shortly after its sensitivity upgrade in September last year, the universe gave LIGO a gravitational gift.
Gravitational waves are generated when massive objects accelerate, collide or explode. If we can measure the gravitational wave signature of these events, we can learn a lot about their properties and even use them in a new era of gravitational wave astronomy.
It would be a paradigm shift away from “regular” astronomy that studies cosmic radiation from the electromagnetic spectrum; the gravitational wave spectrum can reveal the “dark” universe, where black holes collide and neutron stars quietly orbit one another in the night.
And on Sept. 14, 2015, one of the biggest eruptions of gravitational waves thought possible to occur in the universe was detected by LIGO’s freshly upgraded laser system.
Approximately 1.3 billion light-years away, two black holes became trapped in their mutual gravitational well and started to rapidly orbit one another. Binary black holes are thought to be an inevitable part of the cosmic landscape, but this is the first time that we have directly detected two black holes — 29 and 36 times the mass of our sun — just before merging. Watch a computer visualization of merging black holes:
Their gravitational battle was short-lived, however. In a fraction of a second, the two rapidly spinning objects touched and unleashed a fury of energy. In an instant, the black holes lost 3 times the mass of our sun — this mass was converted into pure gravitational energy, peaking in power output 50 times the output of all the stars in the universe combined.
If we remember our “tossing a pebble in a pond” analogy for the generation of gravitational waves, this black hole merger was like throwing a brick into that pond; a gargantuan surge of gravitational wave energy erupted, blasting these spacetime ripples into the universe.
Then, 1.3 billion years later, these waves reached Earth. And it just so happened that LIGO had only just been upgraded to detect this ancient event. It’s cosmic serendipity at its finest.
Theory suggests that when two black holes collide and merge, their gravitational wave signal should produce a very fast pulse and, if slowed down sufficiently, the rapid spinning and collision should be represented in the waves as a “chirp.” And guess what? Almost exactly as predicted by computer models based on Einstein’s 100-year-old equations of general relativity, the black hole merger "chirped." Listen:
What you just heard were two black holes spiraling into one another and merging as one — an event that is thought to occur somewhere in the universe once every 15 minutes. This is the physical embodiment of a quest that has taken 100 years for physicists to achieve.
There is no question about the validity of the signal, this is the cutting edge of physics theory meeting observation; the chirp was predicted and this chirp represents black hole rebirth.
When you sit down and really think about it, it’s hard to comprehend that this signal, from two black holes, washed through our planet. In fact, it washed through each and every person on the planet. This has happened countless times and will continue to do so in the future, it’s only just now that we have the right “hearing aid” installed to actually detect these signals.
What’s more, from the timing of the same signal received by the two LIGO stations, we know the rough direction from which the signal came. The Livingston station (Louisiana) “heard” the chirp 7 milliseconds before the Hanford station (Washington), meaning the signal came from Southern Hemisphere skies.
The fact that we have a very rough idea about where the waves came from gives us a glimpse of a potentially very “bright” future for gravitational wave astronomy; add more stations to a LIGO-like network and you can add more signal delays to the mix, which means we can begin to pinpoint the locations of some of the most gravitationally active regions of the universe.
If you can pinpoint them, you can map them and then we have the potential to see the gravitational landscape of the cosmos. Who knows what mysteries we’ll uncover.
“With this discovery, we humans are embarking on a marvelous new quest: the quest to explore the warped side of the universe -- objects and phenomena that are made from warped spacetime. Colliding black holes and gravitational waves are our first beautiful examples,” — Kip Thorne, theoretical physicist and LIGO co-founder.