A few hours ago the most eagerly-awaited press conference in physics this century was held in the US by the LIGO Scientific Collaboration (LSC). It was widely anticipated that the consortium would announce the first direct detection of gravitational waves with the Advanced LIGO interferometers - and they did!!!
The tension in the astronomical and physics communities has been palpable. Rumours have been circulating for some six months now, but it was only a few days ago that the journal Science published the “woohoo!” email, allegedly from a Canadian department chair (and circulated to his whole department!!!), describing the juicy details of LIGO’s alleged detection of two incredibly heavy black holes merging in a death spiral in the distant Universe. Wow!
The leak turned out to be incredibly accurate and raises all sorts of issues about journals we can discuss some other time.
What are gravitational waves?
Gravitational waves are a direct consequence of Einstein’s General Theory of Relativity. Einstein himself thought that their impact on the Earth would be so minor that they would never be detected, as they cause incredibly minor displacements of normal matter that are so small that physicists have always failed to detect them (well for the first 100 years after they were first predicted at least).
Indeed, two neutron stars (weighing a collective total of 1 million Earth masses!) orbiting each other 1000 times a second (that is not a typo) only generate a gravitational wave signal that displaces a mirror at the end of LIGO’s 4km long tunnels by a pathetic fraction (<1/1,000th) of the diameter of an atomic nucleus! But such vibrations are now apparently within LIGO’s reach, dramatically confirmed by today’s announcement of the merger of two black holes. That represents an amazing scientific and engineering triumph.
Do sources of gravitational waves exist?
Astronomers have been studying gravity for centuries now, but our ability to test gravity has been restricted to small deviations from Newtonian physics.
Some thirty years ago pulsar astronomers first noticed that neutron star pairs have orbits that shrink at the precise rate predicted by Einstein if they give off gravitational waves (resulting in the 1993 Nobel prize to Hulse and Taylor), so we’ve known for a few decades that gravitational waves exist, they just needed to be detected.
Indeed any two objects that orbit each other give off gravitational waves, and this causes their orbits to shrink - as in the binary pulsars. As the stars get closer to each other, the power being expelled in gravitational waves grows, and a positive feedback loop ensues. For most binaries it is completely irrelevant and of little consequence. But not so for black holes and neutron stars in close orbits.
Finally, after millions or billions of years there is an enormous burst of radiation liberated at audio frequencies (up to about 1kHz) as the two stars tear each other apart. In the special case that the objects are black holes, a new solitary black hole is created.
Massive black hole binaries, like the one LIGO detected, can be seen to much greater distances than neutron star pairs. They are probably rarer, but may end up being detected by LIGO more often because we can detect them so much further afield.
Astronomers think that every few minutes, somewhere in the Universe, relativistic stars tear each other to shreds due to the emission of gravitational radiation, but most are too far away to be seen - even by LIGO (or more strictly speaking, Advanced LIGO, which when complete will scan about 1000x more volume than its predecessor, initial LIGO).
What’s the big deal?
The incredible thing about this discovery is that no electromagnetic light was detected from the event, instead gravitational waves were used. Like light, these waves propagate at the speed of light and stretch and squeeze matter as they pass through it.
But gravitational waves are not light, they are a brand new communication medium that travel unimpeded through gas, dust and interstellar space. They are only detectable by looking for almost infinitesimal vibrations in suspended test masses in two orthogonal arms of a laser interferometer (LIGO = Laser Interferometer Gravitational-wave Observatory), or one day, possibly by using observations of distant neutron stars.
Gravitational wave detectors offer the chance to watch gravity at its most extreme, where velocities of stars approach the speed of light, and all the most amazing features of General Relativity, like time dilation, light bending, the emission of gravitational waves and the creation of black holes come to the fore. It is the stuff of science fiction, and yet very real. See Professor Blair’s nice summary of the result here.
But will this extraordinary discovery have an impact?
The death of science journalism
Long ago, newspapers could afford to have science journalists on their staff but nowadays many just cut and paste press releases. The downside of this is that there is next to no scrutiny of science stories, and press officers in universities and research labs end up effectively writing their own propaganda.
For the trusting public, this makes it appear as though every few days some amazing scientific discovery has just been made. This might give us all our 15 minutes of fame, but means the public get science breakthrough fatigue.
Eventually, this leads to science agnostism, then cynicism. When landmark discoveries like this appear, they’re lost in the fluff. This destruction of journalism is not only happening in science, but all throughout the media. Everyone is now suspicious of the motives behind any story, and with good reason. This has a number of unfortunate consequences. When scientists tell us that the world is getting hotter they’re ignored. People can choose to believe in whatever they want, whether it is a 7,000-year old Earth, the world’s immunity to rising CO₂ levels, and even Donald Trump.
Let’s hope that the gravitational wave astronomers and engineer’s triumph is appropriately recognised. Many of them have dedicated their entire careers towards this discovery and for once we stand at the dawn of a new era in astronomy, that of gravitational wave astrophysics that has amazing potential for scientific discovery.
For now I’m fortunate enough to have been invited to the press conference at Parliament house to celebrate with the sizeable Australian gravitational wave contingent that have worked towards this discovery and work out how astronomers can help them look through this new window to Einstein’s Universe.