The first direct detection of gravitational waves is without doubt one of the most remarkable breakthroughs of our time. The Advanced LIGO laboratories in the US states of Washington and Louisiana have traced the warping of space from the merger of two black holes about 1.3 billion light-years from Earth.
It represents the last great confirmation of Einstein's ideas, and opens the door to a completely new way to investigate the Universe. Astronomy and other fields of science are now entering a new era.
So, what exactly are gravitational waves?
According to Einstein's General Theory of Relativity, any accelerating mass should produce ripples in the fabric of space and time. The effect is very weak, however, and only the biggest masses, moving under the greatest accelerations, are expected to warp their surroundings to any appreciable degree. Put in this category the explosion of giant stars, the collision of ultra-dense dead ones, and the coming together of black holes. All these events should radiate gravitational energy at the speed of light.
When you say "weak", just how small is the effect?
When gravitational waves pass through the Earth, the space and time our planet occupies should alternately stretch and squeeze. Think of a pair of stockings: when you pull on them repeatedly, they elongate and narrow. The Advanced LIGO interferometers have been searching for this stretching and squeezing for over a decade, gradually improving the sensitivity of their equipment. The expectation was that their experiments would need to detect disturbances no bigger than a fraction of the width of a proton, the particle that makes up the nucleus of all atoms.
That's an astonishing capability; how is it done?
The idea is to split a high-powered laser beam and send separate light paths down two long vacuum tunnels that are arranged in an L-shaped configuration. The two paths are bounced back and forth by mirrors, before eventually returning to their starting point. The beam is then reconstructed and sent to detectors. If gravitational waves have passed through the lab, the light paths will have been ever so slightly offset, and this will be evident in the analysis. The approach is called laser interferometry.
But surely any signal is swamped by noise?
It's true - even when damping the equipment, by hanging the mirrors on special suspensions for example, the whole set-up is still moving. Even the stillest object you can imagine is vibrating on the smallest scales; and then there are quakes and even the general hum of the Earth (ocean waves crashing on coasts worldwide!) to contend with. But years of research have simulated what gravitational wave signals ought to look like, and supercomputers can trawl the noise looking for these specific patterns. The waves will have telltale frequencies.
How can we have confidence that the detection is real?
First, the recorded data fits perfectly with the modelled expectation for this type of black hole merger. Second, it was seen in both LIGO machines at virtually the same time. The small delay in detection between the two is explained by the stations' 3,000km separation. This is all reminiscent of the Higgs boson discovery. You will recall that the detection was only claimed when (and because) two separate experiments at the Large Hadron Collider saw exactly the same thing in the data. The two LIGO facilities will eventually operate in tandem with a third lab in Italy, called Virgo. All three should then be recording future events together, but researchers will be able to use their different positions and signal timings to determine, much more precisely, the location in the sky of the source.
Why is the discovery so significant?
Consider for a moment just the black hole part of this story. Our knowledge that these objects exist is actually pretty indirect. As everyone knows, the gravitational influence of black holes is so great that not even light can escape their grip; they don't shine for our telescopes, unfortunately. But we know them to be out there because we can see the light coming from material being torn apart or accelerated to high speed as it gets very close to a black hole. Gravitational waves, on the other hand, are a signal that comes right from these objects themselves and carries information about them. In this sense, you can argue that we have just made the first direct detection of black holes as well.
So we have a new way to probe black holes?
Not just black holes, but the "dark" Universe in general. So much of what we theorise to be out there does not radiate light in any of its forms - from gamma-rays to the ultraviolet, from the visible to radio waves - or emit particles. And unlike light or particles, gravitational waves cannot be blocked or deflected; they will pass through any and all obstructions unhindered. And that makes them a free pass to begin exploring phenomena that were previously off limits. We know, for example, that it is impossible to see across space to before 380,000 years after the Big Bang - the Universe hadn't cooled sufficiently until that point to permit light to propagate. But, theoretically, there should still be background gravitational waves washing over us from the very earliest moments of the expanding cosmos. If future spaceborne gravitational-wave observatories can detect this remnant signal, it will bring us closer than ever to understanding what happened at "time equals zero".
So what could this all lead to?
It's easy to speculate that the biggest revelations will come in areas where we didn't even know what the question was - the unknown unknowns. That's always been the case when new observational techniques become available. But dwell for now on the theory of gravity itself. As brilliant as Einstein was, we know his ideas to be incomplete. General Relativity describes the Universe very well on the largest scales, but on the smallest domains we resort instead to quantum theories. As yet, a quantised theory of gravity does not exist. To get there we will need to investigate places with extreme gravity: those black holes. It is there that a route to more complete explanations may be found, in the deviations that observed gravitational waves make from modelled expectations.
Will this detection win a Nobel Prize?
It is nailed on. A certainty. As ever, the debate will be about the recipients and their place in the chain of discovery. Who will be regarded as having made the most significant contribution? Will the recipients be theorists or experimentalists in that chain? One thing is clear: it is in the nature of science today that the really big questions tend to be answered with the aid of really big machines. And without the LIGO Collaboration's many hundreds of participants, who work across diverse fields on a range of complex technologies, this moment would never have come.