The experiment that should finally detect ripples in the fabric of space-time is up and running.
Labs in the US states of Washington and Louisiana began "listening" on Friday for the gravitational waves that are predicted to flow through the Earth when violent events occur in space.
The Advanced Ligo facilities have just completed a major upgrade.
Scientists believe this will now give them the sensitivity needed to pick up what should be a very subtle signal.
The theoretical physicist Kip Thorne, one of the pioneers behind the experiment, went so far as to say that it would be "quite surprising" if the labs made no detection.
"We are there; we are in the ball park now. It's clear that this is going to be pulled off," he confidently told The Documentary programme on the BBC World Service.
Gravitational waves are a prediction of Einstein's Theory of General Relativity.
They describe the warping of space-time that occurs when masses accelerate.
Sources that Advanced Ligo might observe include merging black holes and neutron stars (very dense, burnt-out stars), and, with luck, some exploding giant stars (supernovae).
Ripples in the fabric of space-time
- GWs are an inevitable consequence of the Theory of General Relativity
- Their existence has been inferred by science but not yet directly detected
- They are ripples in the fabric of space and time produced by violent events
- Accelerating masses will produce waves that propagate at the speed of light
- Detectable sources ought to include merging black holes and neutron stars
- Ligo fires lasers into long, L-shaped tunnels; weak GWs should disturb the light
- Detecting the waves opens up the Universe to completely new investigations
The technique being employed is laser interferometry.
Both Ligo labs work by splitting a light beam and sending the two halves down separate, 4km-long, evacuated tunnels.
The beams are bounced back and forth by mirrors before being recombined at their starting point and sent to detectors.
If the delicate gravitational waves pass through the set-up, the laser light should show evidence of having been ever so slightly disturbed - either lengthened or shortened.
Advanced Ligo is looking for changes in laser-arm distance that are on the order of one one-thousandth of the width of a proton.
The equipment's peak sensitivity will be to waves with a frequency of around 100 Hertz, which in auditory terms is at the low end of what humans can hear. And it is for this reason that gravitational wave detection is often described as trying to pick up the "sounds of the cosmos".
"These detectors are like microphones where we're listening to the Universe," said Jamie Rollins from the California Institute of Technology, one of the project's lead institutions.
"It's sort of like we've been deaf to the Universe until now, and turning on these detectors is like turning on our ears."
The Ligo labs first began hunting for gravitational waves in 2002. They were then switched off in 2010 to undergo their more than $200m upgrade.
The improvements suppress further the "noise" in the instrumentation that would otherwise swamp real signals.
Important contributions have been made by the project's international partners, which include the UK.
British scientists provided the technology to keep the mirrors rock steady.
"The mirrors have to be super-quiet so that they're not disturbed by anything other than a gravitational wave. So they're suspended on four pendulum stages, the last stage being ultra-pure glass fibres," explained Sheila Rowan from the University of Glasgow.
The labs have been running in an engineering mode for some weeks, but they were switched to begin formal science observations at 15:00 GMT on Friday.
Like all instruments, they will need to be tuned to gain further improvements in sensitivity.
The upgrade was designed to give the detectors a 10-fold jump in performance over the old set-up. Refinements should achieve another factor of three by the end of the decade.
"The one thing I personally like about Ligo is it's used every bit of physics I know, from the most arcane solid-state and surface physics to lasers, electronics, quantum mechanics - everything," said Vern Sandberg, the lead scientist at the Hanford lab.
"And it's a very gratifying area to work in, a very frustrating area, too. Hopefully, we will know in a few months whether it's very fulfilling," he told The Documentary.
Scientists must now wait for the Universe to comply - for two black holes to spiral into each other, or, perhaps, for a supernova to go off in our Milky Way Galaxy.
Supercomputers will be sifting constantly the data for patterns that match the expectations from simulations - and even for those signals that stand out as so unexpected they may hint at something that goes totally beyond current understanding.
"Recording a gravitational wave for the first time has never been a big motivation for Ligo," said Kip Thorne.
"The motivation has always been to open a new window on the Universe, to see what I like to call the warped side of the Universe - an aspect of the Universe we've never seen before, objects and phenomena that are made either entirely from warped space and time, or partially from warped space and time. And it's going to be fantastic when we do."