The race is on to detect ripples from the most massive events in the universe: spinning, orbiting, exploding or colliding ultra-dense objects like black holes and neutron stars.

In 1918, Albert Einstein predicted these cosmic events would radiate a propagating distortion of space and time: gravitational waves. After spending hundreds of millions of dollars to detect them, scientists have come up empty.

But don't write off the hunt just yet. Physicists worldwide have been fine-tuning enormous, multimillion-dollar machines to filter out background noise so they can observe the unique signatures of a gravitation wave. Before the decade is out, they believe they'll record the percussive crash of colliding black holes or the vibrant hum of a pulsar – a discovery that would be the proverbial shot heard around the scientific world.

"I tell students they're lucky," said Rana Adhikari, a principal investigator at the Caltech-MIT Laser Interferometer Gravitational-Wave Observatory. "They're getting in at the right time – it's right before we see something."

The first concrete proof that gravitational waves exist will not only verify a key tenet of relativity theory, but provide unprecedented insight into the mysterious lives of black holes, neutron stars, quark stars (if these controversial objects exist), cosmic strings (also controversial) and probably other as-yet unimagined treasures.

Scientists have spent more than a generation tinkering patiently, coming up empty again and again, but in the process creating increasingly powerful tools.

The DIY set has even gotten into the act. A scientist at the University of Massachusetts at Dartmouth has strung together eight Sony PlayStation 3s to form a supercomputer powering a search for gravitational waves.

Other groups on the hunt have let loose much bigger machines. Stefano Foffa of the University of Geneva is a member of a leading gravitational-wave-detection team, which includes 33 other scientists from Switzerland and Italy. They recently submitted a report to Classical and Quantum Gravity that details their so-far fruitless attempts at observing tiny gravitational tugs and distortions on Explorer, a supercooled, 3-meter-long aluminum bar at the CERN particle physics lab in Switzerland.

Explorer is particularly well-tuned to sense spinning neutron stars, also known as pulsars, Foffa said. He and his colleagues estimate that some 200,000 of these spinning, super-dense objects – so dense that a just sugar cube-sized amount weighs as much as the entire human race – are scattered throughout the Milky Way.

But the thermal noise of even supercooled atoms is greater than the momentary twang the bar's atoms would experience when being plucked by a passing gravitational wave. So the Explorer group must use sensitive superconducting circuits to coax out a signal. It's an art that's still being perfected.

LIGO, the Caltech-MIT observatory, is an even bigger and more ambitious project than Explorer. To someone flying overhead, LIGO looks like an unfinished oil pipeline, with two mile-and-a-half long tubes jutting in perpendicular directions from a central building. The pipes (one in Livingston, Louisiana, and the other in >Richmond, Washington), contain sensitive optics in which laser light bounces back and forth 100 times, then combines, allowing physicists to compare the two beams to monitor the space-time through which the light traveled.

The interference patterns from LIGO's two perpendicular laser beams sometimes momentarily jostle. If the same jostling happens at both LIGO's Louisiana and Washington detectors, and no earthquakes can explain the anomaly, then the source may well be a gravitational wave.

It's the million-dollar moment that hasn't happened.

Then again, LIGO has produced mountains of data since it first began operating in 2002. One popular distributed computing project, Einstein@Home, sifts through these databases to check for signals that might have been missed.

Merging black holes, otherwise invisible to science, are primary targets for detectors like Explorer and LIGO, Adhikari said.

Before last year, however, the echoes of a black-hole collision were too shrouded in complicated mathematics for scientists to even begin hunting for. But in 2006 three separate teams cracked the numerical code to calculate the gravitational crashing sound that merging black holes would make.

And now LIGO scientists have begun searching their data for this gravitational wave signature. If scientists continue to detect nothing, however, Einstein's theories may well need modifying.

"If we don't see anything in four years," Foffa said, "then it will be the time to start questioning."