PARIS • When Albert Einstein forged the bedrock theory of modern physics 100 years ago, he had no computer, no Internet, no printer - ballpoint pens and pocket calculators did not exist and few homes had telephones.

Yet, it took one of the most sophisticated scientific tools ever built, at a cost of hundreds of millions of dollars, to prove an idea the scientist had crafted with little more than paper, a fountain pen, hard work and a mind sharper than most.

On Thursday, physicists announced they had detected gravitational waves - hitherto a key unproven element of Einstein's general theory of relativity.

The thesis was published 100 years ago this year, when the world was a very different place, inhabited by a man way ahead of his time.

Radios had been invented, but not yet entered people's homes. The Eiffel Tower was the tallest building in the world.

The team of physicists told the world they had heard and recorded the sound of two black holes colliding one billion light-years away, little more than a fleeting chirp.

But that faint rising tone, physicists said, is the first direct evidence of gravitational waves, the ripples in the fabric of space-time that Einstein predicted.

It completes his vision of a universe in which space and time are interwoven and dynamic, able to stretch, shrink and jiggle.

And it is a ringing confirmation of the nature of black holes, the bottomless gravitational pits from which not even light can escape, which were the most foreboding part of his theory.

"We are really witnessing the opening of a new tool for doing astronomy," Massachusetts Institute of Technology (MIT) astrophysicist Nergis Mavalvala said.

"We have turned on a new sense. We have been able to see and now we will be able to hear as well."

Conveyed by these gravitational waves, power 50 times greater than the output of all the stars in the universe combined vibrated a pair of L-shaped antennas in Washington state and Louisiana known as a Laser Interferometer Gravitational-wave Observatory, or Ligo, on Sept 14 last year.

If replicated by future experiments, that simple chirp seems destined to take its place among the great sound bites of science, ranking with Alexander Graham Bell's "Mr Watson - come here" and Sputnik's first beeps from orbit.

Experts said the first direct detection of gravitational waves is likely to earn a Nobel Prize.

Detectable gravitational waves open exciting new avenues in astronomy - allowing measurements of faraway stars, galaxies and black holes based on the waves they make.

Scientists said gravitational waves should help them gain knowledge about enigmatic objects like black holes and neutron stars.

The waves also may provide insight into the mysterious nature of the very early universe.

The scientists said that because gravitational waves are so radically different from electromagnetic waves, they expect them to reveal big surprises about the universe.

Everything we knew until now about the cosmos stemmed from electromagnetic waves such as radio waves, visible light, infrared light, X-rays and gamma rays.

Because such waves encounter interference as they travel across the universe, they can tell only part of the story.

Gravitational waves experience no such barriers, meaning they offer a wealth of additional information. Black holes, for example, do not emit light or radio waves but can be studied through gravitational waves.

When Einstein announced his theory, he rewrote the rules regarding space and time that had prevailed for more than 200 years, since the time of Newton, which stipulated a static and fixed framework for the universe.

Instead, Einstein said, matter and energy distort the geometry of the universe in the way a heavy sleeper causes a mattress to sag, producing the effect we call gravity.

A century on, scientists finally have the equipment sensitive enough to detect the waves.

Ligo's antennas are L-shaped, with perpendicular arms 4km long.

The lasers can detect changes in the length of one of those arms as small as 0.0001 (one ten-thousandth) the diameter of a proton - a subatomic particle too small to be seen by even the most powerful microscopes - as a gravitational wave sweeps through.

The discovery is a vindication for the US National Science Foundation, which spent about US$1.1 billion (S$1.5 billion) over more than 40 years, facing down criticism that sources of gravitational waves were not plentiful or loud enough to justify the cost. Now one of the key parts of Einstein's work has been proven and it could change astronomy and how we view the universe forever.

AGENCE FRANCE-PRESSE, NEW YORK TIMES, REUTERS