by Patrizia Madiona
First proposed 100 years ago by Albert Einstein, as a key prediction of his Theory of General Relativity, gravitational waves were finally detected by both the US instruments on the 14th September 2015, at 9:50:45 universal time—4:50 a.m. in Louisiana and 2:50 a.m. in Washington, which together make up the Laser Interferometer Gravitational Wave Observatory (Ligo).
So what are gravitational waves?
Gravitational waves are concentric ripples that squeeze and stretch the fabric of space-time. They are caused by the movement of mass, but most are so weak that they have no measurable effect. For current technology to glimpse them, we need to find the waves – still incredibly subtle – that are radiating across the cosmos from extremely violent events, like explosions or collisions involving stars or black holes.
They carry information about their dramatic origins and about the nature of gravity that cannot be obtained from elsewhere. Physicists have concluded that the detected gravitational waves were produced during the final fraction of a second of the merger of two black holes to produce a single, more massive spinning black hole.
Ligo’s two stations are 3,000km apart in the states of Louisiana and Washington – If a gravitational wave throbs past, light in one of those beams will travel further than light in the other – by a tiny, tiny fraction of the width of an atom. And this is indeed what was recorded – the oscillation emerged at a frequency of 35 cycles per second, or Hertz, and sped up to 250 Hz before disappearing 0.25 seconds later. The 0.007-second delay between the signals in Louisiana and Washington is the right timing for a light-speed wave zipping across both detectors. This discovery was made possible only after a $200m upgrade that saw the facility redubbed Advanced Ligo and dramatically boosted its sensitivity.
What does this imply?
As part of his Theory of General Relativity, Einstein had predicted that that gravitational waves should carry away orbital energy, and indeed, these pulsars’ orbits spiral inward at exactly the rate relativity predicts.
“The driving force of the universe is gravity,” said Tuck Stebbins, Gravitational Astrophysics Lab Chief at NASA’s Goddard Space Flight Center. “These waves are streaming to you all the time and if you could see them, you could see back to the first one trillionth of a second of the Big Bang,” he added.
“This detection is the beginning of a new era: The field of gravitational wave astronomy is now a reality,” says Gabriela González, LSC spokesperson and professor of physics and astronomy at Louisiana State University.
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The research article in Physical Review Letters is here: