The physics world is abuzz with discussions about gravitational waves.
What are they, and why should you care? What are gravitational waves?
Albert Einstein predicted gravitational waves in his general theory of relativity a century ago.
They are ripples in space time, the very fabric of the Universe
The theory states that mass warps space and time, much like placing a bowling ball on a trampoline. Other objects on the surface will 'fall' towards centre — a metaphor for gravity in which the trampoline is space-time
When objects accelerate, they send ripples along the curved spacetime fabric at the speed of light — the more massive the object, the larger the wave and the easier it would be for scientists to detect.
Gravitational waves do not interact with matter and travel through the Universe completely unimpeded.
The strongest waves are caused by the most cataclysmic processes in the Universe — two black holes colliding, massive stars exploding or the very birth of the Universe some 13.8 billion years ago.
Why would detection of gravitational waves be important?
When gravitational waves become detectable, this opens up exciting new avenues in astronomy — allowing measurements of faraway stars, galaxies and black holes based on the waves they make.
So-called primordial gravitational waves, the hardest kind to detect, would boost another leading theory of cosmology, that of 'inflation', or exponential expansion of the infant Universe.
Primordial waves are theorised to still be resonating throughout the Universe today, though feebly.
If they are found, they would tell us about the energy scale at which infl ation ocurred, shedding light on the Big Bang itself.
Why are they so hard to find?
Waves coming from tens of millions of light years away would stretch and squeeze a 4-km light beam such as the ones used at LIGO by about the width of a proton
Ripples emitted by a pair of orbiting black holes, for example, would stretch a onemillionkilometre ruler on Earth by less than the size of an atom
Gravitational waves were predicted by Einstein in his general theory of relativity in 1915, the theory that proposed space-time as a concept. The waves are a distortion of space-time.
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However, in order for us to detect them, they needed to be created by a mammoth event -- for example, the collision of two black holes.
Black holes are a holy grail of the gravitational wave concept. To date, we'd been able only to see their aftereffects. Black holes themselves were a conjecture.
"There's been a lot of indirect evidence for their existence," says Shoemaker, an expert in black holes. "But this is the first time we actually detect two black holes merging and we know the only thing that predicts that (is) gravitational radiation, (which) comes from a binary black hole merging. There's no other way we could have seen that but gravitationally."
LIGO is described as "a system of two identical detectors" -- one located in Livingston, Louisiana, the other in Hanford, Washington -- "carefully constructed to detect incredibly tiny vibrations from passing gravitational waves." The project was created by scientists from Caltech and MIT and funded by the National Science Foundation.
Szabolcs Marka, a physicist at Columbia University who is leader of the LIGO member Columbia Experimental Gravity Group, said you could think of it as "a cosmic microphone."
'Now we can listen to the universe'
But is LIGO correct? Have we really detected gravitational waves?
Scientists have what they call a "five-sigma" standard of proof, and LIGO's researchers say the gravitational wave discovery exceeds that.
"It took six months of convincing ourselves that it was correct," says Shoemaker. "It goes beyond that five-sigma to proving that nothing was happening with the equipment that couldn't be understood."
She's thrilled with the possibilities.
"Imagine having never been able to hear before and all you can do is see," she says. "Now we can listen to the universe where we were deaf before. It's a different spectrum (from the electromagnetic spectrum). It's unlike anything we've ever detected before."
"What's really exciting is what comes next," said Reitze at the announcement. "I think we're opening a window on the universe -- a window of gravitational wave astronomy."
The 'chirp' of black holes colliding
The gravitational waves stretched and compressed space around Earth "like Jell-O," said Reitze.
However, the waves are so small that it takes a detector like LIGO, capable of measuring distortions one-thousandth the size of a proton, to observe them. They were observed on September 14, 2015.
Scientists heard the sound of the black holes colliding as a "chirp" lasting one-fifth of a second. Though gravitational waves aren't sound waves, the increase in frequency the collision exhibited in its last milliseconds -- when the black holes were mere kilometers apart and growing closer -- is a frequency we can hear, said Deirdre Shoemaker, a Georgia Tech physicist who works on LIGO.Historic Gravitational Waves Discovery