Gravitational waves: discovery hailed as breakthrough of the century
Scientists announce discovery of clear gravitational wave signal, ripples in spacetime first predicted by Albert Einstein
By Tim Radford

Physicists have announced the discovery of gravitational waves, ripples in spacetime first anticipated by Albert Einstein a century ago.

“We have detected gravitational waves. We did it,” said David Reitze, executive director of the Laser Interferometer Gravitational Wave Observatory (Ligo), at a press conference in Washington.

The announcement is the climax of a century of speculation, 50 years of trial and error, and 25 years perfecting a set of instruments so sensitive they could identify a distortion in spacetime a thousandth the diameter of one atomic nucleus across a 4km strip of laserbeam and mirror.

The phenomenon was detected by the collision of two black holes. Using the world’s most sophisticated detector, the project scientists listened for 20 thousandths of a second as the two giant black holes, one 35 times the mass of the sun, the other slightly smaller, circled around each other.

At the beginning of the signal, their calculations told them how stars perish: the two objects had begun by circling each other 30 times a second. By the end of the 20 millisecond snatch of data, the two had accelerated to 250 times a second before the final collision and dark merger.

The observation signals the opening of a new window onto the universe.

“This is transformational,” said Professor Alberto Vecchio, of the University of Birmingham, and one of the researchers working on Ligo. “This observation is truly incredible science and marks three milestones for physics: the direct detection of gravitational waves, the first detection of a binary black hole, and the most convincing evidence to date that nature’s black holes are the objects predicted by Einstein’s theory.”

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Gravitational Waves

Somewhere very far away, a long time ago, two black holes smashed into each other.

One was around 36 times more massive than the Sun, and the other 29 times more massive.

So devastatingly powerful was this event that it did something that might not even be obvious to most of us: it sent a sort of ‘quake’ through the fabric of spacetime.

The power radiated by the combining of the black holes is estimated to be more than the combined light power of all the stars and galaxies in the observable universe.

This ripple event is something known as a ‘gravitational wave’ and we’ve known about them for a very long time ~ sort of.

Einstein predicted their existence long ago as a consequence of the theory of general relativity, but up until now we’ve never had a direct observation of them.

A team of researchers from an international collaboration known as LIGO (Laser Interferometry Gravitational-Wave Observatory) seems to have been the first to observe.

Using lasers, LIGO found a subtle stretching and squeezing of spacetime itself was going on. How this happened is actually a remarkably simple concept:

First they shot a laser beam into a tunnel, that got split into two directions:

Here’s an ‘L’ shape to help you imagine the two tunnels it split into.

Next, once both lasers reached the end of their respective tunnels, they bounced back towards the spot where they split so that they could recombine.

A way to think about this is both lasers racing towards the lower-left corner of the ‘L’ again.

Here’s the rub:

Light can be thought of as a wave, with ripples and peaks etc. The waveform of these two laser beams, when combined add into each other.

If the two laser beams have the same wavelength (as they should if there’s no gravitational waves disturbing spacetime) the two split beams will recombine again into the original beam. It looks like this:

If the two laser beams get somehow disturbed and the waves peak on one as the other crests, the resulting combined beam will be that they simply cancel out:

So in the end, if the LIGO researchers detect alterations to their laser when the two beams recombine, they can tell if spacetime’s subtle ripples have morphed the lasers.

The consequences of this discovery are profound.

It, in a sense, opens up the universe to an entire new branch of physics: the universe of gravity.

Ever hear of dark matter? How about dark energy?

These two things are bound to get close scrutiny now as they’re both a part of what’s known as the ‘dark universe’ - basically neither phenomena interact with light (meaning one can’t see them), making it tough to learn much about them.

Yet much of the universe seems to be comprised of these ‘unseeable’ things.

If this discovery holds up, there’s almost certainly a Nobel Prize in the works.

Why? They may have - and I do mean maybe, not did - well…

The folks at LIGO may have just illuminated the 96% of the universe that’s been invisible to our senses for so long. We’ll have to wait and see.

(Image credit: NASA, NSF/LIGO and Brews Ohare respectively)


The first detection of gravitational waves!

“And what we’ve seen, for the first time, is not just one of the greatest predictions of Einstein’s General Relativity, although we did just verify that. And it isn’t just that we took our first step into the world of gravitational wave astronomy, although LIGO will doubtlessly start seeing more of these signals over the coming years; this is as exciting for astronomy as Galileo’s invention of the telescope, as we’re seeing the Universe in a new way for the first time. But the biggest news of all is that we’ve just detected two merging black holes for the first time, tested their physics, found a tremendous agreement with Einstein, and seen evidence that this happens over a billion light years away across the Universe.”

More than 100 years after Einstein’s relativity came out, one of its last great predictions — the existence of gravitational radiation — has been directly experimentally confirmed! The LIGO collaboration has observed two ~30 solar mass black holes merging together, producing a slightly less massive final black hole as three sun’s worth of mass was converted into energy via Einstein’s E = mc^2. This type of event, although quite serendipitous for the LIGO collaboration, is expected to occur between 2 and 4 times per year within the range of what LIGO can reach. Additionally, other types of mergers should be within the reach of what LIGO can see. Not only have we seen our first gravitational wave event, but we’re poised to truly begin the era of gravitational wave astronomy, as a new type of telescope is finally capable of seeing what’s happening in our Universe.


LIGO - What is a Gravitational Wave?

In less than an hour we’ll know more about gravitational waves, well, we’ll know the same more or less, but will get direct evidence of them. Check it out in this NSF channel webcast.

Also read:

  • The wave nature of simple gravitational waves (Einstein Online)
  • Gravitational Waves Explained (PHD Comics)
  • Gravitational waves: 6 cosmic questions they can tackle (Nature)
  • LIGO wows: black holes heavy as 36+29 merge to 62 Suns + 3 Suns of gravitational waves (The Reference Frame)

(An artist’s impression of binary black holes. Image: NASA. From “Binary black holes found verging on merging”, Astronomy Now)


LIGO Gravitational Wave Chirp - Chirp pattern of gravitational waves detected by LIGO on September 14, 2015.
Credit: LIGO