What are Gravitational Waves?

Today, the National Science Foundation (NSF) announced the detection of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of ground-based observatories. But…what are gravitational waves? Let us explain:

Gravitational waves are disturbances in space-time, the very fabric of the universe, that travel at the speed of light. The waves are emitted by any mass that is changing speed or direction. The simplest example is a binary system, where a pair of stars or compact objects (like black holes) orbit their common center of mass.

We can think of gravitational effects as curvatures in space-time. Earth’s gravity is constant and produces a static curve in space-time. A gravitational wave is a curvature that moves through space-time much like a water wave moves across the surface of a lake. It is generated only when masses are speeding up, slowing down or changing direction.

Did you know Earth also gives off gravitational waves? Earth orbits the sun, which means its direction is always changing, so it does generate gravitational waves, although extremely weak and faint.

What do we learn from these waves?

Observing gravitational waves would be a huge step forward in our understanding of the evolution of the universe, and how large-scale structures, like galaxies and galaxy clusters, are formed.

Gravitational waves can travel across the universe without being impeded by intervening dust and gas. These waves could also provide information about massive objects, such as black holes, that do not themselves emit light and would be undetectable with traditional telescopes.

Just as we need both ground-based and space-based optical telescopes, we need both kinds of gravitational wave observatories to study different wavelengths. Each type compliments the other.

Ground-based: For optical telescopes, Earth’s atmosphere prevents some wavelengths from reaching the ground and distorts the light that does.

Space-based: Telescopes in space have a clear, steady view. That said, telescopes on the ground can be much larger than anything ever launched into space, so they can capture more light from faint objects.

How does this relate to Einstein’s theory of relativity?

The direct detection of gravitational waves is the last major prediction of Einstein’s theory to be proven. Direct detection of these waves will allow scientists to test specific predictions of the theory under conditions that have not been observed to date, such as in very strong gravitational fields.

In everyday language, “theory” means something different than it does to scientists. For scientists, the word refers to a system of ideas that explains observations and experimental results through independent general principles. Isaac Newton’s theory of gravity has limitations we can measure by, say, long-term observations of the motion of the planet Mercury. Einstein’s relativity theory explains these and other measurements. We recognize that Newton’s theory is incomplete when we make sufficiently sensitive measurements. This is likely also true for relativity, and gravitational waves may help us understand where it becomes incomplete.

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One hundred years after Albert Einstein predicted the existence of gravitational waves, they have been detected directly.

In a highly anticipated announcement, physicists with LIGO revealed today, on 11 February, that their twin detectors have heard the gravitational ‘ringing’ produced by the collision of two black holes about 1.3 billion light-years from Earth.

This means we now have a new tool for studying the Universe. For example, waves from the Big Bang would tell us a little more about how the universe formed.
read more here

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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By now you’ve probably heard the news that gravitational waves have been directly observed for the first time ever. Are you excited?

Our friends at PBS Space Time are pretty excited about it too, and they’ve put together an awesome video explaining the physics behind this discovery and why it’s so important. 

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.


Today, a team of physicists at LIGO announced that they have confirmed the existence of gravitational waves. Read more from the NY Times. Looking for some background on Einstein’s Theory of General Relativity, gravitational waves, and LIGO? Watch the above video or click here for more resources

Gravitational Waves Exist: The Inside Story of How Scientists Finally Found Them

Just over a billion years ago, many millions of galaxies from here, a pair of black holes collided. They had been circling each other for aeons, in a sort of mating dance, gathering pace with each orbit, hurtling closer and closer. By the time they were a few hundred miles apart, they were whipping around at nearly the speed of light, releasing great shudders of gravitational energy. Space and time became distorted, like water at a rolling boil. In the fraction of a second that it took for the black holes to finally merge, they radiated a hundred times more energy than all the stars in the universe combined. They formed a new black hole, sixty-two times as heavy as our sun and almost as wide across as the state of Maine. As it smoothed itself out, assuming the shape of a slightly flattened sphere, a few last quivers of energy escaped. Then space and time became silent again.

The waves rippled outward in every direction, weakening as they went. On Earth, dinosaurs arose, evolved, and went extinct. The waves kept going. About fifty thousand years ago, they entered our own Milky Way galaxy, just as Homo sapiens were beginning to replace our Neanderthal cousins as the planet’s dominant species of ape. A hundred years ago, Albert Einstein, one of the more advanced members of the species, predicted the waves’ existence, inspiring decades of speculation and fruitless searching. Twenty-two years ago, construction began on an enormous detector, the Laser Interferometer Gravitational-Wave Observatory (LIGO). Then, on September 14, 2015, at just before eleven in the morning, Central European Time, the waves reached Earth. Marco Drago, a thirty-two-year-old Italian postdoctoral student and a member of the LIGO Scientific Collaboration, was the first person to notice them. He was sitting in front of his computer at the Albert Einstein Institute, in Hannover, Germany, viewing the LIGO data remotely. The waves appeared on his screen as a compressed squiggle, but the most exquisite ears in the universe, attuned to vibrations of less than a trillionth of an inch, would have heard what astronomers call a chirp—a faint whooping from low to high. This morning, in a press conference in Washington, D.C., the LIGO team announced that the signal constitutes the first direct observation of gravitational waves… [+]

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