The first direct detection of gravitational waves is now widely expected to be announced on February 11 by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO). Using LIGO's twin giant detectors—one in Livingston, Louisiana, and the other in Hanford, Washington—researchers are said to have measured ripples in space-time produced by a collision between two black holes.

Such an announcement would vindicate Albert Einstein’s prediction of gravitational waves, which he made almost exactly 100 years ago as part of his general theory of relativity—but it would also have much further significance. As vibrations in the fabric of space-time, gravitational waves are often compared to sound, and have even been converted into sound snippets. In effect, gravitational-wave telescopes allow scientists to ‘hear’ phenomena at the same time as light-based telescopes ‘see’ them. (Already, members of LIGO and its smaller counterpart Virgo in Pisa, Italy have set up a system for alerting communities working on other types of telescope).

When LIGO fought to get US government funding in the early 1990s, its major opponents at congressional hearings were astronomers. “The general view was that LIGO didn’t have much to do with astronomy,” says Clifford Will, a general-relativity theorist at the University of Florida in Gainesville and an early LIGO supporter. But things have changed now, he says.

Welcome to the field of gravitational-wave astronomy: we take a look at the questions and phenomena that it can explore.
Do black holes actually exist?



The signal that LIGO is expected to announce on Thursday is rumoured to have been produced by two merging black holes. Such events are the most energetic known; the power of the gravitational waves that they emit can briefly rival that of all the stars in the observable Universe combined.Black-hole mergers are also among the cleanest gravitational-wave signals to interpret.

A black-hole merger occurs when two black holes start to spiral towards each other, radiating energy as gravitational waves. These waves should have a characteristic sound called a chirp, which can be used to measure the masses of the two objects. Next, the black holes actually fuse. “It’s as if you get two soap bubbles so close that they form one bubble. Initially, the bigger bubble is deformed,” says Thibault Damour, a gravity theorist at the Institute of Advanced Scientific Studies near Paris. The resulting single black hole will settle into a perfectly spherical shape, but first it is predicted to radiate gravitational waves in a pattern called a ringdown.

One of the most important scientific consequences of detecting a black-hole merger would be confirmation that black holes really do exist—at least as the perfectly round objects made of pure, empty, warped space-time that are predicted by general relativity. Another would be that mergers proceed as predicted. Astronomers already have plenty of circumstantial evidence for these phenomena, but so far that has come from observations of the stars and super-heated gas that orbit black holes, not of black holes themselves.

“The scientific community, including myself, has become very blasé about black holes. We have taken them for granted,” says Frans Pretorius, a specialist in general-relativity simulations at Princeton University in New Jersey. “But if you think of what an astonishing prediction it is, we really need astonishing evidence.”
Do gravitational waves travel at the speed of light?

When scientists start to compare observations from LIGO with those from other types of telescope, one of the first things that they will check is whether the signals arrive at the same time. Physicists hypothesize that gravity is transmitted by particles called gravitons, the gravitational analogue of photons. If, like photons, these particles have no mass, then gravitational waves would travel at the speed of light, matching the prediction of the speed of gravitational waves in classical general relativity. (Their speed can be affected by the accelerating expansion of the Universe, but that should manifest only over distances much greater than LIGO can probe).

But it is possible that gravitons have a slight mass, which would mean that gravitational waves would travel at less than the speed of light. So if, say, LIGO and Virgo were to detect gravitational waves from a cosmic event, and find that the waves took slightly longer to arrive at Earth than the associated burst of γ-rays detected by a more conventional telescope, that could have momentous consequences for fundamental physics.
Is space-time made of cosmic strings?

A simulation of cosmic strings.
Mark Hindmarsh/University of Sussex.

An even weirder discovery would occur if bursts of gravitational waves were detected coming from ‘cosmic strings’. These hypothetical defects in the curvature of space-time, which may or may not be related to string theory, would be infinitesimally thin but would stretch across cosmic distances. Researchers have predicted that cosmic strings, if they exist, might occasionally develop kinks; if a string snapped, it would suddenly release a burst of gravitational waves, which detectors such as LIGO and Virgo could measure.
Are neutron stars rugged?

Neutron stars are the remnants of bigger stars that collapsed under their own weight, becoming so dense that they pushed their constituent electrons and protons to fuse into neutrons. Their extreme physics is poorly understood, but gravitational waves could provide unique insights. For example, the intense gravity at their surface tends to make neutron stars almost perfectly spherical. But some researchers have theorized that there could still be ‘mountains’—at most a few millimetres high—that make these dense objects, themselves about 10 kilometres in diameter, slightly asymmetrical. Neutron stars usually spin very rapidly, so the asymmetric distribution of mass would deform space-time and produce a continuous gravitational-wave signal in the shape of a sine wave, which would radiate energy and slow down the star’s spin.

Pairs of neutron stars that orbit each other would also produce a continuous signal. Just like black holes, the stars would spiral into each other and eventually merge, sometimes producing an audible chirp. But their final instants would differ dramatically from those of black holes. “You have a zoo of possibilities, depending on masses and how much pressure neutron-dense matter can exert,” says Pretorius. For example, the resulting merged star could be a huge neutron star, or it could immediately collapse and turn into a black hole.
What makes stars explode?

Black holes and neutron stars form when massive stars stop shining and collapse in on themselves. Astrophysicists think that this process is what powers a common type of supernova explosion, known as Type II. Simulations of such supernovae have not yet clearly explained what ignites them, but listening to the gravitational-wave bursts that real supernova are expected to produce could help to provide an answer. Depending on what the bursts’ waveforms look like, how loud the bursts are, how frequent they are and how they correlate with the supernovae as seen with electromagnetic telescopes, the data could help to validate or discard various, existing models.
Cassiopeia A, a supernova remnant.
NASA/CXC/SAO
How fast is the Universe expanding?

The expansion of the Universe means that distant objects that are receding from our Galaxy look redder than they really are, because the light that they emit stretches as it travels. Cosmologists estimate the rate of the Universe’s expansion by comparing this redshift of galaxies with how far the galaxies are from us. But that distance is usually gauged from the brightness of ‘Type Ia’ supernovae—a technique that leaves large uncertainties.

If several gravitational-wave detectors across the world detect signals from the same neutron-star merger, together they will be able to provide an estimate of the absolute loudness of the signal, which will reveal how far away the merger occurred. They will also be able to estimate the direction it came from; astronomers could then deduce which galaxy hosted the merger. Comparing that galaxy’s redshift with the distance of the merger as measured by the loudness of the gravitational waves could provide an independent estimate of the rate of cosmic expansion, possibly more accurate than current methods.

the power of the gravitational waves that they emit can briefly rival that of all the stars in the observable Universe combined

 whembly wrote:
What's the application of such technology?

 jasper76 wrote:
Not sure I understand the question. Gravitational waves, if they indeed exist, are not technology.

Scientists announced Thursday that, after decades of effort, they have succeeded in detecting gravitational waves from the violent merging of two black holes in deep space. The detection was hailed as a triumph for a controversial, exquisitely crafted, billion-dollar physics experiment and as confirmation of a key prediction of Albert Einstein's General Theory of Relativity.

It may inaugurate a new era of astronomy in which gravitational waves are tools for studying the most mysterious and exotic objects in the universe.
From 'natural place' to gravitational waves: Gravity in 90 seconds
Play Video1:34
From Aristotle to Einstein, the world's greatest minds have long theorized about gravity. Here are the highlights, and where the study of gravity is headed next. (Gillian Brockell,Joel Achenbach/TWP)

"Ladies and gentlemen, we have detected gravitational waves. We did it!" declared David Reitze, the executive director of the Laser Interferometer Gravitational-wave Observatory (LIGO), drawing applause from a packed audience at the National Press Club that included many of the luminaries of the physics world.

Some of the scientists gathered for the announcement had spent decades conceiving and constructing LIGO.

“For me, this was really my dream. It’s the golden signal for me," said Alessandra Buonanno, who started working on the problem of gravitational waves as a postdoctoral student in 2000 and is now a theoretical physicist at Germany's Max Planck Institute for Gravitational Physics.

The observatory, described as "the most precise measuring device ever built," is actually two facilities in Livingston, La., and Hanford, Wash. They were built and operated with funding from the National Science Foundation, which has spent $1.1 billion on LIGO over the course of several decades. The project is led by scientists from the California Institute of Technology and the Massachusetts Institute of Technology, and is supported by an international consortium of scientists and institutions.

LIGO survived years of management and funding turmoil, and then finally began operations in 2002. Throughout the first observational run, lasting until 2010, the universe declined to cooperate. LIGO detected nothing.

Then came a major upgrade of the detectors. LIGO became more sensitive. On Sept. 14, the signal arrived.

Though only a "chirp," it was a clear, compelling signal of two black holes coalescing, LIGO scientists said. It lasted only half a second, but it captured, for the very first time, the endgame of two black holes spiraling together.

"This was truly a scientific moonshot," Reitze said during the announcement. "I really believe that. And we did it. We landed on the moon."

These black holes were each about the diameter of a major metropolis. They orbited one another at a furious pace at the very end, speeding up to about 75 orbits per second — warping the space around them like a blender cranked to infinity — until finally the two black holes became one.

The pattern of the resulting gravitational waves contained information about the nature of the black holes. Most significantly, the signal closely matched what scientists expected based on Einstein's relativity equations. The physicists knew, in advance, what gravitational waves from merging black holes ought to look like — with a rising frequency, culminating in that chirp, followed by a "ring-down" as the waves settle.

Gabriela Gonzalez, a physics professor at Louisiana State University who is the spokesperson for LIGO, revealed images of the waves picked up by the two detectors and then played an audio version of the same signal.

"Did you hear the chirp? There's a rumbling noise, and then there's a chirp," she told the Press Club audience. "That's the chirp we've been looking for.

And that's what they saw and heard, both in Louisiana and Washington state. It was such a strong signal that everyone knew it was either a real detection of a black hole merger, or "somebody had injected a signal into the interferometers and not properly flagged it into the data set. It turned out that fortunately that wasn’t the case,” as Reitze put it in advance of the news conference.

He said the team, knowing the checkered history of gravitational wave detections that were later discredited, took special care to have the results verified and peer-reviewed prior to the big announcement. The scientists even looked for the possible handiwork of a computer hacker, Reitze said. All reviews held up.

The LIGO success has been a poorly kept secret in the physics world, but the scientists kept their historic paper detailing the exact results secret until Thursday morning.

[Why is this famous physicist tweeting rumors about gravitational waves?]

There is no obvious, immediate consequence of this physics experiment, but the scientists are ecstatic and say this opens a new window on the universe. Until now, astronomy has been almost exclusively a visual enterprise: Scientists have relied on light, visible and otherwise, to observe the cosmos. But now gravitational waves can be used as well.

A bird's eye view of LIGO Hanford's laser and vacuum equipment area, which houses the pre-stabilized laser, beam splitter, input test masses and other equipment. (Caltech/MIT/LIGO Lab)

Gravitational waves are the ripples in the pond of spacetime. The gravity of large objects warps space and time, or “spacetime” as physicists call it, the way a bowling ball changes the shape of a trampoline as it rolls around on it. Smaller objects will move differently as a result — like marbles spiraling toward a bowling-ball-sized dent in a trampoline instead of sitting on a flat surface.

These waves will be particularly useful for studying black holes (the existence of which was first implied by Einstein's theory) and other dark objects, because they'll give scientists a bright beacon to search for even when objects don't emit actual light. Mapping the abundance of black holes and frequency of their mergers could get a lot easier.

[European probe launched in a search for ripples in space-time]

Since they pass through matter without interacting with it, gravitational waves would come to Earth carrying undistorted information about their origin. They could also improve methods for estimating the distances to other galaxies.

LIGO scientists, speaking to The Washington Post in advance of Thursday's news conference, say they saw a weaker signal from a black-hole merger about a week after the first detection.

“The geometry of spacetime gives a burp at the end of [the merger],” said Rainer Weiss, an MIT professor of physics emeritus who has labored on LIGO since the 1970s.

Power recycling optic 2. (LIGO)

No one had ever seen direct evidence of “binary” black holes – two black holes paired together and then merging. The Sept. 14 signal came from about 1.3 billion light years away, though that's a very approximate estimate. That places the black hole merger in very deep space; the signal that arrived in September came from an event that happened before there were any multicellular organisms on Earth.

The reason that gravitational waves have been so difficult to detect is that their effects are tinier than tiny. In fact, the signals they produce are so small that scientists struggle to remove enough background noise to confirm them.

LIGO detects gravitational waves by looking for tiny changes in the path of a long laser beam. In each of the lab's two facilities, a laser beam is split in two and sent down two perpendicular tubes 2.5 miles long. Each arm of the beam bounces off a mirror and heads back to the starting point. If nothing interferes, these two arms recombine at the starting point and cancel each other out.

But a photodetector is waiting in case something goes wrong. If the vibration of a gravitational wave warps the path of one of the lasers, making the two beams almost infinitesimally misaligned, the laser will hit the photodetector and alert the scientists.

To catch movement that small, scientists have to filter out ambient vibrations all the time. And sometimes even seemingly perfect results can end in disappointment: To prevent false positives, LIGO has an elaborate system in place to occasionally inject ersatz signals. Only three scientists on the team know the truth in such cases, and in at least one instance their colleagues were prepared to publish the results when they finally revealed the ruse.

The control room of the LIGO Hanford detector site near Hanford, Wash. (Caltech/MIT/LIGO Lab)

This fail-safe gave pause to many scientists when rumors about the LIGO detection began to circulate in recent months. But the team confidently confirmed that its readings were not falsely injected – it really spotted a pair of black holes.

One of the two black holes had a mass about 36 times greater than our sun. The other registered at 29 solar masses. Both were rather massive as black holes go -- 10 solar masses is more typical.

[This is what it looks like when a black hole tears a star apart]

“For the first time we have a signature of the heavy black hole forming. That was a surprise,” said Vicky Kalogera, a Northwestern University astrophysicist who has been with LIGO for 15 years. “It wasn’t a vanilla-type of black hole that we had expected.”

When the two black holes came together – spiraling in gradually rather than colliding suddenly in a linear crash – the resulting black hole was not the 65 solar masses you'd expect from basic arithmetic, but only 62. The rest was converted to energy that radiated across space in a grand gravitational burp.

That burp first reached the LIGO facility in Louisiana, then the one in Washington state just 7 milliseconds later. The sequence is important, as it allowed physicists to chart the black-hole collision back to somewhere in the southern sky. And the incredibly brief time delay supports something that theorists have long believed about gravitational waves: They move at the speed of light.

“This is the most direct test of our concepts of black holes,” said David Spergel, an astrophysicist at Princeton who was not part of the LIGO team.

The scientists are scrutinizing their data for signs of other violent cosmic events. LIGO's sensitivity continues to improve, and meanwhile other labs will work to catch up to their findings.

“This is such a fantastic new window into the universe – all the rules are different,” said Michael Turner, a University of Chicago cosmologist who also was not involved with the new discovery. “This is the Galileo moment of gravity waves.”

A black-hole collision sounds like a dramatic event, but it’s not really the big news for the physicists. The headline is that LIGO finally worked. Success in detecting gravitational waves is a win for Big Science and for the institutions that backed the project.

“It had a very rough beginning,” Weiss said. “The [National Science Foundation] had a tough time explaining to other people why they would back such a crazy thing.”

Einstein’s theory led to the prediction of gravitational waves, but, as Weiss noted, “Even Einstein wasn’t very sure about this.”

 Sigvatr wrote:
Uhm...what are the implications of that proof? Any follow-up?

 Xenomancers wrote:
You guys heads are going to explode when gravity waves, dark energy, dark matter, and black holes, and even space-time, are all proven to be figments of the imagination. Quantum physics is the biggest waste of human ingenuity to ever be conceived and astro-physics is bordering on religious dogma at this point. Actual science doesn't take place anymore. You should probably prove black holes exist with some solid evidence before you make theories and observations about black holes merging and creating gravity waves. I am really excited to see what discoveries NASA's new telescope is going to find - maybe we will be able to stop guessing about whats going on in the cosmos - but that's probably a long way off. Until then these PHD's can keep going on about their imagined universe and wasting grant money on fruitless experiments that can't even offer meaningful or useful data even if they are successful.

 Ketara wrote:
I suppose the significance is that gravity is no longer a theory.

 Xenomancers wrote:
You guys heads are going to explode when gravity waves, dark energy, dark matter, and black holes, and even space-time, are all proven to be figments of the imagination. Quantum physics is the biggest waste of human ingenuity to ever be conceived and astro-physics is bordering on religious dogma at this point. Actual science doesn't take place anymore. You should probably prove black holes exist with some solid evidence before you make theories and observations about black holes merging and creating gravity waves. I am really excited to see what discoveries NASA's new telescope is going to find - maybe we will be able to stop guessing about whats going on in the cosmos - but that's probably a long way off. Until then these PHD's can keep going on about their imagined universe and wasting grant money on fruitless experiments that can't even offer meaningful or useful data even if they are successful.

 Ketara wrote:

 Sigvatr wrote:
Uhm...what are the implications of that proof? Any follow-up?

I suppose the significance is that gravity is no longer a theory. It's a detectable effect in line with magnetism. Up until now, there were several other potential explanations for matter being drawn to other bits of matter (most wildly implausible, admittedly), but this more or less rules them out.

I suppose the next step is that now we can detect these 'waves', we need to find a way to generate them under controlled circumstances. Once we can do that, all sorts of theoretical weaponisations and endeavours like space travel become feasible.

 jasper76 wrote:

 Xenomancers wrote:
You guys heads are going to explode when gravity waves, dark energy, dark matter, and black holes, and even space-time, are all proven to be figments of the imagination. Quantum physics is the biggest waste of human ingenuity to ever be conceived and astro-physics is bordering on religious dogma at this point. Actual science doesn't take place anymore. You should probably prove black holes exist with some solid evidence before you make theories and observations about black holes merging and creating gravity waves. I am really excited to see what discoveries NASA's new telescope is going to find - maybe we will be able to stop guessing about whats going on in the cosmos - but that's probably a long way off. Until then these PHD's can keep going on about their imagined universe and wasting grant money on fruitless experiments that can't even offer meaningful or useful data even if they are successful.

That's the spirit! It's not like quantum mechanics is already being exploited for practical use or anything.

Xenomancers wrote:You guys heads are going to explode when gravity waves, dark energy, dark matter, and black holes, and even space-time, are all proven to be figments of the imagination.

Actual science doesn't take place anymore.

Henry wrote:

 Ketara wrote:
I suppose the significance is that gravity is no longer a theory.

I'm going to be awkward here, but purely because most people use the word "theory" in science incorrectly. This discovery does make gravity a fact yet it still remains a theory too. Theory doesn't mean a hunch or guess, or something that doesn't yet have conclusive evidence. A theory is a description of a scientific event or interaction of forces using scientific laws and evidence.
So this experiment shows that gravity is a fact. We use the laws of gravity to make predictions in specific circumstances and we use the theory of gravity to help us understand the interaction of forces, explain events and make predictions. All remain extant.

 Sigvatr wrote:
Uhm...what are the implications of that proof?

 Sigvatr wrote:
Uhm...what are the implications of that proof? Any follow-up?

 Ketara wrote:

Xenomancers wrote:You guys heads are going to explode when gravity waves, dark energy, dark matter, and black holes, and even space-time, are all proven to be figments of the imagination.

Alright. I'll bite. If gravity, space and time all figments of fertile imaginations, and we concede you are literally smarter than Einstein, what do you plan to replace them with?

Actual science doesn't take place anymore.

Actual science being defined as what? Just so we can test your assertion.

Henry wrote:

 Ketara wrote:
I suppose the significance is that gravity is no longer a theory.

I'm going to be awkward here, but purely because most people use the word "theory" in science incorrectly. This discovery does make gravity a fact yet it still remains a theory too. Theory doesn't mean a hunch or guess, or something that doesn't yet have conclusive evidence. A theory is a description of a scientific event or interaction of forces using scientific laws and evidence.
So this experiment shows that gravity is a fact. We use the laws of gravity to make predictions in specific circumstances and we use the theory of gravity to help us understand the interaction of forces, explain events and make predictions. All remain extant.

Fair enough,

 Xenomancers wrote:

Actual science requires experiment in controlled conditions.
Take this article for example. There's nothing controlled about this experiment - "background noise" from billions of light years of signals coming in from this region are literally indistinguishable from the results they are looking for. Why can't this experiment be reproduced at a smaller scale in a more controlled setting?

Why is the experiment taking so long

in this amount of time it's statistically likely you could accidentally find proof of your hypothesis considering how ambiguous it is?

no answers ever come from this junk science - just more questions.

Their answer -well - this is really really hard to do. Okay fair enough - how much more time and money do you need to prove things like dark matter, dark energy, and gravity waves are real?

Science is in such a need of a break through they'll feed your ripe minds anything to keep the money flowing.