LIGO

As gravitational waves sweep past Earth, they alternately stretch and compress the arms of Advanced LIGO’s twin detectors, located in Hanford, Wash. (pictured), and Livingston, La.

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Ripples in space-time caused by the violent mergers of black holes have been detected, 100 years after these “gravitational waves” were predicted by Albert Einstein’s theory of general relativity and half a century after physicists set out to look for them.

The landmark discovery was reported today by the Advanced Laser Interferometer Gravitational-Wave Observatory (Advanced LIGO) team, confirming months of rumors that have surrounded the group’s analysis of its first round of data. Astrophysicists say the detection of gravitational waves opens up a new window on the universe, revealing faraway events that can’t be seen by optical telescopes, but whose faint tremors can be felt, even heard, across the cosmos.

“We have detected gravitational waves. We did it!” announced David Reitze, executive director of the 1,000-member team, at a National Science Foundation press conference today in Washington, D.C.

Gravitational waves are perhaps the most elusive prediction of Einstein’s theory, one that he and his contemporaries debated for decades. According to his theory, space and time form a stretchy fabric that bends under heavy objects, and to feel gravity is to fall along the fabric’s curves. But can the “space-time” fabric ripple like the skin of a drum? Einstein flip-flopped, confused as to what his equations implied. But even steadfast believers assumed that, in any case, gravitational waves would be too weak to observe. They cascade outward from certain cataclysmic events, alternately stretching and squeezing space-time as they go. But by the time the waves reach Earth from these remote sources, they typically stretch and squeeze each mile of space by a minuscule fraction of the width of an atomic nucleus.

M. Pössel/Einstein Online

Gravitational waves alternately stretch and squeeze space-time both vertically and horizontally as they propagate.

Perceiving the waves took patience and a delicate touch. Advanced LIGO bounced laser beams back and forth along the four-kilometer arms of two L-shaped detectors — one in Hanford, Wash., the other in Livingston, La. — looking for coincident expansions and contractions of their arms caused by gravitational waves as they passed. Using state-of-the-art stabilizers, vacuums and thousands of sensors, the scientists measured changes in the arms’ lengths as tiny as one thousandth the width of a proton. This sensitivity would have been unimaginable a century ago, and struck many as implausible in 1968, when Rainer Weiss of the Massachusetts Institute of Technology conceived the experiment that became LIGO.

“The great wonder is they did finally pull it off; they managed to detect these little boogers!” said Daniel Kennefick, a theoretical physicist at the University of Arkansas and author of the 2007 book Traveling at the Speed of Thought: Einstein and the Quest for Gravitational Waves.

The detection ushers in a new era of gravitational-wave astronomy that is expected to deliver a better understanding of the formation, population and galactic role of black holes — super-dense balls of mass that curve space-time so steeply that even light cannot escape. When black holes spiral toward each other and merge, they emit a “chirp”: space-time ripples that grow higher in pitch and amplitude before abruptly ending. The chirps that LIGO can detect happen to fall in the audible range, although they are far too quiet to be heard by the unaided ear. You can re-create the sound by running your finger along a piano’s keys. “Start from the lowest note on the piano and go to middle C,” Weiss said. “That’s what we hear.”

NSF

Audio: The “chirp” of gravitational waves recorded by the LIGO team.

Physicists are already surprised by the number and strength of the signals detected so far, which imply that there are more black holes out there than expected. “We got lucky, but I was always expecting us to be somewhat lucky,” said Kip Thorne, a theoretical physicist at the California Institute of Technology who founded LIGO with Weiss and Ronald Drever, who is also at Caltech. “This usually happens when a whole new window’s been opened up on the universe.”

C. Henze/ NASA

Video: A simulation of two black holes merging and the resulting emission of gravitational radiation.

Eavesdropping on gravitational waves could reshape our view of the cosmos in other ways, perhaps uncovering unimagined cosmic happenings.

“I liken this to the first time we pointed a telescope at the sky,” said Janna Levin, a theoretical astrophysicist at Barnard College of Columbia University. “People realized there was something to see out there, but didn’t foresee the huge, incredible range of possibilities that exist in the universe.” Similarly, Levin said, gravitational-wave detections might possibly reveal that “the universe is full of dark stuff that we simply can’t detect in a telescope.”

The story of the first gravitational-wave detection began on a Monday morning in September, and it started with a bang: a signal so loud and clear that Weiss thought, “This is crap. It’s gotta be no good.”

Fever Pitch

That first gravitational wave swept across Advanced LIGO’s detectors — first at Livingston, then at Hanford seven milliseconds later — during a mock run in the early hours of Sept. 14, two days before data collection was officially scheduled to begin.

The detectors were just firing up again after a five-year, 200-million upgrade, which equipped them with new noise-damping mirror suspensions and an active feedback system for canceling out extraneous vibrations in real time. The upgrades gave Advanced LIGO a major sensitivity boost over its predecessor, “initial LIGO,” which from 2002 to 2010 had detected “a good clean zero,” as Weiss put it. When the big signal arrived in September, scientists in Europe, where it was morning, frantically emailed their American colleagues. As the rest of the team awoke, the news quickly spread. According to Weiss, practically everyone was skeptical — especially when they saw the signal. It was such a textbook chirp that many suspected the data had been hacked. William Widmer for Quanta Magazine From left: A four-kilometer arm of the LIGO Livingston Observatory, the control room, and a schematic diagram of the detector’s “optical layout.” Mistaken claims in the search for gravitational waves have a long history, starting in the late 1960s when Joseph Weber of the University of Maryland thought he observed aluminum bars resonating in response to the waves. Most recently, in 2014, an experiment called BICEP2 reported the detection of primordial gravitational waves — space-time ripples from the Big Bang that would now be stretched and permanently frozen into the geometry of the universe. The BICEP2 team went public with great fanfare before their results were peer-reviewed, and then got burned when their signal turned out to have come from space dust. When Lawrence Krauss, a cosmologist at Arizona State University, got wind of the Advanced LIGO detection, “the first thought is that it was a blind injection,” he said. During initial LIGO, simulated signals had been secretly inserted into the data streams to test the response, unbeknownst to most of the team. When Krauss heard from an inside source that it wasn’t a blind injection this time, he could hardly contain his excitement. On Sept. 25, he tweeted to his 200,000 followers: “Rumor of a gravitational wave detection at LIGO detector. Amazing if true. Will post details if it survives.” Then, on Jan. 11: “My earlier rumor about LIGO has been confirmed by independent sources. Stay tuned! Gravitational waves may have been discovered!” LIGO The first gravitational wave signal was observed seven milliseconds apart on Sept. 14 at Advanced LIGO’s Hanford and Livingston detectors. The team’s official stance was to keep quiet about their signal until they were dead sure. Thorne, bound by a vow of secrecy, didn’t even tell his wife. “I celebrated in private,” he said. The team’s first step was to go back and analyze in excruciating detail how the signal had propagated through the detectors’ thousands of different measurement channels, and to see whether anything strange had happened at the moment the signal was seen. They found nothing unusual. They also ruled out hackers, who would have had to know more than anyone about the experiment’s thousands of data streams. “Even the team that does the blind injections have not perfected their injections well enough not to leave behind lots of fingerprints,” Thorne said. “And there were no fingerprints.” Another, weaker chirp showed up in the weeks that followed. The scientists analyzed these first two signals as even more swept in, and they submitted their paper to Physical Review Letters in January; it appeared online today. Their estimate of the statistical significance of the first, biggest signal is above “5-sigma,” meaning the scientists are 99.9999 percent sure it’s real. Listening for Gravity Einstein’s equations of general relativity are so complex that it took 40 years for most physicists to agree that gravitational waves exist and are detectable — even in theory. Einstein first thought that objects cannot shed energy in the form of gravitational radiation, then changed his mind. He showed in a seminal 1918 paper which ones could: Dumbbell-like systems that rotate about two axes at once, such as binary stars and supernovas popping like firecrackers, can make waves in space-time. Courtesy of Kip Thorne Rainer Weiss, a professor of physics at the Massachusetts Institute of Technology, around 1970. Still, Einstein and his colleagues continued to waffle. Some physicists argued that even if the waves exist, the world will oscillate with them and they cannot be felt. It wasn’t until 1957 that Richard Feynman put that question to rest, with a thought experiment demonstrating that, if gravitational waves exist, they are theoretically detectable. But nobody knew how common those dumbbell-like sources might be in our cosmic neighborhood, or how strong or weak the resulting waves would be. “There was that ultimate question of: Will we ever really detect them?” Kennefick said. In 1968, “Rai” Weiss was a young professor at MIT who had been roped into teaching a class on general relativity — a theory that he, as an experimentalist, knew little about — when news broke that Joseph Weber had detected gravitational waves. Weber had set up a trio of desk-size aluminum bars in two different U.S. states, and he reported that gravitational waves had set them all ringing. Weiss’ students asked him to explain gravitational waves and weigh in about the news. Looking into it, he was intimidated by the complex mathematics. “I couldn’t figure out what the hell [Weber] was doing — how the bar interacted with the gravitational wave.” He sat for a long time, asking himself, “What’s the most primitive thing I can think of that will detect gravitational waves?” An idea came to him that he calls the “conceptual basis of LIGO.” Imagine three objects sitting in space-time — say, mirrors at the corners of a triangle. “Send light from one to the other,” Weiss said. “Look at the time it takes to go from one mass to another, and see if the time has changed.” It turns out, he said, “you can do that quickly. I gave it to [my students] as a problem. Virtually the whole class was able to do that calculation.” In the next few years, as other researchers tried and failed to replicate the results of Weber’s resonance-bar experiments (what he observed remains unclear, but it wasn’t gravitational waves), Weiss began plotting a much more precise and ambitious experiment: a gravitational-wave interferometer. Laser light would bounce between three mirrors in an L-shaped arrangement, forming two beams. The spacing of the peaks and troughs of the light waves would precisely measure the lengths of the two arms, creating what could be thought of as x and y axes for space-time. When the grid was still, the two light waves would bounce back to the corner and cancel each other out, producing a null signal in a detector. But if a gravitational wave swept across Earth, it would stretch the length of one arm and compress the length of the other (and vice versa in an alternating pattern). The off-alignment of the two light beams would create a signal in the detector, revealing a fleeting tremor in space and time. Courtesy of the Archives, California Institute of Technology From left: Kip Thorne, Ron Drever and Robbie Vogt, the first director of the LIGO project, with a 40-meter prototype of the LIGO detectors at the California Institute of Technology in 1990. Fellow physicists were skeptical at first, but the experiment soon found a champion in Thorne, whose theory group at Caltech studied black holes and other potential gravitational-wave sources and the signals they would produce. Thorne had been inspired by Weber’s experiment and similar efforts by Russian physicists; after speaking with Weiss at a conference in 1975, “I began to believe that gravitational-wave detection would succeed,” Thorne said, “and I wanted Caltech to be involved.” He had Caltech hire the Scottish experimentalist Ronald Drever, who had also been clamoring to build a gravitational-wave interferometer. Thorne, Drever and Weiss eventually began working as a team, each taking on a share of the countless problems that had to be solved to develop a feasible experiment. The trio founded LIGO in 1984, and, after building prototypes and collaborating with a growing team, banked more than100 million in NSF funding in the early 1990s. Blueprints were drawn up for a pair of giant L-shaped detectors. A decade later, the detectors went online.

In Hanford and Livingston, vacuums run down the center of each detector’s four-kilometer arms, keeping the laser, the beam path and the mirrors as isolated as possible from the planet’s constant trembling. Not taking any chances, LIGO scientists monitor their detectors with thousands of instruments during each data run, measuring everything they can: seismic activity, atmospheric pressure, lightning, the arrival of cosmic rays, vibrations of the equipment, sounds near the laser beam, and so on. They then cleanse their data of these various sources of background noise. Perhaps most importantly, having two detectors allows them to cross-check their data, looking for coincident signals.

Inside the vacuum, even with isolated and stabilized lasers and mirrors, “strange signals happen all the time,” said Marco Cavaglià, assistant spokesperson for the LIGO collaboration. The scientists must trace these “koi fish,” “ghosts,” “fringy sea monsters” and other rogue vibrational patterns back to their sources so the culprits can be removed. One tough case occurred during the testing phase, said Jessica McIver, a postdoctoral researcher and one of the team’s foremost glitch detectives. It was a string of periodic, single-frequency artifacts that appeared every so often in the data. When she and her colleagues converted the mirror vibrations into an audio file, “you could clearly hear the ring-ring-ring of a telephone,” McIver said. “It turned out to be telemarketers calling the phone inside the laser enclosure.”

The sensitivity of Advanced LIGO’s detectors will continue to improve over the next couple of years, and a third interferometer called Advanced Virgo will come online in Italy. One question the data might help answer is how black holes form. Are they products of implosions of the earliest, massive stars, or do they originate from collisions inside tight clusters of stars? “Those are just two ideas; I bet there will be several more before the dust settles,” Weiss said. As LIGO tallies new statistics in future runs, scientists will be listening for whispers of these black-hole origin stories.

Judging by its shape and size, that first, loudest chirp originated about 1.3 billion light-years away from the location where two black holes, each of roughly 30 solar masses, finally merged after slow-dancing under mutual gravitational attraction for eons. The black holes spiraled toward each other faster and faster as the end drew near, like water in a drain, shedding three suns’ worth of energy to gravitational waves in roughly the blink of an eye. The merger is the most energetic event ever detected.

“It’s as though we had never seen the ocean in a storm,” Thorne said. He has been waiting for a storm in space-time ever since the 1960s. The feeling he experienced when the waves finally rolled in wasn’t excitement, he said, but something else: profound satisfaction.

• Joseph Ratliff says:

History in the making.

Terrific story, Natalie!

• Keith Tinkler says:

Dare one suggest that the Nobel Committee consider the award an Honorary Posthumous Century Nobel Prize to Einstein himself? After all he never got a Nobel for his relativity work! And it wouldn't cost them a penny (in anyone's currency).

• Kevin Fries says:

Amazing news. Great story. I just watched the news conference where they announced the findings live. This will be one of those events where I will always remember where I was when I heard it, although I imagine I might find this more exciting than the public at large. Still, super exciting.

• Bob Zimmerman says:

What does theory say about the speed of propagation?

• Natalie Wolchover says:

Bob: gravitational waves (like light waves) are predicted to travel at speed c. This is because gravitons (like photons) are theoretically massless. By observing how quickly the signal got from Livingston to Hanford, LIGO was able to place an upper limit on the mass of the graviton of 1.2e-22 electron-Volts. Shows the mass probably is 0.

• Alastair Dick says:

As an amateur, I had a belief in Einstein's mathmatical paradoxies/ predictions.
This Sigma 5+ is a wonderful result. Congratulations to the team.

• rick fetters says:

The best write up of today's announcement I have read; well done Natalie.

• CFT says:

Ok,
I'll bite.
How do they know what generated the signal? The article really doesn't say. What connects the signal they claim their device detected with a specific source? In which direction? This sounds like BICEP all over again.

• Ken Crost says:

I was wondering if another universe bumped into our universe, would that produced gravity waves that could be detected by LIGO? And if detected, would we be able to understand that they came from a bump from another universe?
And this really was a wonderful article.

• DS says:

I don't understand how the waves are persistent. If an event occurs at a point in time, we can see the consequences of the event and then it's gone. If the sun were to 'shutdown', I'd see no more light from it and that would be that. So why are the gravity waves persistent? Wouldn't they simply go through the earth leaving nothing to detect? Confused!

• wakefield says:

Does this get us any closer to understanding dark matter? Is that what is moving in the wave?

• Steve Demuth says:

@CFT: We "know" what generated the signal in the following sense: physicists have simulated what the gravitational waves generated by black holes merging would look like if measured for a variety of parameterized (by size of the respective black holes, e.g.) scenarios, and the signal detected matched to those predicted measurements for two black holes of specific masses spiraling into each other. The fact that it was detected in the same form in two detectors that are separated in space by 7 thousands of a light second, 7ms apart further enhances the detection, since an anomolous signal would be highly unlikely to appear in both in the same form.

• jrh says:

Natalie (and feel free to edit most of this out of existence),
The artist’s conception by M. Pössel of the gravitational wave movement, while simple, was elegant. I offer my appreciation to the artist for providing a visual of this complex phenomenon differing from the traditional bowling ball/net image. Of course, your writing style contributed greatly towards my immense reading pleasure. My hat is off.
This is what you may fling away. The sources of the gravitational waves being observed is suggested to be the result of the collision of two black holes. In itself a rarity (to us), there also exists the extremely more remote possibility of the collision of two black holes followed in (short) order by that of a third massive object resulting in a progressively complex catastrophic event depending upon several contributing factors such as timing, mass and paths to impact measurements. That's a handful. The probability of this occurring is immeasurably small. I argue that this possibility exists and should be maintained on the back shelf to be consulted upon the arrival of valid but inordinately puzzling data. The waveform for this would be exciting.
jrh

• dean day says:

Congratulations. How did you get rid of the telemarketers?

• Ken in CT says:

DS, I believe the event was not persistent. It was just a brief event whose effects coincidentally reached Earth a short time after we turned on the new detector, registered in the instruments, and then continued past us, outward bound from its source at the speed of light. It basically had no lasting effect other than in these instruments so carefully constructed to *be* affected by it. There will be other events, similarly brief, that will blip the instruments in the same way. The characteristics of each blip should tell us what sort of thing created it, much like a sonar operator can distinguish a whale chirp from a torpedo tube being opened. There will be other events that happened to occur X billion years ago, exactly X billion light-years away, and those should be detected, too.

• gustavo sarmiento says:

So my question is: is the fabric of space time so rigid it needs 2 black holes to merge and produce the GW? Or is the fabric of space time so weak that it needs to black holes to create a GW
and the other questions is how come space time fabric doesn't rip with the blasts

• CW says:

Comment by Natalie Wolchover: "… gravitational waves (like light waves) are predicted to travel at speed c. This is because gravitons (like photons) are theoretically massless."

This is not wrong, but one could argue that invoking gravitons is a bit of cheat. The notion of a graviton has no place in classical general relativity; it comes out of attempts to state key features of a quantum field theory based on linearized gravity, i.e., a linear approximation to full classical general relativity. The conclusion that gravitational waves travel at the speed of light can be derived from classical general relativity without reference to gravitons.

It also typically relies on a linear approximation, and the assumption that the wave amplitude is quite small. (Something roughly similar to this can be said about calculating the speed of sound waves in a medium, although with gravity—deviations of spacetime curvature from flatness—there is no medium in the usual sense.)

• Dave in SAN says:

I printed this article to read it off-line, but the font is so small as to make it unreadable. This only seems to happen with your longer articles. Please do not use the smaller font.

Thank you for the many excellent articles.

• Richard says:

I could be wrong, but I believe a major problem in fMRI (functional magnetic resonance imaging) is the corruption of signals due to the miniscule* disturbances originating in tiny movements of the subject's head. (N.B. I believe that these perturbations may occur even when people are doing their level best to keep their head perfectly still.)
Perhaps the techniques evolved by the LIGO team to insulate their apparatus from external disturbances (as well as those employed for the purpose of "cleans[ing] their data of… various sources of background noise") might be effectively adapted to aid in the production of less noisy fMRI's.

* I * know * the usual spelling is "minuscule": I just kinda * like * "miniscule". (It seems somehow "minier" to me.)

• James says:

Now that we can detect the waves, has anyone modeled the signature of gravitational lens on gravity waves?
One can imagine an very early gravity wave passing through a super cluster of galaxies and providing time separated signals.

What would be the expected delay of the signals?

• S Krishna says:

I have a doubt. I am not a physicist, but a retired structural engineer.
As both the light and gravitational waves travel at the same speed, when LIGO recorded the waves, could not the merging of the two black holes be seen? Were they seen? If not, why not?

• TP Leao says:

Is there a paper in Arxiv or elsewhere yet?

• Michael Moyer says:

@TP Leao:
Here's a link to the paper (it's also linked in the article):
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102

• [email protected] says:

This is a really wonderful discovery if it is true.

I guess all the discussion about "fringy sea monsters" and other spurious signals in the data left me a bit less confident in the result because they suggest that the experiment was not so well isolated from its environment as to eliminate such effects. Indeed, we now know that the sound of a telephone can inject signals into the system! Subtracting the signals out after they have contaminated the data is surely less satisfactory.

Any electromagnetic disturbance will (obviously) propagate at the speed of light, so if these can disturb the equipment, the fact that the signal was detected twice with a suitable time delay might not be as significant as it sounds!

• JonW says:

So if detecting a ripple in the Higgs Field meant we have discovered the Higgs boson (not "we have discovered Higgs waves"), does this new discovery of a ripple in space-time mean–via wave/particle duality–that we have discovered the graviton? I'm guessing not or it would appear in all the coverage… but why not, what's the difference?

• Chris Cantrell says:

"I have a doubt. I am not a physicist, but a retired structural engineer.
As both the light and gravitational waves travel at the same speed, when LIGO recorded the waves, could not the merging of the two black holes be seen? Were they seen? If not, why not?"

Thanks for voicing this question, S Krishna. The same question occurred to me. What would be nice is if the Advanced LIGO team could either enhance the experiment to detect from which direction the disturbance arrived, or complement the Advanced LIGO detections with wide-field observations of the sky, since it's unlikely a narrow field image would happen to be directed in the right place at the right time. I don't know enough about observation, though, to know whether or not a wide-field observation would have the power to resolve a black hole collision so far away.

• Dileep Sathe says:

S-T Fabric: Is it sensible?

The ecstasy among scientists on the discovery of gravitational waves is obvious. Although Albert Einstein, the Guru of all present physicists, predicted the gravitational waves 100 years ago some physicists are against the existence of such waves. For example, 15-year old conflict (and a bet also) between Stephen Hawking and Neil Turok, on the out-come of BICEP2 experiment.

Let me pose a question, which can come from a common man after a popular lecture. We can see the fabric, even the massive ball is not there – as shown usually in the picture. Then, can we see the fabric without ripples of gravitational waves. This question is very likely to be posed by a common-man because human-beings do not have sensory organs to sense time and space. That is why we have to rely on instruments or astronomical phenomena for counting time. We do not have an orgar for sensing space. It should be noted that we can find out the volume of a body – that is part of space occupied by the body BUT we cannot sense space.

Therefore now physicists have to launch a search for the space-time fabric without gravity’s ripples. This is similar to looking for luminiferous aether, required by Huygen’s wave theory of light.

• Robert Matthews says:

"Their estimate of the statistical significance of the first, biggest signal is above “5-sigma,” meaning the scientists are 99.9999 percent sure it’s real".

Pleeeeeease, Natalie – can you stop whoever it is who edits your stuff from appending these "explanations" to your use of sigma levels. Just tell them – yet again, no doubt – that it means the chances of getting at least as impressive a signal by fluke are just 1 in [fill in figure appropriate to sigma value probability].

As the Bicep team discovered, just because the chances of getting a result by fluke alone are staggeringly low says zip about the chances you've been fooled by some other cause – say, dust.

• jrh says:

Would there have been an advantage to constructing a second array so that one of itsd legs was perpendicular to the plane of the Washington site?

• Larry Anta says:

Dean Day asks, "How did you get rid of the telemarketers?"

They forwarded the phone to CERN.

• Brian W says:

S Krishna and Chris Cantrell, it seems directionality of waves is detectable (basically along the detector or not), but without a way to focus gravitational radiation, an image can't be formed. The detector only knows if it is hit by radiation or not, so it's roughly analogous to a photographic plate with no camera around it—no lens, pinhole, or parabolic reflector—so you just get 'light or no light' style detection, possibly with intensity and direction but definitely nothing like an image. I have no idea if some kind of interferometry is possible (that is, gravitational interferometry, not the light interferometry within the detector itself!).

• Carter Bancroft says:

I had a similar thought to that of S. Krishna, but instead about the possibility of testing the prediction that gravitational waves travel at the speed of light. Couldn't this prediction have been investigated in these studies (if some telescope(s) did in fact detect the collision of the two black holes), by comparing the time when light from the collision was observed, with the time of the detection of this event by LIGO? If gravitational waves do travel at light speed, these two observations should have occurred simultaneously (unless the speeds of light and gravitational waves might have been differentially affected during their travel to Earth through a distance of a billion light-years).

Carter: Black holes are, well, black. There would have been no light from the collision. At a billion light years distance, the odds of being able to discern them at all is nil.

I do have a question of my own, which should be answerable by someone in the know. What effect would these waves have on matter in the vicinity of the merger? Gravity waves couple weakly to matter, but then, 3 solar masses of energy in a few minutes is a truly phenomenal flux. Which "wins out"?

• James says:

So, if we hadn't tried to observe these waves would they have presented as particles ?

• Carter Bancroft says:

Blink: black holes themselves are of course black. But they are surrounded by accretion disks that release powerful X-rays and gamma rays. And I would think that the collision itself would yield additional high energy light. I think the main problem with my suggestion above is actually, as Brian W described so well above, that we know only the general direction from which the gravity waves caused by the collision arrived, so we wouldn't really know where to look for the source of the accompanying light. But I do agree that at a distance of a billion light years, it would be difficult to detect this light.

• kdl says:

I have concerns about Earth Tide by moon and sun (not ocean tide) effect and how the LIGO team dealt with it. This can’t be found in the PRL paper. Search the online pdf, only found Earth once, no tide or tidal. Earth tide effect can be calculated for the LIGO location based on the position of moon, sun and other factors, and I would hope the background noises mentioned in PRL paper include these in some detail to convince people that Earth tide effects are eliminated in the data analysis.

• EM says:

This is a good question, Blink.
The gravitational waves near their source were intense.
They would have distorted space and time immensely and ripped apart any normal matter we are familiar with.

• Dale Williams says:

Could someone please explain to me the medium these gravity waves propagate through?
Is it coincident with the Higgs field?
If this field is non-zero, what is it and can it be measured?

• rob snyder says:

From ordinary experience (that's all I have), the idea that a distance between two points in the world can be fixed, stable, seems unnatural. A very well engineered device may partially isolate two points from environmental movements caused by fluctuating temperatures and shifting ground and other conditions. There must be many kinds of environmental conditions that cause movement greater than 1/1000th the width of a proton. All of these known conditions have to be controlled for by sensors precise enough to account for 1/1000th the width of a proton, I guess? Is that so? With all of the known conditions accounted for with the necessary precision, what about unknown conditions that are not accounted for? With those two questions answered (explanation would be really interesting), another question is: what about the isolating contraption that stabilizes the measured points?

The idea of any actual thing (a mirror, a structure, a plane) having its position and rotation fixed to this precision is hard to comprehend. If the contraption that isolates the points from environmental conditions isolates effectively, how does it not then introduce some other kind of free movement? Thinking of Foucault's pendulum, an engineered joint isolates the pendulum from the earth, but that freedom is translated precisely into movement of a different kind. So I wonder, if a mirror's position is isolated from the motions of the surface of the earth, how is it that this freedom is not also translated into an expectation of some other kinds of movement within the contraption itself; how is it not a contradiction (or rather, how is it done?) that two points are isolated from the earth, but also are not free to move otherwise, not even 1/1000th the width of a proton?

• Chris D says:

As a layperson, I have so many questions that I can't find answers to. Also, I think the media often mis-reports science…

* I have seen articles that claim that we have definitively discovered two circling black holes at a distance outside of our observable universe. Is this true, or is this only theoretical? Ie, have we discovered both effect (gravitational waves) and cause (black holes) — or have we only discovered the effect and have inferred the cause?
* Can this detector determine exactly where the waves originated from? If so, how in the world does it do this? It's easy imagining pointing a telescope at a certain part of the sky — how do you "point" LIGO in a direction?
* Have we proven there is such a thing as a graviton or not? This is super confusing. Is the warping of spacetime a field akin to the Higgs with a wave/particle duality, or is gravity something else entirely?
* Should I be skeptical of this result? When I hear about subtracting of sources of interference, results matching computer models, and a black hole merger magically being detected during the first trial run — I become dubious that we have discovered anything at all. Even the same team that made the discovery also calculating their own sigma level feels a little like the fox watching the henhouse.

• Yubal Masalker says:

What are the wavelengths of the detected gravitational waves ??
Are those detectors (LIGO) still continuing to detect those specific gravitational waves ??

• QFan says:

Can anyone explain where the energy contained in the gravitational waves ends up. That is, is the energy eventually converted to heat energy in some way, or do the gravitational waves continue through space-time without losing energy? It seems that even the minuscule compression and expansion of the Earth as the wave propagates through would cause some energy loss. Three solar masses worth of energy has to end up somewhere. It would be interesting to calculate the total mass-energy contained in gravitational waves in the universe.

• Yubal Masalker says:

Dale Williams,

The medium is the space itself. I don't think these waves have anything with the Higgs field (as far as we know today). As Einstein has claimed in his General Relativity in 1916, and as it was verified in the measurements in 1919 by Eddington, the space itself is a real but non-materialistic fabric which gets deformed by static masses. 100 years after the General Relativity publication, it is seemingly getting verified that the space also gets vibrating around dynamic (moving) masses as was also predicted by Einstein through his General Relativity. These vibrations of space spread over the entire fabric of space in the universe.

• Harry Seldon says:

In the ligo communication they say: “Based on the observed signals, LIGO scientists estimate that the black holes for this event were about 29 and 36 times the mass of the sun, and the event took place 1.3 billion years ago. About 3 times the mass of the sun was converted into gravitational waves in a fraction of a second—with a peak power output about 50 times that of the whole visible universe.”

Does someone know what is the exact phenomenon in place for the “three suns’ worth of energy”?
Is it “only” a conversion of kinetic energy in a kind of elastic shock, is it something close to a nuclear fusion, or something completely different?
What happened at the time of the “explosion” that caused the gravitational waves?

• Tom Murphy says:

Brian: "I have no idea if some kind of interferometry is possible (that is, gravitational interferometry, not the light interferometry within the detector itself!)."

By way of wild speculation, might a large spherical structure in space be capable of rendering a 3d image from passing gravitational waves? Perhaps a spoked central core akin to the LIGO detector.

• Neil Trombly says:

For a single point to momentarily outproduce the energy release of the entire known universe by 50x raises a question for me. Is there no upper limit to the energy conductance capacity of space-time? Why is there no sign of overload that might cause a tear in space-time? What does this tell us about the mind-blowing "surface tension" of space-time that it could conduct away so much energy and, incredibly, perfectly preserve intact the original orderly signal? The Big Bang must have been really big to break out of subservience to space-time the way it did.

• Neil Trombly says:

Could it be that the Big Bang was the collision of two incomprehensively massive black holes providing an energy release so large that the surface tension of space-time was exceeded and the universe started over?

• Jamie says:

So is this datapoint (assuming the noise has been properly cleaned) also proof of Lorentz's Aether theory, the math of which is the basis of relativity?
Could also be seen to prove quantization within a VR theory?
Space folding within Bohms theory?
Is there a reason this is presented as proof by the media, when it hasn't been replicated, and is not a direct measurement of space-time, but rather length?
Like a lot of scientific discoveries, too much big talk, too soon methinks. What is exciting is that _maybe_ this shows length contracting and expanding. Which is fascinating, and a great datapoint for many models to try and account for. What has caused the phenomena, if it proves to be real, however is unknown – on the level of either black holes, or space curving.

• Friedwardt Winterberg, Professor of Physics, Academician IAA Paris France says:

Here is a problem:
The PRL article says: "… we report the first direct observation of a binary black hole system merging to form a single black hole". But according to the general theory of relativity, seen from an outside observer, as is the LIGO, the time to form even a single black hole is infinite with the time standing still at the event horizon. Therefore, where do the two merging black holes come from?

• David Mitchell says:

I assume that the signal delay between the Hanford and Livingston sites allows determination of the wavefront angle with respect to the baseline between the two interferometers, by using simple trigonometry based calculations.
Am I correct in assuming that the straight-line path (rather than the ground based distance of 3,001 km) should be used for this calculation? Would the Earth offer perceptible impedance to gravitational waves, and would it produce any waveform distortion?

• David Mitchell says:

@ S Krishna – The baseline between the Hanford and Livinston sites allowed only a location to be determined within 590 square degrees of the sky (with 90% probability).
There is another interferometer in Italy (Virgo) which is in collaboration with LIGO, but I believe it is undergoing upgrades which would increase its sensitivity.
Other observatories are located in Germany and Japan, and there are plans for other wordwide facilities. A synchronised wordwide network of GW observatories would give the long baselines necessary to increase angular resolution, and thus to localize the sources within a reasonable search area, which would allow optical and radio observatories to identify and further study GW sources.

• Yubal Masalker says:

I’m afraid that with the huge excitement coming up with this discovery, one major point is missed by the LIGO publications and by the other Physicists in their explanations to the wide public. They are too much emphasizing the Black Hole stuff in their explanations about the Gravitational Waves and by doing this they mislead the wide public into thinking that Gravitational Waves are only related with black holes and their collisions. Their publications don’t emphasize enough the real revolutionary aspect of this discovery.

The real revolutionary aspect of this discovery is that *moving mass DOES create vibrations of the space itself*. It’s about ANY mass and not just about Black Holes mass. It’s about the vibrations OF the space itself and NOT about any vibrations IN the space. It’s the space itself which gets wavy (vibrating) and NOT any other external wave moving in the space. Any moving mass, no matter how small it is, makes ALWAYS the space nearby it to become wavy, let it be even the tiniest wavy form of space caused by any tiny moving mass. This local wavy form of space, nearby any moving mass, spread all over the space in the form of the so called Gravitational Waves, by the speed of light.

The importance of the black holes and their collision in this discovery is just in their immense massiveness and in the abrupt collision (half of the speed of light at the moment of collision) of such immense masses, which created all together gravitational waves which are just strong enough to be hardly measurable by such sensitive and sophisticated installation called LIGO from such a huge distance (1.3 billion light years away).

• Chris D says:

If someone could answer my questions above, I would greatly appreciate it.

There really is a gap between surface level knowledge (that the media reports) and the hard science (published papers). I want that next level down below the surface, but unfortunately no one seems to report on that. I'm always left frustrated, with no answers…

• marten says:

"Ripples in space-time have been detected…"

"The first gravitationel wave signal was observed seven milliseconds apart…."

I have two questions: how many signals were observed after the first one and how many ripples were counted in total?

• David Mitchell says:

In answer to marten. LIGO recorded just one signal, firstly at Livingston on the East coast, then a delayed version of the same signal (6.9 millisecond later) at Hanford on the West coast.
The measured delay (from my understanding) gives the angle of the gravitational wave wrt the line between the two LIGO interferometers. As for ripples, I believe that refers to the peaks and troughs in the GW, which correspond to the waveform peaks, as visible in the waveform diagram shown in the article.

• David Mitchell says:

As follow up to my earlier response to marten.
Clean images of the signal waveform are contained in the LIGO report, which was published in Physical Review Letters, and is available as free download.
Cardiff University in UK has put out a "GW150914 FactSheet" which gives more information about the GW received by LIGO. This gives 12 cycles (ripples) over a 0.2 second period for the GW150914 event, as well as other details which have been inferred from signal analysis.
Interestingly, the GW wavelength at peak intensity is given as 2,000 km !

• Daniel N. Russell, physicist says:

I am not convinced! My problem with this alleged gravity wave detection by LIGO is that it is assumed too quickly that gravity waves must travel at light speed, and that the event seems to be too short-lived to represent a cataclysmic merger of black holes orbiting each other at nearly the speed of light. That this wave travelled from one LIGO antenna array to the other at exactly the speed of light should be a clue that this may have been a type of electromagnetic wave burst, such an X-ray burst that was red-shifted enough to interact with the LIGO antenna.
I suggest, astronomers look carefully at any recordings of X-ray bursts or other E-M bursts occurring at the time of this alleged gravity wave detection reported by LIGO.
Yes, LIGO detected something. But, let us not leap to the conclusion that it could not have been anything other than a gravity wave, without deep scrutiny.

• marten says:

@ David Mitchell

Thanks

• Serg says:

Any wave stretches in some physical environment. In what physical environment the gravitational wave stretches? What fluctuates?

• Man of Mytery says:

Space – specifically the space between the mirrors at each end of the LIGO 4 kilometre arms compresses and stretches in response to gravitational waves from merging black holes.

• Mikey says:

Yay! Big Einstein fan, and this is a score for him, and all scientists!

• Harry Schneider says:

Been following HORIZON programs for years now. Got quite lost last night about the DARK ENERGY thing. My understanding is that the speed that galaxies are doing right now are actually slowing down. After all, the Andromeda galaxy is coming closer to the Milky Way just now and not accelerating away. The observation that galaxies are accelerating away from each other is a function of their past location. The explosive nature of the early universe meant that the galaxies appeared to be accelerating away from each other then. As they are not accelerating from each other just now there is therefore no need for us to consider the existence of DARK ENERGY at all.