As expected, gravitational waves, which were predicted by Albert Einstein a century ago, have been detected by the LIGO Collaboration:
Scientists are claiming a stunning discovery in their quest to fully understand gravity. They have observed the warping of space-time generated by the collision of two black holes more than a billion light-years from Earth. The international team says the first detection of these gravitational waves will usher in a new era for astronomy.
It is the culmination of decades of searching and could ultimately offer a window on the Big Bang. The research, by the LIGO Collaboration, has been accepted for publication in the journal Physical Review Letters. The collaboration operates a number of labs around the world that fire lasers through long tunnels, trying to sense ripples in the fabric of space-time. Expected signals are extremely subtle, and disturb the machines, known as interferometers, by just fractions of the width of an atom. But the black hole merger was picked up by two widely separated LIGO facilities in the US.
The historic paper in question: Observation of Gravitational Waves from a Binary Black Hole Merger (open, DOI: 10.1103/PhysRevLett.116.061102)
Archived video of the press conference webcast will be available here.
NASA provided an infographic for their Astronomy Picture of the Day feature with details about the discovery.
Also at NPR, NYT, Scientific American, and Ars Technica Live, The New Yorker. BBC's Jonathan Amos offers an analysis of the discovery.
Related Stories
Thursday Feb 11th will likely go down in scientific history as the formal announcement of the widely-leaked and hinted-at first detection of gravitational waves.
The LIGO gravitational wave team is having a press conference on Thursday at 10:30am EST to announce the widely expected to result in Nobel Prizes first detection of gravitational waves.
The LIGO team's press release notes:
(Washington, DC) -- Journalists are invited to join the National Science Foundation as it brings together the scientists from Caltech, MIT and the LIGO Scientific Collaboration (LSC) this Thursday at 10:30 a.m. at the National Press Club for a status report on the effort to detect gravitational waves - or ripples in the fabric of spacetime - using the Laser Interferometer Gravitational-wave Observatory (LIGO).
Do any Soylentils have the "secret" URL for the webcast? Please don't do anything stupid on this historic occasion, but it would be cool to watch history being made. Its kind of the physics equivalent of a moon rocket launch. It's very widely leaked that history will be made Thursday morning... wouldn't you like to see it?
Backreaction has everything you need to know about gravitational waves for preparation for the webcast.
It's an exciting time to be alive! On the other hand, if the endless leaks and insinuations are bogus, its also an exciting time to be pissed off, too.
The European Space Agency's LISA Pathfinder (LPF) will begin testing gravitational wave detection in space, in preparation for a full observatory to be launched later:
The formal test programme has begun on the technologies required to detect gravitational waves in space. Europe's Lisa Pathfinder (LPF) probe is engaging in a series of experiments roughly 1.5 million km from Earth. The project has heightened interest, of course, because of the first sampling of the "cosmic ripples" made by ground-based detectors last September. A successful demo for LPF would pave the way for a fully operational orbiting observatory in the 2030s. This would likely be known simply as Lisa — the Laser Interferometer Space Antenna.
"It's a wonderful time right now," said Paul McNamara, the European Space Agency's (Esa) project scientist on Lisa Pathfinder. "I've spent my entire career in this endeavour, and for years we were told — even ridiculed in some cases — that gravitational waves don't exist, or that we'd never find them. Well, now we have found them, and we're about to take the next big, big step towards building a mission that could detect them in space," he told BBC News.
The Earth-bound laser interferometers sited at the Advanced Ligo facilities in the US are sensitive to the gravitational waves generated in "smaller" cosmic events. Back in September, they observed the signal produced at the moment two black holes, each about 30 times the mass of our Sun, whirled around one another and merged. A space-based laser interferometer would chase much more massive targets - the monster black holes, millions of times the mass of our Sun, that coalesce when galaxies collide, for example.
Previously on SoylentNews:
Advanced Ligo Facilities Begin Search for Gravitational Waves
Gravitational Waves Detected From Black Hole Merger
Physicist Brian Greene of Columbia University appeared on Wednesday's The Late Show with Stephen Colbert. Bringing a laser interferometer, he explained gravitational waves and the LIGO experiment.
Without missing a beat, Greene jumped right in.
"Spacetime is a four dimensional Hausdorff differential manifold on which a metric tensor is imposed that solves the Einstein field equations and that metric tensor gives rise to geodesics and objects that are not experiencing any other force move along the geodesic described by the metric."
Stephen Colbert's assessment: "That's the s**t right there!"
Previously: Gravitational Waves Detected From Black Hole Merger
Gravitational waves were the most controversial and difficult to verify prediction of Albert Einstein's Theory of General Relativity, so much so that at one point even Einstein himself thought that they might just be an artefact of the mathematics. It wasn't until the 1970s that careful observations of binary pulsar systems showed the indirect effects of gravitational waves, and not until 2016 that LIGO, an extremely sensitive instrument designed to detect gravitational waves directly, managed to detect the gravitational waves from the merger of two black holes. It has made two more gravitational wave detections since. However, a new analysis of the LIGO data by an independent team led by Prof. Andrew D. Jackson at the Niels Bohr Institute in Copenhagen has cast doubt on the detections, hinting that they might just be seeing patterns in the noise. Sabine Hossenfelder has an article on this:
A team of five researchers — James Creswell, Sebastian von Hausegger, Andrew D. Jackson, Hao Liu, and Pavel Naselsky — from the Niels Bohr Institute in Copenhagen, presented their own analysis of the openly available LIGO data. And, unlike the LIGO collaboration itself, they come to a disturbing conclusion: that these gravitational waves might not be signals at all, but rather patterns in the noise that have hoodwinked even the best scientists working on this puzzle.
The LIGO gravitational wave observatory consists of two experimental sites – one in Livingston, Louisiana and one in Hanford, Washington – each of which is a laser interferometer with arms that are several kilometers in length. Even for these super-sensitive detectors, however, gravitational waves are difficult to measure. The problem isn't so much the absolute weakness of the waves, the problem is that there are many other disturbances that also wiggle the interferometer. The challenge, thus, is to tell the signal from the noise.
[...] The Danish group found, however, that the noise at both detector sites — and puzzlingly, between the two supposedly independent detectors — is also correlated. And worse, the correlation time is similar to the time-lag between the recorded signals, for each of the three so-far confirmed events. According to Andrew Jackson, the leader of the Danish group,
"If the correlation properties of signal and the noise are similar, how is one to know precisely what is signal and what is noise?"
That's a really important realization. A correlation in the noise would not affect the individual signals at each of the sites. But in order to achieve a highly significant signal between the detectors, the LIGO collaboration takes into account how both signals are correlated. If this correlation were not reliable, because (for example) there was the possibility that noise correlations contaminated their data, the statistical significance of the detection would be reduced. In other words, what appears to be a signal might actually be caused merely by fluctuations. How much the statistical significance would be affected, however, the Danish researchers have not quantified.
(Score: 2) by isostatic on Thursday February 11 2016, @04:27PM
Why are things so heavy in the future? Is there a problem with the Earth's gravitational pull?
(Score: 2) by takyon on Thursday February 11 2016, @04:30PM
Gravitational memes detected in YouTube live chat [youtube.com].
[SIG] 10/28/2017: Soylent Upgrade v14 [soylentnews.org]
(Score: 2) by Tork on Thursday February 11 2016, @04:41PM
🏳️🌈 Proud Ally 🏳️🌈
(Score: 2) by bob_super on Thursday February 11 2016, @06:35PM
I've had too many coworkers which could be detected by the earthquake waves created when they moved around. My old boss could visibly shake my monitor from about 20 feet away, just by walking his 300++ pounds down the corridor.
Jokes about gravity waves are not new.
(Score: 1, Insightful) by Anonymous Coward on Thursday February 11 2016, @04:44PM
great scott!
(Score: 2) by Bobs on Thursday February 11 2016, @04:37PM
I am still working to grok how gravity is actually a warp in space/time and not some sort of attractive energy so I can better explain it to my kids. We are not being “pulled down” by gravity - space is just bent in that direction. Or something...
Glad this sort of research is happening. Warms the cockles of my soul.
(Score: 5, Informative) by NotSanguine on Thursday February 11 2016, @04:50PM
Another Soylentil posted this comic [phdcomics.com] the other day.
Perhaps it will help.
No, no, you're not thinking; you're just being logical. --Niels Bohr
(Score: 3, Interesting) by maxwell demon on Thursday February 11 2016, @06:28PM
I'm not happy with this rubber sheet explanation. Here's why. [soylentnews.org]
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by NotSanguine on Thursday February 11 2016, @06:48PM
I suggest you contact the author [jorgecham.com] of the comic to express your displeasure. I'm sure he'd appreciate your feedback.
No, no, you're not thinking; you're just being logical. --Niels Bohr
(Score: 5, Interesting) by AnonymousCowardNoMore on Thursday February 11 2016, @07:17PM
All analogies are imperfect. The rubber sheet analogy is great for intellectual levels ranging from toddlers to journalists up to say, early teenagers. But I think one can really do better with the more educated readership of PhD or Soylent.
Imagine you're on the surface of a (sufficient approximation of a) sphere. Now stop imagining 'cause you are. Have a buddy stand next to you, then the two of you face directly east. Start walking in a straight line (i.e. great circle) and eventually you'll collide*. Not because of any rubbery stuff under your feet, nor any attractive force between you, but because the geometry which you are constrained to move under does not allow parallel lines.
In the same way, gravity is interpreted not as a force but as geometry. One could then say that the Earth is not held in orbit around the sun by a gravitational force, but is in fact moving in a straight line within a geometry that is determined by gravity. Once inside a black hole, things are extreme enough to be similar to the sphere example: it is impossible for two bodies to not collide at the singularity, no matter their speed or direction of motion, if any.
Marrying this interpretation with quantum theory is left as an exercise for the reader.
* But before that you'll get bored and go home.
(Score: 1, Interesting) by Anonymous Coward on Thursday February 11 2016, @08:12PM
Geometry is indeed the right way to think about the 'distortion of space time' discussed in general relativity. This site [pitt.edu] does a good job developing the concept in an understandable way (despite odd choices for text layout).
The 'curved sheet' diagrams are one way to visualize the stretching of space. What's important to understand is that the out-of-plane distance is not meant to be taken literally (space is not stretching into some other dimension), but is rather a way of describing how patches of space are progressively getting bigger than they 'ought to be' (i.e. bigger than the naive Euclidian flat space concept). So the 'force' of gravity is simply the curvature of spacetime. Objects are 'trying' to move in straights lines; but the stretching/curvature of space means that 'lines of minimum distance' are actually curved, not straight. This 'curving' of objects looks like an attractive force (free-falling objects curve towards sources of mass, like planets).
One can also think of this as the tilting of the local timelike direction (which is orthogonal to the spacelike directions of the sheet). In this sense, objects are just naturally propagating 'into the future', but the (local) direction of this inherent propagation tilts (owing to curvature of spacetime). So near a massive body (planet, star), the timelike direction is slightly 'tilted' towards the mass. So an object will inherently propagate in that direction.
All of these words/images are just ways to try to visualize what's really going on: spacetime is a geometric construct, with the goemetry being influenced by the local presence of matter/energy. Matter and energy tell spacetime how to curve. The curvature of spacetime tells matter and energy how to move.
(Score: 2) by Bobs on Friday February 12 2016, @04:44AM
Thanks - I realize now that I having a classical "map is not the territory" (Bateson [wikiquote.org]) problem.
The rubber sheet model makes sense to me for orbital mechanics.
Where I seem to be stumbling how to visualize the warping of space as I move into the earth?
For example, if I am standing on the surface of the earth holding a ball, and I release the ball, there is a gravitational effect towards the center of mass.
But as I move through the earth, how do I visualize the offsetting change in effect as more mass is overhead versus under my feet?
Then once I reach the center and the Earth's gravitational effect on the ball when I release it: The ball doesn't move - the "force" of the earth's gravity is at an equilibrium. How do I visualize the warp of space/time to account for this?
I can visualize/represent the gravitational effect at the center of the earth as a series of offsetting vector / force arrows. But that is not an accurate model of how space is warped by gravity.
Can someone please suggest how to visualize / model the warping of space while moving within a gravity well? I think that is whereI am getting stuck. (I am sure somebody has made an elegant visualization but I haven't found it yet in all the stacks of rubber sheets.)
I am feeling confused.
Thanks for any insights.
FYI: I have found a bunch of videos like this - Gravity from Newton to Einstein [youtube.com] but they all seem to use the same rubber sheet model.
And this: I don't see how it explains the model of the warping of space within the mass of the earth (for example): http://www.einstein-online.info/elementary/generalRT/GeomGravity [einstein-online.info]
(Score: 0) by Anonymous Coward on Friday February 12 2016, @02:13PM
Also, how is this geometric model at all compatible with the idea of a graviton?
(Score: 0) by Anonymous Coward on Friday February 12 2016, @09:33AM
I watched the video several times but it still didn't help me to understand women.
(Score: 2) by wonkey_monkey on Thursday February 11 2016, @05:18PM
Rubber sheet - or maybe just a taut bedsheet - and a couple of balls should do it. Tap the sheet and you might be able to see ripples spreading out.
Bubbles in the tub cause dents in the water and draw each other together at close range, too. Get your kids to fart for science!
systemd is Roko's Basilisk
(Score: 0) by Anonymous Coward on Thursday February 11 2016, @05:39PM
Think of a swimming pool. Two people are standing about 3 feet from each other. One person swims between them perpendicularly. Each of the bystanders are temporarily drawn towards each other.
Gravity does the same thing. There is also matter between objects in space and everywhere else. The larger the disturbance/perturbance the easier it is to perceive, but so called dark matter[really just smaller than us, stuff] is there binding us to everything else.
(Score: 2) by maxwell demon on Thursday February 11 2016, @06:31PM
Sorry, but that's definitely not was General Relativity tells us.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by PartTimeZombie on Thursday February 11 2016, @09:22PM
Sorry, but that's definitely not was General Relativity tells us.
OK, then try this:
A pizza delivery guy knocks on a door. An attractive woman wearing only a diaphanous nighty answers the door...
(Score: 2) by HiThere on Thursday February 11 2016, @07:55PM
One way of thinking about it is that the position of small thing is uncertain, and gravity tends to slow time, so random variations of position tend to cause things (especially lighter things, electrons, neutrinos, photons, etc.) to drift into slower time. Since any large thing is build out of absolutely huge numbers of the smaller things, it gets difficult to move away from the "attraction".
I'm not certain that this is 100% accurate, but it seems to work. There's probably lots of details that need to be fleshed out, though.
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 0) by Anonymous Coward on Thursday February 11 2016, @09:09PM
Think of it like centrifugal force (the one they say is fake). If are spun around a point quickly, you feel a pull outward, but it is simply inertia sending you outward. The centripetal force keeping you from flying out is the actual force being applied to you.
Likewise, the earth curves space-time into itself, and inertia sends you to its center. The real force is the ground pushing you upwards, but you feel like you are being pulled down because the ground is accelerating you upwards fast enough to keep you from the center of the earth.
At least, I think that is how it works.
(Score: 2) by takyon on Thursday February 11 2016, @04:54PM
Why did YouTube and BBC end their live streams? It's still up at Reuters, but I had to scramble to find it:
http://live.reuters.com/Event/Outer_Space [reuters.com]
[SIG] 10/28/2017: Soylent Upgrade v14 [soylentnews.org]
(Score: 3, Informative) by Anonymous Coward on Thursday February 11 2016, @05:13PM
The link to the Astronomy Picture of the Day points to the changing-every-day page. It should probably be replaced with the permalink: http://apod.nasa.gov/apod/ap160211.html [nasa.gov]
(Score: 2) by takyon on Thursday February 11 2016, @05:34PM
Done, thanks.
[SIG] 10/28/2017: Soylent Upgrade v14 [soylentnews.org]
(Score: 2) by Zinho on Thursday February 11 2016, @06:15PM
Does this result give a final answer to the question of whether gravity propagates at speed of light or not?
For those of us dabbling in the subject it would be good to have someone who is "in the know" weight in.
"Space Exploration is not endless circles in low earth orbit." -Buzz Aldrin
(Score: 0) by Anonymous Coward on Thursday February 11 2016, @06:38PM
I thought gravity propagates faster than speed of light... however gravity waves are not necessarily gravity.
(Score: 2, Informative) by Anonymous Coward on Thursday February 11 2016, @07:24PM
In every meaningful sense, gravity propagates at c (the speed-of-light). Gravitational waves propagate at c. A static gravitational field doesn't really have any 'speed' associated with it (it is static, unchanging in time). However, any change to a gravitational field (e.g. a mass changing velocity) propagates outward as gravitational waves (again, travelling at c). So gravity only 'updates' at the speed of light. The usual thought-example is: if the sun magically disappeared, the Earth would continue orbiting around that empty spot for 8 minutes, which is how long it takes light (and gravitational changes) to reach Earth.
[Note that the above is based upon the predictions of general relativity, which has been experimentally tested innumerable times. Theory and experiment have always agreed perfectly. Of course, there is always the possibility of us discovering some new physics that contradicts our current understanding. But, as far as we can tell, gravity propagates at the speed-of-light.]
(Score: 0) by Anonymous Coward on Friday February 12 2016, @01:55PM
Mod the parent up. Basically, take one step back. Things that can change gravity field can only happen at c at the fastest. Assuming instant reflection in the gravity field, that means any change in the gravity field, i.e., propagation, is also limited to c.
(Score: 0) by Anonymous Coward on Thursday February 11 2016, @06:48PM
This detection didn't independently measure the speed of propagation of gravitational waves--but it essentially indirectly confirms that they propagate at c (speed-of-light). This detection is perfectly in-line with the predictions of general relativity, within which gravitational waves propagate at c.
The concordance between the theoretical prediction and the experimental result is truly awe-inspiring. (Especially when you consider that the theory was worked out over 100 years ago.)
(Score: 0) by Anonymous Coward on Thursday February 11 2016, @07:00PM
well they also measure the propagation speed as c, since the two detectors are at opposite corners of the US. get the PRL .pdf itself, it has nice pictures.
(Score: 5, Interesting) by Anonymous Coward on Thursday February 11 2016, @07:17PM
They measured a delay between the detection at the two LIGO stations that is consistent with c. But they can't use this to directly measure the propagation speed. The delay between the two detectors is based on the propagation speed of the wave, but also the angle at which the wave is approaching Earth. E.g. if the wave comes from directly 'in between' the two LIGO stations, then they will detect the wave simultaneously (which tells you nothing about propagation speed).
This single event had a slight delay between the two detections, which allows us to place an upper bound on the propagation velocity of gravitational waves (we certainly confirmed that they do not travel at infinite speed). As LIGO continues collecting data, it will be able to add more and more constraints, and so in a statistical sense it should be able to confirm that gravitational waves propagate at the speed-of-light.
There are plans for additional detectors to come on-line at different locations on Earth. Once we have more than 2 stations, we will be able to triangulate (at least roughly) the direction from which waves arrive, and measure their speed much more accurately. Exciting times ahead!
(Score: 2, Interesting) by dogvomit on Thursday February 11 2016, @08:11PM
Specifically, the delay between the two events was 7.1 ms. So this proves that gravitational radiation cannot travel faster than 1.4c, but we don't get any lower bound from this single measurement. Of course, we all know it is c :-).
(Score: 1, Interesting) by Anonymous Coward on Thursday February 11 2016, @08:18PM
Maybe you can clarify. From my reading it sounds like they filtered out signals that did not correlate between sites. Since this correlation was calculated using a time offset calculated using the speed of light, they could only detect gravity waves traveling at that speed.
(Score: 0) by Anonymous Coward on Friday February 12 2016, @09:01PM
I'm sure the allowed the time offset to vary... but maybe they missed a trick when spending the last 20 years designing the experiment, you should definitely email them.
(Score: 0) by Anonymous Coward on Friday February 12 2016, @09:22PM
This is what was written down in the paper:
Are you claiming that is inaccurate and they actually did something else?
(Score: 2) by maxwell demon on Thursday February 11 2016, @07:34PM
Was the event also visible in the EM spectrum (through its effects on matter around the black holes)? If so, comparison of the time of arrival of the EM and the gravitational waves should give quite tight constraints to the speed of gravitational waves.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 1) by dogvomit on Thursday February 11 2016, @07:49PM
No, nothing was observed for this one. A colleague of mine is one of the many coauthors on a follow-up paper about that and described it to me this morning. If I can get a citation, I'll follow up with it.
(Score: 0) by Anonymous Coward on Friday February 12 2016, @12:19AM
https://en.wikipedia.org/wiki/Speed_of_gravity#Possible_experimental_measurements
(Score: 2, Funny) by Anonymous Coward on Thursday February 11 2016, @06:22PM
Chris Christie dropping out of the candidate race.
(Score: 4, Interesting) by dogvomit on Thursday February 11 2016, @07:43PM
There are great details in their Phys. Rev. Lett. [ligo.org] which is actually surprisingly readable, even though it is the longest one I have ever seen or heard of.
But just thinking of three solar masses of matter converting to energy over 20 milliseconds! Wow. Not quite ten-to-the-fiftieth watts, but still. Holy crap!
—G
(Score: 0) by Anonymous Coward on Thursday February 11 2016, @08:05PM
This seems like the important info. They go over how other possible explanations were ruled out:
https://dcc.ligo.org/LIGO-P1500238/public/main [ligo.org]
(Score: 0) by Anonymous Coward on Thursday February 11 2016, @08:25PM
Well, there goes my chance of getting my humble paper published in Phys. Rev. Lett.
I'll start considering other options [universalrejection.org].
(Score: 0) by Anonymous Coward on Friday February 12 2016, @12:29AM
Compare their signal to what was expected a priori due to a "burst". This looks a lot more similar than the actual signal to "inspiral", which is the preferred explanation. Why? What is wrong with burst? To my laymans eye, It looks like they arbitrarily chose inspiral.
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102# [aps.org]
http://www.ligo.org/science/GW-Inspiral.php [ligo.org]
http://www.ligo.org/science/GW-Burst.php [ligo.org]
(Score: 1) by loki on Friday February 12 2016, @07:11AM
The signal they observed had an increasing amplitude, and a reducing wavelength, which corresponds well with the inspiral model. The model is that two large objects were orbiting each other faster and faster, eventually colliding (well, merging). The amplitude of gravity waves increased up until they hit, and the speed at which they were orbiting each other also increased until that point. (For an analogy, consider those donation buckets at museums where you put a coin in and it rolls down and around, getting faster and faster as it approaches the hole at the bottom, except both the hole at the bottom and the coin both have thirty times the mass of our sun).
By comparison, the burst is a sheer guess at what we might see from a supernova or some other source. The example they gave does the opposite of the signal that's been observed - a big blip at the start, then a reduction in amplitude towards noise. To be fair, there's a bit of that reduction (ringdown) at the end of the inspiral signal too, except the example computed graph doesn't show that bit. The detected signal apparently shows some ringdown as the newly merged black hole rings, shedding some graviational energy as it settles into a spherical shape.
(Score: 0) by Anonymous Coward on Friday February 12 2016, @02:03PM
Thanks, I guess what I'd need is some idea of the range of possible inspiral signals consistent with GR and the same for these "bursts". I did notice the signal looked like a "reverse burst" rather than the example shown, but that seemed to lead down the path of gravity waves travelling backward in time.
(Score: 2) by el_oscuro on Friday February 12 2016, @02:34AM
xkcd [xkcd.com]
SoylentNews is Bacon! [nueskes.com]
(Score: 2) by aristarchus on Friday February 12 2016, @05:39AM
That is one Obligatory XKCD. Just saying.
(Score: 0) by Anonymous Coward on Friday February 12 2016, @01:26PM
1) They misinterpret the p-value:
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102 [aps.org]
The p value is not a false alarm probability. It is the probability of seeing at least as extreme a deviation given their model of background is true and nothing else is going on. This is well known, see for example here: http://www.statisticsdonewrong.com/p-value.html [statisticsdonewrong.com]
2) They do not correct for all multiple comparisons:
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102 [aps.org]
Here we see they do correct for checking the data three different ways, but they only consider this one run:
https://dcc.ligo.org/LIGO-P1500238/public/main [ligo.org]
They need to correct for all the previous runs where they looked for signals. Since this was only based on six weeks, that will probably mean multiplying that p-value by a factor of ~8 for each year these experiments have been running. This is not some pedantic complaint, using what I have gathered is their method of statistical analysis they are bound to see such a signal eventually just due to background and they have underestimated this possibility by something like 10x-100x. I don't see the exact calculations they did published anywhere so maybe they did something different from what is described in the papers.
Now, personally I don't think this would affect their findings. They don't actually seem to need this statistical reasoning, it is just irrelevant. They ranked all the possible signals according to how similar they were to that predicted by theory and took a closer look at the best one. They ruled out every other explanation for this signal (that people have come up with so far), and found their theory could explain the signal very well.
(Score: 1) by khallow on Saturday February 13 2016, @05:22PM
They need to correct for all the previous runs where they looked for signals.
At the same sensitivity and precision? From the article,
The Ligo laser interferometers in Hanford, in Washington, and Livingston, in Louisiana, were only recently refurbished and had just come back online when they sensed the signal from the collision. This occurred at 10.51 GMT on 14 September last year.
That apparently was a significant improvement [ieee.org].
When the detectors ramp up to their full sensitivity, Advanced LIGO is expected to be able to pick up gravitational wave-creating events–in particular the violent, inspiraling merger of two neutron stars—in a volume of space 1000 times bigger than what the initial incarnation of LIGO could probe. Some are quite optimistic about the new experiment’s chances. According to a report in Nature, some physicists peg the chances of a detection in the next three months as high as 1 in 3.
At a guess, it sounds like the system improved in sensitivity by an order of magnitude.
Since this was only based on six weeks, that will probably mean multiplying that p-value by a factor of ~8 for each year these experiments have been running.
You still end up with a p value of around 10^-5 even if we ignore the improvements in the experiment last year.
(Score: 0) by Anonymous Coward on Sunday February 14 2016, @06:49AM
If you follow to the nature you link you see that 1000x improvement refers to something supposed to happen later this year:
http://www.nature.com/news/hunt-for-gravitational-waves-to-resume-after-massive-upgrade-1.18359 [nature.com]
Also, interesting they say the expected range for "regular" detections after the future improvements is ~ 0.1 Gpc. In a sister paper it is estimated that this signal came from 0.2-0.6 Gpc:
http://iopscience.iop.org/article/10.3847/2041-8205/818/2/L22 [iop.org]
I'm not sure what would make this detection irregular. Perhaps LIGO was operating at higher than usual sensitivity. Later in that same paper they report previous estimates for such events range from ~0.1-1000 per Gpc^3 per year:
That means after the future upgrade we would expect these events in the regularly detectable range to occur from 10^-2 to 10^2 times a year (divide by ten since the range is about 0.1 Gpc). The probability of seeing such a signal from the background was estimated at ~10^-7. Now, they won't detect every such event but I have no idea what percentage they currently think they are getting. Let's say it is 1/1000, then we would expect 10^-5 to 10^-1 detections per year.
If that is the case, the odds are about 1/100 to 1/1,000,000 that this was a false positive (eg 10^-7 divided by 10^-5 or 10^-1). Multiply 10^-7 by 100 to correct for the extra multiple comparisons to get odds of 1/1 to 1/10,000.
Like I said, I have no idea what percent of events they are currently capable of detecting and don't even know if it is possible to estimate this. However, that is the type of calculation we would want to get an idea of whether this is a false positive. As it currently stands the article is misleading on this point.
(Score: 0) by Anonymous Coward on Sunday February 14 2016, @07:13AM
Also interesting. It took them 6 weeks to get 16 days of baseline data for this signal which means only 38% duty factor. This is half of what they were considering a good rate in 2007:
https://labcit.ligo.caltech.edu/~ll_news/s5_news/s5article.htm [caltech.edu]
I wonder if this is because they were extra careful about filtering the data after seeing the signal. That would cause problems as well since the signal was not sampled from the same distribution as the baseline.