17 ms·

Second Gravitational Wave Detected at LIGO

621 points by specialp 10y ago

devy 10y ago
Can someone who has expertise explain the significance of this in plain English please?
- cheez 10y ago
  First time might be a fluke, second time might be a fluke, third time... The theory might be right. Basic science. Remember, this was imagined by some dude 100 years ago. That's nucking futs.
- jofer 10y ago
  I'm definitely not a good person to explain or comment on this, but in a nutshell: Black hole collisions were expected to be a fairly common occurrence, but we had no way of knowing for certain how frequent they are when only one had been detected. Now that another one has been detected a fairly short time afterwards, it supports the prediction that we should observe several of them every year. If black hole collisions were very rare events, it would be harder to use them as a natural laboratory. It's much easier to demonstrate that some small deviation from theory is real when you have multiple observations of it. Therefore, this second observation in a short time confirms that gravitational waves are likely to be a useful tool to explore fundamental physical principles.
  - batbomb 10y ago
    > Therefore, this second observation in a short time confirms that gravitational waves are likely to be a useful tool to explore fundamental physical principles. No, it just means there's a high probability that these events occur on a regular basis. Of course, "regular basis" really means we might be able to approach gaussian statistics within the lifetime of the experiment.
  - ISL 10y ago
    Agreed. The most important single fact (though there are many to be extracted here) is an estimate of the detectable merger rate.
  - misnome 10y ago
    Also, this event is much closer to what they would "Expect" e.g. a much more common predicted size of black holes; the first event was rather on the extreme end of things (which is why it was so much easier to see in the data).
- nabaraz 10y ago
  The end target is to measure gravitational waves of Big Bang and learn about the origin of universe. PS: Watch this brilliant video [1] if you want to know what gravitational waves are. [1] https://www.youtube.com/watch?v=4GbWfNHtHRg https://www.youtube.com/watch?v=4GbWfNHtHRg
- CamperBob2 10y ago
  They didn't expect these events to be very common, so it was suspicious that they got a signal almost as soon as they turned the interferometer on. Receiving additional events will help reassure people who were afraid that the first result was a glitch of some kind. The more you read about how this particular instrument is constructed, the more incredulous you'll be that it could possibly work at all, or if it does, that it could pick up anything but noise. So it's very good news that these events are relatively common. (Well, good for the research teams at the LIGO installations, but bad for anyone within a few hundred light years of the events being detected.)
- fraserharris 10y ago
  The largest gravitational waves, emanating from black hole collisions, have now repeatedly been detected. Scientists can iterate on the design to build more sensitive detectors. This will give us a new way of studying the universe that doesn't rely on electromagnetic radiation (light, x-rays, etc). Perhaps it will lead to answers about the missing matter in the universe.
kirykl 10y ago
Some nice visuals to explain the concept http://www.npr.org/sections/thetwo-way/2016/06/15/481934630/gravitational-waves-from-colliding-black-holes-shake-scientists-detectors-again http://www.npr.org/sections/thetwo-way/2016/06/15/481934630/...
heegemcgee 10y ago
>he signal persisted in the LIGO frequency band for approximately 1 s, increasing in frequency and amplitude over about 55 cycles from 35 to 450 Hz Would love to hear an audio facsimile of what this might "sound" like.
- saboot 10y ago
  https://www.youtube.com/watch?v=QyDcTbR-kEA https://www.youtube.com/watch?v=QyDcTbR-kEA From the first detection
- eaq 10y ago
  For this particular event, see https://losc.ligo.org/events/GW151226/ https://losc.ligo.org/events/GW151226/, near the bottom of the page
  - dev1n 10y ago
    It's truly incredible that I can sit here, plug my headphones into this little electrical box and "hear" the gravitational sound of two black holes colliding. If only Mr. Einstein could be here to listen.
    - pavel_lishin 10y ago
      I love that the data-as-sound is so cute: https://losc.ligo.org/s/events/GW151226/GW151226_template_shifted.wav https://losc.ligo.org/s/events/GW151226/GW151226_template_sh...
      - jerf 10y ago
        Truly an unbelievably jaunty little sound considering that it represents the entire mass of our solar system being converted into energy. Yow.
        lisper 10y ago
        Indeed. And to put that into perspective: "The amount of matter converted to energy in the atomic bomb dropped on Hiroshima was about 700 milligrams, less than one-third the mass of a U.S. dime." (From http://discovermagazine.com/2010/jul-aug/24-numbers-nuclear-weapons-bomb-stockpile-peace http://discovermagazine.com/2010/jul-aug/24-numbers-nuclear-...)
macintux 10y ago
The article linked from that page is more useful to the physics-challenged such as myself: http://link.aps.org/doi/10.1103/Physics.9.68 http://link.aps.org/doi/10.1103/Physics.9.68
rubidium 10y ago
"“It is very significant that these black holes were much less massive than those observed in the first detection,” Gabriela Gonzalez, LIGO's spokesperson, said in a statement. “Because of their lighter masses compared to the first detection, they spent more time—about one second—in the sensitive band of the detectors. It is a promising start to mapping the populations of black holes in our Universe.”" from http://arstechnica.com/science/2016/06/ligo-data-includes-at-least-one-more-black-hole-merger/ http://arstechnica.com/science/2016/06/ligo-data-includes-at...
scrumper 10y ago
What does the last line of the abstract refer to, about "deviations from general relativity"? Is it simply a statement that this (and other) observations of gravity waves are another tool for verifying GR's predictions?
- jlebar 10y ago
  > Is it simply a statement that this (and other) observations of gravity waves are another tool for verifying GR's predictions? That is how I read it, yes.
- ISL 10y ago
  Yes. These gravitational wave detections are the only direct measurements anyone's ever made in regions of such strong gravity. If strong-field gravity betrays any failing of the theory of GR, we'd expect to find it in gravitational-wave waveforms. That these waveforms (the first one, in particular) appear to follow the basic GR plan at the time of the merger is actually kind of a bummer. As a physicist who makes precision tests of gravity for a living, I'd really hoped that we'd see something unexpected. Instead, the new thing that we learned is that GR continues to be an accurate model, something that was perhaps unexpected on its own.
  - eaq 10y ago
    I'm not so surprised that there are no GR deviations in these early days of detections that are just becoming significant over the instrument noise. GW physicists are hard at work on the future generations of GW detectors... In 50 years, a signal like GW150914 might have an SNR of >1000, so we'll see all kinds of detail that may hold more information than what we've seen so far.
    - ISL 10y ago
      People have been testing GR really hard for a century, especially in the last fifty years. Nobody's convincingly found a single chink in the armor. Clifford Will's "The Confrontation between General Relativity and Experiment" is an excellent introduction to the state of the art (this is the stock reference for everyone in the field). http://relativity.livingreviews.org/Articles/lrr-2014-4/ http://relativity.livingreviews.org/Articles/lrr-2014-4/
etrautmann 10y ago
Interesting to note that the y-scale on figure 1 is 10^-21 (units of strain). Measurement at that scale is absolutely insane. The power of good engineering, incredible optics, and lots of averaging.
- ISL 10y ago
  There's minimal averaging, as the signals are transient.
  - antognini 10y ago
    Depends on what you mean by averaging I suppose, but the signal is a measure of the average path length from several hundred reflections in the cavity. Furthermore, the signals are generated from the average interference of many photons. Even still, shot noise from the limited number of photons in the cavity is a major source of uncertainty.
    - ISL 10y ago
      All true. To give perspective on where I was coming from: from the (human) perspective of looking at noise in 1/sqrt(Hz) units, the signals are actually less-than-averaged if they're less than a second in duration. It's not like a continuous-wave source where one can integrate for months and hammer down the uncertainty to far lower strain values.
  - pdabbadabba 10y ago
    I suppose one could rely on averaging the raw inputs of a large number of less reliable sensors, but I don't believe that's what the researchers have actually done here.
  - etrautmann 10y ago
    It's averaging in the sense of a long integration time. The signals of interest are glacial w/r/t speed of light, in the few Hz range, so you can integrate the optical signal over a longer period of time to improve SNR and beat shot noise.
BenoitP 10y ago
How come: 14.2 + 7.5 == 21.7 != 20.8 ? Does the gravitational wave contain 0.9 solar masses of energy?
- antognini 10y ago
  Yes. Gravitational waves transmit both energy (i.e., mass) and angular momentum.
  - markus2012 10y ago
    So... build more detectors and pinpoint the direction the gravity wave is coming from?
    - infogulch 10y ago
      Yes! There are 2 now, so they can give a very coarse estimate of the direction with a precision of 'somewhere in this swath of the sky' (using the relative location, the wave propagation speed [light speed], and the arrival times relative to each detector). As more detectors come online they will be able to triangulate more accurately. The next one is supposed to come online in 2018 iirc.
    - antognini 10y ago
      That's the idea! I seem to remember that there are plans to build another detector in India. I'm not sure what its status is. http://gw-indigo.org/tiki-index.php?page=LIGO-India http://gw-indigo.org/tiki-index.php?page=LIGO-India
      - ahnitz 10y ago
        Funding for its construction was approved by India. We are hoping that it might coming online in ~ 5 years. It takes a long time to build everything from the ground up.
      - ziotom78 10y ago
        Note that the VIRGO detector, located near Pisa (Italy), has already taken a set of measurements a few years ago, and it would have been sensitive enough to detect waves like the one from last September. Unfortunately, Virgo was down for upgrades when GW150914 came. I recall that they plan to restart Virgo by early 2017, so it should be operational well before Ligo India. (And I like to underline that Ligo and Virgo joined together in the Ligo/Virgo collaboration and thus work together: as a matter of fact, the first guy who saw GW150914 in the raw data was from Virgo, and the papers presenting the detection were signed by people in both collaborations.)
Jerry2 10y ago
How do we know that these waves they're detecting are from collisions of black holes? How do they locate these black holes and how do they make these conclusions that what they detect is coming from black hole Alpha & Beta colliding?
- eaq 10y ago
  The waveforms are predicted by our knowledge of general relativity; specifically what the gravitational wave profile would be for two objects of certain masses spiraling into each other. A semi-hand-wavy way we know that the objects in this particular inspired are black holes is that for them to be orbiting each other this fast, they must be very close. For them to be this close, they must be very small. Together, this implied the objects have a certain size which is smaller than their Schwarzschild radius, which means they are probably black holes.
- ajkjk 10y ago
  We're pretty sure they come from black holes because the signals are uncannily similar to what theory says signals from black holes looks like. So either they're black holds or the theory is wrong or there's something else that acts very much like a black hole.
- deleted 10y ago
  [deleted]
- femto113 10y ago
  It's mainly derived from the frequency and magnitude of the wave. The frequency derives from the period of the binary system, which in turn derives from the masses of the two black holes. Once the period has been determined it's possible to solve for the original masses. Given those we can estimate the energy produced in the collision, compare that to the magnitude of the wave we received, and then estimate the distance (similar to estimating how far away a sound is by how loud it was). LIGO can also roughly locate the direction the waves come from by comparing when the signals are received at the two different detectors (LA and WA). If we add a third observatory at some point we should be able to triangulate much more accurately.
eaq 10y ago
Data and audio files relevant to this event are available to the public at https://losc.ligo.org/events/GW151226/ https://losc.ligo.org/events/GW151226/ There are also Jupyter tutorials on processing GW signals at https://losc.ligo.org/tutorials/ https://losc.ligo.org/tutorials/
yread 10y ago
My wife has this really stupid (or brilliant?) question - if during the collision one solar mass turned into gravitational waves is it possible to create mass from gravitational waves?
- CaptSpify 10y ago
  Doesn't seem like a dumb question to me! AFAIK, our models of physics work forwards, as well as backwards. So in theory: yes IANA physicist, and have 0 training in physics, so someone please tell me if I'm wrong
- antognini 10y ago
  In principle, yes. The Einstein field equations are time symmetric, so you could reverse the situation by pumping in gravitational waves from a great distance in a spherically symmetric way, and have them converge in some central region in such a way as to increase the mass of a black hole at the center, or have it split up into two more massive black holes.
  - iaw 10y ago
    I think the practical limitation here is generating and concentrating/focusing the gravitational waves. EDIT: And there would probably have to be some control of the relative phase of each component wave as well.
    - deleted 10y ago
      [deleted]
  - dandare 10y ago
    Isn't it in principle equally difficult as generating concentrial waves on the surface of a swimming pool in such way as to eject a swimmer back on the podium where he jumped in from?
    - antognini 10y ago
      Yes, it's almost an exactly analogous situation. The difficulty is that you're going against the second law of thermodynamics in both cases.
lpage 10y ago
For those curious about the future of LIGO... At present there are two LIGO facilities - one in Hanford, Washington and another in Livingston, Louisiana. This is necessary for both denoising (it's unlikely that a seismic event or random perturbation would effect both simultaneously) and triangulation via parallax. Right now having just two facilities (that are relatively close together) limits localization to broad regions of the sky. Additional facilities are under way/in discussion for Europe, Japan, and India. This would significantly improve both the sensitivity of the array and its ability to localize events in a smaller region of the sky. Hopefully these projects get funded. LIGO stands to resolve some of the biggest open questions we have in cosmology.
- specialp 10y ago
  There is also an effort to get these detectors in space with eLISA: http://physics.aps.org/articles/v9/63 http://physics.aps.org/articles/v9/63 This will be able to detect wave frequencies that would normally be drowned out by terrestrial sources.
  - nerfhammer 10y ago
    terrestrial sources of gravitational waves?
    - deleted 10y ago
      [deleted]
    - infogulch 10y ago
      Terrestrial sources of run-of-the-mill vibrations. The detector can pick up both.
  - mino 10y ago
    I recommend the two animations on the wiki page of eLISA, they are worth a thousands words to understand the (crazy!) orbit of this contraption: https://en.wikipedia.org/wiki/Evolved_Laser_Interferometer_Space_Antenna https://en.wikipedia.org/wiki/Evolved_Laser_Interferometer_S...
mudil 10y ago
When gravitational waves are released from the merger of two black holes, the combined mass of the resultant black hole is less than sum of its component black holes, because some of the mass was released in the form gravitational waves. Questions. What are the implications of this on black hole entropy and temperature? Can black holes evaporate from gravitational radiation alone without Hawkins radiation? What are the implications of this on the mass of any object in the universe, since all objects are related to each other gravitationally and the universe is expanding? Does it mean objects are constantly loosing mass and the universe is filled with energy from this release? Can dark energy be related to this process? Can universe be expanding because matter is constantly lost into the gravitational waves? What are the implications of gravitational waves on the fabric of spacetime, if objects are constantly leaking gravitational waves in a nonstatic universe?
- akuchling 10y ago
  My understanding is that gravitational waves are only radiated when there's a dipole moment: two (or more, I guess) objects in rotation around each other. A black hole just sitting on its own doesn't radiate any gravitational waves at all. According to http://www.ast.cam.ac.uk/public/ask/2519 http://www.ast.cam.ac.uk/public/ask/2519 , the Earth and Moon should also be radiating gravitational waves (and therefore losing a little bit of energy) but this is surely dwarfed by other effects and not measurable.
  - mudil 10y ago
    But in a nonstatic, expanding universe any two objects, no matter how far apart, are gravitationally related and have dipole moment. Isn't it true?
  - ISL 10y ago
    Important pedantic point: There are no gravitational dipoles, as there is no repulsive gravitational interaction. A dumbbell-shaped mass is the sum of a gravitational monopole and a gravitational quadrupole. That gravitational radiation originates only from quadrupolar sources (which are far less-efficient radiators than an equal-sized dipole) is one reason that gravitational waves are hard to detect.
nonbel 10y ago
This signal was reported on months ago. Can anyone explain what they did to move the GW151226 signal from 2-sigma to >5-sigma? "The data is in fact completely open and you could analyse it yourself! In addition to the GW150914 event there are also two others that rise somewhat above the background ("GW151012" and "GW151226"). You can see them by eye in the above plot. They are clearly not statistically significant enough to announce a discovery alone, but still they are tantalising... with room for improvement to design sensitivity (by a factor of ~2 which increases the spatial reach by 2^3) and the construction of a third detector in India to triangulate the signal, the future of gravitational wave astronomy is exciting." http://syymmetries.blogspot.com/2016_03_01_archive.html http://syymmetries.blogspot.com/2016_03_01_archive.html
- dandare 10y ago
  Gosh, wake me up in 50 years, I can't wait to learn what is the nature of dark energy and what lays beyond standard model.
  - j1vms 10y ago
    Wow 50 years, by then it might be that we find we were asking the wrong questions at this point. And even with the right questions, the answers were... unexpected.
- xxxxxxxxxxxxx 10y ago
  Their background analysis is data-driven; the more data they take the better they know their background noise. They are now at the point where they can definitely say that what looked like an unusual fluctuation above the background noise months ago was in fact very unusual. Enough for discovery significance.
  - hcs 10y ago
    Citation, as it says in the article: "Two matched-filter searches used coincident observations between the two LIGO detectors from September 12, 2015 to January 19, 2016 to estimate the significance of GW151226. One of these searches was the off-line version of the online search discussed previously. The off-line searches benefit from improved calibration and refined data quality information not available to online searches." http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.241103#fulltext http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116...
ars 10y ago
Something I've asked before but got no answer - at these extreme masses, velocities, and forces time dilation has got to be immense. Yet the article makes no mention of this whatsoever.
- ahnitz 10y ago
  For the most part, we concentrate on quantities far away from the sources as these are what we have a chance to measure and can more easily intuit. There are however, extremely precise simulations of GR that can numerically simulate the spacetime around close to mergers which of course includes all of these effects.
- jerf 10y ago
  It's referenced in the abstract in the phrase "source-frame". For the target audience, belaboring the time-dilation point would be a waste of time; it's basic knowledge to the intended readers of the abstract. We here on HN are by-and-large just sight-seeing.
- dsqrt 10y ago
  Something to keep in mind is that gravitational waves are linearized solutions to Einstein equations. That is, they can only be meaningfully defined far away from those sources. What happens close to the black holes cannot be described in terms of waves. You should not think of the gravitational waves as being emitted directly by the black holes. Instead what happens is that the black holes distort spacetime in complex non-linear ways, which turn into gravitational waves at large distances (hundreds of Schwarzschild radii). As such, the time dilation imprinted in the waves depends on the motion of the "center of mass of the binary" and the cosmological redshift.
  - bobbles 10y ago
    I keep trying to picture how this would work using a 'drain in a bathtub' image in my head where the water rushing down the plug is like a black hole pulling things in with gravity. Is there any way thinking like that I can get an approximate idea of what the gravitational wave would be in that scenario?
jayess 10y ago
Could black holes lose their mass through the propagation (generation?) of gravitational waves?
- roywiggins 10y ago
  It seems that it happens when they merge, since: > The cataclysmic event saw the black holes, one eight times more massive than the sun, the other 14 times more massive, merge into one about 21 times heavier than the sun. In the process, energy equivalent to the mass of the sun radiated into space as gravitational waves. So the final black hole had rather less mass than the sum of the original two. https://www.theguardian.com/science/2016/jun/15/gravitational-waves-detected-from-collision-of-second-set-of-black-holes-ligo https://www.theguardian.com/science/2016/jun/15/gravitationa...
- cyphar 10y ago
  Yes, that's where the energy for gravitational waves comes from. In fact, if you check the original LIGO paper, that black hole merger lost something like 3 solar masses of energy.
peter303 10y ago
I thought the shaking was Santa on the roof!
an_account_name 10y ago
So, I remember hearing about LIGO when the first wave was detected, and was excited by it - but I had never heard of it before that. What other experiments are running that would generate similar excitement from the science community that I probably haven't heard of yet?
JumpCrisscross 10y ago
One of my favourite things to do, when I'm back in California, is attend SLAC Public Lectures [1]. (There is a disturbing paucity of scientific cultural institutions in New York.) The most recent one was by Dr. Brian Lantz about LIGO [2]. [1] https://www6.slac.stanford.edu/community/public-lectures.aspx https://www6.slac.stanford.edu/community/public-lectures.asp... [2] https://www.youtube.com/watch?v=EMzoQAmK8Dc https://www.youtube.com/watch?v=EMzoQAmK8Dc
cyphar 10y ago
I was lucky enough to be in an astrophysics faculty (doing a research project) when the LIGO paper was published. Everyone was super excited and was printing off papers and discussing the experiment, results and future of astronomy. It was really something else to see that many clever minds so excited about a result that many scientists involved in the field of cosmology and relativity didn't live to see.
ksec 10y ago
How far are we from creating Gravity Shockwave Generating Division Tool? A.k.a Goldion Crusher.
jakeogh 10y ago
Gem YT chan: https://www.youtube.com/watch?v=MOVArup3jfg https://www.youtube.com/watch?v=MOVArup3jfg
noselfrighteous 10y ago
I have a side question. If gravity (i.e. warping of space time) propagates at the speed of light. Then does that mean that the Alcubierre warp drive is fundamentally incapable of supra-light speeds?
eggy 10y ago
Is there any correlation between the fundamental vibrating of atoms in a vacuum at absolute zero, which I think is known as the zero-energy state, and gravitational waves? Meaning, I know quantum mechanics describes why they vibrate, but do gravity waves have any play in this, or effect? So exciting to have lived long enough to see some more theory become real with measurements!

621 points by specialp 10y ago

devy 10y ago

Can someone who has expertise explain the significance of this in plain English please?

cheez 10y ago
First time might be a fluke, second time might be a fluke, third time... The theory might be right. Basic science. Remember, this was imagined by some dude 100 years ago. That's nucking futs.
jofer 10y ago
I'm definitely not a good person to explain or comment on this, but in a nutshell: Black hole collisions were expected to be a fairly common occurrence, but we had no way of knowing for certain how frequent they are when only one had been detected. Now that another one has been detected a fairly short time afterwards, it supports the prediction that we should observe several of them every year. If black hole collisions were very rare events, it would be harder to use them as a natural laboratory. It's much easier to demonstrate that some small deviation from theory is real when you have multiple observations of it. Therefore, this second observation in a short time confirms that gravitational waves are likely to be a useful tool to explore fundamental physical principles.
- batbomb 10y ago
  > Therefore, this second observation in a short time confirms that gravitational waves are likely to be a useful tool to explore fundamental physical principles. No, it just means there's a high probability that these events occur on a regular basis. Of course, "regular basis" really means we might be able to approach gaussian statistics within the lifetime of the experiment.
- ISL 10y ago
  Agreed. The most important single fact (though there are many to be extracted here) is an estimate of the detectable merger rate.
- misnome 10y ago
  Also, this event is much closer to what they would "Expect" e.g. a much more common predicted size of black holes; the first event was rather on the extreme end of things (which is why it was so much easier to see in the data).
nabaraz 10y ago
The end target is to measure gravitational waves of Big Bang and learn about the origin of universe. PS: Watch this brilliant video [1] if you want to know what gravitational waves are. [1] https://www.youtube.com/watch?v=4GbWfNHtHRg https://www.youtube.com/watch?v=4GbWfNHtHRg
CamperBob2 10y ago
They didn't expect these events to be very common, so it was suspicious that they got a signal almost as soon as they turned the interferometer on. Receiving additional events will help reassure people who were afraid that the first result was a glitch of some kind. The more you read about how this particular instrument is constructed, the more incredulous you'll be that it could possibly work at all, or if it does, that it could pick up anything but noise. So it's very good news that these events are relatively common. (Well, good for the research teams at the LIGO installations, but bad for anyone within a few hundred light years of the events being detected.)
fraserharris 10y ago
The largest gravitational waves, emanating from black hole collisions, have now repeatedly been detected. Scientists can iterate on the design to build more sensitive detectors. This will give us a new way of studying the universe that doesn't rely on electromagnetic radiation (light, x-rays, etc). Perhaps it will lead to answers about the missing matter in the universe.

kirykl 10y ago

Some nice visuals to explain the concept http://www.npr.org/sections/thetwo-way/2016/06/15/481934630/gravitational-waves-from-colliding-black-holes-shake-scientists-detectors-again http://www.npr.org/sections/thetwo-way/2016/06/15/481934630/...

heegemcgee 10y ago

>he signal persisted in the LIGO frequency band for approximately 1 s, increasing in frequency and amplitude over about 55 cycles from 35 to 450 Hz Would love to hear an audio facsimile of what this might "sound" like.

saboot 10y ago
https://www.youtube.com/watch?v=QyDcTbR-kEA https://www.youtube.com/watch?v=QyDcTbR-kEA From the first detection
eaq 10y ago
For this particular event, see https://losc.ligo.org/events/GW151226/ https://losc.ligo.org/events/GW151226/, near the bottom of the page
- dev1n 10y ago
  It's truly incredible that I can sit here, plug my headphones into this little electrical box and "hear" the gravitational sound of two black holes colliding. If only Mr. Einstein could be here to listen.
  - pavel_lishin 10y ago
    I love that the data-as-sound is so cute: https://losc.ligo.org/s/events/GW151226/GW151226_template_shifted.wav https://losc.ligo.org/s/events/GW151226/GW151226_template_sh...
    - jerf 10y ago
      Truly an unbelievably jaunty little sound considering that it represents the entire mass of our solar system being converted into energy. Yow.
      - lisper 10y ago
        Indeed. And to put that into perspective: "The amount of matter converted to energy in the atomic bomb dropped on Hiroshima was about 700 milligrams, less than one-third the mass of a U.S. dime." (From http://discovermagazine.com/2010/jul-aug/24-numbers-nuclear-weapons-bomb-stockpile-peace http://discovermagazine.com/2010/jul-aug/24-numbers-nuclear-...)

macintux 10y ago

The article linked from that page is more useful to the physics-challenged such as myself: http://link.aps.org/doi/10.1103/Physics.9.68 http://link.aps.org/doi/10.1103/Physics.9.68

rubidium 10y ago

"“It is very significant that these black holes were much less massive than those observed in the first detection,” Gabriela Gonzalez, LIGO's spokesperson, said in a statement. “Because of their lighter masses compared to the first detection, they spent more time—about one second—in the sensitive band of the detectors. It is a promising start to mapping the populations of black holes in our Universe.”" from http://arstechnica.com/science/2016/06/ligo-data-includes-at-least-one-more-black-hole-merger/ http://arstechnica.com/science/2016/06/ligo-data-includes-at...

scrumper 10y ago

What does the last line of the abstract refer to, about "deviations from general relativity"? Is it simply a statement that this (and other) observations of gravity waves are another tool for verifying GR's predictions?

jlebar 10y ago
> Is it simply a statement that this (and other) observations of gravity waves are another tool for verifying GR's predictions? That is how I read it, yes.
ISL 10y ago
Yes. These gravitational wave detections are the only direct measurements anyone's ever made in regions of such strong gravity. If strong-field gravity betrays any failing of the theory of GR, we'd expect to find it in gravitational-wave waveforms. That these waveforms (the first one, in particular) appear to follow the basic GR plan at the time of the merger is actually kind of a bummer. As a physicist who makes precision tests of gravity for a living, I'd really hoped that we'd see something unexpected. Instead, the new thing that we learned is that GR continues to be an accurate model, something that was perhaps unexpected on its own.
- eaq 10y ago
  I'm not so surprised that there are no GR deviations in these early days of detections that are just becoming significant over the instrument noise. GW physicists are hard at work on the future generations of GW detectors... In 50 years, a signal like GW150914 might have an SNR of >1000, so we'll see all kinds of detail that may hold more information than what we've seen so far.
  - ISL 10y ago
    People have been testing GR really hard for a century, especially in the last fifty years. Nobody's convincingly found a single chink in the armor. Clifford Will's "The Confrontation between General Relativity and Experiment" is an excellent introduction to the state of the art (this is the stock reference for everyone in the field). http://relativity.livingreviews.org/Articles/lrr-2014-4/ http://relativity.livingreviews.org/Articles/lrr-2014-4/

etrautmann 10y ago

Interesting to note that the y-scale on figure 1 is 10^-21 (units of strain). Measurement at that scale is absolutely insane. The power of good engineering, incredible optics, and lots of averaging.

ISL 10y ago
There's minimal averaging, as the signals are transient.
- antognini 10y ago
  Depends on what you mean by averaging I suppose, but the signal is a measure of the average path length from several hundred reflections in the cavity. Furthermore, the signals are generated from the average interference of many photons. Even still, shot noise from the limited number of photons in the cavity is a major source of uncertainty.
  - ISL 10y ago
    All true. To give perspective on where I was coming from: from the (human) perspective of looking at noise in 1/sqrt(Hz) units, the signals are actually less-than-averaged if they're less than a second in duration. It's not like a continuous-wave source where one can integrate for months and hammer down the uncertainty to far lower strain values.
- pdabbadabba 10y ago
  I suppose one could rely on averaging the raw inputs of a large number of less reliable sensors, but I don't believe that's what the researchers have actually done here.
- etrautmann 10y ago
  It's averaging in the sense of a long integration time. The signals of interest are glacial w/r/t speed of light, in the few Hz range, so you can integrate the optical signal over a longer period of time to improve SNR and beat shot noise.

BenoitP 10y ago

How come: 14.2 + 7.5 == 21.7 != 20.8 ? Does the gravitational wave contain 0.9 solar masses of energy?

antognini 10y ago
Yes. Gravitational waves transmit both energy (i.e., mass) and angular momentum.
- markus2012 10y ago
  So... build more detectors and pinpoint the direction the gravity wave is coming from?
  - infogulch 10y ago
    Yes! There are 2 now, so they can give a very coarse estimate of the direction with a precision of 'somewhere in this swath of the sky' (using the relative location, the wave propagation speed [light speed], and the arrival times relative to each detector). As more detectors come online they will be able to triangulate more accurately. The next one is supposed to come online in 2018 iirc.
  - antognini 10y ago
    That's the idea! I seem to remember that there are plans to build another detector in India. I'm not sure what its status is. http://gw-indigo.org/tiki-index.php?page=LIGO-India http://gw-indigo.org/tiki-index.php?page=LIGO-India
    - ahnitz 10y ago
      Funding for its construction was approved by India. We are hoping that it might coming online in ~ 5 years. It takes a long time to build everything from the ground up.
    - ziotom78 10y ago
      Note that the VIRGO detector, located near Pisa (Italy), has already taken a set of measurements a few years ago, and it would have been sensitive enough to detect waves like the one from last September. Unfortunately, Virgo was down for upgrades when GW150914 came. I recall that they plan to restart Virgo by early 2017, so it should be operational well before Ligo India. (And I like to underline that Ligo and Virgo joined together in the Ligo/Virgo collaboration and thus work together: as a matter of fact, the first guy who saw GW150914 in the raw data was from Virgo, and the papers presenting the detection were signed by people in both collaborations.)

Jerry2 10y ago

How do we know that these waves they're detecting are from collisions of black holes? How do they locate these black holes and how do they make these conclusions that what they detect is coming from black hole Alpha & Beta colliding?

eaq 10y ago
The waveforms are predicted by our knowledge of general relativity; specifically what the gravitational wave profile would be for two objects of certain masses spiraling into each other. A semi-hand-wavy way we know that the objects in this particular inspired are black holes is that for them to be orbiting each other this fast, they must be very close. For them to be this close, they must be very small. Together, this implied the objects have a certain size which is smaller than their Schwarzschild radius, which means they are probably black holes.
ajkjk 10y ago
We're pretty sure they come from black holes because the signals are uncannily similar to what theory says signals from black holes looks like. So either they're black holds or the theory is wrong or there's something else that acts very much like a black hole.
deleted 10y ago
[deleted]
femto113 10y ago
It's mainly derived from the frequency and magnitude of the wave. The frequency derives from the period of the binary system, which in turn derives from the masses of the two black holes. Once the period has been determined it's possible to solve for the original masses. Given those we can estimate the energy produced in the collision, compare that to the magnitude of the wave we received, and then estimate the distance (similar to estimating how far away a sound is by how loud it was). LIGO can also roughly locate the direction the waves come from by comparing when the signals are received at the two different detectors (LA and WA). If we add a third observatory at some point we should be able to triangulate much more accurately.

eaq 10y ago

Data and audio files relevant to this event are available to the public at https://losc.ligo.org/events/GW151226/ https://losc.ligo.org/events/GW151226/ There are also Jupyter tutorials on processing GW signals at https://losc.ligo.org/tutorials/ https://losc.ligo.org/tutorials/

yread 10y ago

My wife has this really stupid (or brilliant?) question - if during the collision one solar mass turned into gravitational waves is it possible to create mass from gravitational waves?

CaptSpify 10y ago
Doesn't seem like a dumb question to me! AFAIK, our models of physics work forwards, as well as backwards. So in theory: yes IANA physicist, and have 0 training in physics, so someone please tell me if I'm wrong
antognini 10y ago
In principle, yes. The Einstein field equations are time symmetric, so you could reverse the situation by pumping in gravitational waves from a great distance in a spherically symmetric way, and have them converge in some central region in such a way as to increase the mass of a black hole at the center, or have it split up into two more massive black holes.
- iaw 10y ago
  I think the practical limitation here is generating and concentrating/focusing the gravitational waves. EDIT: And there would probably have to be some control of the relative phase of each component wave as well.
  - deleted 10y ago
    [deleted]
- dandare 10y ago
  Isn't it in principle equally difficult as generating concentrial waves on the surface of a swimming pool in such way as to eject a swimmer back on the podium where he jumped in from?
  - antognini 10y ago
    Yes, it's almost an exactly analogous situation. The difficulty is that you're going against the second law of thermodynamics in both cases.

lpage 10y ago

For those curious about the future of LIGO... At present there are two LIGO facilities - one in Hanford, Washington and another in Livingston, Louisiana. This is necessary for both denoising (it's unlikely that a seismic event or random perturbation would effect both simultaneously) and triangulation via parallax. Right now having just two facilities (that are relatively close together) limits localization to broad regions of the sky. Additional facilities are under way/in discussion for Europe, Japan, and India. This would significantly improve both the sensitivity of the array and its ability to localize events in a smaller region of the sky. Hopefully these projects get funded. LIGO stands to resolve some of the biggest open questions we have in cosmology.

specialp 10y ago
There is also an effort to get these detectors in space with eLISA: http://physics.aps.org/articles/v9/63 http://physics.aps.org/articles/v9/63 This will be able to detect wave frequencies that would normally be drowned out by terrestrial sources.
- nerfhammer 10y ago
  terrestrial sources of gravitational waves?
  - deleted 10y ago
    [deleted]
  - infogulch 10y ago
    Terrestrial sources of run-of-the-mill vibrations. The detector can pick up both.
- mino 10y ago
  I recommend the two animations on the wiki page of eLISA, they are worth a thousands words to understand the (crazy!) orbit of this contraption: https://en.wikipedia.org/wiki/Evolved_Laser_Interferometer_Space_Antenna https://en.wikipedia.org/wiki/Evolved_Laser_Interferometer_S...

mudil 10y ago

When gravitational waves are released from the merger of two black holes, the combined mass of the resultant black hole is less than sum of its component black holes, because some of the mass was released in the form gravitational waves. Questions. What are the implications of this on black hole entropy and temperature? Can black holes evaporate from gravitational radiation alone without Hawkins radiation? What are the implications of this on the mass of any object in the universe, since all objects are related to each other gravitationally and the universe is expanding? Does it mean objects are constantly loosing mass and the universe is filled with energy from this release? Can dark energy be related to this process? Can universe be expanding because matter is constantly lost into the gravitational waves? What are the implications of gravitational waves on the fabric of spacetime, if objects are constantly leaking gravitational waves in a nonstatic universe?

akuchling 10y ago
My understanding is that gravitational waves are only radiated when there's a dipole moment: two (or more, I guess) objects in rotation around each other. A black hole just sitting on its own doesn't radiate any gravitational waves at all. According to http://www.ast.cam.ac.uk/public/ask/2519 http://www.ast.cam.ac.uk/public/ask/2519 , the Earth and Moon should also be radiating gravitational waves (and therefore losing a little bit of energy) but this is surely dwarfed by other effects and not measurable.
- mudil 10y ago
  But in a nonstatic, expanding universe any two objects, no matter how far apart, are gravitationally related and have dipole moment. Isn't it true?
- ISL 10y ago
  Important pedantic point: There are no gravitational dipoles, as there is no repulsive gravitational interaction. A dumbbell-shaped mass is the sum of a gravitational monopole and a gravitational quadrupole. That gravitational radiation originates only from quadrupolar sources (which are far less-efficient radiators than an equal-sized dipole) is one reason that gravitational waves are hard to detect.

nonbel 10y ago

This signal was reported on months ago. Can anyone explain what they did to move the GW151226 signal from 2-sigma to >5-sigma? "The data is in fact completely open and you could analyse it yourself! In addition to the GW150914 event there are also two others that rise somewhat above the background ("GW151012" and "GW151226"). You can see them by eye in the above plot. They are clearly not statistically significant enough to announce a discovery alone, but still they are tantalising... with room for improvement to design sensitivity (by a factor of ~2 which increases the spatial reach by 2^3) and the construction of a third detector in India to triangulate the signal, the future of gravitational wave astronomy is exciting." http://syymmetries.blogspot.com/2016_03_01_archive.html http://syymmetries.blogspot.com/2016_03_01_archive.html

dandare 10y ago
Gosh, wake me up in 50 years, I can't wait to learn what is the nature of dark energy and what lays beyond standard model.
- j1vms 10y ago
  Wow 50 years, by then it might be that we find we were asking the wrong questions at this point. And even with the right questions, the answers were... unexpected.
xxxxxxxxxxxxx 10y ago
Their background analysis is data-driven; the more data they take the better they know their background noise. They are now at the point where they can definitely say that what looked like an unusual fluctuation above the background noise months ago was in fact very unusual. Enough for discovery significance.
- hcs 10y ago
  Citation, as it says in the article: "Two matched-filter searches used coincident observations between the two LIGO detectors from September 12, 2015 to January 19, 2016 to estimate the significance of GW151226. One of these searches was the off-line version of the online search discussed previously. The off-line searches benefit from improved calibration and refined data quality information not available to online searches." http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.241103#fulltext http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116...

ars 10y ago

Something I've asked before but got no answer - at these extreme masses, velocities, and forces time dilation has got to be immense. Yet the article makes no mention of this whatsoever.

ahnitz 10y ago
For the most part, we concentrate on quantities far away from the sources as these are what we have a chance to measure and can more easily intuit. There are however, extremely precise simulations of GR that can numerically simulate the spacetime around close to mergers which of course includes all of these effects.
jerf 10y ago
It's referenced in the abstract in the phrase "source-frame". For the target audience, belaboring the time-dilation point would be a waste of time; it's basic knowledge to the intended readers of the abstract. We here on HN are by-and-large just sight-seeing.
dsqrt 10y ago
Something to keep in mind is that gravitational waves are linearized solutions to Einstein equations. That is, they can only be meaningfully defined far away from those sources. What happens close to the black holes cannot be described in terms of waves. You should not think of the gravitational waves as being emitted directly by the black holes. Instead what happens is that the black holes distort spacetime in complex non-linear ways, which turn into gravitational waves at large distances (hundreds of Schwarzschild radii). As such, the time dilation imprinted in the waves depends on the motion of the "center of mass of the binary" and the cosmological redshift.
- bobbles 10y ago
  I keep trying to picture how this would work using a 'drain in a bathtub' image in my head where the water rushing down the plug is like a black hole pulling things in with gravity. Is there any way thinking like that I can get an approximate idea of what the gravitational wave would be in that scenario?

jayess 10y ago

Could black holes lose their mass through the propagation (generation?) of gravitational waves?

roywiggins 10y ago
It seems that it happens when they merge, since: > The cataclysmic event saw the black holes, one eight times more massive than the sun, the other 14 times more massive, merge into one about 21 times heavier than the sun. In the process, energy equivalent to the mass of the sun radiated into space as gravitational waves. So the final black hole had rather less mass than the sum of the original two. https://www.theguardian.com/science/2016/jun/15/gravitational-waves-detected-from-collision-of-second-set-of-black-holes-ligo https://www.theguardian.com/science/2016/jun/15/gravitationa...
cyphar 10y ago
Yes, that's where the energy for gravitational waves comes from. In fact, if you check the original LIGO paper, that black hole merger lost something like 3 solar masses of energy.

peter303 10y ago

I thought the shaking was Santa on the roof!

an_account_name 10y ago

So, I remember hearing about LIGO when the first wave was detected, and was excited by it - but I had never heard of it before that. What other experiments are running that would generate similar excitement from the science community that I probably haven't heard of yet?

JumpCrisscross 10y ago

One of my favourite things to do, when I'm back in California, is attend SLAC Public Lectures [1]. (There is a disturbing paucity of scientific cultural institutions in New York.) The most recent one was by Dr. Brian Lantz about LIGO [2]. [1] https://www6.slac.stanford.edu/community/public-lectures.aspx https://www6.slac.stanford.edu/community/public-lectures.asp... [2] https://www.youtube.com/watch?v=EMzoQAmK8Dc https://www.youtube.com/watch?v=EMzoQAmK8Dc

cyphar 10y ago

I was lucky enough to be in an astrophysics faculty (doing a research project) when the LIGO paper was published. Everyone was super excited and was printing off papers and discussing the experiment, results and future of astronomy. It was really something else to see that many clever minds so excited about a result that many scientists involved in the field of cosmology and relativity didn't live to see.

ksec 10y ago

How far are we from creating Gravity Shockwave Generating Division Tool? A.k.a Goldion Crusher.

jakeogh 10y ago

Gem YT chan: https://www.youtube.com/watch?v=MOVArup3jfg https://www.youtube.com/watch?v=MOVArup3jfg

noselfrighteous 10y ago

I have a side question. If gravity (i.e. warping of space time) propagates at the speed of light. Then does that mean that the Alcubierre warp drive is fundamentally incapable of supra-light speeds?

eggy 10y ago

Is there any correlation between the fundamental vibrating of atoms in a vacuum at absolute zero, which I think is known as the zero-energy state, and gravitational waves? Meaning, I know quantum mechanics describes why they vibrate, but do gravity waves have any play in this, or effect? So exciting to have lived long enough to see some more theory become real with measurements!