We are the LIGO Scientific Collaboration and the Virgo Collaboration, and we have made a second gravitational wave detection. Ask Us Anything!

my 9 year old would like to know what is the coolest thing about your discovery? and what should he study at school to get to do your job in future? and what happens to the waves after they go out into space?

EDIT: I cant spell

The coolest thing is now we have a new way of finding out about awesome things in space - mostly really huge stars that we don't know very much about yet.

He should study maths, physics and computing in school. We have teams of physicists, engineers and computing scientists that work for us. I'm a physicist.

The waves are released really far away in space and travel towards us for over a billion years before our detectors 'see' them.

Edit: JW, grad student, experimental interferometry

The coolest thing is discovering the unexpected. We were not even expecting to detect binary black holes the first time round. Now we are learning about a population of systems (binary black holes) about which we know very little - how and when they form, what determines the masses and timescales involved, etc. After we detect them, the waves continue to travel to the the most distant reaches of the universe, getting weaker and weaker as they go.

Tell your 9 year old that asking the question is already a good start. Apart from the courses already mentioned, you could encourage your child to be inquisitive, to make things, play with things and take things apart to discover how they work, to try to understand why the things (s)he can know and see in everyday experience are why they are the way they are. Inquisitiveness is so valuable for good science to function and succeed. It is great to have caring parents posing questions here for their interested children. Thank you.

BFW, Professor of Physics, gravitational wave research and data analysis.

How will these discoveries change the way we live today?

Up until LIGO came on line, almost everything we know about the Universe has come to us from light, either from just looking up at the night sky, to advanced space telescopes, and everything in between. Now, through gravitational wave observations, we have a brand new way of receiving information from the Universe. It's as if we have gained a new "sense" to explore our surroundings, and we are bound to learn things about how the Universe works that we couldn't have otherwise.

TBL, Research Scientist, Data Analysis / Astrophysics

I think these sorts of questions are disingenuous.

Einstein didn't theorise General Relatively and go "Now people 100 years from now will be able to discuss the validity of my working out over a global internetwork thanks to satellites in orbit around the Earth which wouldn't have been possible without my theory of GR to begin with."

We can't begin to understand the future and what we can make happen of the discoveries of the present. We can only continue going forward and keep building on our knowledge.

As it is, the technology and practises used in setting up LIGO could help pave the way for advancements in quantum computing which could have lots of practical knock on effects.

Measuring gravitational waves is an incredible achievement and it took decades of inventing new technologies to create observatories sensitive enough to make these discoveries. While gravitational waves themselves probably won't make faster phones or less stressful commutes, the esoteric science goals set the technology requirements, and then the worlds best minds create the machines needed to get the job done. We maintain a list of spinoff technologies that have resulted from the push to reach the performance goals of the LIGO observatories that you can check out at https://advancedligo.mit.edu.

TBL, Research Scientist, Data Analysis / Astrophysics

What counts as a black hole merging? When the singularities combine or when the event horizons touch each other?

Generally it is the latter, the touching of event horizons. However we could also see the ringing of final black hole which could provide some information about what happens after the touching of event horizons. The physics of it is still in early stages.

SK, postdoc, data analyst/instrumentalist

Does this ringing have to do with spacetime, or is the black hole getting larger and smaller? If the latter, does this cause the density of the black hole to decrease and would light then be able to escape?

The ringing is basically comes from the settling of event horizon of the new black hole. It doesn't affect the black hole in other ways.

Sk, postdoc, data analyst/instrumentalist

As a retired physicist who has worked with interferometers I am blown away with your accuracy, It seems to be orders of magnitude more than I am familiar with. I looked at your paper, http://arxiv.org/pdf/1604.00439v1.pdf, but still do not get where the improvement comes from. Is it some breakthrough, or just doing what was done before but much more precisely?

Basically, by using several clever tricks. :) The Fabry-Perot optical cavities in the arms, which keep most of the light bouncing back and forth in the cavity, boost our sensitivity by about two order of magnitude compared to having simple arms bouncing the beams back to the beam splitter. Power recycling recaptures light that would otherwise exit the interferometer, helping us build up lots of laser power -- and having so many photons keeps the quantum noise relatively low. Signal recycling is a little subtle, but helps us achieve good sensitivity over the frequency band we want. We stabilize the laser in a few ways, and keep mechanical and electronic noise sources down through careful design and engineering. So yes, there are some fancy techniques in there, but mostly it is just putting it all together very carefully.

PS, professor, data analysis and astrophysics (mostly)

What do you think the biggest misconception is that the average person has about your industry?

Perhaps, the biggest misconception is that we are all similar, boring physicists. This detection required the expertise and skills of people from all levels of education, from all over the world – most of which are truly passionate about opening this new window to the universe.

JR, engineer, instrumentalist

Generally people think of LIGO detectors as optical telescopes and hence think that with single detector we can pin-point the location of sources easily. But that is not the case. The LIGO detectors are like microphones which can hear sound coming from any direction, but not good at identifying the source location. But with multiple detectors we can use triangulation to locate the sources. SK, postdoc, data analyst/instrumentalist.

How does it feel knowing that you're basically listening to the universe's dubstep mixtape?

Whether it is dubstep or not, it feels great!

KAS, professor, instrumentation.

Seeing as they're called gravitational "waves," do they possess some of the wavelike properties we've learned about in high school? For example, light and sound waves can be refracted, reflected, diffracted, polarized, and so on. Do gravitational waves have these traits, and if so could you explain them a little?

Yes, gravitational waves can have all of the same types of interactions as other waves, as long as you are considering the correct limit. For instance, light waves occur at much higher frequencies than are typical for gravitational waves. We are used to dealing with light in the so-called "geometric optics" limit, where the object doing the refracting (i.e. a lens) is much larger than the wavelength of the light. Since gravitational waves can have much longer wavelengths, they will only behave in a totally analogous way to light if the object doing the lensing is correspondingly larger. Otherwise, the interaction won't be in the right limit, so the wave will behave differently, but the same would be true if light interacted with a lens comparable to or smaller than its wavelength, for example.

STM, Professor, source modeling and astrophysics

How helpful is gravityspy.org #citizenscience, since finding #gravitationalwaves relies on cleaning out noise, like a needle in a haystack?

Since LIGO is so sensitive to minuscule changes in distance, it is also susceptible to many instrumental and environmental sources of noise. LIGO relies on having two detectors to look for coincident signals between the detectors - noise is unlikely to happen at both detectors at about the same time whereas gravitational waves should hit both the detectors at nearly the same time (since they go right through our planet). However, even with the two detector failsafe similar noise features can hit both the detectors at the same, and at times when there are strong noise triggers it makes that data essentially useless (if a GW hits the detector when noise hits the detector, would it make a sound? :)

This is especially detrimental in looking for unmodelled GW signals, since we do not have great models for how they are supposed to look to compare with the data, like we do with compact binary inspirals. Gravity Spy uses human intuition and machine learning to classify and characterize many of these sources of noise, which will help LIGO scientists identify what is causing the noise and remove it from the data or the instrument all together, and making it even more sensitive to gravitational waves.

MZ, doctoral student, Astrophysics / Detector Characterization

Here's cool article on the detective work done to catch these glitches : https://cqgplus.com/2016/06/06/how-do-we-know-ligo-detected-gravitational-waves/

NM, PhD Student, Detector Characterization/Astrophysics

What implications does this have in our search for dark energy?

The concept of Dark Energy is an attempt to explain why the expansion of the universe is accelerating. Imagine the universe is painted on the surface of a balloon: An expanding Universe would be a balloon that is inflating steadily. An accelerating expansion would be a balloon that is inflating faster as it gets larger. Our understanding of how the universe as a whole works comes from Einstein's General Theory of Relativity and Dark Energy is basically a modification to Einstein's original equations to account for the observed accelerated expansion. But perhaps Einstein's equations are not perfect. By measuring gravitational waves, we are able to test Einstein's theory in new ways. So far our theoretical predictions from General Relativity have matched our gravitational wave observations, but we will continue to test them with ever-increasing sensitivity as we measure more and stronger gravitational wave signals. Eventually we might uncover a scenario where General Relativity does not accurately explain our measurement, at which point we may gain new insights into some of the consequences of that theory, such as Dark Energy.

TBL, Research Scientist, Data Analysis / Astrophysics

Aside the background noise, what are the greatest sources of imprecision in your measures?

"Background noise" could mean a few different things. If you mean noise from outside the detector -- like, vibrations and stray electric/magnetic fields -- then yes, those are a source of imprecision, especially at low frequencies. We also have noise sources that are internal to the detectors. The fundamental quantum noise of the laser beams used to measure the distances between mirrors, due to the fact that light is carried by discrete photons, is our main noise source at high frequencies, above ~100 Hz. Thermal noise -- the random motion of the individual atoms making up parts of the detectors, like the mirror coatings and the fibers and springs that the mirrors are hanging from -- also contributes some at medium frequencies. Finally, even if we understand all the sources of random noise, there's a limit to how well we know the calibration of our detectors; with a lot of careful work we can pin it down within several percent, but that's still an uncertainty we have to account for when analyzing our data.

PS, professor, data analysis and astrophysics

How does space-time behave across a gravitational wave, relative to normal space?

The gravitational waves that we have detected alternately stretch and compress distances measured in spacetime by a really tiny amount - less than 1 part in 10 to the power 21. So space is barely disturbed at all. That's why it has taken decades of effort and extreme technology to see the first signals.

A feature of the waves is that when one direction is compressed, at 90 degrees to that space is stretched (and a half-cycle of the wave later the situation is reversed). That's why we use L shaped detectors.

KAS, professor, instrumentalist.

Which color or wavelength is the laser in the inferometers?

The wavelength currently used is 1064 nm. That's twice as long as the wavelength of green light. Comparing it to the rainbows you can see in the sky: 400 nm is violet and 650 nm is red.

Prior to 1995, most of the experiments used green lasers. Some of the ideas for future interferometers entail using a laser with a 1500-2200 nm wavelength due to material science issues.

RXA, experimental physicist

We use several laser wavelengths for different purposes. The main science laser, which is used for sensing the gravitational waves, is at 1064 nm (nanometers). This is in the infrared, so the laser light is invisible to human eyes. In addition, we use 532 nm lasers (green) for the process of bringing the interferometers to an operational state, and we use 10 um (micron) infrared lasers to change the shape of the main test mass mirrors by selectively heating them.

RLW, instrumentation and data analysis

Given the 2 events observed (or 3, including LVT151012), and given the expected increase in sensitivity over the coming year, how many events can be expect in O2?

From these events, we expect to see ~50 events per year. There still significant uncertainties in these numbers (10 events per year is very likely). The table II in our paper link provides better numbers with uncertainties.

SK, Postdoc, data analyst/instrumentalist

Why do you think the black holes collided?

Black holes are very massive objects. This means they exert a very strong pull on other objects due to gravity. This pull or force causes them to orbit each other. As they do they lose energy as gravitational waves and the orbits get smaller and smaller until they collide.

JW, grad student, experimental interferometry

It seems like Weiss, Drever, and Thorne have won every major prize in astronomy this year. Is the collaboration hopefully for a Nobel in the near future?

The LIGO founders have certainly been receiving the recognition they deserve in response to the gravitational wave discoveries. Hulse and Taylor won the Nobel Prize in 1993 in part for "demonstration of gravitational waves" by the orbital decay of a binary pulsar system in our galaxy, so there is precedent for this kind of science being recognized by the Nobel committee. While the prize is not the motivation for LIGO's work, it would be a nice way to cap the incredibly successful first observing run.

TBL, Research Scientist, Data Analysis / Astrophysics

Thanks! Follow-up question, what is everyone doing with their share of the Breakthrough prize?

It will take some time reach us, so we aren't thinking about it now :)

My daughter is obsessed with Hamilton and would dearly like to see it on Broadway. We figure the prize money might buy 2 or 3 tickets...

If CERN were to discover the currently theoretical graviton particle tomorrow, what would that mean for LIGO's research?

That would be awesome!

We can actually think of gravity in terms of fields, in which case a gravitational wave is a disturbance travelling in a gravitational wave field that interacts with the light waves in our detector. This is the 'wave' picture.

Alternatively we can think of it in terms of particles interacting with each other in which case a gravitational wave is actually gravitons interacting with the laser photons in the detector. This is the 'particle' picture.

We usually use the 'wave' way of thinking about gravitational waves as we don't know enough about gravitons and how they interact to use the 'particle' explanation.

If CERN discovered these then it would give us a whole new way of doing some of our theory calculations and this might give us new insight into how gravity itself works.

Because of the discovery of the photon, we understand how light works a lot more, it would be great if this happened with the graviton.

JW, grad student, experimental interferometry

Do you have the capability to detect a unique polarization pattern that would result from the colliding of bubble galaxies?

EDIT: meant to say bubble universes

The gravitational waves from colliding galaxies will be of very low frequency and sensitivity of LIGO detectors are not good at low frequency to see them. However LIGO can in principle to see different polarizations than predicted by GR if the frequencies of signals are in the sensitive band. But that would require different signal processing than we are currently doing to look for binary coalescence signals.

SK, postdoc, data analyst/instrumentalist.

Binary coalescence signals would result from galaxy core collisions, if the cores were black holes?

Sorry, about the original question typo, I intended to ask about unique waves from bubble universe collisions. I am referencing " Cosmic bubble and domain wall instabilities III: the role of oscillons in three-dimensional bubble collisions " (PDF) by Laura Mersini-Houghton, J. Richard Bond and Jonathan Braden

Yes, we would see very low frequency binary coalescence signals from galaxy mergers (as most of the galaxies are expected to have huge black holes at their center). Yes, interaction of bubbles with different vacuum expected to produce gravitational waves. Some theories of early universe suggest such scenarios. There are also theories that suggest that many such collisions happened in the early universe and producing a background of gravitational waves. At this point they are simply theories, but if they are true and the signals are strong enough LIGO will be able to see them.

SK, postdoc, data analyst/instrumentalist.

Gravitational waves are said to be the only potential way to communicate with another universe (if another universe exists).

How much energy would be required for humans to engineer a wave of the magnitude you observed from the merging black holes?

The gravitational waves we have detected so far came pairs of black holes that orbit one another. As they orbit they lose energy in the form of gravitational waves until they eventually merge. The amount of energy lost to gravitational radiation depends on the original masses of the black holes. For the September detection, GW150914, the system radiated about 3 solar masses worth of energy. That is, take 3 times the mass of the sun, and convert that into pure energy using Einstein's handy E=mc² equation. Give it a shot, but you might break your calculator. For comparison, turning a single mosquito into pure energy would be enough to power an average home for a year. The December event, GW151226, was lower in mass, emitting about 1 solar mass worth of energy.

So, for humans to generate this sort of signal, we would have to annihilate the entire solar system. I wouldn't recommend that experiment...

TBL, Research Scientist, Data Analysis / Astrophysics

Hi! If ever you add up another batch of instruments that could detect gravitational waves, plus further increase the sensitivity of LIGO and others, how do you foresee the likelihood of discovering gravitational waves? Would it be very frequent? And also, could LIGO detect other potential sources of gravitational waves such as neutron star collisions or neutron star-black hole binaries, or is LIGO not yet sensitive for that?

The more gravitational wave detectors we have the better! Over the next few years, more ground-based facilities will be joining LIGO in searching for gravitational waves - such as Virgo in Italy, LIGO India, and Kagra in Japan. In the future, we'll also send gravitational wave observatories into space (such as eLISA), which will be able to look at gravitational waves at different frequencies than ground-based detectors and see different types of objects and events.

One of the greatest benefits of having more detectors in the network is localizing gravitational wave events better (constrain what part of the sky they came from). Check out this image to see how well we're able to localize the events as of now: http://ligo.org/detections/images/gw150914-lvt151012-gw151226.jpg

As LIGO advances and upgrades, it becomes more sensitive to gravitational waves, which means it is able to explore a larger volume of the Universe. By doing so, the rate at which we see gravitational wave events should increase!

LIGO can certainly detect things other than black hole-black hole mergers. The mergers of two neutron stars or a neutron star-black hole binary are expected to be detected, though since they are lower mass systems we can't see them quite as far away in the Universe than black hole-black hole mergers. So LIGO is sensitive enough to detect those systems, we just need to catch ones that are slightly closer to us. It will be very exciting when we find a system containing a neutron star, because they're expected to emit light as well as gravitational waves.

MZ, doctoral student, astrophysics / detector characterization

Since you mention eLISA, I had a question related to that project: Do you know, is there a reason why it has a triangular configuration rather than tetrahedral? A tetrahedron would require double the emitters/detectors, but my (admittedly uneducated) guess is that as a result it would be better at locating events in 3D space?

I'm not a LISA expert, but I think it is a cost-performance trade-off. Going from two arms to three brings better source location ability (especially for some signal frequency ranges) and gives a fall-back option if there is a problem on one spacecraft. Going from three arms to four does not bring nearly as much of an advantage.

KAS, professor, instrumentalist.

Edit - fixed typo.

You are right that more is better, but there is also an issue with the orbits of each spacecraft. The (e)LISA concept has each spacecraft orbiting the sun in ~free fall. The separation between spacecraft has to be stable enough to do the interferometry. More complicated constellations of spacecraft would require each one "station keep" i.e. perform maneuvers to maintain the right separation. That means fuel sloshing around which means too much noise for gravitational wave detection. The triangle configuration doesn't require anything more than Kepler's Laws which are pretty good, so the only maneuvers the spacecraft have to do is to keep pointing in the right direction.

There are advanced mission concepts including the Big Bang Observatory (BBO) which involve 6 spacecraft, all in the same plane, forming a star-of-David-like configuration. The best you could do localizing sources with a space-based detector is have one constellation of three spacecraft leading the Earth, and one constellation trailing the Earth. Then you have a multi-million mile baseline to triangulate the source location.

TBL, Research Scientist, Data Analysis / Astrophysics / LISA evangalist

Does gravitational wave astrophysics purely incorporate Einstein's relativity? Or does it have any implications in quantum mechanics or physics beyond the standard model? If so, how and why?

At this point we're delighted to have been able to check the GW150914 wave against GR. It is a strong signal and the fit is good*. It will probably be a while until we get a better black-hole signal to probe any deeper. When we start to detect gravitational waves from neutron stars either as pulsars, or when they are being ripped apart in a merger with another neutron star or a black hole, we might start to get some clues about their structure which depends on the strong force. That's an interesting regime to probe. KAS, professor, instrumentalist.

*We wrote a paper on that Phys. Rev. Lett. 116, 221101 (2016)

Are the recently detected gravitational waves already in the limits of sensitivity of LIGO's detectors, or can the detectors still detect far fainter signals? If so, what are the limits of sensitivity? How do you upgrade LIGO?

Fortunately, yes, both GW events were within LIGO's sensitivity limits! Although, because the detectors aren't perfect we are indeed limited by certain types of noise sources. For example, at low frequencies, we're usually limited by seismic noise from things like earthquakes! We're limited by many other different types of noise sources as well, but as we improve the detector we'll hopefully continue to see these limits decrease and possibly detect even fainter signals.

HG, Research Assistant, Detector Characterization

About how many times have you asked yourselves, "Have we really detected this, guys?"? Hahaha Any possible plans for collaboration with CERN? Could the black holes merging or the resulting black hole formed have an accretion disk for EM based astronomers to observe them?

I think when we made the first detection and while the analysis was being carried out, most of the collaboration was asking themselves that question. We were sure once we released the first detection paper, as sure as scientists ever are when they publish. There is always the chance that other scientists could look at your research and find something wrong with your maths or your method, or even that they could carry out another experiment and disprove your research.

For the second detection I think we were all a bit more confident as we were further is the analysis for the first detection at this point and the detectors has been observing for a longer period.

We don't plan to collaborate with CERN at the moment. Mainly because we are a telescope so we look for events that are already occurring in the universe, we don't create events to observe.

CERN is an experiment where they create events in their accelerator and then look at the results. So we are different types of experiments.

There might be a possibility that we work together on theory in the future if CERN discovered something about gravity.

Indeed the whole community of scientists around the world in every field of research is meant to work together. Thats why we have conferences and scientific journals, to discuss each others' work.

There is a possibility that there would be an accretion disk around one of the black holes that is merging but this is unlikely to still be there around the final black hole due to the high forces involved in the merger possibly ripping it apart.

We do send out detection alerts to EM astronomers at space-based and ground-based telescopes every time we see something that has a good chance of being a real signal.

JW, grad student, experimental interferometry

Dark matter is considered to be the "pulling force" which has the universe accelerating.. How do we know that it is not just a super massive black hole we are heading towards?

The acceleration of the universe is due to what we call dark energy, as opposed to dark matter. Dark energy might just be a manifestation of a particular allowed solution of Einstein's theory, or it could be due to some new physics, we don't really know at this point. Dark matter, on the other hand, is related to the fact that if the only matter in the universe was made up of things that we see/that emit light, then it turns out that galaxies could never have formed the way they have, and they wouldn't rotate in quite the same way that we observe them to. We need an extra amount of matter to cause galaxies to clump together in the early universe, and to be spread amongst the regular matter in galaxies, so that it changes the orbits of the regular matter (e.g. stars) that we do directly observe. If the dark matter was made up of supermassive black hole, then we would be able to observe them in the cosmic microwave background (CMB). Dark matter needs to be primordial, meaning created in the very early universe, in order to explain the subsequent formation of galaxies, but if it was made up of supermassive black holes, then those black holes would accrete the surrounding matter and emit highly energetic radiation. This would change the appearance of the CMB in a predictable way, but in a way that we don't actually observe, so we know that dark matter can't be made up of supermassive black holes.

STM, Professor, source modeling and astrophysics

Is there any other event that could theoretically produce more gravitational waves than 2 black holes merging? If not what else could come close, if anything?

The strength of the gravitational waves emitted is proportional to the masses involved and the speed at which they move. Since black holes are generally massive objects in the universe and also move relatively fast they expected to produce strong gravitational waves and hence considered one of the efficient sources of gravitational waves. However there are theories that suggest much stranger objects, at least in the early part of universe, such as domain walls, bubble universes collision of whom could produce much stronger signals. Since they expected to be happened long time ago (and far away) they would still only produce small effects on LIGO detectors compared to closer black hole binaries.

SK, postdoc, data analyst/instrumentalist

Why should we care what LIGO does (serious question)? What practical usefulness is it to us here on earth?

Check out this thread from earlier today: https://www.reddit.com/r/IAmA/comments/4odza8/we_are_the_ligo_scientific_collaboration_and_the/d4bpn8n

I know it's not the same field as what the LIGO team study, but I'm curious as to your thoughts on the recently published paper regarding the proposed EM drive's 'exhaust', explaining how it doesn't break Newton's third law?

http://scitation.aip.org/content/aip/journal/adva/6/6/10.1063/1.4953807

P.S. - Thank you for all that you do! You're opening up a new sensory organ for the human race to explore the universe with.

As you've said, this paper is on a very different topic. What I can say, after having a very quick look, is that it is certainly true that momentum conservation and Newton's third law are obeyed, so any claim to the contrary is going to be wrong. That said, based on the description given here, such an "EM drive" should not generate thrust. Perhaps this is just due to their description, and the actual device (which I know nothing about) operates differently? The only other thing I can add is that their description of how two photons interact is very inconsistent with my own understanding, in which two photons don't interact at all in the simplest approximation at which they seem to be trying to work (called linear order, which by definition is the order at which the electromagnetic field does not interact with itself). Therefore, the momentum and energetic properties of a system of two photons won't depend at all on where they are, even if they are located at the same place at some instant. Now, photons can interact at higher orders, for example in certain astronomical settings, but that requires vastly higher energy photons than what, I gather, is being considered here.

STM, Professor, source modeling and astrophysics

Do you have plans for the next generation of gravity wave detectors and what kind of resolution would they have?

Yes, there are plans for several future generations of ground-based detectors with ever improving sensitivity, first by improving the components at existing sites, then by building completely new facilities with longer arms and improved components. There is also a plan to launch a detector into space (called LISA or eLISA, which was discussed above), which would allow us to get away from the noisy Earth and measure signals at lower frequencies. In terms of resolution, that's actually a fairly involved question, as it doesn't only depend on the detector sensitivity, but also on the number of detectors, since the more detectors that are spread around the world (or throughout space :-) ), the better we can localize. The goal would be to eventually localize sources to individual galaxies, which will require pinpointing to arc seconds on the sky, but we have a long way to go to reach that point.

STM, Professor, source modeling and astrophysics

I'm currently a sophomore physics major and I am curious into the application of gravitational waves to look into the early universe, maybe for research in the near future. I believe an application of detecting gravitational waves is to look far back into the early universe right after the Big Bang. Could using "gravitational wave telescopes" help us look beyond the Cosmic Microwave Background, and into the very early universe before it was opaque? If so, how and when do you think you would be capable of doing that?

Good question! Yes, we do search for gravitational waves left over from the early universe using data collected by LIGO and other gravitational wave detectors. If we can detect it, it tells us about the flow of matter and energy in the early universe, when a lot was happening rapidly. And yes, it does look beyond the time when the cosmic microwave background we see today was emitted, about 380,000 years after the big bang. That makes it a unique probe of the early universe. Here's a summary of an article we wrote about it a few years ago: http://www.ligo.org/science/Publication-S6VSR23StochIso/index.php .

However, it might be the case that the gravitational waves from the early universe are simply too week for LIGO to ever detect; the standard "slow-roll inflation" model for the evolution of the universe would've produced waves that are extremely weak today. On the other hand, we don't know if that is the right model and we want to test it; there are some alternative models that could lead to detectable signals. So we will continue searching as we improve our detectors and collect more data.

PS, professor, data analysis and astrophysics

So is it anything like how earth quakes can help study the earths interior? I was wondering if it could tell you anything about the interior of the black hole because what confuses me is if these things are spheres does stuff just spiral down equally to an infinite point at the centre... Please excuse my infinite stupidity I have a very basic understanding of what is what out there

The thing about black holes is that we can't find out any information about the centre. This is all 'hidden' from us inside the 'event horizon' of the black hole. This is the perimeter from beyond which no light or mass can escape the gravity of the black hole.

The 'singularity', what I think you mean by 'infinite point,' is inside this horizon at the centre of the black hole, but the horizon itself has a size dependent on the mass of the black hole.

If we detected gravitational waves from another type of object without the strange properties of a black hole, we might be able to use the signals to discover things about the interior of the star.

We hope to detect merging neutron star binaries with Advanced LIGO and seeing gravitational wave signals from these would tell us about their structure.

JW, grad student, experimental interferometry

How do the gravitational waves affect the universe..?

Gravitational waves are disturbances in space-time itself, and we measure them by carefully measuring the distances between objects.
All accelerating masses generate gravitational waves and as those gravitational waves pass by, the space between objects shrinks and stretches imperceptibly. Only the most violent events, like the mergers of black holes, disturb space-time enough that those changes in distance are measurable. Adding together the tiny contributions from all of the immeasurable gravitational waves, and there should be a background "hiss" of gravitational waves--as if the whole Universe was continuously, randomly, vibrating. This background of gravitational waves is a very interesting source to try and measure, and there are a variety of experiments (pulsar timing, polarization of the cosmic background radiation) working to detect different parts of this spectrum. LIGO searches for a background as well, and it is possible that when we reach full sensitivity we will detect a background form countless black hole mergers that are too quiet to individually measure.

TBL, Research Scientist, Data Analysis / Astrophysics

Would a giant alien spaceship (1/4 mass of moon) suddenly appearing at the Moon's distance cause a gravity wave event detectable with your instruments?

Interesting question. The first problem is that we don't know (in physics, rather than science fiction) how to make a spacecraft suddenly appear. If we try to calculate a more realistic approach into an orbit near the moon, the signals would be too low in frequency for our detectors.

The signals we see come from black holes only 10s of km across moving at close to the speed of light. With less exotic systems it is hard to make gravitational waves reach the frequencies we can detect.

KAS, professor, instrumentalist.

Cheers! How do you think the transition from sight-based to sound-based astronomy will affect science (since we're so visually driven)? Saluti!

Observing and gathering information through multiple windows (Visible light, X-rays, Radio Waves, Neutrinos... and now Gravitational Waves) will allow astronomers to get a better and more vivid picture of the Cosmos ... So its rather a transition to the era of Multi-messenger Astronomy

NM, PhD student, Detector Characterization/Astrophysics

The "light" or other EM waves we see comes mostly from the surface of objects. The gravitational "sound" we hear comes from their bulk movements and the vibrations of that sound tell us quite directly how the objects move. In truth though we'll learn most when we put sight and sound together. KAS, professor, instrumentalist

Agreed, the more senses we can integrate, the better. Like you say, the 'light' is electromagnetic waves. Would you explain more about the 'sound' -- not on the EM spectrum, since typical sound can't travel through a vacuum (like, space)? Thank you.

The analogy to 'sound' should be taken with a grain of salt. With gravitational waves we 'sense' the changes in the space-time that we live in. Since gravitational waves changes the space-time they travel through similar to 'sound' changing the medium it travels through (or it needs a medium to travel through), we often make that analogy.

SK, postdoc, data analyst/instrumentalist.

In fact we are not just sight driven. The human ear is remarkably effective at picking up weak signals buried in random or ambient noise. Luckily for us, the the signals LIGO-like detectors are sensitive to are in the audio band and so we also have a chance to hear them.

BFW, Professor of Physics, gravitational wave research and data analysis.

If the black hole merger happened, say, only about 1000 light years away, how "large" of a distortion would be the effect of gravitational waves on us and Earth?

The effect of GWs on our detectors is proportional to their amplitude which means the signal rises in proportion as the distance falls. (It is not like detecting photons, which is a measurement of the signal power, when we get the familiar 1/r² law.) The first events are - in rough terms - 1 billion ly distant, you ask about 1 millionth of that distance, so the peak strain would be 1 million times what we saw at around 1 part in 10 to the power 15. That's huge for us, but still tiny - just picometres of movement of the detector's mirrors. Your height and mine would oscillate by a couple of proton diameters as the wave passed through. Nothing to worry about.

The chance of that happening is around 1 per hundred million billion years, so I don't plan to hold my breath.

KAS, professor, instrumentalist.

What is the significance of turning the gravitational wave into a sound? What do the sounds represent exactly?

Listening to the interferometer has a great diagnostic power. When we are trying to debug the behavior or understand the noise, our ear is able to recognize patterns of misbehavior quite well. I have often been able to diagnose malfunctions or mistunings by just hearing the interferometer squeal. RXA, experimental physicist

On the significance: We convert the gravitational wave signals into sound to help both us and the public connect to the observations in a more human way than looking at some plot in some paper. The sounds do not have much diagnostic value (though in earlier generations of LIGO we did actually listen to the data trying to classify different disturbances called "glitches," but check out gravityspy.org for a more modern version that you can help with!)

This process of taking scientific data and turning it into something that we can perceive happens all the time in traditional astronomy. Spend some time on http://apod.nasa.gov and you will see images from radio, infrared, ultraviolet, x-ray, and even gamma-ray telescopes. We can't see those different kinds of light, so the astronomers create "false color images"--assigning visible light colors to different parts of their data sets--so that we can experience the data in a familiar way.

TBL, Research Scientist, Data Analysis / Astrophysics

There is an answer to a similar question here https://www.reddit.com/r/IAmA/comments/4odza8/we_are_the_ligo_scientific_collaboration_and_the/d4bsxso

There may be more to follow on this from a colleague.

KAS, professor, instrumentalist. edit - tidy up

Congratulations on another detection! I have two questions: How much will the localization improve with the next generation instruments? Also do you expect to see a GW event where an electromagnetic (i.e. photons, light) counterpart is predicted soon?

Localization: The majority of our localization information comes from the differences in arrival time between detectors. Therefore, the best way to improve localization is to increase the number of observatories around the world. During the next observing run we expect the European observatory, Virgo, to begin taking data. In the early part of the next decade, detectors in India and Japan will join the world-wide network. By then typical localizations will go from 100s of square degrees down to a few. You can have a look at https://arxiv.org/abs/1304.0670 which is occasionally updated with new forecasts.

Electromagnetic: We are all certainly looking forward to observing the same source with gravitational waves and photons. We work closely with electromagnetic observing partners around the world to follow-up gravitational wave candidates with telescopes. As the sensitivity of the detectors improves so too will the chance for joint observations, because we will increase the volume of space where we are sensitive. . We would be ecstatic for that to happen during the next observing run starting this Fall, where we will be probing a factor of ~2 larger volume, but there are no guarantees. We'll have to be patient and let the Universe decide when we get the chance to begin the era of multimessenger astronomy.

TBL, Research Scientist, Data Analysis / Astrophysics

So you didn't get a neutron star merger during the O1 run. This must be pushing the event rate way down. What does this mean for models that assume short GRBs are produced by neutron star mergers?

Is there friction between the event rate we could expect given the observed rate of sGRBs (assuming they are produced by NSMs) and the lack of events observed by LIGO during the O1 run?

Not yet - the range of rates allowed by population models is huge. There is a way to go before we constrain these rates to any significant extent. Remember how far away most GRBs are - we can't see neutron stars mergers anything like as far away as the black hole mergers. KAS, professor, instrumentalist.

What is your favorite pizza?

With over 1000 international members in our collaboration, there is, by definition, 1000s of favorites! JR, engineer, instrumentalist

What do we physically know about 'gravitational waves' in terms of how we translate that to sound since sound can't travel through a vacuum (like, space), for example, are there similarities to electromagnetic waves? Best in your work!

If the gravitational waves happen to oscilate at a frequency such as, for example, low C we can play the signal through a loudspeaker and hear the familiar sound. The detectors are in a sense like ultra-sensitive microphones, though it is not vibrating air molecules we are picking up, but vibrations in spacetime. One similarity with EM waves is they travel at the same speed as gravitational waves. I prefer to say that light travels at the speed of gravity, rather than gravity travels at the speed of light. KAS, professor, instrumentalist.

Do you wear lab coats?

While working in and around the vacuum chambers, in which are optics reside, we wear special scrubs and cleanroom garb (“bunny suits”) that cover us from head to toe. We need to do this to reduce the particulate and contamination that would make its way to the optics and cause absorption when we are pumped down to 10^-9 torr and lasing.

JR, engineer, instrumentalist

I thought LIGO+VIRGO was supposed to be able to triangulate the position of gravitational wave sources. Why is the position estimate still so large?

The Virgo detector is not up and running quite yet so for these two detections we have only had two detectors with the correct sensitivity range to see these sources (Advanced LIGO Hanford and Advanced LIGO Livingston). Once Advanced Virgo has finished its installation and is also looking with the Advanced LIGO detectors this will improve the position estimate.

JW, grad student, experimental interferometry

Advanced Virgo is still not running, but they collaborate with us in interpreting and analyzing our data.

BFW, Professor of Physics, gravitational wave research and data analysis.

I've read that some scientists (not stated whether they are with LIGO or not) believe this recent sighting may have accidentally shown us some dark matter. Is there any truth to this statement?

Technically speaking, black holes are "dark matter" in that they account for part of the total mass of the Universe but do not emit any light. That being said, the question is whether or not there are enough black holes to account for the total amount of dark matter we need to explain the observed Universe. Smaller black holes, like the ones we observed in December, have already been ruled out as a significant contribution to dark matter. The larger black holes, like we found in September, could, be a larger contributor but more observations from LIGO and other experiments will help support or rule out that possibility.

TBL, Research Scientist, Data Analysis / Astrophysics

Why is it important the detection of gravitational waves?

Thanks a lot for taking your time here.

There are probably a range of opinions among the LIGO and Virgo scientists about this. I think its important because we have basically proven that we can build a new type of telescope that can look at the universe via signals that we have never been able to use to observe things before. This means we can find out information about objects that don't really emit enough energy in electromagnetic signals to be seen with conventional telescopes. For instance the detections we have made already tell us about black holes. These were objects that had never been directly observed before because they don't emit light (electromagnetic radiation). I mean, scientists believed they did exist because we could see their effect of other objects, but not the black holes themselves. JW, grad student, experimental interferometry

Edit: 'see their effect on other objects'

When a gravitational wave arrives at an angle to the plane of the interferometer other than perpendicular, don't they oscillate the test masses in directions other than that of the laser? Wouldn't the vertical component of its motion couple with its horizonal motion due to the way it's suspended?

The detectors can only sense in the plane of the "L" shape. Even if the mirrors were to move perpendicular to the plane, that would not (to first order) change the length of the arm and that's what we measure. Actually vertical seismic noise can be a problem if it gets though our isolation systems to the mirrors, but that's because "vertical" is not exactly the same direction at the two ends of a 4km arm due to the curvature of the earth's surface, so there is a component of the motion in the plane of the detector. KAS, professor, instrumentalist

Why is a chirp signal associated with gravitational wave detection?

A gravitational wave detection does not need to be a chirp. Any gravitational wave signal will cause the LIGO detectors (and eventually Virgo and others around the world) to respond together in a predictable way. We search for gravitational waves in the data by looking for a "pattern" in the data that is found in multiple detectors. The chirp signal is the tell tale sign that the gravitational wave came from the merger of two dense objects and not something else (like a supernova, spinning neutron star, etc.). How long the chirp lasts, and at what frequency (or pitch) the chirp ends, tells us about the masses of the two objects that merged. Because our two discoveries were both relatively short and low frequency, we are able to confirm that they came from the mergers of black holes. Longer-lasting signals, and higher frequency chirps, would imply that the objects were lower in mass. A very long, very high frequency chirp would be caused by merging neutron stars instead of black holes, which would be another moment of great excitement for gravitational wave astronomers and astrophysicists.

TBL, Research Scientist, Data Analysis / Astrophysics

Interesting, thank you! I have a couple more questions. Is the range of frequencies in a chirp signal somehow related to the Doppler Effect, as masses spin faster and faster around each other? Similarly, if a sinusoid with a constant frequency is observed from orbiting masses, does that mean the space surrounding the masses is stationary with respect to the Earth, with the exception of the ripples from the masses?

The chirp itself is because, as the black holes get closer together, they orbit faster and faster, making the frequency of the gravitational wave signal increase. Just before merger they are moving at around half the speed of light, which is mind boggling.

The entire signal is frequency shifted due to the expansion of the Universe (something astronomers typically refer to as redshift). The sources we are measuring are close enough that the redshift is a small effect, but as we reach design sensitivity it will be more pronounced.

Space-time is all stirred up by the merger of the black holes and it's that disturbance in space-time that propagates to us as gravitational waves. Checkout this simulation of the December detection, GW151226, to see what the curvature of spacetime looks like around the black holes: https://www.youtube.com/watch?v=3pK5oenm5gw

TBL, Research Scientist, Data Analysis / Astrophysics

Congratulations on the second detection! How do we proceed from here? What's next for LIGO after this second detection of gravitational waves? Thanks! You guys are awesome.

Lots of hard work is already underway to make the detectors even better, ready for 6 months observing starting this Fall. Then more upgrading and data taking until we reach full design sensitivity - around 3x better than in O1, seeing 3x farther out, which means we should see about 27 times the rate of events in that volume. Add Virgo, Kagra and LIGO India detectors and we'll have an amazing observatory, black holes seen every week or so, and hopefully many other signals too. KAS, professor, instrumentalist.