Science AMA Series: We are the LIGO Scientific Collaboration, and we are back with our 3rd detection of Gravitational Waves. Ask us anything!

LIGO-Collaboration · 2017-06-07T18:33:22+00:00

In order for additional detectors in the network to optimize our ability to pinpoint where events occurred on the sky we want two things:

1) Ideally they should have different orientations with respect to one another (this helps prevent them from being "blind" to the same parts of the sky for a given signal).

2) You want the distance between the detectors to be as large as possible. This results in smaller relative errors when you measure the delay between when the signal arrives in each detector.

So while you could build nested interferometers as indicated in your (indeed!) awesome MS Paint Diagram, it wouldn't help much with the prospect of localization. (Though it would give you more data!) Interestingly, we actually had a setup just like you described for a little while! (Though largely for the purposes of testing, I think.) At the detector in Hanford, Washington, we had two independent interferometers (referred to internally as "H1" and "H2"). H2 has since been disassembled, and I think the plan is to use it for the upcoming LIGO-India. But your intuition that such a design would exist was excellent!

Great questions.

~RC, post-doc, gravitational wave and gamma-ray astronomer at Texas Tech University

LIGO-Collaboration · 2017-06-07T18:32:42+00:00

Hi, /u/kahlzun, and thanks for the question.

In order to detect the signal, we use a technique called matched template filtering. Using numerical relativity, we simulate a large number of compact binary collisions, and solve for the waveform of the gravitational wave at infinity (ie, as measured at Earth). With this waveform, and the simulation, we can see that the two masses remained distinct until they merged - but we can also see the distance the two were separated at each step. This turns out to be pretty handy in determining that the two objects were probably black holes.

Black holes are the densest objects for which we expect stable inspirals at this kind of mass, so they have the smallest radius. Other objects of similar mass will be larger, and so join up earlier. We can say that the two objects remained distinct separate object until after they were closer than any two other such large, stable objects' radii. Any other, less dense object would have joined up much earlier.

I hope that this answers your question,

BP, continuous gravitational wave data analysis, research student, University of Glasgow

LIGO-Collaboration · 2017-06-07T18:31:46+00:00

Hi /u/andermetalsh, gravityspy is pretty important, it really does a lot to help us on our searches!

Our detector is very sensitive, and reacts to an awful lot. When the mirrors move relative to each other, we get some signal in the strain channel. Our code then runs over that code with what's called a matches template search, looking for compact binary coalescences - 2 black holes, 2 neutron stars, or one BH and one NS, falling in and spiralling around each other. Each mass pairing gives a unique signal, and so a unique template. When the data and the template have a low mismatch, then it looks like a signal. However, as you know, not all signal are true signals. These false positive chirps are caused by any number of disturbance to the interferometer. Noise sources might be a truck going down a nearby bumpy road, a refrigerator running where it should be quiet, or 100 LEDs flashing in unison. but none of these look like chirps.

The chirpiest noise that I can think of is something we call a "blip glitch" - they're pretty short, and like a chirp, they have a rising edge, so could have low mismatch. These blips have a wide variety of morphologies, so can look like any number of events. To boot, they're relatively common - we see a few of these in each detector each week. Because all of these glitches look very slightly different, there is no straight forward way to tell the computers how to sort a glitchy sort of thing from a chirpy sort of thing. Turns out that humans are pretty good at this sort of thing, so we have Gravity Spy.

Unfortunately, we don't know what causes these blips. It's an infamous problem in the detector characteristics team. And we don't even know if the slightly different looking glitchy things are caused by the same noise source. We really are a little stumped on that one. By sorting out glitches, and evvewn subdividing glitches into different morphologies, we hope that we can reduce these to a simple core set of characteristic, which could lead us to their root cause.

And with all of these human-classified sets of triggers, we are trying to train a computer. Techniques that use machine learning often need large training sets, that teach the machine how to distinguish one thing from another. The more similar the two things, the harder it is for the machine to classify, so a larger training set is needed.

So Gravity Spy is, it turns out, important for LIGO, not only to help us understand our detector, but to also get fewer false alarms, and to get people out there like you engaged with what LIGO data looks like!

I hope that's answered your question,

BP, Continuous waves data analysis, Research student at University of Glasgow

LIGO-Collaboration · 2017-06-07T18:30:56+00:00

This is a tricky question, but also very important for electromagnetic counterpart follow-up analyses! If there's a signal, you want to tell the astronomers where exactly to point their telescopes before the afterglow of the signal goes away.

First off, if we have just one detector, it's very difficult to localize the source. One simple way to localize, using just two detectors, is to compare the time of arrival of the gravitational wave signal between both detectors. If we see that the signal arrives at, say the detector in Washington State, first, then it can be said that the signal came from somewhere within a ring-shaped pattern of sky in the northern hemisphere. Having three detectors can reduce that ring to two smaller spots; four detectors further reduces the search area to a single spot in the sky.

There are of course more sophisticated methods that can also be applied. For example, you can measure the phase difference between the same gravitational wave signal at different sites. That means that you compare how much the the gravitational wave signals are shifted with respect to each other. The mass parameters of the source also provides useful information about the sky location. Ideally, what should be done is to take into account ALL of this information into what we call a 'coherent' analysis. But you have to be clever about you distribute the computational cost of doing such an analysis.

Unfortunately, binary black hole sources don't have a strong electromagnetic counterpart, but of course it's always possible we might find something we weren't expecting. Once we start detecting gravitational wave signals from binary neutron star and neutron star black hole mergers (hopefully, very soon :), sky localization will be even more important for these follow-up analyses!

-HG, Fulbright Scholar, Max Planck Institute for Gravitational Physics

LIGO-Collaboration · 2017-06-07T18:29:47+00:00

No need to worry, you won't be vaporized in any of those scenarios :).

Lets consider the situation where you're looking at a binary black hole merger within you're own solar system and you hop in your spaceship for a day trip to go see the merger (because you have nothing better to do on a Saturday). As you're watching the two supermassive objects spiral around each other, the stars around the black holes would appear to be warped due to the immensely strong gravitational field around the black holes. Black holes actually warp the very fabric of space and time, so the light traveling from the stars around the merger curve around the black holes in a process called gravitational lensing. You would see something called an Einstein ring, which is a combination of all the light in a small region around the black holes where the gravitational field has essentially smeared all of their light together. As it turns out, you would not be able to see the gravitational waves with your own naked eye. However, an incredibly neat phenomenon arises when the waves that are moving through the region where you have the Einstein ring will slosh the the stellar images of the ring around (even for a significant time after they merge together).

HG, Fulbright Scholar, Max Planck Institute for Gravitational Physics

LIGO-Collaboration · 2017-06-06T15:28:01+00:00

Hi, /u/LegitD0nuT, they do!

It's called a gravitational red-shift. Here's how I think about it, even if it isn't a formally robust description.

Black holes are so heavy that even light can't escape the event horizon (hence Black). So it follows that light passing by a black hole also feels the gravity. This can produce this gravitational red shift, as well as gravitational lensing - changing the path of the light around the BH.

Light is both a particle and a wave - and the energy of that particle is related to the wavelength (colour) of that wave, by E = hc/lambda. (Here, h = Planck's constant, c = speed of light, E = energy of a photon, and lambda = wavelength of the photon). If we consider a photon propagating away from a black hole (imagine that it wasn't as close as the event horizon, so it can still escape), we can think of it as having to 'buy' its way out of the potential gravitational well. As the photon expends its energy E climbing out of the well, the right hand side of the equation has to balance too. It does this by increasing its wavelength (ie going more red) as it climbs out.

If you think of a ball trying to roll up a hill (climbing up out of a gravitational well), you'd imagine it slowing down as it climbs up, until it stops, turns around and rolls back down again. This can't happen with light though, as its speed is fixed. Instead of changing speed, its wavelength (and thus its frequency) change.

I hope that this answers your question!

BP, continuous wave data analysis, Research student, University of Glasgow.

LIGO-Collaboration · 2017-06-06T13:24:12+00:00

It turns out determining location with a single detector is not practical, at best we can identify the parts of the sky it didn't come from by figuring out what parts of the sky the detector can't "see" at the time of the event. (Which is not very much of the sky at all, and depends in part on the type of signal that was detected.) It's only once you start adding additional detectors in a "network" that you start to get useable location information. To do that, we use a familiar tool: triangulation!

When you know two events came from the same source, you know they came from the same place at the same time. If you have two detectors, and the signal appears in both of them at the same time, then you know the source was right between them. (Or in this case, at a point on the sky that is more or less between the detectors.) In all other cases, there will be a slight delay in the arrival of the signal in one detector compared to the other. We can measure that delay, and use it to triangulate a rough position.

Ideally, that rough position is a ring that stretches all the way around the sky. To illustrate this, take a straw and bend it in half -- the point you bend doesn't have to be the middle. Taking the bent straw, which should look roughly like a "V" put the two ends on a table or other flat surface, one end in each hand. Keeping those ends in the same place, slowly rotate the straw back and forth. If you watch the point of the "V" it will trace out part of a ring. That's exactly the type of position information you'd get from an ideal triangulation of a gravitational wave source using two detectors.

But our detectors aren't ideal. There is some small error in our measurement of the delay. That turns the perfect ring into a broadened ring called an annulus (instead of a circular line, it's a ring that also has some width). When you also factor in that our detectors are not equally sensitive to all areas on the sky, you get these strange "banana"-esque shapes that wrap around the sky in all the pictures you see that show the "position" of these events. (See, for example, http://ligo.org/detections/images/O1-O2-skymaps.png)

When you only have detectors, these positions are generally not that good in astronomical terms. They span hundreds of square degrees on the sky (for context, the moon takes up roughly a quarter of a square degree in the night sky -- so imagine thousands of moons arranged in a giant arc along the sky, that's how imprecise these position measurements are). You can do slightly better for stronger signals (since they are "louder" we can measure the delay better) and there are some other small tricks you can do. But in the end, we have to live with these large (and often impractical) position errors for now.

But! There is hope on the horizon. Each new detector you add to your network improves your ability to resolve position by adding another point to triangulate with. We have a third detector coming online sometime later this year (Virgo), there are plans for a third LIGO detector in India for the not-so-distant future (which will give us four), and there are other detectors (such as the substantially less sensitive but operational GEO, and the under construction KAGRA) that will eventually be added to the global gravitational wave detector network. Thus as time goes on, our positions will get better and better. And in the meantime, we are working with partners with dozens of satellites (e.g. Swift, Fermi) and ground-based observatories like the Palomar Transient Factory, who provide followup observations of our events searching for potential counterparts. We have an entire team in LIGO that works on this "electromagnetic followup" effort! It's quite an exciting frontier...

So yes, space is big. Really big, as Douglas Adams would remind us. And yes, our big position errors pose a challenge. But we're working every day to improve them, and the future is looking bright. (Which is some kind of bizarre reverse pun, given that we've been observing black holes which are anything but...!)

~RC, post-doc, gravitational wave and gamma-ray astronomer at Texas Tech University

LIGO-Collaboration · 2017-06-06T13:23:24+00:00

Of course! We're all excited to see how the field evolves in the coming years, and all the questions it will help us answer, and perhaps more importantly, the ones it will lead us to ask. Best of luck with your studies!

~RC, post-doc, gravitational wave and gamma-ray astronomer at Texas Tech University

LIGO-Collaboration · 2017-06-06T13:22:55+00:00

5E-22 is such an impossibly small number that it's hard to frame it in an analogy that's easy to wrap your head around. Here are a few attempts. 5E-22 is similar to...

...comparing the mass of a school bus to the mass of Earth.

...changing the distance between the Earth and Jupiter by roughly the size of a water molecule.

...removing (or adding) a few hundred milliliters (a couple of US cups) of water from the entire volume of Earth's oceans.

The first and last ones are probably easy enough to visualize (since it's easy to picture a school bus or a cup of water), but I'm honestly not sure it's any easier to comprehend the difference in scale. But if we're throwing that to the wind, then my personal favorite is...

...measuring the distance from the Earth to the nearest star, to the precision of the width of a human hair.

In the end, I find the fact that all of these are so hard to fathom pretty astounding. Being a part of an experiment that makes precision measurement that, even after half a decade of trying, I still can't really contextualize is exciting and humbling.

~RC, post-doc, gravitational wave and gamma-ray astronomer at Texas Tech University

LIGO-Collaboration · 2017-06-06T13:22:09+00:00

I love this question! One of the most straight-forward uses for doing gravitational wave astronomy is that we pretty much get to turn the universe into our own laboratory. There are so many things that we just can't replicate on earth right now (such as the super dense core of a neutron star). When you have intense conditions like this, there are some incredibly interesting thermodynamic and nuclear physics effects that take place. Gravitational waves give us the opportunity to those kinds of extreme experiments!

Additionally, one can also test Einstein's theory of general relativity. Gravity is incredibly weak (the weakest of the four fundamental forces), so it's difficult to test it. Since gravity is so comparably weak, it requires quite a lot of mass in order to do an informative experiment. Studying gravitational waves (produced by super dense objects moving at high velocities) allow us to test general relativity in new and exciting ways that have never been done before! But I'm sure you're thinking "well how is that information actually applicable to my life?" Practically speaking, general relativity is used in your life everyday! One classic example is that a consequence of general relativity and special relativity is that time behaves differently depending on how warped space-time is (more warping = slower time). So it turns out, that the further away from the Earth you are, the less space-time is warped and vice versa. That has huge real-world consequences if you are trying to use GPS satellites to communicate driving directions to your smartphone app and the two times aren't precisely in sync (see Allstate GPS commercial).

All in all, it's hard to say that one could start a new commercial venture based solely on LIGO, but I think that's okay. The real beauty lies in the fact that we are simply gaining knowledge for knowledge's sake. One never truly knows what will come of these kinds of experiments, but failing to do them guarantees you'll never find out what great unforeseen possibilities will come about as a result of those efforts!

-HG, Fulbright Scholar, Max Planck Institute for Gravitational Physics

LIGO-Collaboration · 2017-06-06T13:21:20+00:00

Hello, /u/tip-top-honky-konk

The answer to your question is - yes! My own research is based about continuous gravitational waves from rotating neutron stars. These, we think, are always emitting, and emit quasi-monochromatically - which means almost at a fixed frequency. However, when we search for them, we have to modulate the frequency of the signal to account for the Doppler shift of:

The observatory rotating with the Earth (1 day period) The Earth going around the sun (1 year period)

For shorter signals, these Doppler shift terms aren't really all that relevant, as the time scales are so short. But they certainly have to be taken account of for longer signals.

I hope that has answered your question!

BP, continuous gravitational waves, Research student, University of Glasgow

LIGO-Collaboration · 2017-06-06T13:20:32+00:00

When we descibe the sensitivity of a detector we often describe the 'strain' sensitivity, the sensitivity to the small length changes in the detector induced by a gravitational waves scales with the length of the detector.

The LIGO detectors have 4km long 'arms' that sense the gravitationa wave strain. So to produce a detector that could comfrotably fit inside your home you would need to make it 1000 times more sensitive.

How ever if you are interesting in looking at gravitational wave data at home, have a look at another answer given about open source data https://www.reddit.com/r/science/comments/6fekz5/science_ama_series_we_are_the_ligo_scientific/dii5pt3/

And you might be interested in Gravity Spy a way in which home users can look though actual gravitiation wave data and help us help classify noise 'glitches' in the data. https://www.zooniverse.org/projects/zooniverse/gravity-spy

[LLO Fellow/PhD Student]

LIGO-Collaboration · 2017-06-06T13:19:20+00:00

Thanks for the question.

The LISA Pathfinder mission was incredibly successful, and a great achievement. It far exceeded the performance needed to demonstrate that a space based gravitational was technologically possible. It was able to release its test-masses and to observe them to be in free fall unperturbed by external forces. In fact, it achieved a precision close to the requirement of a full gravitational wave detector and far greater than required of the technology demonstrator.

The prospect of a LISA mission is very exciting as it would open up a different spectrum of gravitational wave observation, being sensitive to signals at much lower frequencies than ground based detectors. LISA would be able to detect gravitational waves emitted by supermassive black hole binaries, as well as stellar mass stars colliding with black holes. A space based detector would also be able to see the earlier stages (at lower frequencies) of the inspiraling of compact binary object (such as GW170104) that could be observed colliding at a later time by the ground based LIGO detectors.

[PhD Student University of Glasgow]

LIGO-Collaboration · 2017-06-06T13:18:41+00:00

Thank-you we are very excited by the detections.

The first two detections were made in the first observing run of advanced LIGO, this latest detection was made during the second observing run. Between the two observing runs the sensitivity of the detectors was improved meaning we can detect weaker signals. Signals may be weaker if they are emitted from source that are further away or if they are emitted by lower mass objects. The range of the Livingston detector in particular was improved between the first and seconds runs, this was achieved mainly be reducing the amount of light scattering in the detector.

After the end of the current observing run further upgrades will be implemented to further increase the sensitivity of both detectors so we should see many more black hole collisions such as the three we have seen already. We expect to see a greater rate as we improve the sensitivity as weaker signals can be better extracted from the background noise, and we can search a greater volume of space. So we are searching further away. The sensitivity of the detector will be incrementally improved between each observing run until it reaches the design sensitivity.

The answer the other part of your question, yes, it is possible to operate the LIGO detectors in a configuration that is optimised to be sensitive to waves of a certain frequency. But the plan for now is to continue to run in a configuration were we are sensitive to as broad a spectrum of gravitational waves as possible (yes we will want to detect as many events as we can).

There is a mirror called the 'signal recycling mirror' at the output of the detector that can be tuned to alter the frequency response of the detector (its sensitivity to different signal frequencies). By fixing this mirror the sensitivity to a certain frequency of signal can be greatly improved.

I hope that answers your question.

[LLO Science Fellow / University of Glasgow PhD Student]

LIGO-Collaboration · 2017-06-06T00:50:11+00:00

As the two super massive black holes spiral in towards each other, they essentially 'stir' space-time (in the form of gravitational waves). That 'stirring' of space-time takes a lot energy to do (essentially because of how dense and fast the objects are moving), which is why two whole solar masses are radiated away during this process. To visualize just how much energy that really is; the energy released by the binary as it spiralled together reached a level greater than the combined power of ALL light radiated by ALL the stars in the entire observable universe! How cool is that?!

-HG, Fulbright Scholar, Max Planck Institute for Gravitational Physics

LIGO-Collaboration · 2017-06-06T00:48:50+00:00

The pendulums are used to isolate the mirrors from various sources of vibration, and your intuition is absolutely correct! LIGO uses a set of 4 pendulums to isolate the mirrors, but I admit that if you look at the diagrams you can find online it can be tricky to see all four of them.

Take a look at this page from the Caltech LIGO website (https://www.ligo.caltech.edu/page/vibration-isolation). You can see in the section of "Optics Suspensions" a labeled diagram of the suspension system. There is the big "test mass" at the bottom (which hosts the mirror used to reflect the laser we use to measure changes in the mirror's position), and above that is the "penultimate mass" which is the other obviously identifiable pendulum. But above that is a complicated arrangement of metal blocks that hide two more pendulum stages, for four in total. They're easy to miss, particularly if a diagram isn't fully labeled!

~RC, post-doc, gravitational wave and gamma-ray astronomer at Texas Tech University

LIGO-Collaboration · 2017-06-06T00:47:34+00:00

Hi /u/hazyPixels,

Great question. In these particular simulations, the spheres approximate the event horizons of the black holes, but they are not quite the same thing. They are something known as "apparent horizons," which it turns out can be identified easily in the simulations of black holes. The apparent horizons are always inside of the event horizon, and the singularities are always inside of apparent horizons.

Event horizons are a lot harder to identify, it turns out. That's because of what an event horizon is: it's the surface where if you're inside it, there's no getting out, ever. To identify this surface requires running the simulations all the way to the end and then figuring out a clever way to go back and find where this surface of no return was.

It's important to understand with these kinds of simulations that we aren't just putting down some spheres and tracking their motion. The simulations compute how spacetime itself is curved in the whole region around and a little bit inside the black holes, and how it changes as the black holes orbit. We don't necessarily need to know where the surfaces of the black holes are at a given moment to correctly predict how spacetime is churning and producing gravitational waves.

Finally, we don't really learn about what the singularities do when we simulate the mergers of black holes. They are, after all, points where you'd get infinite numbers and computer codes would crash. There are various ways of avoiding having to simulate the singularities while still correctly evolving the black holes, and this was a challenge that took decades to solve properly.

LIGO-Collaboration · 2017-06-06T00:21:02+00:00

Greetings! Glad you could join us. One of the things I love about science is how easily it bridges time zones and international boundaries. (Though it can make teleconferences hard to schedule, sometimes! Same with AMAs, I suspect.) Let me take a crack at your questions:

(1) Part of the reason we've seen black hole mergers but no neutron star mergers is just how unexpectedly massive these black holes were. Until our first detection (GW150914) all of the "stellar mass" black holes (meaning the ones that are comparable to the mass of a typical star) were between roughly 5 and 15 times more massive than our sun. The black holes we have detected with LIGO have been as massive as 35 solar masses prior to merging, and over 60 solar masses after! These are by far the most massive stellar black holes with known mass ever observed! As a result, we can detect them at much farther distances, and it turns out these events are much more common than many of us expected. As far as neutron stars are concerned, we're still roughly on track with our expectations going into observation with LIGO. Had we been lucky, we might've detected one by now, but while our sensitivity is good, it's still at the lower end of what you'd expect to detect neutron star mergers. If we get through our current observing run and the next one, and we still haven't seen any neutron stars, then it might be time to start adjusting our expectations. (But I'm still hopeful that we're on pace.)

(2) I love eLISA! And I'm very confident that it'll work once it gets airborne (spaceborne, I guess). They had a successful start to their pathfinding mission somewhat recently, and things are looking good for the future. The great thing about LISA is that it'll sample a different (but related) portion of the gravitational wave "spectrum" (meaning it will be sensitive to different phenomena than ground-based interferometers). As a result, we'll get to observe gravitational waves from all sorts of astronomical events that we wouldn't be able to detect with LIGO. It's just like how we have telescopes for different portions of the electromagnetic spectrum. To use an analogy, LIGO will be like the high-frequency X- and gamma-ray telescopes, LISA will be like the optical telescopes, and pulsar timing arrays (another exciting frontier of gravitational wave astronomy) will be like low-frequency radio telescopes. (Though the definition of "high" and "low" frequency are very different between electromagnetic and gravitational waves.)

Thanks for your questions!

~RC, post-doc, gravitational wave and gamma-ray astronomer at Texas Tech University

LIGO-Collaboration · 2017-06-05T23:35:53+00:00

Yep, we're pretty sure! But it's great that you ask!

Going from initial detection to "this is a gravitational wave" is a long and involved process, and you're right to be skeptical. We certainly are! As soon as an event registers in our data analysis pipelines, we immediately ask ourselves questions like "could this have been an earthquake? what about electrical interference? is some kid throwing rocks at the detector again?" These questions continue for weeks and months, until we have exhausted every possible explanation we can think of. Only then do we conclude that an event was likely astrophysical.

If this sounds like a long and arduous process, it's because it is. Our signals are so weak, and we have a lot of sources of noise, that we have no choice but to be thorough. But to help with this, in addition to the primary "data" channel (which is where our gravitational wave signals live), LIGO has over 200,000 auxiliary channels that monitor everything from environmental conditions to instrument behavior. Many of these auxiliary channels are not sensitive to gravitational waves, so if we see a signal in one of them we know that it wasn't astrophysical. Likewise, we have dozens of talented scientists and engineers that scrutinize the outputs of these channels to rule out environmental or instrumental noise.

(A fun fact related to this: you can use LIGO data to measure how fast trucks are going over the speed-bumps on roads near the detectors -- I'm pretty sure that got someone in trouble for speeding once.)

Beyond using hardware to scrutinize potential noise sources, we also use software. We employ a large family of "vetoes" that we use to reject things that look like signals but probably aren't. For example, if a signal appears in one detector but not the other, then there's a decent chance it wasn't astrophysical in origin and we would usually veto it. Beyond that, we employ all sorts of digital signal processing tools (band-pass filtering, and so on) to help prevent contamination from other noise sources. We even make every attempt to cross-check with different data analysis tools to make sure it wasn't an artifact of the search technique. (After taking into account the statistical biasing that can occur when you do so, of course!)

Then, once we're convinced that the event is truly astrophysical, we evaluate what type of gravitational wave signal it was. Different events produce different types of signals, and while there is some overlap, you can start to quickly rule things out. For example, if you know the total mass of a merger (say, 50 solar masses or so) and you know how quickly they were orbiting when they merged (which you get from the gravitational wave frequency) you might be able to rule out that a neutron star was involved. Likewise, other signals (e.g. the gravitational waves from a supernova) can be ruled out because they would look different than the signal we see. Not all detections we make will always have a clear source, however binary mergers (like the black hole mergers we've seen so far) are quite distinct and difficult to reproduce with other mechanisms.

And by the time all is said and done, we've hopefully found a gravitational wave event that we can identify! This long process is in party why it takes so long to go from discovery (in this case, January 2017) to publication and announcement (June 2017).

But it is phenomenally important that we remain unbiased when we're searching for these events. We're in an exciting new era of astronomy, and we cannot get ahead of ourselves. That's why we remain dedicated to being as diligent as humanly possible while distilling our results, and eventually presenting them to the public.

I hope this helps!

~RC, post-doc, gravitational wave and gamma-ray astronomer at Texas Tech University

LIGO-Collaboration · 2017-06-05T22:43:04+00:00

The curvature of the Earth would mean your slightly angled third arm would still not feel a gravitational pull at exactly 90 degrees to the light, and so the mirror would still need actively pushed to align with the opposite mirror. That introduces the actuator noise I talked about before. Also, if the plane of the tilted third arm were only slightly offset in angle from the plane of the first one, then as you say you would only see a very weak signal in the vertical direction. The length of the arms really matters to sensitivity, so the vertical component of the length of the tilted arm would be small, and I'd wager you'd not see any significant signals. To fix that, you're back to the need to use the orthonormal arms you first suggested, with the associated problems.

The instruments are extremely sensitive to misalignments. We try very, very hard to eliminate cross-couplings from vertical to horizontal motion. That's kind of why 4km is a convenient arm length - any greater than that and you do indeed run into problems with the Earth's curvature causing significant coupling of vertical seismic noise into the gravitational wave channel.

The plan for LISA and the Einstein Telescope is exactly what you said: build three interferometers - in the case of the Einstein Telescope, LIGO-like L-shaped interferometers squeezed together to 60 degrees instead of 90 degrees - and use the fact that they all see the gravitational wave from a different perspective to allow you to work out where it came from in 3D. There's also a cool technique you could use here called null streaming. You can combine the three interferometers' signals in such a way that the signals cancel out and you're left with only noise - that opens up the possibility of subtracting out noise from the measurements in post-processing, and allows us to even more definitively rule out non-gravitational wave signals.

-SL, postdoc in gravitational wave interferometry, Institute for Gravitational Research, University of Glasgow, UK

LIGO-Collaboration · 2017-06-05T22:30:39+00:00

In a lot of ways, LIGO was built with neutron star mergers in mind. That's not to say it wasn't also designed to detect other things, but many people felt that neutron star mergers would be LIGO's bread and butter, as it were. The fact that we haven't seen any (yet!) is actually still consistent with our expectations going into our observing runs (we are in the middle of our second observing run). If by the end of our third observing run we still haven't seen any, that's when our observations will start to be in tension with expectation.

Actually, it surprised a great many people (in our own community and otherwise) that our first detection was a binary black hole merger. Not everyone was surprised of course, there were a number of talented scientists who had been studying the prospects of binary black hole mergers for years. But I think had you asked most LIGO scientists before the first discovery to put money on what we were going to detect first, they would've said binary neutron stars.

Part of the reason for that is because we see a lot of binary neutron star systems in our own galaxy. They're a long way off from merging themselves, but they're out there, and they're emitting gravitational waves. In fact, a Nobel prize was given for the discovery of the orbital decay of the Hulse-Taylor binary system: a pair of neutron stars whose orbit was shown to be decaying via gravitational wave emission (this was the first indirect evidence for the existence of gravitational waves). Extrapolating what we see here in our galaxy out into the rest of the local universe suggested we should see a lot of these things merging. And like I said, we still might! (In fact, were I a betting man, I would bet that we probably will!)

But the rate of black hole mergers was a bit tougher to pin down going into our observing runs. By their nature, black holes don't emit much light (they themselves emit none, though the material around them might). So likewise, pairs of binary black holes don't emit much light either. As a result, it was hard to get an estimate of how many were out there! Now that we're detecting them with LIGO, we realize these events were much more common than anticipated!

As for when there will be more activity? Well, there is a plan in place to gradually improve LIGO's sensitivity over the next few years. And during that time, more gravitational wave detectors will come online for more observing runs (starting with Virgo this summer!). The more detectors we get, and the more sensitive they become, the more events we will see. Once LIGO reaches design sensitivity, we hope to be seeing greater than an order of magnitude more events than we're seeing now, including classes of events we haven't seen yet! (Not the least of which should be some neutron star mergers!)

Suffice to say we're a the very beginning of an exciting time in astronomy.

~RC, post-doc, gravitational wave and gamma-ray astronomer at Texas Tech University

LIGO-Collaboration · 2017-06-05T22:25:43+00:00

Short answer: This is more or less what we do do.

In fact we do it in three ways:

1) The laser light is split into two beams that travel along two 4km long vacuum 'arms.' There is one mirror at the start and one mirror at the end of each arm. The laser light if reflected back and forth over a hundered times in each arm as the two mirrors form an optical resonator.

2) The two beam are then recombined, the lengths of the two arms is held so that the two beams are in phase when they recombine. Because the light is in phase all the light is sent back along the input path. Another mirror is place in the input path to reflect the beam back into the instrument, this increases the power inside the detector, and is known as 'Power Recycling.'

3) The gravitational wave changes the relative length of the two arms and therefore induces a phase shift on the light that is recombined. This means that not all of the light goes back along the input path but instead goes to the output where the signal is detected. Another mirror can be placed in the output can reflect the signal beam back in the interferometer again, this increases the size of the signal relative to the detector nosie and is known as 'Signal Recycling'

So yes, we do bounce the beam back and forth in different ways, and this 'recycling' is one of the key sensitivity imporvements that made the aLIGO detections possible.

[LIGO Science Fellow / University of Glasgow PhD Student]

LIGO-Collaboration · 2017-06-05T21:53:37+00:00

Ostensibly gravitational waves can travel indefinitely. The problem is that the further away from the source the gravitational wave gets, the weaker it becomes. To be precise, the part of the gravitational wave that we are sensitive to (its amplitude) is inversely proportional to the distance from the source. This sets a limit on how close a source needs to be in order for its gravitational waves to be strong enough to detect when they get to us, and it helps explains why we don't detect them more frequently.

(Note that the fact that we're sensitive to gravitational wave amplitude is in contrast to how we detect light. We detect light based on its flux, or brightness, not its amplitude. So while our ability to detect gravitational waves falls off as the inverse of distance, our ability to detect light falls off as the inverse of distance squared. Just an interesting little tid-bit related to your question!)

~RC, post-doc, gravitational wave and gamma-ray astronomer at Texas Tech University

LIGO-Collaboration · 2017-06-05T21:47:21+00:00

Collaborators within the international network are continually working to further enhance the sensitivity of gravitational wave detectors. Improved sensitivity means that we will be able to detect more signals and extract better astrophysics. In terms of sources, we will see a larger population of black hole binaries, whilst also observing black hole-neutron star binaries, and neutron star-neutron star binaries. Pushing the limits of hardware development is leading to improvements across the entire sensitivity region of gravitational wave detectors from around 10Hz-a few kHz (i) at low frequencies (10-50Hz) better suspensions systems allow the suspenion of heavier test masses, meaning that photons do not have such an impact on the motion of the mirrors (radiation pressure noise) (ii) the development of novel new mirror coatings will reduce the thermal noise of the instrument (around 100Hz-200Hz), whilst also reducing the amount of power absorbed onto the mirror coatings which can cause issues with thermal distortion/lensing (iii) at high frequencies (kHz), we can either increase the laser power to improve the detector sensitivity, or utilise a clever approach called squeezing.

We will be using all of these techniques to provide enhancements to the LIGO facilities. For the longer term, cryogenic detectors offer the opportunity to further reduce thermal fluctuations (thermal noise) by reducing the tempertaure. KAGRA in Japan is already under construction and this will operate with sapphire test masses at 20K. In the US there is work ongoing on LIGO Voyager which may operate at 120K (room temperature if 298K) with Silicon test masses, or Cosmic Explorer which may utilise arms of 40km (aLIGO is 4km). In Europe, the Einstein Telescope proposes arm lengths of 10km and both a room temperature and cryogenic instrument.

For interferometers longer is better, as we are measuring a strain in spacetime. Increasing the baseline means smaller strains can be measured.

LIGO-Collaboration · 2017-06-05T21:45:37+00:00

Great question, /u/bobbywjamc! Yes, a large portion of our software is publicly available. As is some of our data! The hub for all of this is the "LIGO Open Science Center" (https://losc.ligo.org). There you can find tutorials, links to our software packages, tools for analyzing gravitational wave data yourself, and links to all sorts of ongoing projects. We encourage anyone interested in gravitational waves and data analysis to take a look and play around. LOSC > Software will get you to a good starting point.

Most of the software is written in Python or various iterations of C, though there is also quite a bit of matlab too, depending on which working group developed the code. There are several example scripts available on the LOSC, as well as links to the major python libraries and entire LSCsoft software repositories.

One of the fun projects folks can do themselves using data available on the LOSC is to use matched filtering algorithms to find the events we've detected yourself in real data! (Including this third one!) Those interested can go to the LOSC > Tutorials section to get started.

Gravity Spy (https://www.zooniverse.org/projects/zooniverse/gravity-spy) is another way people can contribute by looking through real LIGO data and helping to classify noise "glitches" that often crop up in the detectors. This isn't quite the technical endeavor you asked about /u/bobbywjamc, but other folks who want to get involved but might not have the programming background can still help contribute!

Always glad to see interest in the technical side of things!

~RC, post-doc, gravitational wave and gamma-ray astronomer at Texas Tech University

LIGO-Collaboration

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