# Which things can LIGO see that LISA can't, and vice-versa?

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CNET's Astronomers discover two ferociously fast stars locked in a death spiral quotes Kevin Burdge, lead author on the new paper in Nature General relativistic orbital decay in a seven-minute-orbital-period eclipsing binary system (no ArXiv?) and a Ph.D. candidate in physics at Caltech as saying:

LIGO can see things that LISA can't, and LISA can see some things that LIGO can't, and there are a handful of things that might switch from being visible in LISA to LIGO over time"

Is there a simple way to understand which "things" fall into which categories?

LIGO is a ground-based gravitational wave observatory and is used together with Virgo, whereas LISA stands for Laser Interferometer Space Antenna and will be a GW sensing inteferometer based on a trio of satellites in an Earth-trailing heliocentric orbit.

GIF Source

LISA spacecrafts orbitography and interferometer -yearly-periodic revolution in heliocentric orbit.

Gravitational wave detectors have a frequency range that they are sensitive to.

In the case of LIGO it is about 10Hz to 1kHz. The lower limit is imposed by seismic noise, the upper limit by "shot noise" (basically not having enough photons to sample the interferometer path difference at high frequencies).

LISA is in space and doesn't have the problem with seismic noise. However, there is still an upper limit determined by shot noise. As I understand it, LISA will not/can not use the same sort of resonant cavity arrangement that LIGO uses to boost the effective laser power and hence number of photons in the apparatus. Thus the upper frequency limit for LISA is more like 1Hz.

So now to your question. What LIGO can see that LISA can not, are gravitational waves with frequencies of 10-1000 Hz. The astronomical phenomena that lead to such waves are the merger of compact binary systems with stellar masses, rapidly rotating, asymmetric pulsars and perhaps supernova explosions.

Lower frequency GWs can only be seen with LISA. This would include stellar binary systems with orbital periods longer than about 10 seconds, merging supermassive black holes and maybe GWs from the big bang.

What could move from being observable in LISA to being observable in LIGO? In principle, the merger of any binary system results in a gradually increasing frequency and amplitude. Stellar mass, neutron star or black hole binaries can get to frequencies around 1kHz before they merge, but sweep through lower frequencies before that, but with much lower amplitudes. Perhaps the best bet would be the merger of intermediate mass black holes with masses of $$10^3$$ to $$10^4$$ solar masses, which would have merger frequencies of tens of Hz, but a significant amplitude at much lower frequencies prior to merger.

I suppose the implication of the quote you gave is that this newly discovered inspiralling white dwarf binary might be another possibility. I don't think that is correct. It has a 7 minute orbital period, which puts it in LISAs frequency domain (the GWs emitted have twice the orbital frequency), but because white dwarfs are physically larger than neutron stars, the peak frequency at merger (some time in the future) will only be a few Hz and not visible to LIGO.

## Which things can LIGO see that LISA can't, and vice-versa? - Astronomy

Anyone writing that Einstein was fundamentally wrong has not understood General Relativity. There are really two different things which are General Relativity. There are the principles that define General Relativity in particular the Einstein Equivalence Principle. Then there are mathematical realizations of those principles like the Einstein Hilbert Action, from which we derive predictions from the big bang, to light bending due to the Sun, to Black Holes, and gravitational waves. So if you see a headline saying Einstein was wrong the answer is no. His theories were incomplete descriptions of nature. We seek to extend this with modified theories, and test those with experiments. Any such theories would include general relavity within their structure. This is similar to the way Einstein's theory reduces to Newtons laws at smaller masses and lower speeds.

This mathematical realization has been remarkably successful and passed every test thrown at it. However, we still test it to look for alternatives and extensions to it. For example, we look for various ways to extend our mathematical formulation of General Relativity by adding new fields, or new interactions between gravity and those fields.

The most robust approach holds that instead of gravity being described by the Ricci curvature R it is instead a function, f, of the Ricci curvature R. f(R) in the traditional mathematical formulation of general relativity f(R)=R. This formulation has gotten us all the modern astrophysics you have ever heard of.

There are many other models of f(R). I developed a few of my own which I either gave talks about or published however those models were extraordinarily complex and, in a way, mostly academic. These days the most interesting f(R) is due to Alexi Starobinsky f(R)=R+bR^2 . This model is the simplest function of R possible other than adding a constant such as the cosmological constant. This model gives cosmic inflation and preserves all the predictions of the traditional formulation. However, how does one test for this?

This is where gravitational wave observations may be instructive. Extreme mass ratio in spirals. This is where a black hole interacts with an object which is much less massive. This could be a super massive black hole and a neutron star or a black hole and an ordinary planet. The LISA probe to be launched by the European Space Agency with a little cooperation with NASA is going to investigate this and many other things.

LIGO is not sensitive to EMRI interactions as it does not have the scale to be sensitive to them. These low frequency interactions just are not visible to LIGO.

In simple terms think of these devices as being like tuning forks. Strike a tuning fork on one side of the room. An identical tuning fork will resonate with the sound of the one you struck. The frequency of the tuning fork depends on the size of the tuning fork. LIGO is not sensitive to EMRI interactions as it does not have the scale to be sensitive to them. These low frequency interactions just are not visible to LIGO but they will be to LISA which will be much larger.

None of this even considers issues of how to incorporate gravity into quantum mechanics or perhaps vice versa.

Einstein can be built upon, and physicists will do just that. Einstein and Hilbert were not perfect. However, if anyone says, &ldquoEinstein was wrong&rdquo, at this point, they are a quack.

Opinion and views articles that I post at Science 2.0 will be available on my Substack for at least a day before they are here. If you like what you see here check out my feed there. It is free for the time being.

Currently I am an adjunct professor at the College of DuPage. My research focuses on astrophysics from massive star formation to astroparticle physics.

## LIGO's Director Explains What It's Like To Find A Gravitational Wave

Image Credit: SXS, the Simulating eXtreme Spacetimes (SXS) project (http://www.black-holes.org).

On September 14th, 2015, less than 72 hours after it began operation at it's current sensitivity, an incredible event unfolded at each of the twin LIGO detectors in Washington and Louisiana: an event consistent with a gravitational wave signal from the merger of two massive black holes was observed! This direct detection -- the very first for gravitational waves of any type -- ushered in the dawn of a new kind of astronomy. It was the first time black holes of these masses, 29 and 36 solar masses, merging to form one of 62 solar masses, was ever observed. And it was a convincing, robust detection at greater than a 5-sigma significance match in each detector, independently. The fact that both detectors saw the exact same thing leaves very little doubt that this was, in fact, a gravitational wave signal.

Image credit: Observation of Gravitational Waves from a Binary Black Hole Merger B. P. Abbott et . [+] al., (LIGO Scientific Collaboration and Virgo Collaboration), Physical Review Letters 116, 061102 (2016).

While there's plenty to say about this, there's simply no substitute for going straight to the source. In this case, that means going directly to Dr. Dave Reitze, scientist, professor and the executive director of LIGO!

Image Credit: T. Pyle/Caltech/MIT/LIGO Lab.

Ethan Siegel: A lot has been written about this discovery, but it must have been very different back in September when this signal first showed up just a few days after it started taking data. When these waves first came in, was it what you expected to see, or was it a surprise?

Dave Reitze: It was a surprise in terms of its amplitude: this was a very strong, loud signal. It was black holes, very few people would've predicted that binary black holes would've been the first thing we would've detected. It was black holes that are heavier than any other stellar mass black holes that have been observationally recorded. There are so many elements that are just, sort of, so out there!

Image credit: LIGO collaboration.

ES: What do you wish everybody knew about LIGO that hasn't been given its due yet?

DR: I think one of the things that hasn't gotten as much airplay as it should've is not so much about LIGO, but is about other detectors that are coming online and the roles they're going to play. There are other detectors coming online: one is in Italy, the VIRGO detector, which hopefully will be online sometime this year, there's a detector in the Kamioka mines [in Japan] called KAGRA coming online hopefully in 2019, and then India announced that they wanted to build a gravitational wave detector, which is something we've been pursuing for about four years.

Having those detectors come online will be crucial, because it will be the things that allows us to couple gravitational wave astronomy with [traditional astronomy done in the] electromagnetic. That's the next step: to simultaneously see [gravitational waves] with three, four or five interferometers, localize it quickly, within minutes, and have other observatories catch it instantly, and catch it in the optical or the X-ray bands. That's going to provide a whole new understanding in these cataclysmic events. It's not just what happens now, it's how much more rich this discovery space will be once these detectors come online. LIGO is great, but when all these detectors come online, that's something that's really going to be super great.

Image credit: R. Hurt - Caltech/JPL.

ES: The Advanced LIGO upgrade isn't complete yet. When do you expect it to be finished, and how much more sensitive will it be than it currently is?

DR: We have a science design goal for our sensitivity as a function of frequency. By some measure, we're roughly a third of the way from most of that design goal over different frequency spaces. We have this metric we call the binary neutron star inspiral range, the range at which we could see the binary merger of a neutron star, and where we're operating now we're somewhere between 70 and 80 Mpc. We want to get to 200 Mpc. Where I think the tough part is, in terms of making the detectors work right, is that at low frequency we have probably a factor of 10-15-20 (to improve) depending on where you are, and that opens up a whole new spectrum of black holes we could detect. And that probably is pushed out to 2018-2019-2020 in terms of reaching that design sensitivity. It turned out that nature was very kind, and there appear to be many of these black holes in the Universe and we were lucky enough to see one.

Image credit: Bohn et al 2015, SXS team, of two merging black holes and how they alter the . [+] appearance of the background spacetime in General Relativity.

ES: The first announced event was estimated to have occurred at a distance of 1.3 billion light years. How far can LIGO realistically reach?

DR: With advanced LIGO, for these stellar mass black holes, we should be able to see out beyond 2 or even 3 Gigaparsecs, so call that 9 or 10 billion light years. For 100, 200 or 300 solar mass black holes, that range falls down again, because we're losing sensitivity as the frequency gets lower. The neutron stars are higher frequencies, and those are also less sensitive: out to about 700 million light years. What do we do next? If we can make our instruments, say, ten times more sensitive over Advanced LIGO, we could see ten times as far.

Image credit: Caltech/MIT/LIGO Lab, of the Advanced LIGO search range.

ES: What are the prospects for probing to the limits of the observable Universe (

DR: For a future detector that could see a factor of ten over Advanced LIGO, you could pretty much see the whole Universe in terms of black holes, and could see neutrons stars merging out for billions of light years, out to near where the first stars formed. There are plans in place where we're trying to build detectors -- they're at least 15 years away -- but the prospects are good for building in the next generation of detectors. I think the future's bright.

ES: People don't typically appreciate the precision of the lasers, the vacuum through which they travel, the cooling apparatus or the insulation from noise that needs to occur for LIGO to work. What can you tell us about them?

DR: LIGO is a tour de force both in precision measurement and also in engineering. Being able to do experiments to demonstrate that you can measure things to the limits of a tiny, tiny fraction the diameter of a proton, to engineer that so you can do it day in and day out robustly, that's a whole other level of effort. The interferometer is made up of different sub-systems: you need a laser, you need the mirrors, the beam splitter, a vacuum to put the interferometer in, the control systems to sense and control the positions of the mirrors, and then the angle, how you position the laser light so it's aligned. There are also seismic isolation systems, because you have to filter out by about a factor of a trillion the seismic noise, both from the Earth's natural movement and because there's manmade noise.

Image credit: public domain / US Government, of a schematic of how LIGO works. Modifications made by . [+] Krzysztof Zajączkowski.

So let me pick one and talk about the input optics. The input optics is basically the first part of the optics for the interferometer, and it plays a very special role. The laser that we use is very stable, it's the most stable laser in the world. But you can't just put the laser light into the interferometer, because the laser beam isn't the right size, it's still too noisy -- everybody thinks of laser light as being the purest light you can get but it's not there are different levels of purity -- and to do the interferometry and measure those displacements of 10^-18/10^-19 meters, we need to do further purification. And we also have to change the character of the laser and put something called "side bands" on, so instead of having one monochromatic laser we have slightly different colors to have sensing light to read out some of the positions of the mirrors. You have to blow the beam up from the thickness of a pencil to maybe 6-7 cm, and then at the heart of it there's something called the mode cleaner. It makes the light more stable in terms of frequency, amplitude and also in something called pointing, which control the angular fluctuations. The input optics does all of those things. It's not one of the sexiest subsystems in terms of the interferometer, but it's the most complicated part of the interferometer in that it interfaces with all the other parts of it. And that's what University of Florida has contributed, and it works remarkably well.

ES: There are many things that can make gravitational waves at the high frequencies LIGO is sensitive to: black hole-black hole mergers, neutron star-black hole mergers, neutron star-neutron star mergers, supernovae and gamma ray bursts. But do any, other than black hole-black hole mergers, have a chance of being seen with their anticipated amplitudes?

DR: Certainly the black hole-neutron star source is one that we really do hope to see. There's no observational support for it so far, even though that is supposed to be a candidate source for gamma-ray bursts, as are the binary neutron star mergers. The rate for those is highly non-constrained, which means until we see one or two, we really don't know. The supernovae are a really interesting case. When LIGO was first conceived in the late 1970s and 1980s, supernovae were thought to be one of the really good sources of gravitational waves. But as people began to model supernovae better and understand core collapse and the subsequent shockwave and blow-off of the outer layers, they turned out to be rather poor radiators. So Advanced LIGO and even with the next generation, we might be unlikely to detect supernovae outside of our own galaxy.

An artist's impression of two stars orbiting each other and progressing (from left to right) to . [+] merger with resulting gravitational waves. This is the suspected origin of short-period gamma ray bursts. Image credit: NASA/CXC/GSFC/T.Strohmayer.

ES: Are there any unexpected surprises LIGO might find, or would we not see anything we don't have a template for?

DR: The other interesting source -- and if we saw it, it'd be really cool, but it's a tougher source to see -- we search for gravitational waves from isolated neutron stars, from pulsars. If there's a mechanism that breaks the sphericity, that puts a time-dependent quadrupole mass moment (e.g., a crustal deformation, an elliptical shape to the neutron star, etc.), it will spin in such a way that there's a wobble as its rotating around its axis. These gravitational waves will be very weak, but they'll have the advantage that they're very monochromatic, since neutron stars are very precisely clocked. We search for those over days, months and years, and we just keep integrating over time. If there is a signal that pops up above the background, eventually, if you integrate long enough, we'll see it. Seeing something like that would be really exciting, because then you could say that gravitational waves contribute to the spinning down, to the slowing down of an isolated neutron star, of a pulsar.

Illustration of a starquake occurring on the surface of a neutron star, one cause of a pulsar . [+] "glitch." Image credit: NASA.

ES: So if we had a pulsar glitch within our galaxy, would LIGO have a shot?

DR: We absolutely could! It would have to be close, and it would have to be a pretty big glitch, but we search for those, actually. A glitch would be a burst-type event, where all the energy would be emitted at once, rather than a small signal that you integrated over a long time as in the above example. The pulsars are expected to spin down over perhaps billions of years, seeing a slow rate of change, and those searches are hard. The nice thing about a pulsar is that we have the radio information from pulsar timing: we know what the spin frequency is and what the gravitational wave frequency is and where they are in the sky. We have a much narrower parameter space, so we know what we're looking for. I think the odds are long for Advanced LIGO, but you never know and that's why we look.

ES: Steve Detweiler, our friend and colleague, just passed away suddenly of a heart attack last month. Is there anything you'd like to share about his role or impact on numerical relativity and on LIGO in particular?

DR: That was a shame it was very sudden. Steve wrote one of the seminal papers for another type of gravitational wave detection on pulsar timing. He was always a little bit skeptical of LIGO I'd see him in the hallway and he'd go, "Oh, so how's LIGO going?" I'd say, "Oh, it's going great!" He'd say, "When are you going to detect gravitational waves?" I'd say, "Oh, about five years," and then he'd say, "yeah, everyone's been saying that for 20-30 years!" The last time I saw him was five years ago, and I said, "this time it is going to be five years, it's not going to be any longer than that."

Image credit: David Champion, of an illustration of how many pulsars monitored in a timing array . [+] could detect a gravitational wave signal as spacetime is perturbed by the waves.

But he theorized that you could detect gravitational waves from pulsar timing using radio astronomy. You'd have to look not for days or weeks but years, and even 5-10 years. If you had enough pulsars located over points in the sky, you should be able to look at a difference in timing from those pulsars. From that difference in timing, you could infer the existence of a gravitational wave background at extremely low frequency gravitational waves: in the nanoHertz range. This is an experiment going on right now. There are a number of these experiments working together, the NANOGrav collaboration in the United States, one in Europe called the European Pulsar Timing Array and one in Australia called the Parkes Pulsar Timing Array, and they all share data and work together. They are potentially on the verge of making a discovery of these low-frequency waves using a method that was first proposed by Steve Detweiler, so in some sense I think Steve was a real pioneer there. Steve made really a seminal contribution to the field.

LIGO's sensitivity as a function of time, compared with design sensitivity and the design of . [+] Advanced LIGO. The "spikes" are from various sources of noise. Image credit: Amber Stuver of Living LIGO, via http://stuver.blogspot.com/2012/06/what-do-gravitational-waves-sound-like.html.

ES: Other than going into space, what are the prospects for increasing our sensitivity to gravitational waves via experiment?

DR: A lot of what we're thinking about for making a new ground-based gravitational wave detector goes into thinking about how you suppress low-frequency noise: the noise that comes from the Earth. It's really hard to envision how to build an Earth-based detector that goes below 1 Hz with any degree of precision. The Earth's motion gets to you, but there's also gravity gradient noise, which is also called Newtonian noise. Any time you have an object that's moving, it's changing the local gravitational field. The atmosphere is moving, the Earth is moving as there are surface waves going through it, people are driving cars and things like that. The problem with gravity is there's no way to shield it gravity goes through everything. In order to try and beat this Newtonian noise, you have to actually measure the stuff that's moving around using seismometers and things like that, and then you have to account for it. I think we're at a position where we can consider what type of monitoring network you'd need to weed out that noise, and. it's a challenge. If you want to go below 1 Hz, you really do want to think about going into space.

Artist's impression of eLISA. Image credit: AEI/MM/exozet.

ES: What is your great hope for the future of gravitational wave astronomy, given the successes of LIGO so far?

DR: Oh! I think it's all about cosmology. I think you want to get back to a bigger, better version of LISA. I think if there's some path for NASA and ESA to join back together with some really significant contributions from NASA, you could envision a mission to do cosmology with some sort of distance ladder with gravitational waves. Gravitational waves have this property that they scale with the baseline of your detector -- if you make your detector 10 times larger, you make it 10 times more sensitive -- then if you make a ground-based detector with 40 km arms rather than [LIGO's] 4 km arms, you can start to do experiments where you can start to see out far enough in the Universe then you can start to maybe measure cosmological parameters like w, the dark energy equation of state. I think ultimately, you'd like to see the cosmological gravitational wave background. I think there are a number of experiments that are thinking about how you could look in different frequency bands, and get a glimpse of the primordial gravitational wave background. I think that would be really revolutionary, because that would be your first glimpse at the very first instant of our Universe.

Image credit: National Science Foundation (NASA, JPL, Keck Foundation, Moore Foundation, related) – . [+] Funded BICEP2 Program.

ES: And if we could see that, because gravitational waves from inflation are generated by an inherently quantum process, that would be a "smoking gun" signal that gravitation is an inherently quantum force, and that there must be a truly quantum theory of gravity out there.

DR: Right! Exactly! You've put it perfectly, that's a perfect way to say that.

ES: What's on the horizon for you, personally, now that LIGO's finally detected its first gravitational wave event?

DR: Continue making our detectors better and seeing a lot more of them. I think that's really the name of the game now: to show that LIGO can deliver on its promise of viewing the Universe with this new kind of tool, this new kind of detector, and start to see not only things we expect to see, but things we don't expect to see. I think for me, it's clear: I'm going to do my job to get the gravitational wave detectors working better, even beyond their current sensitivity states, and to start working more closely with astronomers to do this multi-messenger type of astronomy.

Image credit: M. Pössel/Einstein Online.

Another way to say this is that people who've been in this field have been wandering in the desert for 40 years -- and I've been in it for 20 -- and we've just entered the promised land. I'm sure there are going to be things we knew we were going to see but also things that we don't, so let's continue what I'm doing and get more excited as we see more things.

ES: And finally, what message would you most like to share with the general public who might be interested in gravitational wave physics, but doesn't necessarily have expertise in it?

DR: There are a couple of messages. One message is the beauty of fundamental science and understanding our Universe. Even though gravitational waves are a very esoteric feature of a very complicated mathematical theory called General Relativity, which happens to work extraordinary well at explaining the way gravity works, even if you don't understand the details, I think people can understand the wonder that comes with using these gravitational waves as messengers of understanding some of the most interesting phenomena in the Universe. Looking at two black holes colliding, you don't expect to be able to observe them, in a general sense, in any other way. So I think there's an exciting aspect to this, that we're going to learn more about the Universe and how awe-inspiring it is, using gravitational waves.

Kip Thorne, Ron Drever and Robbie Vogt, the first director of LIGO. Image credit: the Archives, . [+] California Institute of Technology.

I think the other message is that the tool we've developed, and I want to point out that there are a couple of people who deserve credit for this -- Rainier (Rai) Weiss at MIT, one of the first people who conceived of using interferometers to detect gravitational waves Kip Thorne, who had the vision to realize this could be a new field of astronomy and hunted for people who were interested in building these sorts of detectors Ron Drever who also made a lot of seminal contributions in terms of ideas for making interferometers -- they came up with a tool that is really, really amazing technologically. It's gotten to the point where we're capable of making these mind-bogglingly tiny measurements of displacement, and from that inferring something about the nature of the distant Universe and black holes. When you look at it from the perspective of making a measurement that's highly precise to measure a displacement of a fraction of an atom's nucleus, from the standpoint that that's what you need to do see these things like black holes, and the technology you need to develop, that's awe-inspiring too. For me, as a scientist, that's the kind of thing that gets me jazzed, that gets me excited.

## Gravitational Waves

But what of the gravitational waves emitted by our ill-fated dance partners? These ripples in distance, in the very fabric of space and time, travel outwards from their source at the speed of light. Space is large and empty and it is mostly a lonely journey. Perhaps they pass through a cloud of gas and dust. Perhaps they don’t. If they do, the distortions of distance move the gas. Some gas particles move apart, some together. The gravitational waves might move a ring of gas particles, as shown in figure 2.

Figure 2. A ring of particles distorted as a gravitational wave passes by. Distances stretch in one direction and then shrink in another. Source: Wikimedia Commons.

The effect is small if the gas cloud were a few kilometres in width, the gas particles would move a distance less than one one-thousandth of the width of a proton. But they would move. And if they moved enough (they don’t) they would make a sound—the sound of the merging black holes:

## Which things can LIGO see that LISA can't, and vice-versa? - Astronomy

Hello Reddit, we will be answering questions starting at 1 PM EST. We have a large team of scientists from many different timezones, so we will continue answering questions throughout the week. Keep the questions coming!

On January 4, 2017 the LIGO twin detectors detected gravitational waves for the third time. The gravitational waves detected this time came from the merger of 2 intermediate mass black holes about 3 billion lightyears away! This is the furthest detection yet, and it confirms the existence of stellar-mass black holes. The black holes were about 32 solar masses and 19 solar masses which merged to form a black hole of about 49 solar masses. This means that 2 suns worth of energy was dispersed in all directions as gravitational waves (think of dropping a stone in water)!

Simulations and graphics:

Thanks for the great response!

When I was an undergrad I did some work with the ICECUBE group (big giant neutrino detector buried in Antarctica). We of course had tons of noise to sort through as well, but luckily not to many thing look like super high energy cosmic rays unless they're super high energy cosmic rays. Sadly I don't really do that stuff anymore.

Anyway, a follow up question if I may.

Given the layout of the detector how accurately can you determine the direction a signal has come from? Accurate enough to go to the astromers and say "hey, look at X point in the sky".* Space is big after all, you would need to be pretty exact.

*Maybe not for a black hole collision since even if you knew exactly where to look you have trouble seeing it. But say something like a fast orbiting neutron star.

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 youɽ 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

## ALIGO and eLISA: Tuning The Instrument

Oh, it’s good to see Big News in hard science get big attention in Big Media. The LIGO story and Columbia’s Dr Brian Greene even made it to the Stephen Colbert Late Show. Everyone chuckled at the final “boowee-POP” audio recording (simulation at 7:30 into this clip get for-real traces and audio from this one).

There’s some serious science in those chirps, not to mention serious trouble for any alien civilization that happened to be too close to the astronomical event giving rise to them.

Adapted from the announcement paper by Abbot et al

The peaks and valleys in the top LIGO traces represent successive spatial compression cycles generated by two massive bodies orbiting each other. There’s one trace for each of the two LIGO installations. The spectrograms beneath show relative intensity at each frequency. Peaks arrived more rapidly in the last 100 milliseconds and the simulated sound rose in pitch because the orbits grew smaller and faster. The audio’s final POP is what you get from a brief but big disturbance, like the one you hear when you plug a speaker into a live sound system. This POP announced two black holes merging into one, converting the mass-energy of three suns into a gravitational jolt to the Universe.

Scientists have mentioned in interviews that LIGO has given us “an ear to the Universe.” That’s true in several different <ahem> senses. First, we’ve seen in earlier posts that gravitational physics is completely different from the electromagnetism that illuminates every kind of telescope that astronomers have ever used. Second, black hole collisions generate signals in frequencies that are within our auditory range. Finally, LIGO was purposely constructed to have peak sensitivity in just that frequency range.

Virtually every kind of phenomenon that physicists study has a characteristic size range and a characteristic frequency/duration range. Sound waves, for instance, are in the audiophile’s beloved 󈬄 to 20,000” cycles per second (Hz). Put another way, one cycle of a sound wave will last something between 1/20 and 1/20,000 second (0.05-0.000 05 second). The speed of sound is roughly 340 meters per second which puts sound’s characteristic wavelength range between 17 meters and 17 millimeters.

No physicist would be surprised to learn that humans evolved to be sensitive to sound-making things in that size range. We can locate an oncoming predator by its roar or by the snapping twig it stepped on but we have to look around to spot a pesky but tiny mosquito.

So the greenish box in the chart below is all about sound waves. The yellowish box gathers together the classes of phenomena scientists study using the electromagnetic spectrum. For instance, we use infra-red light (characteristic time range 10 -15 -10 -12 second) to look at (or cause) molecular vibrations.

We can investigate things that take longer than an instrument’s characteristic time by making repeated measurements, but we can’t use the instrument to resolve successive events that happen more quickly than that. We also can’t resolve events that take place much closer together than the instrument’s characteristic length.

The electromagnetic spectrum serves us well, but it has its limitations. The most important is that there are classes of objects out there that neither emit nor absorb light in any of its forms. Black holes, for one. They’re potentially crucial to the birth and development of galaxies. However, we have little hard data on them against which to test the plethora of ideas the theoreticians have come up with.

Dark matter is another. We know it’s subject to gravity, but to our knowledge the only way it interacts with light is by gravitational lensing. Most scientists working on dark matter wield Occam’s Razor to conclude it’s pretty simple stuff. Harvard cosmologist Dr Lisa Randall has suggested that there may be two kinds, one of which collects in disks that clothe themselves in galaxies.

That’s where LIGO and its successors in the gray box will help. Their sensitivity to gravitational effects will be crucial to our understanding of dark objects. Characteristic times in tens and thousands of seconds are no problem nor are event sizes measured in kilometers, because astronomical bodies are big.

Gravitational instrumentation, from Christopher Berry’s blog and Web page

This is only the beginning, folks, we ain’t seen nothin’ yet.

Scientifically at a first analysis it would make only a small difference. The reason is that there are other sources of noise besides the terrestrial and man made noise, that together are about the same amount of noise.

It's easy to see it in the following summary of the LIGO amplitude spectral density sensitivity of about $10^<-23>$ per sqrt(Hz). http://ligo.org/science/Publication-O1Noise/flyer.pdf See Figure 3 specifically, the seismic and Netwon noise is one of the the plotted curves, it seems the dark grey one, but in any case of approximately the same order of magnitude as more than 10 other noise sources that have to do with the measuring apparatus, and not external noise limited. The red curve is the measured noise, the purple one is the expected noise (probably RSS of the others). It is clear that it is not mainly external noise limited.

So if you placed this Ligo on the Moon, and were able to control all the Moon Related problems as well as on Earth, the sensitivity would not change, for the most part.

Now, it is possible that many of the non-seismic noises were designed that way because it was useless to do better on those and have it all be seismic dominated. You'd have to read detailed papers and designs of the apparatus. It took a long time to design it and build it, this second generation took 3-5 years, the first more than 10.

LISA will definitely do much better. It is much longer so a strain would cause a displacement larger by the ratio of the lengths (see below, this is true for longer wavelengths). LISA, the original space based interferometer was to have 5 million km legs (compare with a few Kms for LIGO), and distances and measurements would be done by an advanced design as well. The latest LISA is proposed to be 2 detectors instead of 3, so you loose a little, but still much better than LIGO because of the much longer lengths. You can see the NASA website or Google it. See the NASA site at https://lisa.nasa.gov. Arm lengths may also be a little less.

There is a review, but I can't locate it, that shows the sensitivities and spectral range, as well as what kind of object emit those frequencies. It might be a Living Review of Relativity, but can't confirm it.

EDIT/ADDITION ON LISA SENSITIVITY AND INTERFEROMETER MEASURE IN SPACE

First, changed the name of the interferometer 'legs' to 'arms' to make it consistent with standard usage on the topic.

More importantly added more on LISA.

I am adding this response to a very good question in the comment by @robert bristow-johnson below. He asked how does the interferometer work in space since the arm lengths do not appear to be rigidly fixed. In fact there are a few things that are done to try to measure the path length changes due to ONLY the gravitational forces (or changes in spacetime curvature, equivalently). The first part is that a drag-free satellite is used so that non-gravitational effects (like solar wind and light pressure ) are eliminated (hugely reduced). A drag-free satellite uses the satellite itself as a container, but lets the detector test masses float inside the satellite and follow spacetime geodesics, i.e. freely floating trajectories. Sensors and small jets keep the container, the satellite, centered around the test masses. See the descriptions of such things in https://en.wikipedia.org/wiki/Zero-drag_satellite.

There's more that they have to do, besides as you'd think besides making sure the sensors and jets don't affect the test masses too much. The arms are not rigidly locked, and they have to keep track and try to offset long term effects (such as changes due to planetary movement), which would be in essence pseudo-static and not gravitational waves. They measure the arms lengths it with lasers, actually keeping track of how many millions of wavelengths are changing constinuously, over short and longer time periods. They separate those changes in the frequency domain and offset and filter out the long period changes while using the rest to look for the gravitational waves.

## 19 SEASON 25 - “BRICK LIKE ME”

The Simpsons goes LEGO on season 25’s “Brick Like Me.” This very fun and creative episode was brought to life through a LEGO style of animation where Springfield and the Simpsons family were shown in the form of bricks.

Despite the fact that the episode looks and feels nothing like The LEGO Movie franchise, the episode does reference those movies at the end. All in all, “Brick Like Me” will forever be remembered as one of the best times that the franchise explored a different animation style.

The out of universe explanation is they managed to snag Richard Gere [a rather famous Buddhist] for an episode.

In-universe, they can deal with Lisa'a permanent conversion to Buddhism more easily than atheism, by never really having to mention it again afterwards - though she does shout "Free Tibet" in a later episode.

Gere informs her that while Buddhism is about one finding inner peace, it is also about respecting the diversity of other religions based on love and compassion. Thus, Lisa is free to celebrate any holiday with her family, including Christmas. Lisa goes back home, falling asleep beside the Christmas tree and tells everyone that she will be celebrating Christmas with them and continue paying lip service to Christianity while practicing Buddhism for the rest of her life.

Unlike several other episodes in the series in which a character undergoes a change in their personality, Lisa has remained a Buddhist since this episode, much like her conversion to vegetarianism in Lisa the Vegetarian

The episode features actor Richard Gere as himself. Gere agreed to guest star under two conditions, the first being that Buddhism should be portrayed accurately, and his second and strongest request being that Lisa should say "Free Tibet" in the episode.

Buddhism, from the 'get out of jail free' card given to her by Gere, has a certain long-term ignorability. Atheism would have been a much more challenging structure to hang onto in subsequent episodes. Every time the family goes to church the topic will rear its head again. The writers would have to be constantly vigilant for potential plot continuity errors.

Out of universe, the 'mentor' to her decision would also have been considerably more controversial - who would you choose, Richard Dawkins, Sam Harris, Christopher Hitchens? …Stephen Fry?
I'm sure there would have been much greater opposition to that from various religious communities.

As the top answer to this question indicates, Buddhism is quite compatible with Lisa's skeptical nature. The second-highest answer on that question has an interesting quote: "Remember that every buddhist text is advice, not doctrine." (it's a long answer, and worth reading) Lisa, who has a tendency to question everything, would feel right at home here. Whereas many Christian churches demand an adherence to what is in scripture (especially the one Rev. Lovejoy runs), it seems Buddhism gives a lot more permission for one to explore and find their own path.

Whereas religions like the Abrahamic ones center around the worship of a deity, Buddhism centers around the teachings of an ordinary person. The Buddha is held up, not as a god, but a very wise human teacher. Practitioners seek to emulate the Buddha and find inner peace. Given Lisa's angst and love of learning, it is entirely seemly that this would appeal to her.

I'd argue that Lisa is an atheist.

Atheism is, in the broadest sense, an absence of belief in the existence of deities.[1][2][3][4] Less broadly, atheism is a rejection of the belief that any deities exist.

As Seth R mentions in his answer, Buddhism is based on the teachings of a human, rather than a deity or god. Buddha isn't a god, nor is Siddhartha Gautama. The fact is, Siddhartha isn't the only Buddha, as it's simply a description of the level of consciousness a person has achieved.

A Buddha is one who has attained Bodhi and by Bodhi is meant wisdom, an ideal state of intellectual and ethical perfection which can be achieved by man through purely human means. The term Buddha literally means enlightened one, a knower.

In fact, Buddhism doesn't exactly conform to everyone's idea of what a religion is.

Because Buddhism does not include the idea of worshipping a creator god, some people do not see it as a religion in the normal, Western sense.

The same problem exists for atheism, since it specifically doesn't believe in any gods. I can't find the meme anymore, but I read one a couple weeks ago that states that atheism doesn't really exist as anything other as a way to describe a disbelief in gods to people who do believe in god(s).

The definition of religion is a controversial and complicated subject in religious studies with scholars failing to agree on any one definition. Oxford Dictionaries defines religion as the belief in and worship of a superhuman controlling power, especially a personal God or gods.[1] Others, such as Wilfred Cantwell Smith, have tried to correct a perceived Judeo-Christian and Western bias in the definition and study of religion. Thinkers such as Daniel Dubuisson[2] have doubted that the term religion has any meaning outside of western cultures, while others, such as Ernst Feil[3] even doubt that it has any specific, universal meaning even there.

Heck, I've heard people say they don't believe in the religion of science, but I think I'm getting off topic here.

The fact that Lisa is "allowed" to give lip-service to other religions simply shows that people can disbelieve in a god(s) or faith while still wanting to spend time with their friends and family. Simply not arguing with people about their faith, or lack thereof, is simply a human trait to avoid confrontation when it's either not necessary or useless.

IMO, there's very little difference in many religions, including the god(s) they worship, just in the ways they worship the god(s). So really, there's no reason a person can't be multiple religions, except for the people who refuse to believe the god(s) are the same or that the only difference is in ceremonies. Buddhism really isn't so different from Judaism/Christianity/Catholicism/Islam. (And yes, Judaism, Christianity, Catholicism and Islam all worship the same god.)

Also, most holidays (even the religious ones) are more about coming together as a family and friends than anything else, anymore. Passing around gifts, good food, conversation, and generally good times is more about community than a specific deity. This fits well with Buddhism, Atheism, and a whole host of other religions.

What Lisa really needs is the same thing a lot of people need (and likely why there's an episode about it): that's a way of understanding that their beliefs or lack of belief in something is just as valid as anyone else, and that a disbelief in gods isn't demeaning nor does it mean that you have to argue with everyone about religion all the time. There really is a way to co-exist.

The only problem for that comes from religions that refuse to believe in facts or the rights of others, but that's far beyond the scope of this Question. I'll only say that Lisa being Buddhist is slightly more acceptable to Christians than her being an Atheist is because of their misconceptions that Buddha is a god and somehow believing in a god makes it more likely they can be converted back.

## The Final Parsec Problem

The final parsec problem is central to our understanding of binary black hole mergers. It’s a theoretical problem that says when two black holes approach each other, their excessive orbital energy stops them from merging. They can get to within a few light years, then the merging process stalls.

When two black holes initially approach each other, their hyperbolic trajectories carry them right past each other. Over time, as the two holes interact with stars in their vicinity, they slingshot the stars gravitationally, transferring some of their orbital energy to a star each time they do it. The emission of gravitational waves also decreases the black holes’ energy.

Eventually the two black holes shed enough orbital energy to slow down and approach each other more closely, and come to within just a few parsecs of each other. The problem is, as they close the distance, more and more matter is ejected from their vicinity via sling-shotting. That means there’s no more matter for the black holes to interact with and shed more orbital energy. At that point, the merging process stalls. Or it should.

Yet astrophysicists know that black holes merge because they’ve witnessed the powerful gravitational waves. In fact, LIGO (Laser Interferometry Gravitational-Wave Observatory) is discovering a black hole merger about once a week. How they merge with each other at the end is called the final parsec problem.

The team behind this study thinks that they might have an answer. They think that a third black hole, like they’ve observed in this system, could provide the boost needed to get two holes to merge. As a pair of black holes in a trinary system approach each other, the third hole could influence them to close the final parsec and merge.

According to computer simulations, about 16% of pairs of supermassive black holes in colliding galaxies will have interacted with a third supermassive black hole before they merge. Those mergers would produce gravitational waves, but the problem is that those waves would be too low-frequency for LIGO or the VIRGO observatory to detect.

The spectrum of gravitational waves and the instruments that observe them. LISA is a space interferometer and can detect things that LIGO can’t. Image Credit: ESA/NASA/LISA

To detect those, scientists may have to rely on future observatories like LISA, ESA/NASA’s Laser Interferometer Space Antenna. LISA will observe lower frequency gravitational waves than LIGO or VIRGO and is better-equipped to find super-massive black holes merging.

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