Tag Archives: gravitational waves

A Cosmic Collection

by Shane L. Larson

The Cosmos wheels above our heads, far out of reach but well within our powers of perception. Always we wonder, what’s happening now and what does it mean? [Image: S. Larson]

The Cosmos is vast beyond ordinary comprehension, and it is always up to something. Astronomy is our most valiant attempt to observe everywhere all at once, to discover all that is discoverable, to know all that is knowable. We are exceptionally good at it, by any standard you can imagine. The store of cosmic knowledge we have amassed just since recorded human history began (only a few millennia) is extraordinary, and has helped push mathematics, physics, and technology forward in dramatic and unexpected ways. In just the last century and a half, technology has expanded our capabilities by leaps and bounds, allowing us to collect exquisite data that is perplexing and mysterious and revealing. Today we live in an era where we can collect so much data, and collect such complex data, that it cannot be absorbed, analyzed, nor understood with only brief consideration. It requires long and sustained study, intense scrutiny, and expansive modeling.

[L to R] Hubble Space Telescope, the ATLAS detector at the Large Hadron Collider, and LIGO-Livingston. Exquisite technology expands our ability to observe the Universe around us.

Modern science, particularly at the frontiers of knowledge, requires a lot of human brains to make great discoveries. It begins with the great machines themselves. Building something like the Hubble Space Telescope, or the Large Hadron Collider, or LIGO and Virgo requires vast teams of engineers, physicists, materials scientists, construction engineers, titanium welders, chemists, geologists, and a thousand other professions just to build the experiments. Once we start collecting data, there are thousands of others in physics, computer science, signal processing, image analysis, information technology, visualization, and a thousand other professions needed to understand the data!

Big discoveries emerge almost immediately, because the Universe is always up to something, and always up to something that is dramatic and stunning to behold. If you build an exquisite experiment, you’re going to discover something. Such was the case of Hubble’s discovery of the existence of other galaxies, when we constructed the 100-inch Hooker Telescope on Mount Wilson. Such was the case of Rosalind Franklin’s discovery of double-helix structure of DNA with the development of x-ray crystallography. Such was the case of the discovery of the Higgs Boson with the construction of the Large Hadron Collider. Such was the case with LIGO and Virgo, which over the past three years have witnessed six different gravitational wave events.

My personal accounting of every known gravitational wave event, accurate and complete up through GW170817. When we announced GW170608, my page was too narrow to include it!

Today, the LIGO-Virgo Scientific Collaboration announced our first catalog of gravitational wave events — GWTC-1 (Gravitational Wave Transient Catalog). It is the current complete list of every event we’ve discovered in our data. Some of them you know about, because we have talked about them before (even here on this blog: GW150914, GW151226, GW170104, GW170814, GW170817). But since then, we’ve been sifting through the data, looking at every feature, comparing it to our astrophysical predictions, cross-checking it against monitors that tell us the health of the instruments, determining if it appears in all the detectors, and using our most robust (but slow-running) super-computer analysis codes. 

The result is the catalog before you (if you’re curious, you can see the catalog at the Gravitational-wave Open Science Center), that has improved values for the properties of all the previously announced sources, and four new binary black hole sources that were in the data: GW170729, GW170809, GW170818, and GW170823. Additionally a source previously known as LVT151012 (“LIGO-Virgo Trigger“) has been renamed GW151012.

A screen cap of GWTC-1, the first “Gravitational Wave Transient Catalog of Compact Binary Mergers” as it appeared today. The number of events, the amount of data about what the Cosmos is doing, is growing. [Image: LIGO-Virgo Collaboration]

Astronomers are collectors. Every event has an identity, and a long list of everything that we know about it, but there are always going to be a few that are well known and remembered above all the others. GW150914 is always going to be “The First.” GW151226 (“Boxing Day“) was the second and will always represent the moment we all realized this endeavour really was going to be astronomy, not just a single one-time experiment. GW170817 is always going to be remembered as the first multi-messenger gravitational wave detection of a binary neutron star.

But today when you look at the long list of events it strikes me, for the first time, that this is a huge and ever-growing collection. We’ve always known that would be the case, but there is something viscerally pleasing about watching it happen right before your eyes. It is clear that the list is now long enough that it would be challenging to memorize!

We don’t have images of the gravitational wave events, but our artists can imagine what the members of our collection might have looked like at the moment we observed them. [Images: Aurore Simonnet/LIGO-Virgo Collaboration/Sonoma State University]

From the perspective of astronomy, this is a good thing. Having a collection of events is how we learn things about the Universe that can’t be learned from just a few observations. Let’s examine an analogy to explain the necessity of collections. Suppose you were an extraterrestrial visitor who landed on Earth to learn about “humans” and visited someone’s book-club, perhaps five people. What could be learned by just observing five people? A few obvious things might pop out immediately. Humans have five projections from their bodies (two arms, two legs, a head). They have two eyes and two ears. But depending on the five people you may not learn that there is a wide range of hair or eye colors (any redheads in your reading group? anyone with grey hair? what about blue or green eyes?). You may or may not know that there are multiple sexes, nor that there are smaller and larger humans.  Your knowledge would be completely defined by the size of your collected observations.

This is absolutely the case in astronomy — sometimes we have many observations, sometimes we have only a few, but we always want more. Having many observations is paramount to understanding the Cosmos because observations are the only things we have. We are confined to observing the Universe from this small world on which we live, and what we know is built completely on our few, meager observations.

What stands out the most in the new LIGO catalog? We are still letting the implications settle in, but the most important thing the new events do is it makes our estimate of the popuatlion of black holes in the Universe more accurate, and we’ve started to examine those implications is a new study that is being released in tandem with this announcement. But let me highlight the things that personally catch my attention the most.

This shows all the known masses of black holes and neutron stars, detected both by traditional telescopes and using gravitational waves. I’ve highlighted the new black holes in the catalog in green. You can explore this plot with an interactive we’ve created at CIERA. [Image: LIGO-Virgo/Frank Elavsky/Northwestern]

First, remember that every gravitational-wave detection by LIGO-Virgo is not just one black hole, but three — the two black holes that came together, and the black hole that resulted from their merger. That is very important because it means we have three new measurements of the possible masses that black holes can have. If you look at our black hole mass plot you see that black holes come in all masses between five solar masses and 80 solar masses. In fact the new event, GW170729, produced the heaviest stellar origin black hole known to humans, at 80.3 times the mass of the Sun!

Second, it is interesting to look at the black holes that merged and consider how they are different from one another. From the existent data, it looks like the black holes that merge are always close to the same mass. So far, we’ve never seen a smaller black hole fall into another black hole that is five or ten times larger. Does that mean it never happens in Nature? Or does it mean it happens rarely? Or does it mean we’re not good at seeing or recognizing such events yet? The answer is an important one because the sizes of the black holes before they merge tells us something about how they form and grow together. That question is of intense interest to astronomers since black hole formation is tied to stellar evolution, and stellar evolution is tied to how all the stuff around us is made.

Lastly, the trend continues to show that LIGO and Virgo are sensitive to heavier black holes than those that have been previously known from traditional telescopes. The dramatic demonstration that there are stellar-origin black holes near 100-solar masses is stimulating dramatic conversations among astronomers (particularly theoretical astronomers like my group, who study stellar evolution) about how the Cosmos creates these large black holes. 

Left to Right: LIGO-Hanford (Hanford, Washington), LIGO-Livingston (Livingston, Louisiana), and Virgo (Pisa, Italy). All three detectors are currently working toward the start of our new observing run (“O3”) in the Spring of 2019. When new data begins to flow, the catalog is going to start growing once again.

Perhaps the most exciting thing to me, is this is just the beginning. LIGO and Virgo are currently in a maintenance phase, but our third observing run (“O3”) will begin in the spring of 2019. The instruments will be performing at higher precision than ever before, and there are going to be more detections that will make this catalog grow even larger. Our questions are swirling, the anticipation is palpable. But even more importantly, there is a dedicated group of scientists, particularly those who work in signal analysis, computer science, and machine learning, who are developing new and improved techniques for finding signals in data. There are great practical applications to such endeavours (like how do you separate the 25 zillion text messages sent by teenagers every five minutes), but it will once again help grow our gravitational wave catalog, expanding our understanding of the stellar graveyard of the Universe.

Once new data is being collected, the data from our previous observing runs will sit there in the open data archives, waiting for someone to come back and look at it again. Historically, there have always been discoveries made in archived astronomical data long after it was collected. Data is simply too complex to understand everything in it, and we are simply too naive about everything that is going on in the Universe to recognize everything in our data the first time we work with it. There is certainly more in the LIGO-Virgo data than even this catalog. But progress is slow, and only the future will show us what is yet to be discovered, in an every growing tree of knowledge, dividing and growing from our previous discoveries.

Examples of Lichtenberg figures, created by electrical discharges and discovered by the father of experimental physics, Georg Christoph LIchtenberg. Knowledge, like these figures, branch and grow continuously from each other. [Images: Wikimedia Commons]

One of the great physicists of the 16th Century was Georg Christoph Lichtenberg, widely recognized as the first great designer and builder of experiments in physics, our distant ancestor in this game. Today he is most well known for an artform known as “Lichtenberg figures”, the branching shapes burned in materials by surges in electricity — a most suitable metaphor for our growing branches of knowledge. Lichtenberg fully understood the staggering and surging process of scientific discovery, writing “Nothing puts a greater obstacle in the way of the progress of knowledge than thinking that one knows what one does not yet know.” Today’s announcement is just the beginning of what we do not know.

So today, please join us in basking in the glow of new discovery, reveling in the joy that this is just the beginning, and there is no end. Congratulations to my colleagues and friends in LIGO and Virgo; we’ll do this again sometime soon!

Several of my colleagues in LIGO and Virgo have also written about the new catalog — please check out their posts as well!

Songs from the Stellar Graveyard (GW170817)

by Shane L. Larson

Bernie Capax meets Death in Brief Lives, by Neil Gaiman.

In Neil Gaiman’s transcendent literary comics series The Sandman,  the Endless are echoes of the patterns of force and existence that define the Universe. Among them is Death, who at the end of our lives, collects us and escorts us from this Universe. As she says to Bernie Capax, who had walked the world for some 15,000 years, “You lived what anybody gets… you got a lifetime.”  (issue 43, contained in the collection “Brief Lives“).

If there is any truth in astronomy that we have learned over the last few centuries, it is that the Universe itself evolves. The stars are born, they live their long lives, and ultimately they perish and decay away. Death waits for them too. The galaxy is littered with the remains of stars that once were. From our vantage point here on Earth, we peer out into the Cosmos and glean what we can with the meager view we have in our telescopes. We have mapped billions of stars, and millions of galaxies. But in the stellar graveyard, we have only seen a handful of objects — we know precious little about the skeletons of the stars, because they simply don’t emit much light.

On 17 August 2017, at 7:41:04 am CDT, a faint whisper from the stellar graveyard washed across the shores of Earth. It showed up first in the LIGO-Virgo gravitational wave network, which was deep in our second observing run (what we call “O2”). At that particular moment, we were all wound up and celebrating because just three days before, we had made our first joint detection with LIGO and Virgo together (a pair of black holes called GW170814). When signals register in our network, the automated software (we call them “pipelines“) generates initial numbers about what the source might be, and that morning we knew we had something special. Our group lead at Northwestern was spinning us all up to start doing computer simulations, and in an early email to us she said what we all knew: this is life changing.

On the first day, we were sending emails that had the inkling already of how important this discovery was.

Why? Because the mass of the objects in the new signal were smaller than anything we had seen in gravitational waves before — all together less than about 3 times the mass of the Sun. Our predisposition from all our years of experience in astronomy said that could mean only one thing: the LIGO-Virgo network had just detected the first binary neutron star merger in history. Today, we call this event GW170817.

Spectrograms show how the frequency of the signal (vertical axis) changes in time (horizontal axis) in each of the three detectors. The long swoop up and to the right is called a chirp. [Image: LIGO-Virgo]

But the story gets better. 1.7 seconds after the gravitational wave signature, the Fermi Gamma-ray Burst Monitor (GBM), in orbit high over the Earth, registered an event — a short gamma-ray burst, now called GRB170817A. This was hugely significant, because we have often speculated about what causes gamma-ray bursts. For short gamma-ray bursts we’ve long thought it must be colliding neutron stars.

The discovery of GRB170817A by Fermi-GBM. [Image: NASA/Fermi]

What are these neutron stars? They are the dead skeletons of stars, one possible outcome of a colossal stellar explosion known as a supernova. They are extreme objects. They have about one and a half times the mass of the Sun packed inside a sphere about 20 kilometers across (about the size of a city). That means they are extraordinarily dense — a tablespoon of neutron star matter would weigh 10 billion tonnesabout 30 times the mass of all the humans on planet Earth. Gravity on the surface is outrageously strong — about 190 billion times the strength of gravity on the surface of the Earth; if you had the misfortune of falling off a 1 millimeter high cliff, you would be travelling almost 220,000 kilometers per hour when you hit bottom (136,000 mph).

A neutron star (diameter 20 km) scaled to the Chicago skyline. [Image: LIGO-Virgo/Daniel Schwen/Northwestern]

One thing we know about the lives of the stars is that many of them live together with a partner, orbiting one another in a fashion similar to the Earth orbiting the Sun. Like human life partners, one star inevitably reaches the end of its life first, and expires in a supernova. Some such stars become neutron stars. Eventually, the second star in the pair also dies, and if it supernovas, then one end state is two neutron stars, left in an orbital dance with the skeleton of their partner. One might think that is the end of the story for such stars, but there is still one final chapter in this tale from the stellar graveyard. The orbit of the two neutron stars can and will shrink over time through emission of gravitational waves. Of course, we’ve detected gravitational waves before (GW150914, GW151226, GW170104, GW170814), but this time it’s different. Why? We’re talking about neutron stars instead of black holes, which means there can be light, and indeed there was.

The collision of the neutron stars smashes all the matter together, and under such energetic circumstances, matter generates light. The gamma-ray burst was only the beginning. The collision sheds matter into a volume around the merging pair. This matter, suddenly free of the strong nuclear forces involved in the dense matter of the neutron star, recombines and makes heavy elements (physicists call this “r-process nucleosynthesis“). This recombination also creates light, and is called a kilonova. Following the gamma-ray burst there is also a long term afterglow, from the energetic jet of the gamma-ray burst blasting through the surrounding interstellar medium.

Different phases of emission of electromagnetic radiation from the binary neutron star merger. (L) The initial gamma ray burst. (C) The kilonova from nucleosynthesis. (R) Long term afterglow from energized material around the event. [Images: NASA-GSFC SVS]

The LIGO error ellipses plotted on a skymap of the Hydra-Virgo region. The galaxy NGC 4993 is visible in amateur telescopes. [Image: S. Larson]

Together, the LIGO and Virgo detectors can determine where on the sky a source comes from, though not perfectly. They can point to a region called the gravitational-wave error ellipse. For Gw170817, the ellipse on the sky was narrowed down to just about 30 square degrees — an area about the size and shape of a small banana held out at arms length. The error ellipse spans the boundary between the constellations Hydra and Virgo, with a little tail that stretches into Corvus. This was a difficult position in the sky because at the time of the discovery in mid-August, it sets very shortly after sunset. Never-the-less, telescopes around the world began an intensive imaging search, and just 10.9 hours after the detection of GW170817 and GRB170817A, an optical signal was discovered by the Swope telescope in Chile — a pinpoint of light on the outer fringes of the galaxy NGC 4993 that was not there before. Over the course of the next 10 days, the kilonova faded away; in the end more than 70 observatories worldwide imaged and measured the kilonova.  It has been a historic discovery and observing campaign. This is the beginning of multi-messenger astronomy with gravitational waves.

An image of the kilonova associated with GW170817; the fuzzy blog is NGC 4993. [Image: TOROS Collaboration, M. Diaz]

So what can you do with gravity and light together? As it turns out, an awesome amount of science! Today there is a virtual raft of papers being published (the first wave of many, I expect) outlining what we have learned so far. There are too many to explain them all here, but let me just outline a few that stand out to me.

Probably the most important outcome is the confirmation of the connection between short gamma-ray bursts and binary neutron star mergers. Gamma-ray bursts have been a mystery for more than 40 years. First discovered in the 1960s by the military using satellites meant to monitor nuclear weapon tests, the discovery that they were of cosmic origin was revealed to the scientific community in the 1970s. Since then many ideas and models to explain their origin and intense energy have been explored, but none have been confirmed because the engines — the astrophysical systems that drive them — are far too tiny to resolve in telescopes. The LIGO-Virgo detection of gravitational waves confirms that neutron star binaries are the progenitor of short gamma-ray bursts.

The result that I’m most excited about is we used GW170817 to measure the expansion of the Universe. The expansion of the Universe was first noted by Hubble in 1929, by measuring the distances to other galaxies. This was being done just 5 years after the discovery that there were other galaxies! Fast forward 88 years to 2017 — we’ve measured the expansion of the Universe independently using the distance to a galaxy with gravitational waves and light from telescopic observations together. This measurement comes just two years after the discovery of gravitational waves!  It gives me no small amount of pleasure to echo that historic discovery so close to the beginning of this new era of astronomy. 🙂

Top shows Hubble’s original 1929 diagram (from PNAS, 168, 73[1929]); bottom shows the location on this diagram of the GW170817 measurements, at the + mark. [Image: W. Farr/LIGO-Virgo]

We try and write public accessible versions of our papers in the LIGO-Virgo Collaboration. If you’d like to explore some of the science we’ve been doing, try out some of our science summaries.

There are of course many things we still don’t know about the discovery. Foremost among them is this: what is the thing that formed after the merger of the two neutron stars? Some of us in the astrophysics community think it might be some kind of exotic super-neutron-star, larger than any neutron star ever detected. Some of us in the astrophysics community think it  might be some kind of exotic light-black-hole, smaller than any black hole ever detected. Whatever it is, it lies within a very fuzzy range of masses that we call the mass gap — a range of masses where we have never seen any stellar remnant. What is the lightest black hole Nature can create in the Universe?  What is the heaviest neutron star Nature allows? These are questions we would very much like to know the answer to. At least for the moment, it seems we may not learn the answer from GW170817, but with future detections of binary neutron star mergers we may.

The masses of known stellar remnants discovered by both electromagnetic and gravitational wave observations. Between the black holes and the neutron stars is the “mass gap.” [Image: LIGO-Virgo/Frank Elavsky/Northwestern]

So here we are. We are all simultaneously exhilarated, relieved, joyous, and eager for more discoveries to be made. We’re very tired from late nights analyzing data, arguing about results, writing papers, and furiously preparing ways to tell our story to the world.

We could all use a nap. And a pizza.

Because this is only the beginning, the culmination of decades of hard work, difficult hardships, and anticipation. And the best is yet to come. I’m so happy that I’ve seen these days. Being tired doesn’t bother me, all the struggles getting to this point don’t bother me either, because I got to watch it unfold. As Death said, we get what anyone gets; we get a lifetime. These are the moments, the discoveries, that are filling that lifetime up.  Onward to the next one.


This post is the latest in a long series that I’ve written about all the LIGO detections up to now.  You can read those previous posts here:

The Harmonies of Spacetime – GW150914

My Brain is Melting – GW150914 (part 2)

The Cosmic Classroom on Boxing Day (GW151226)

New Astronomy at the New Year (GW170104)

Focusing our Gravitational Wave Attention (GW170814)


I have many LIGO and Virgo colleagues who also blog about these kinds of things. You may enjoy some of their posts too!


Focusing Our Gravitational Attention (GW170814)

by Shane L. Larson

Nature guards her secrets jealously, and wresting them from her grasp is an arduous, and frustrating task. One of the great difficulties of the modern world is that knowledge is so easy to pull up, with the flick of a finger across a screen, that we forget how hard it was to obtain that knowledge in the first place. Every bit of knowledge that you and I take for granted was earned, at great cost, by a long line of humans who came before us.

Knowledge is hard to come by, a fact that we often forget in an age where virtually any and all information is readily available on demand with a handheld device. [Image: S. Larson]

For just more than two  years now, we have lived in a new astronomical era, where astronomers have the ability to sense minute deviations in the shape of spacetime and use them to discover the secrets of the Cosmos. We call this science gravitational wave astronomy.

This new branch of observational astronomy burst on the scene with much fanfare in early 2016 when it was announced that the two LIGO gravitational wave observatories had detected a pair of black holes merging far across the Cosmos. We knew roughly where it was in the sky, but only roughly in the same sense that “Kansas is roughly in North America.” The physics of how an instrument like LIGO works means detection is easier than pointingpointing to a gravitational wave source on the sky is hard, because Nature guards her secrets jealously.

We call gravitational wave detectors “observatories“, but they are very different from traditional telescopic facilities that you and I are familiar with. Telescopes work more or less like your eyes — they point in a given direction, and are sensitive to a narrow space in front of them (what astronomers call the “field of view“).

A daytime picture of the Moon, taken by holding my phone up to the eyepiece of my backyard telescope. The field of view is not much bigger than the Moon, which is very small on the sky. [Image: Shane L. Larson]

By contrast, gravitational wave detectors are largely omnidirectional — they can sense gravitational waves from every direction on the sky, though some directions are easier than others.  They are much more like your ears in this way. If you close your eyes, you can hear sounds in front of you, above you, to the sides, or behind you. You can usually point at a source of a sound, but that is because your brain is using both of your ears together to triangulate the position of the source of sound. Here’s an experiment: close your eyes and plug one of your ears. Have one of your friends stand somewhere in the room and sing “The Gambler” (here’s a version I particularly like, by First Aid Kit) and see if you can point to them. It’s not so easy to point with only one ear.

We use this same method of triangulation in gravitational wave astronomy — multiple detectors can point better than single detectors alone. The more detectors, the better a source of gravitational waves can be found on the sky.

The Virgo gravitational wave observatory, outside of Pisa, Italy, looking roughly northward toward the Monte Pisano Hills. [Image: Virgo Collaboration]

For the past two decades, at the same time LIGO was being built, our colleagues in Europe were constructing another gravitational wave observatory outside of Pisa, called Virgo. On 1 August 2017, the Advanced Virgo detector joined the two Advanced LIGO detectors in the search for gravitational waves.

There was much celebration in the LIGO-Virgo Collaboration that day, because gravitational wave detectors are not easy to build. Getting to the moment where all three advanced detectors were online together was a tremendous accomplishment, and one that held much promise. With three detectors, we should be able to pinpoint gravitational wave sources on the sky better than ever before. The holy grail of events would be to make a detection, and narrow the skyview to an area so small that one could reasonably point a telescope there and possibly see a simultaneous signal in light.

Doing directed astronomy with gravitational wave detectors requires a network of many facilities. As time goes on, more are being built around the world.

We held our breath, and dared not hope. That’s the nature of astronomy — it’s a spectator sport. All we can do is turn on our instruments, and sit here on Earth and wait for the Universe to do something awesome.

As it turns out, we didn’t have to wait long for something awesome. On 14 August 2017, all three detectors registered the gravitational wave signature from a pair of merging black holes.  At about 5:30am CDT in the United States (10:30:43 UTC), a signal came sailing through the Earth, ringing off each of the three gravitational wave detectors that were diligently collecting data, hour after hour, minute after minute, waiting for the Cosmos to do something. Nature did not let us down. The signal was a strong series of spacetime ripples, with the same pattern, showing up in each of the three detectors. We call the event GW170814 (here is a LIGO-Virgo factsheet on the event), and it brings the total number of events in the gravitational wave catalog to 4.

The GW170814 signal, as gravitational wave astronomers like to represent it. The top row shows the spectrograms, showing how the frequency (analogous to the pitch of a sound) evolves in time, chirping as you go from left to right. The lower row shows the waveform traces in time from left to right, growing stronger as the black holes approach and merge, then tapering away. [Image: LIGO-Virgo Collaboration, from our paper]

Below, I show a table I keep of events, and it is getting harder to manage! I like to take it out and stare at it sometimes because you can see a story beginning to emerge, and for a scientist there is nothing more exciting. A story is exactly what we’ve been trying to learn from Nature, but you can seldom figure it out from just one astronomical event. It is only the long, slow accumulation of happenings in the Cosmos that lets us begin to see the tantalizing patterns of what is going on. Lots of black holes. We’re beginning to get a sense for some trends in their masses. We’re beginning to figure out how many there might be, and how common they are in the Universe. Scientists, as a general rule, are a cautious lot. It will still be a while before there are definitive statements on Wikipedia or in astronomy textbooks. But buy your favorite gravitational wave astronomer a bag of jelly donuts (I also like Dr. Pepper), and they’ll talk your ear off about what we’re beginning to figure out.

My updated gravitational wave catalogue. [Image by Shane L. Larson]

But the real story of GW170814, is Virgo. Virgo came roaring on the scene, and transformed our ability to point on the sky. The sky location graphic below shows all of the gravitational wave events seen to date (including one interesting signal, called LVT151012 that wasn’t quite strong enough for us to make out perfectly in the data, but looks an awful lot like a black hole pair).  In every previous detection, the source was known to lie in some great banana shaped region of the sky that we call an error ellipse. With the addition of Virgo to the network, and the arrival of GW170814, we see the dramatic and awesome difference it makes, collapsing the giant banana of an error ellipse into a much smaller bubble on the sky. This bubble lies near the southern end of the constellation Eridanus (if you’d like to look at a starmap, it came from an area around RA = 3h 11m, DEC = -44d 47m). At the moment of the event, the source was directly overhead southern Chile.

The sky location of all gravitational wave events to date. [Image: LIGO/Virgo/NASA/Leo Singer (Milky Way image: Axel Mellinger)]

There were no detected signals with light associated with the event, but these were after all, black holes. By definition, black holes emit no light; if there is going to be something for traditional telescopes to see, there is going to have to be some kind of matter involved. And so, we wait for the next one. We can tell we’re on the cusp of a tremendous new era of astronomy. We still haven’t found the holy grail, an event seen with both gravitational waves and light, but we continue to look. With our growing network of detectors, and scientists around the globe, we will eventually make that discovery too.

Until then, my heartfelt congratulations to my colleagues and friends who I work with on LIGO and Virgo — here’s to many more long years of searching the Cosmos. Viva Virgo!


You can read about the previous LIGO detections in my previous posts here:

The Harmonies of Spacetime – GW150914

My Brain is Melting – GW150914 (part 2)

The Cosmic Classroom on Boxing Day (GW151226)

New Astronomy at the New Year (GW170104)


New Astronomy at the New Year (GW170104)

by Shane L. Larson

Newton’s portrait.

January 4 holds a special place in the hearts of scientists — it is Isaac Newton’s birthday (*). Newton stood at the crossroads that led to modern science, and astronomy in particular. He was the first person to build a workable reflecting telescope, a design that now bears his name and for the past 4 centuries has been the dominant type of telescope used by amateurs and professionals alike. Newtonian telescopes have revealed much about the Cosmos to our wondering minds. Newton was also responsible for the first formulation of a physical law that describes the working of gravity, called the Universal Law of Gravitation. Today we use the Universal Law to launch satellites, send astronauts into orbit, convert the force of your feet on the bathroom scale into your “weight“, and a thousand other applications.  There is much to celebrate every January 4.

(L) Aerial view of LIGO-Hanford Observatory [top] and in Google Maps [Bottom]. (R) Aerial view of LIGO-Livingston Observatory [top] and in Google Maps [Bottom].

But on January 4, 2017 the Cosmos celebrated with us, singing in the faint whispers of gravity itself. On January 4, the signal of two black holes catastrophically merging to form a new bigger black hole washed quietly across the shores of Earth, carried on undulating vibrations of space and time. You were very likely unaware of this cosmic event — it happened at 4:11:58.6 am in Chicago. It was a Wednesday morning, and I imagine most people were blissfully asleep. But two of the grandest pieces of experimental apparatus ever built by humans were paying attention – the twin LIGO detectors in the United States.  For only the third time in history, a gravitational wave signal from the deep Cosmos was detected here on Earth.

The signal was the signature of two black holes (a “black hole binary,” in the lingo of the astrophysicists) merging to form a new, bigger black hole. The black holes, by definition, emit no light themselves. However, astronomers know that black holes can often be surrounded by swaths of interstellar gas. The intense gravity and motion of the black holes can stir the gas into a violent froth that can emit light. At the time of the event, the LIGO team sent out alerts to astronomers around the world, who turned their telescopes skyward looking for a tell-tale signature of light bursting from the energized gas. Our best estimate of the location of the event was canvased by 30 groups, in many different kinds of light ranging from radio waves, to optical light, to gamma rays. No tell-tale emissions of light were seen. The only way we were aware of this event is from the LIGO detectors themselves.

An artist’s impression of two black holes insprialling, near merger. [Image by Aurore Simonnet, SSU E/PO]

The Gravitational Wave Signal. We call the event GW170104, named for the date it was detected. The signal from the black holes registered first in the LIGO detector outside Hanford, Washington, and 3 milliseconds later registered at the LIGO detector outside Livingston, Louisiana. All told, it only lasted about 0.3 seconds. The signal exhibited the characteristic chirp shape expected of compact binaries that spiral together and merge — a long sequences of wave peaks that slowly grow in strength and get closer and closer together as the black holes spiral together.

Comparison of the chirp waveforms from the first 3 detected gravitational wave events. LVT151012 was a very quiet event that was not strong enough for LIGO scientists to be confident it was a pair of black holes. [Image: LIGO Collaboration]

During the early inspiral phase of GW170104, where the black holes are independent and distinct, the heavier black hole of the pair was 31 times the mass of the Sun, and the smaller black hole was 19 times the mass of the Sun. Ultimately, they reached a minimum stable distance (in astrophysics lingo: the “innermost stable circular orbit“) and plunged together to form a new bigger black hole. When that plunge happened, the gravitational wave signal peaked in strength, and then rang down and faded to nothing as the black hole pulled itself into the stable shape of single, isolated black hole. For GW170104, this final black hole was 49 times the mass of the Sun.

All of this happened 3 billion lightyears away, twice as far as the most distant LIGO detection to date. Perhaps these numbers impress you (they should) — they tell the story  of events that happened billions of years ago and in a place in the Cosmos that neither you, nor I, nor our descendants will ever visit. We add them today to a very short list of astronomical knowledge: the Gravitational Wave Event Catalogue, the complete list of gravitational wave signals ever detected by human beings. There are only three.

The current Gravitational Wave Catalogue, of all known events [click to make larger].

Take a close look at the list. There are interesting similarities and interesting differences between the three events. They are all black hole binaries. They are all at least a billion light years away from Earth. Some of the black holes are heavier than 20 times the mass of the Sun, and some are lighter than 20 times the mass of the Sun. Astronomers use those comparisons to understand what the Universe does to make black holes and how often.

This is the most important thing about GW170104 — it is a small but significant expansion to this very new, and currently, very limited body of knowledge we have about the Cosmos. These three events are completely changing the way we think about black holes in the Cosmos, forcing us to rethink long held prejudices we have about their masses and origins. We shouldn’t feel bad about that — evolving our knowledge is the purpose of science. LIGO is helping us do exactly what we wanted it to do: it is helping us learn.

What do we know? There are many things we are trying to learn from the meager data contained in these three signals. The new signal from GW170104 in particular has tantalizing evidence for the spin of the black holes, and some neat assessments of how close these astrophysical black holes are to what is predicted by general relativity. But I think the most important thing about the event from the perspective of astronomy is this: the black holes are, once again, heavy. GW170104 is the second most massive stellar mass binary black hole ever observed (GW150904 was the heaviest).

The masses of known black holes. The purple entries are observed by x-ray telescopes, and represent what we knew about the size of black holes before LIGO started making detections. [Image: LIGO Collaboration]

With the first two events we had one pair of heavy black holes (GW150914), and one pair of lighter black holes (GW151226). There is a great mystery hiding there: where do the heavy black holes come from, and how many are there in the Cosmos? Perhaps they are just a fluke, a random creation of Nature that is possibly unique in the Cosmos. But the detection of GW170104 suggests that this is not the case; we’ve once again detected heavy black holes. The race is on to decide how the Cosmos makes them. The answers to those questions are encoded in the properties of the black holes themselves. How many are there? Are they spinning or not? Are they spinning the same direction as one another? How do their masses compare to one another? GW170104 is another piece of the puzzle, and future detections will help solidify what we know.

How can you help? If you’d like to help the LIGO project out, let me direct your attention to one of our Citizen Science projects: GravitySpy. Your brain is capable of doing remarkable things that are difficult to teach a computer. One of those things is recognizing patterns in images. The LIGO detectors are among the most sensitive scientific instruments ever built; they are making measurements at the limit of our capabilities, and there are all kinds of random signals that show up in one detector or the other — we call them glitches.  It is very hard to teach a computer to tell the difference between glitches and interesting astrophysical events, so we have citizens just like you look at glitches and identify them, then we use that information to train the computer. So far citizens like you have helped LIGO classify more than two million glitches, and they put more on the pile every day.

If you’d like to help out too, head over to http://gravityspy.org/ and try it out; you can do it in your web-browser, or on your phone while you’re sitting on the train to work. We have citizens from kids to retirees helping us out. If gravitational waves aren’t your thing, there are more than 50 other projects in science, arts, history and more at http://zooniverse.org/ you can try out!

A representation of the GW170104 signal, from the scientific paper. These are the kinds of images citizens can classify easily, whereas computers sometimes have trouble. [Image: LIGO Collaboration]

PS: For all of you super-nerds out there, let me point something out if you haven’t already noticed. Suppose you were to parse the name of the signal in the following way: 1701 04. Look familiar? The 4th incarnation of 1701; for the cognoscenti, this event shares the designation of the Enterprise-D. 🙂  Until next time, my friends. Live long, and prosper.

(*) When Newton was born, England had not yet switched to the new Gregorian Calendar, which we use today. They were still using the older Julian Calendar, by which Newton was born on December 25; when converted Newton’s birthday falls on January 4 on the Gregorian Calendar.


You can read about the previous LIGO detections in my previous posts here:


Many of my colleagues in the LIGO Virgo Collaboration have also written excellent blog posts about the GW170104 event, and the work we do to make gravitational wave astronomy a reality. You should visit their blogs!

My Brain is Melting — GW150914 (Part 2)

by Shane L. Larson

It has been just more than a week since we told the world about our great discovery. It was a cold winter morning in Washington DC, the temperature hovering just below freezing. In a room at the National Press Club, the world press had gathered, and at the behest of NSF Director, Frances Córdova, LIGO Executive Director, Dave Reitze, took to the podium.

“Ladies and gentlemen. We have detected gravitational waves. We did it!” Mic drop. (Well, he should have; in the movie dramatization, he will. You can watch the moment here on YouTube, or the full press conference.)

Dave Reitze makes the announcement to the world that LIGO had detected gravitational waves.

Dave Reitze makes the announcement to the world that LIGO had detected gravitational waves. “We did it!”

So began a ninety minute press conference delivering the news of the first gravitational wave detection to the world. In the days that followed, social media and press outlets exploded in a veritable tidal wave of excitement and awestruck wonder. On twitter, the hashtags #gravitationalwaves, #LIGO, and #EinsteinWasRight have accumulated more than 70 million tweets in just one week.

Everyone has the same sense that we scientists have — this is a doorway, now open, to a Universe we have only imagined. Beyond the threshold are certainly things we have predicted and speculated about, but also many wonders yet to be found or understood.

We have done our best to explain what we are doing with LIGO, and how it works. We have made a Herculean effort to describe the astrophysical significance of the discovery. We have tried mightily to explain what Einstein’s ideas about spacetime and gravity are all about.

But this is hard stuff to think about, it is hard stuff to understand, and it is hard stuff to explain. It is well outside our normal everyday experience, so it is easy to feel like your brain is melting.


You shouldn’t worry that these things are hard to understand. It took physicists 41 years to even decide gravitational waves were real, and then another 59 years to build an experiment capable of detecting them. There is no doubt these are hard, brain melting matters. But the beauty of the discovery of gravitational waves is that this can be understood!

A large number of my colleagues in LIGO (and myself) have spent the last week collecting and responding to questions emailed to us, asked in public forums, and delivered on social media (if you have more questions, ask in the comments below, or please email question@ligo.org). All of them are thoughtful, genuine, and demonstrate a pleasing curiosity and wonder about the nature and workings of the Cosmos. I am constantly amazed by the questions people ask.

Here are a few of the more common brain-melters we have been asked, and some meager attempt to answer them. The questions are marked in red, to make them easy to find. Some responses are more complicated than others, and you may or may not want to read them all. They are here to help stem the meltdown, if you find your brain is still reeling. 🙂

What does this mean for ordinary folks?  Far and away, this is the most common question I’ve been asked, particularly from the press. What does this mean for the world? How will this help my golf game?

LIGO’s discovery is what we call “fundamental physics.” It is a discovery that tells us something about how the Universe works and why it behaves the way it does. Figuring out how to use knowledge like that to make your life better or turning it into a gadget that’s useful in your kitchen or garage takes time — we’ve only just now made the first detection of gravitational waves, and are trying to wrap our brains around it.  Scientists and engineers will have to think a long time, maybe decades, before they can make this knowledge “useful for everyday life.”  That’s always how it works with scientific discoveries. How it will impact our everyday lives is not for us to know — that is for the future.

In the modern era, many of us navigate using GPS technology, built directly into our smartphones.

In the modern era, many of us navigate using GPS technology, built directly into our smartphones.

That is not to say that there isn’t some amazing future application. We only have to look at the history of general relativity itself to know the truth of this. Einstein worked out general relativity between 1905 and 1915. This was an age before cars and electricity were mainstays in everyday life. Yet Einstein had the where-with-all to understand that gravity could be thought of as the warpage of spacetime, and that one consequence of that warpage is clocks tick at different speeds depending on how strong the gravity is.

Did you know that little, obscure fact of general relativity is used by you and most other people every, single day? It is an essential part of how the GPS in your phone works. It took nearly a hundred years for the “fundamental physics” we call general relativity to be turned into an essential piece of technology that now gets millions of us from place to place in the world every day. Without GPS and general relativity, you’d still be navigating using paper maps. Einstein had to rely on his neighbor to tell him where to find a pub; you have a smartphone.

Is it really that important? I think this is one of the most important discoveries in astronomy in the last 100 years. It is as important as discovering that there are other galaxies beyond the Milky Way, it is as important as discovering the expansion of the Universe, it is as important as discovering the Cosmic Microwave Background.

The reason I think this is just about everything you’ve ever heard about the Universe, or seen a picture of, has been discovered using LIGHT. Telescopes are just instruments that do what your eyes do (collect light), though telescopes collect much more light than your eye or collect light that your eye cannot see (like infrared or ultraviolet light).

Gravitational waves are different — none of us have a “gravitational wave detector” as part of our bodies. Gravitational waves are something that we predicted should exist, and we built an experiment that showed us our ideas were right.  The beginning of gravitational wave observations will change how we see the Universe in ways that we cannot yet imagine.

Dr. France Córdova, Director of the National Science Foundation.

Dr. France Córdova, Director of the National Science Foundation.

As a scientist and a teacher, I can appreciate the importance and utility of the collection of knowledge. But LIGO’s discovery goes far beyond the mere acquisition of yet another fact to post on Wikipedia. What the scientists and engineers working on LIGO have done was often regarded as impossible to do. But as Dr. Córdova intoned at the LIGO press conference, we took a big risk. Through a judicious application of sweat, brains, and stubbornness, we endured a decades long effort to design a machine to do the impossible. We encountered countless challenges and obstacles, and diligently overcame every single one of them to arrive at this day. That should make every person sit up a little bit straighter and prouder. That should make every single person aware that whatever challenges or problems we face on our small world, we have the means to overcome them, if we have the will to commit our time and brains and resources to them.

The black hole collision LIGO observed was more than 50 times brighter than all the stars in the Universe. How can that be?  The comparison is “the gravitational energy released by the merger is about 50 times the energy released by all the stars in the Universe during the same time.”  This is an example of a “Fermi problem” which astrophysicists use all the time to figure out if our numbers are right when we are doing complex calculations.

The night sky over the Pando Forest in central Utah. Pando is an 80,000 year old aspen grove -- it has seen almost 30 million nights like this one, but very little has changed. The constellations change over thousands of years, but the sky is still full of stars, and the Milky Way still arches over the sky, giving the impression that the Universe is unchanging. [Image: Shane L. Larson]

The night sky over the Pando Forest in central Utah. Pando is an 80,000 year old aspen grove — it has seen almost 30 million nights like this one, bathed in the light of the stars of the Milky Way. [Image: Shane L. Larson]

Astrophysicists measure brightness in watts, just like you are used to expressing the brightness of a light bulb in watts — the “wattage” tells you how much energy is released in a fixed amount of time. The higher the wattage, the more energy is released in a given moment, so the brighter the star (or bulb). Astronomers call this the “luminosity.” We can estimate the luminosity of all the stars in the Universe and compare it to what LIGO measured from the black holes. [[I’m going to use some scientific notation here to write some mind-bogglingly big numbers; a number like 106 means a 1 followed by 6 zeroes: 106 = 1,000,000. ]]

If you express the luminosity of the black holes (3 solar masses in just about 20 milliseconds) as a “wattage,” the brightness is about 3.6 x 1049 watts, or about 1023 times brighter than the Sun.

The Hubble Extreme Deep Field (XDF).

The Hubble Extreme Deep Field (XDF). We can use images like this to estimate the total number of stars in the Cosmos.

Now suppose we make the assumption that all the stars in the Universe are just like the Sun. This isn’t true, of course — some are brighter, some are dimmer, but on the average this is a good starting guess. There are about 100 billion stars in a galaxy like the Milky Way, and if you look at an image like the Hubble Extreme Deep Field, there are on order 100 billion galaxies in the Universe. So there are 100 billion x 100 billion = 1022 stars in the Universe. If each one of them is the brightness of the Sun, the total brightness of stars in the Universe is 1022 times the brightness of the Sun.

But we said the black hole merger seen by LIGO was 1023 times brighter than the Sun, so: 1023/1022 = 10. The black hole merger was 10x brighter than all the stars in the Cosmos. With a careful calculation, we could get the 50 number you hear from LIGO, but 10 is pretty close. This is the nature of Fermi problems — they don’t give you the exact number, but they quickly get you close to the exact number so you can understand the Universe.

What do you mean “spacetime is stretching LIGO’s arms?” What is spacetime? Spacetime is the substrate, the matrix upon which everything in the Universe is built — as we like to say, spacetime is the “fabric of the Cosmos.” It is, of course, easy to say that, but difficult to wrap your brain around. We’re used to not thinking about space at all; it is the nothing between everything. But it is exactly that nothing of which we speak — if we were not here, if nothing were here, there is still space.

Imagine a gravitational wave shooting through LIGO, directly out of the screen at you. (A) When there are no waves, the arms are at their fixed lengths. (B) When the wave first hits LIGO, the spacetime in one arm stretches and in the other arm compresses. This changes how long it takes light to go from the corner to the end of the arms and back again. (C) As the wave passes by, the arms change back and forth between stretching and compressing.

Imagine a gravitational wave shooting through LIGO, directly out of the screen at you. (A) When there are no waves, the arms are at their fixed lengths. (B) When the wave first hits LIGO, the spacetime in one arm stretches and in the other arm compresses. This changes how long it takes light to go from the corner to the end of the arms and back again. (C) As the wave passes by, the arms change back and forth between stretching and compressing.

How do you measure the length of something in space? Most of the time we use a ruler or a tape measure. You lay it down along the thing you are interested in, like LIGO’s arms, and you see how many it takes. Imagine that you put down kilometer markers along LIGOs arms, just like you see on the highway — one at 0km, 1km, 2km, 3km and 4km. When spacetime between the ends of LIGO changes, the entire arm stretches. You still think the arm is 4 kilometers long, because the markers are still evenly spaced (the spacing is just larger than it was before, though you may not be aware of it). We need a way to measure the stretching without relying on the kilometer markers.

Visualization of LIGO interferometry. (A) When no gravitational wave is present, the laser timing is set up to make a "dark fringe" at the output [square panel on the right]. (B) At the output, the light is like waves canceling each other out. (C) When a wave stretches or compresses the arms, it changes how the light is added together at the output. [Frames from video by Caltech/LIGO]

Visualization of LIGO interferometry. (A) When no gravitational wave is present, the laser timing is set up to make a “dark fringe” at the output [square panel on the right]. (B) At the output, the light is like waves canceling each other out. (C) When a wave stretches or compresses the arms, it changes how the light is added together at the output. [Frames from video by Caltech/LIGO]

A reliable way to measure the distance in a space that is changing and stretching, is to time a beam of light as it makes its way through the space you are trying to monitor. In LIGO, we use laser light. Imagine two photons, injected into LIGO at the corner, with a photon traveling down each of the two arms (in terms of the the optics, there is an element at the corner called a “beamsplitter” that splits a laser beam and sends part of it down each of the two arms). When there are no gravitational waves distorting LIGO, the two photons arrive back at the beam splitter and are combined to make an interference pattern, which is a brightness pattern that depends on how the photons arrive together. We set it up so the pattern is a “dark fringe” — the two photons cancel each other out (what physicists call “destructive interference”).

A simple demonstration of how sensitive interference can be to small shifts in space. These interference patterns are made with regular circles (as opposed to moving waves) and create Moiré patterns. The difference between the left and right image is a shift of only 0.05 inches, but the pattern difference is easy to see, indicating the shift happened.

A simple demonstration of how sensitive interference can be to small shifts in space. These interference patterns are made with overlapping patterns of regular circles (as opposed to moving waves) and create Moiré patterns. Here the horizontal dark region in the left image is analogous to LIGO’s “dark fringe.” The difference between the left and right image is a shift of only 0.05 inches, but the pattern difference is easy to see. What was a “dark fringe” now has a sliver of white, indicating the shift happened. [Image: S. Larson]

When a gravitational wave goes through LIGO it stretches the spacetime in one arm, and compresses the spacetime in the other arm. That means the photon in the stretched arm arrives back at the beam splitter LATE (it had farther to travel) and the photon in the compressed arm arrives at the beam splitter EARLY (it had less distance to travel). The result is the brightness pattern CHANGES. The changing pattern of brightness is exactly in tandem with the passing gravitational wave, telling us about the shape of the wave as it passes by.

They said the stretching that LIGO measured was a fraction of the width of a proton. But I remember from Chemistry that atoms are always moving, so how can you make such a precise measurement? Remember that LIGO is not measuring the distance shift in single atoms — it is watching the mirror, which is comprised of many atoms, each of which is moving exactly as you remember from Chemistry.

Everyone on a boat is doing their own thing, but they are all moving together as the boat moves on the waves.

Everyone on a boat is doing their own thing, but they are all moving together as the boat moves on the waves.

When we make our measurements, we are looking at the behaviour of many, many photons that have travelled down the arm together, hit the mirrors, and made the return journey. Sure — some of the atoms are going one way, and some are going some other way, but overall they are all moving together, going wherever gravity is pushing the center of mass of the mirror. When we read out the light, we are looking at all of those photons that hit the mirror at the same time and using that information to determine where the mirror is.

It’s a bit like having a big gravitational wave discovery party on a boat. If you are on the shore, watching all the physicists and engineers having a good time, you see they are all going every which way on the deck. But they are all on the boat, which moves them all together in response to the underlying waves of the sea.

Will this help with time travel? quantum gravity?  Einstein’s great discovery with general relativity was the idea that gravity can be described as the interaction of mass with the shape and warpage of spacetime. The unification of space and time into a single entity — spacetime — is a huge conceptual leap that is sometimes hard to come to grips with because of the way we think about space and time.

In our everyday lives we measure space with rulers and car odometers, and we measure time with wristwatches and calendars. If they are the same thing, why don’t we measure them the same way? The idea that the two are connected takes some getting used to, though as I like to remind people: when you go somewhere, you are usually comfortable saying your destination is “25 minutes” away or saying “20 miles” away!

You think about travel as travelling through space or travelling through time without even noticing! You are used to being a "spacetime traveller."

You think about travel as travelling through space or travelling through time without even noticing! You are used to being a “spacetime traveller.”

Since gravitational waves are moving ripples, propagating warpage in spacetime, it is natural to ask: can this discovery can help us understand space and time? Can we understand “time travel” and “warp drive?

Time itself, despite being part of general relativity, is still a great mystery to us. But what we often forget is that we are time travelers. Even as you are reading this, you are traveling through time from this moment, heading toward next Tuesday. It is not possible, so far as we know, to go backward toward last Friday, and that is a great mystery. It appears to be true based on experimental evidence, but we don’t yet understand how the laws of Nature — general relativity — tell us that. So in as much as gravitational waves will dramatically improve our understanding of how spacetime works and behaves, that deeper understanding could lead us down a path of thinking that will ultimately give us more insight into the mystery of time.

In a similar way, the LIGO detection does not address the enduring questions about the microscopic, quantum nature of gravity. The gravitational waves are a “big world” phenomenon, created by strongly gravitating astrophysical objects. But based on our experience with other quantum physics, we expect that there will be a clear (though not now obvious) connection between quantum gravity and general relativity. The more we expand our understanding of general relativity, it becomes more likely we will stumble on the deep connections that would lead to ultimately understanding quantum gravity.


I encourage you to continue asking your favorite physicist your questions and share what you learn. Also, send questions to: question@ligo.org

Remember: no question is a dumb question. If you are wondering something, I’ll bet you a jelly donut someone else has the exact same question!

The Harmonies of Spacetime — GW150914

by Shane L. Larson

I have a good friend, Tyson, whom I don’t get to see nearly often enough. We are both privileged to be among the first generation of scientists who will know the Universe by observing the faint whisper of spacetime, bending under the influence of massive astrophysical systems. We are “gravitational wave astronomers.”

Picking crab with Tyson (far right) and family. [Image: Sabrina Savage]

Picking crab with Tyson (far right) and family. [Image: Sabrina Savage]

A while back we were sitting on his back porch late into the evening, picking crab and talking about everything. It was the kind of common, easy conversation among friends that ranges over movies, politics, family, childhood memories, inside jokes, and so on. But at one point, the conversation drifted back to science and to the near future. Tyson said something that really just kind of made us all stop in shocked silence: “If we’re really going to detect gravitational waves in the next 3 or 4 years, they are already closer than Alpha Centauri and heading right for us.”


Little did we know then how prescient that observation was. We are both part of a project called LIGO — the Laser Interferometer Gravitational-wave Observatory. And this morning our collaboration made the big announcement.

Frame from a visualization of the binary black hole merger seen by LIGO [Visualization by "Simulating Extreme Spacetime" (SXS) Collaboratoin]

Frame from a visualization of the binary black hole merger seen by LIGO [Visualization by “Simulating Extreme Spacetime” (SXS) Collaboration]

On 14 September 2015, the two LIGO observatories detected a very loud gravitational wave event. Our analysis since that day has told us that it was the merger of two black holes — one 29 times the mass of the Sun, the other 36 times the mass of the Sun. The two black holes merged, forming a new, bigger black hole 62 times the mass of the Sun. We named the event after the date: GW150914.

All of this happened about 400 Megaparsecs from Earth (1.3 billion lightyears). If you are adding up the numbers, you see that there are 3 solar masses missing. That is the equivalent mass that was radiating away from the system in the energy of the gravitational waves.

Make no doubt about it — this is one of the most momentous discoveries in the history of astronomy. It will be up to historians of science to place this within context, but I would rank it right up there with the discovery of the nature of the spiral nebulae and the discovery of the Cosmic Microwave Background.

There are many important and stunning parts of this story. Let’s me tell you just a small slice of how we got to today.

LIGO: LIGO is two gravitational wave observatories that work together as a single experiment. The are located 3002 kilometers apart, with one in Hanford, Washington and the other in Livingston, Louisiana. They are enormous, 4 kilometers to a side — so large, they can be seen in satellite photos.

(L) Aerial view of LIGO-Hanford Observatory [top] and in Google Maps [Bottom]. (R) Aerial view of LIGO-Livingston Observatory [top] and in Google Maps [Bottom].

(L) Aerial view of LIGO-Hanford Observatory [top] and in Google Maps [Bottom]. (R) Aerial view of LIGO-Livingston Observatory [top] and in Google Maps [Bottom].

The observatories are “laser interferometers” — laser light is injected into the the detector, and split so it flies up and down each of the two arms. When the light returns back to the splitter, it is recombined. When you combine laser light in this way, it can be combined such that the beams cancel out (making what we call a “dark fringe”) or they combine to make a bright spot (making what we call a “bright fringe”); in between combinations have a full range between bright and dark. We sit on a “dark fringe.”

Schematic of the LIGO interferometers, showing the basic layout of the lasers and optics locations. [Image: S. Larson & LIGO Collaboration]

Schematic of the LIGO interferometers, showing the basic layout of the lasers and optics locations. The lasers travel up and down the two 4 kilometer long arms, and are recombined and detected at the photodetector. [Image: S. Larson & LIGO Collaboration]

When a gravitational wave hits LIGO, it stretches and compresses the arms. The result is that it changes how long it takes the lasers to travel from the splitter to the end mirror and back. If that happens, when the lasers are recombined the brightness of the fringe changes.

What Happened? Both the LIGO detectors run more or less continuously, and we get our primary science data when they are on at the same time. In the early morning hours of 14 September 2015, at 4:50:45am Central Daylight Time, a signal was detected in the Livingston detector. 7 milliseconds later, a signal was also detected in the Hanford detector. These detections are sensed automatically by sophisticated software that looks for things that are “out of the ordinary.” Notable events are logged, and then humans can take a look at them. In this case, we knew almost immediately it was significant because it was in BOTH detectors, and it was a strong signal (we use words like “loud” and “bright” to mean strong, but we don’t really “hear” or “see” the signals in the usual sense; these are descriptive adjectives that are helpful because of the analogy they make with our normal senses).

Spectrograms of the event at Hanford and Livingston. The darker areas are what a "typical" spectrogram might look like; the bright swoops are the (very noticeable) signal! [Image: LIGO Collaboration]

Spectrograms of the event at Hanford and Livingston. The darker areas are what a “typical” spectrogram might look like; the bright swoops are the (very noticeable) signal! [Image: LIGO Collaboration]

One of the easiest ways to see the signal is in a diagram called a “spectrogram” which shows how the signal in the detector changes in time. Once we had the first spectrograms, the emails began to fly.

Finding Out: We all get LOTS of email, so it took a while before everyone in the collaboration actually realized what was going on. I didn’t hear until the night of September 15. AT 9:35pm CST I got an email from Vicky Kalogera, the leader of our group, that said “have you caught any of what’s going on within LIGO?” We had a round of email with unbearably long delays between them, but by 11:35pm, I had our initial understanding/guesses in my hands. That was enough to do what we all do in science — we make some calculations and extrapolations to understand what we have seen, and to plan what we should do next. We want to figure out what the new result might mean! Here’s the page out of my Moleskine, where I started to compute what a detector in space, like LISA, might be able to see from a source like this.

My journal page from the hour after I first found out about the event. [Image: S. Larson]

My journal page from the hour after I first found out about the event. [Image: S. Larson]

The Importance: There are all kinds of reasons why this discovery is important. If you take your favorite gravitational physicist out for pizza, they’ll talk your ear off for hours about exactly why this is important. But let me tell you the two I think the most about.

First, this is the first direct detection of gravitational waves. It is the first time we have built an experiment (LIGO) and that experiment has responded because a gravitational wave passed through it. This is the beginning of gravitational wave astronomy — the study of the Cosmos using gravity, not light.

Second, this is the first time that we have directly detected black holes, not observed their effects on other objects in the Universe (stars or gas).

The Astrophysics: The two black holes, caught in a mutual gravitational embrace, had spent perhaps a million years slowing sliding ever closer together, a long and lonely inspiral that ended with their merger into a single, bigger black hole. This is the first time we know conclusively of the existence of black holes that are tens of solar masses in size. Such black holes have been predicted in theoretical calculations, but never seen in the Cosmos before.

A more technical simulation of the binary black hole merger; gravitational physicsists and astronomers will be comparing the data to their simulations to examine how well we understand "real" black holes. [Image: SXS Collaboration]

A more technical simulation of the binary black hole merger; gravitational physicsists and astronomers will be comparing the data to their simulations to examine how well we understand “real” black holes. [Image: SXS Collaboration]

Our next big question is “how often does this happen?” If it happens a lot, that is a potential clue pointing to where such black holes come from. If it is a rare event, that also tells us something. So now, we wait — this is just the beginning of LIGO observations, and after a few years of listening for more, we’ll know how common these are.

The People: Science is a way of thinking about the Universe, and so often when we talk about science we talk about Nature — all the wonder, all the mystery, the rules of the Cosmos. But science is a uniquely human endeavour and every momentous discovery is the culmination of countless hours of sweat, uncountable failures, and equally uncountable tiny moments of success that culminate at a profound moment of knowing something new. It would not be possible without the dedication of enormous numbers of people. The world gravitational wave community has been working toward this day for decades. More than 1000 authors appear on the discovery paper, and there are thousands of others who have worked and are working on the project, who are not in that list of authors. It has been a heroic effort on the part of physicists, astronomers, optical engineers, data and computer scientists, technical and support staff, professors and students.

Just some of the thousands of people who have made LIGO a reality and the detection of GW150914 possible. [Images from the LIGO Collaboration]

Just some of the thousands of people who have made LIGO a reality and the detection of GW150914 possible. [Images from the LIGO Collaboration]

Teasing out the secrets of Nature is hard. Since before recorded history began, our distant ancestors  have plumbed the mysteries of the Cosmos using tools that Nature gave us — our five senses. Astronomer Edwin Hubble once opined “Equipped with his five senses, man [sic] explores the universe around him and calls the adventure Science.” (Harper’s Magazine 158: 737 [May, 1929]).

Today, we add a new sense to our quest to understand the Cosmos. TODAY the Era of Gravitational Wave Astronomy opens. Within the next few years, we will no longer live in a world where our view of the Cosmos is limited to what light alone can tell us. TODAY, we see the Cosmos anew, with senses attuned to the fabric of space and time itself!


I’ve written about gravitational waves here at WriteScience before. In many of those I’ve explored what the physical description and meaning of gravitational waves are, and what the endeavour to detect them is all about. If you’d like to take a stroll down memory lane, here are links to those old posts:

Many of my colleagues in LIGO are also blogging about this momentous discovery. I will add their links here as they appear, so you can read their accounts as well:


This is just the beginning

by Shane L. Larson

Each morning, I roll out of bed, dutifully feed the three cats that own me, help my fourth-grader get her backpack put together for the day and put my daily secret note in her lunch, enjoy a few brief moments over morning coffee with my spouse, and then it is off to work.

For my day job, I’m a scientist. My friends and I work in a completely new branch of astronomy called gravitational wave astronomy. Our express goal is to detect a phenomenon that was predicted almost a century ago by Einstein: the undulations and propagating ripples in the fabric of spacetime that signify the dynamic motion of matter in the Cosmos.


Gravitational waves are ripples in the fabric of spacetime; propagating disturbances caused by the dynamical motion of heavy masses, like black holes or neutron stars.

Gravitational waves are expected to be a phenomenal probe of the Cosmos because they are readily generated by objects that are otherwise hard to detect by other means. This includes objects of intense interest to astronomers, like neutron stars, stellar mass black holes, white dwarfs, cosmic strings, and supermassive black holes at the hearts of galaxies. Despite their apparent utility in astronomy, the are exceedingly hard to detect. When Einstein first deduced their existence, he famously showed that the waves were so weak he thought we might never be able to measure them. But as is often the case, the future is full of wonders, and with the advent of the Space Age, people began to question that judgement. Maybe, with some cleverness and awesome technology, we could gaze at the Universe with gravity rather than light.

As with many scientific endeavours, gravitational wave detection is a difficult task because we’ve never built machines to do this before. We are learning how to do everything for the first time. You try things out, making your best guess as to how it is all going to work, but when you finally flick the switch to “on” you can debug your experiment because it is right in front of you.  That’s all well and good when your lab is here on planet Earth, but when you shift your experiments to space, it becomes a bit more difficult.


LISA will be a constellation of 3 spacecraft, 5 million kilometers apart, shining lasers at each other. [Image: Astrium]

Someday we want to build a space observatory for measuring gravitational waves, called LISA — the Laser Interferometer Space Antenna. LISA consists of three spacecraft, each about 2 meters in diameter and 50cm deep. They fly in space, 5 million kilometers apart, and shine lasers back and forth between themselves. We time the flight of those lasers (nominally just over 16.6 seconds from one spacecraft to another) and if a gravitational wave blasts through LISA, we see the laser times change.

So how do we go about building new spacecraft for the first time? We take things in stages, just like you and I do when we try to learn something new. When I want to learn to play guitar, I don’t take the stage on Day One with Dr. Brian May; instead I get an old beater guitar out of the basement and I plunk out riffs of “Old Sussanah” until my fingers bleed. Then I work on the guitar solo in “Brighton Rock.”  Building spacecraft is kind of the same thing.


Artists conception of the LISA Pathfinder spacecraft. [Image: European Space Agency]

No space observatory like LISA has ever been built before, so we have to figure out how to do it. How do you build the laser timing system? How do you set up the spacecraft thrusters to respond to external influences like the solar wind? How do you get the whole thing into orbit in one piece, then set it up so it works? How do you control spacecraft temperature to the precision we need?  The best way to answer all of these questions, and to discover all the pitfalls we haven’t imagined, is to build one. This is one of the primary reasons we built a spacecraft called LISA Pathfinder.

LISA Pathfinder is an “almost LISA”. The spacecraft itself is roughly the size and shape of a LISA spacecraft, but it’s guts are slightly different. Deep down inside, it has a linked laser system that is easiest to think of as if it is just an entire LISA arm, shrunk down to fit on a single spacecraft. This is not ideal for doing astrophysical work, but it is perfect for understanding how the spacecraft are going to work in space.


The heart of LISA Pathfinder (the “payload” in spaceflight lingo). A laser system monitors two freely flying “test masses” (2 kg cubes of gold & platinum). [Image: European Space Agency]

Throwing a robot into space is hard. You have to get it to outer space, and get it there in one piece! The usual way you get things into space (so far) is with rockets. Putting aside the fact that they sometimes explode, a rocket ride to space is not the gentlest experience in the world. It’s loud — noise levels in proximity to a typical rocket engine are a million-billion times louder than sound you encounter at home every day. It shakes a LOT — rocket vibrations back and forth across the body of a rocket can be so strong they have led to catastrophic destruction of the rocket itself. The launch forces are enormous — human spaceflight engineers keep launch forces low for crew comfort (the maximum on space shuttle flights was about 3 times Earth gravity), but rockets without human crews regularly reach 5 to 10 times Earth gravity during launch. Add that all together, and the ride to space can be pretty rough. So how do you get a sensitive gravitational wave experiment into space, all in one piece and undamaged, on a rough and tumble rocket ride?

Hucking robots into space is hard, to be sure, but using a robot you threw into space to do science can be even harder. First, everything has to work. When your robot is tens of thousands of kilometers away from the closest space engineer, you can’t tinker with it — there’s no tightening up bolts, no replacing faulty lasers, no kicking stuck gear boxes, nor swapping out new battery packs. Second, the environment of space is harsh — there’s no air, the Sun is constantly blasting and heating one side of your spacecraft while the other side is turned toward the frigid chill darkness of deep space. And all the while, your dedicated space robot is bathing in a constant wash of hard cosmic radiation. Every ultra-sensitive space experiment has to weather through those hardships, while collecting data that would be hard to collect even under controlled laboratory conditions on Earth.

So you take a baby step, and you test everything first on Earth, then in space. This is the purpose of LISA Pathfinder. To teach us how to build a spaceborne gravitational wave detector, then to show we know how to get the thing safely to space, then once we’re in space, we turn it all on to show that we can do the actual experiment we want to do.

VV06 Lisa PathFinder Launch

LISA Pathfinder launch on a Vega rocket (VV06). [Image: European Space Agency]

On December 2, after many years of design and laboratory work, LISA Pathfinder was launched atop a Vega rocket from Kourou Space Center in French Guiana. It has gone through a series of orbital burns that are sending it to a neutral “Lagrange point” between the Earth and Sun, where it will enter a “halo orbit” to test its lasers, thrusters, and spacecraft guidance systems in the very same way that LISA will have to work. So far, the flight has been flawless.


Just a few of the people who worked on LISA Pathfinder, my colleagues Karsten Danzmann (L), Paul McNamara (C), and Stefano Vitale (R). [Image: Paul McNamara]

What constantly amazes me about the people who build these machines is their diligence and tenacious attention to detail. A robot that we huck into space is not just a dumb hunk of metal. It is an amazing complex machine that is capable of thinking and taking care of itself. It conducts experiments that we tell it to do, stores the results of those experiments and faithfully beams the information back to Earth. At the same time, it is surviving one of the most hostile environments known: the vacuum of space. The influence of the Sun produces drastic temperature shifts across your spacecraft. Cosmic radiation is constantly bathing the spacecraft in a wash of seething, energetic particles. And all the while it has to gather and store energy, and all the zillions of parts and components have to work together, flawlessly and seamlessly.

Your car is also an amazingly complex machine. But if some piece of it stops working and leaves you on the side of US Route 50 in Nevada (the Loneliest Highway in America), a passing motorist will still happen along to help you, or you can make a quick call to the motor club to come tow you. There are no such luxuries in the game of space exploration.

awesomeLISAThe scientists and engineers who contemplate these things every day are ingenious and clever. The delivery of LISA Pathfinder was the culmination of a decade long effort by an enormous team of scientists and engineers. And all the while they were designing and building LISA Pathfinder, they were teaching classes, and training new students and young scientists who will go on to do new and awesome things in the future. These are the people who make our modern world go ’round. I have nothing but admiration for my colleagues who have built and flown this marvelous machine.

So, at long last, the beginning has arrived. We are all simultaneously exhilarated, relieved, joyous, and eager for the next bit of news and the latest results to get here. Because this is only the beginning, the culmination of decades of hard work, difficult hardships, and anticipation. The BEST stuff — the detection of gravitational waves from the Cosmos — is yet to come.