Tag Archives: LIGO

The Cosmic Classroom on Boxing Day

by Shane L. Larson

The seas of the Cosmos are vast and deep. From our vantage point here on the shores of Earth, we have seen much that is beautiful, awe-inspiring, frightening, humbling, confusing, and enigmatic. The simple truth of astronomy is that it is a spectator sport. The only thing we can do, is watch the skies and wait for the next Big Thing to happen. We collect events, like bottle-caps or flowers, and add them to our collection. Each new addition is a mystery, a new piece of a puzzle that takes shape ever-so-slowly over time.

On 14 September 2015, the LIGO-Virgo collaboration announced that they had detected the first gravitational waves ever, and that those waves had been created by a pair of merging black holes far across the Cosmos.

Today, we have some more news: LIGO has detected the second gravitational wave event ever, and those waves were also created by a pair of merging black holes far across the Cosmos. But as is often the case with astronomy, we know what we’ve observed, but we still don’t know what it means.

The name of the event is GW151226 (the date of the event), but within the collaboration, we call it “The Boxing Day Event.” On 26 December 2015 (Boxing Day in Europe), the two LIGO detectors responded to the faint ripple of gravitational energy washing across the Earth, the signature of two black holes merging to form a new larger black hole.

LIGO detected the black holes merging at 3:53 UTC in the morning on Boxing Day (it was late in the evening on Christmas Day in the United States, 9:53pm Central Standard Time). The event happened 440 Megaparsecs away — almost 1.4 billion lightyears! As with GW150914 before it, this titanic merger of black holes happened long, long ago, in a galaxy far, far away. It happened before multi-cellular life had ever arisen on Earth, and for a billion years that information has been sailing through the void, until it washed across our shores.

Learning to do astronomy: We can’t do experiments in astronomy, not the way we all learned to do them in middle schoolExperiment. Observe. Fail. Learn. Repeat.

The timeline of LIGO's first Observing run (called O1). The first detection (GW150914) and the second detection (GW151226) are marked. There was also a candidate that looked like a gravitational wave, but was not strong enough for astronomers to confidently say a detection was made.

The timeline of LIGO’s first Observing run (called O1). The first detection (GW150914) and the second detection (GW151226) are marked. There was also a candidate that looked like a gravitational wave, but was not strong enough for astronomers to confidently say a detection was made. [Image: LIGO Collaboration]

In astronomy, all we can do is observe, and hope that when we see something interesting happen, it happens again. Or something similar happens again, so we can start trying to make connections. Since the first LIGO detection, we have been patiently waiting for more detections. It could have been anything: merging neutron stars, a gamma-ray burst with an associated gravitational wave signal, a supernova explosion in the Milky Way, or perhaps other pair of black holes similar to GW150914.  As it turns out, it was the merger of black holes, but somewhat different than the one we observed before. Excellent! A chance to learn something new about the Cosmos!

When you look at the pile of gravitational wave events we’ve seen before (it’s a very small pile — there is only one event there, GW150914), we do the most obvious thing you can imagine: we start to compare them.

sll_blackHoleSummary

Strictly in terms numbers, you see that the Boxing Day black holes are less massive than the GW150914 black holes, by a substantial amount. This tells astronomers something very important: black holes can and do come in a variety of masses. That certainly did not have to be the case; there are many instances in the Cosmos where almost every example of an object is similar to every other object. People are all roughly the same height; grains of sand are almost all roughly the same size; yellow-green stars like the Sun (“Type G2” in astronomer speak) are all roughly the same mass. Though we did not expect it to be true, it could have been the case that all black holes were about the same mass; LIGO is happy to report that black holes come in many different masses.

But this, in and of itself, inspires new questions and new mysteries. The question for astronomers now is where do black holes of different sizes come from? The Boxing Day black holes are “normal size” — we think we understand how black holes in this mass range are made in supernovae explosions. The GW150914 black holes are a much grander mystery — they are larger (by a factor of 2 or 3) than any black holes that we expect to form from stars today. We have some interesting ideas about where they may come from, but those ideas can only be tested with more gravitational wave observations.

Comparison of the size of black holes observed by LIGO, as well as other candidates detected with conventional telescopes. (L) The physical size of the black holes overlaid on a map of the eastern United States. (R) The same image showing the masses on the vertical axis, and the black holes that combined to make larger black holes. [Image: LIGO Collaboration]

Comparison of the size of black holes observed by LIGO, as well as other candidates detected with conventional telescopes. (L) The physical size of the black holes overlaid on a map of the eastern United States. (R) The same image showing the masses on the vertical axis, and the black holes that combined to make larger black holes. [Image: LIGO Collaboration]

Gravitational wave astronomy: Every observation is different, because every source is different. Every set of waves is a unique fingerprint that encodes the physical properties of the objects that made the waves: their masses, how fast they are spinning, what kind of object they are,  how physically big they are, the distance to them, and so on. It’s like looking at the pictures in your high school yearbook — every picture is the same size, and is what we all call a “picture,” but each one uniquely identifies you or your friends. It encodes the color of your hair and eyes, whether you were smiling and wearing braces, the sweater you wore on picture day, and so on.

A typical visualization of a black hole binary. They emit no light, so there are no pictures! [Image: SXS Collaboration]

A typical visualization of a black hole binary. They emit no light, so there are no pictures! [Image: SXS Collaboration]

When we look at our data, we don’t usually show pictures. LIGO is not a telescope, so it does not generate images like we are used to seeing from the Hubble Space Telescope. Most “pictures” you see are simulations or realizations of the data. Instead, we show our data as graphs and plots that represent our data in ways that tell astronomers what LIGO is measuring and how that relates to quantities in physics we understand, like orbit size or energy.

A stereo equalizer display.

A stereo equalizer display.

One common picture we use is something called a “spectragram” — you may have encountered something like a spectragram on a stereo. The equalizers on your stereo tell you how loud the music in terms of whether it is more treble sounding or bass sounding.  In LIGO, we look at our data by looking a spectragram and how it changes over time.  The fact that the Boxing Day black holes and GW150914 are different is immediately obvious when comparing their spectragrams — the fine details of the shape and duration is different in the two cases, but they have the same basic swoopy shape to them. Think about your high school yearbook: the pictures are all kind of the same, but different in the details.

The comparison of spectragrams from GW150914 (top) and the Boxing Day event (bottom). The blue swoop is the gravitational wave signal as it evolves in time (early in the event on the left, and the final merger in the tall swoop on the right). [Images: LIGO Collaboration]

The comparison of spectragrams from GW150914 (top) and the Boxing Day event (bottom). The blue swoop is the gravitational wave signal as it evolves in time (early in the event on the left, and the final merger in the tall swoop on the right). [Images: LIGO Collaboration]

The difference in the gravitational waves LIGO detected is even more obvious if you look at the waveforms themselves. Imagine you are standing on the beach watching waves roll in and crash on the sand. In between waves, the water is calm and relatively low, but at the moment the wave is washing ashore, the height of the water increases subtantially; if you happen to be standing in the wave as it washes by, you might not be able to stand up because the energy carried by the wave is enough to knock you over. In a very similar way, the waveforms illustrate the strength of the gravitational waves as they wash past the Earth. The size of the “up and down” in the waveforms we plot tells us how strong the waves are.  If you compare the Boxing Day black hole waveforms with the GW150914 waveforms, you see they both have a lot of up and down (a measure of strength — they were strong enough for LIGO to detect!), but their overall shape and duration is different.

Comparison of the "waveforms" for GW150914 (top) and the Boxing Day black holes (bottom). The signals are considerably different, and longer in the case of the Boxing Day event. [Images: LIGO Collaboration]

Comparison of the “waveforms” for GW150914 (top) and the Boxing Day black holes (bottom). The signals are considerably different, and longer in the case of the Boxing Day event. [Images: LIGO Collaboration]

Gravitational wave astronomers at LIGO are most excited about the long chain of up-and-downs in the Boxing Day waveforms. This is a part of the black hole evolution we call the insprial — the long, slow time where the orbit is shrinking, the black holes drawing inexorably closer, creeping toward their ultimate fate: the coalesence into a new, single, spinning black hole. The longer the inspiral is visible to LIGO, the longer we can study the black holes with gravitational waves. Once they merge to form a new black hole, they very quickly become quiet, much like a bell fading into silence after being struck by a hammer. The inspiral, and the merger, are the only chance we have to take the measure of these tremendous astrophysical entities.

What now? LIGO has now made two detections of gravitational waves, both during our first observing run (what we call “O1”). In mid-January 2016, we turned LIGO off and have spent the ensuing months combing over the machine and addressing all the problems and difficulties we encountered in O1. In late summer 2016, we’ll start up for “O2.” We’ll turn up the lasers a little bit, and LIGO will be able to see a bit farther into the Cosmos. If our first stint as gravitational wave astronomers is any indication, we will likely see something new; we don’t know, all we can do is observe.  After a few months, we’ll shut down again, tune things up, think hard about how we are working with the machine, and in 2017 expect to come back online with everything at full design specifications.  We are like toddlers, learning to walk. We’ve taken our first few steps, and have discovered there is a tremendous world just waiting to be explored. We’re learning to keep our balance and do things right, but in the not too distant future will be confident and excited in our new found ability to observe and discover a Cosmos that up to now, has been completely hidden from us.  Carpe infinitum!


Many of my colleagues in the LIGO Virgo Collaboration have also written excellent blog posts about the Boxing Day 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.

brainMelt

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.

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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.”

Whoa.

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!

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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:

 

Gravity 12: Listening for the Whispers of Gravity

by Shane L. Larson

The Cosmos is alive with energetic happenings.  Planets barrel along their orbits, unstoppable by anything short of a collision with another planet.  There is a cluster orbiting the black hole at the center of the Milky Way, with stars being flung and slingshot around their orbits like they were nothing more than ping-pong balls. Massive stars, in a last desperate gasp for attention, explode and spew their guts all around the galaxy, leaving a dark, compact skeleton behind. Billions of light years away, the shredded remains of galaxies slowly coalesce to make a larger elliptical galaxy and their central black holes dance together in a deadly inspiral, spewing jets of energetic material outward to mark their titanic struggle.

Gravitational waves are created by the dynamic motion of mass, a common occurrence in the Cosmos: supermassive black holes mergering or eating stars, stars exploding, and compact interacting binaries are all likely sources.

Gravitational waves are created by the dynamic motion of mass, a common occurrence in the Cosmos: supermassive black holes mergering or eating stars, stars exploding, and compact interacting binaries are all likely sources.

All of these examples have one thing in common: huge masses moving in dynamic ways.  The changing gravitational structure in these systems will manifest itself as gravitational waves propagating across the Cosmos, whispering ripples in the structure of space and time.  Encoded in those waves, if we could detect them, is a previously unheard story for the reading.

The “sticky bead experiment,” worked out at the 1957 Chapel Hill conference, taught us the effect of gravitational waves on the world: they change the distance between points in spacetime. Once we knew what physical effect to look for, physicists began to ask “how do we detect it?”  It was straight-forward to compute the size of the distance change caused by gravitational waves, and it was tiny. But seemingly impossible measurements have never stopped physicists and astronomers from trying to imagine clever and imaginative ways to probe Nature’s secrets.

One of the first people to seriously consider how to measure the extremely tiny stretching effect of gravitational waves was Joseph Weber at the University of Maryland. After the Chapel Hill conference he began to think seriously about the problem of gravitational wave detection, and settled on a clever and imaginative idea: if gravitational waves change the distance between any two points in spacetime, it should stretch a physical object as they pass through it. Once the wave goes by, the inter-atomic forces that hold the object together take over, and try to snap it back into its original shape. This kind of snapback motion would set up acoustic waves — sound waves — in the object. If you could detect those tiny, faint sound waves, it would be an indicator of the passage of a gravitational wave.  Weber fashioned such an experiment from a 0.61 meter diameter, 1.5 meter long cylinder of aluminum that massed 1.5 tons. Such a device is now called a Weber Bar.

(L) Joe Weber instrumenting his bar detector with sensors in the 1960's. (R) You can visit the bar, live and in person, at the LIGO-Hanford Observatory.

(L) Joe Weber instrumenting his bar detector with sensors in the 1960’s. (R) You can visit the bar, live and in person, at the LIGO-Hanford Observatory.

There are, of course, many influences and physical effects that can set off acoustic vibrations in a large aluminum bar. Random acoustic vibrations could be mistaken for a gravitational wave, or more likely, hide the putative effect of a passing gravitational wave. Random signals like this are called noise; filtering noise is one of the foremost problems in any experiment. The solution to this difficulty is to have more than one bar; you set them up and wait to see if both bars ring off at the same time. Since noise is random, it is unlikely to influence both bars identically at the same time, so a common signal is most likely a gravitational wave. Weber’s detection program grew to include a second bar at Argonne National Laboratory that operated in coincidence with the bar he had built in Maryland.

By the late 1960’s, Weber’s analysis of his bar data convinced him he was seeing coincident events, which he dutifully reported to the scientific community.  The ensuing debate has been roundly documented (e.g. in Harry Collin’s book “Gravity’s Shadow”), but that tale is not germane to our discussion here. The important point is this: the scientific community suddenly became cognizant of the idea that gravitational waves could be detected through clever, high precision experiments, and Joe Weber set us on that path.

(Top L) The EXPLORER bar at CERN; (Top R) the AURIGA bar in Italy; (Lower L) The NAUTILUS bar in Italy; (Lower R) The new MiniGRAIL detector at Leiden.

(Top L) The EXPLORER bar at CERN; (Top R) the AURIGA bar in Italy; (Lower L) The NAUTILUS bar in Italy; (Lower R) The new MiniGRAIL detector at Leiden.

In the years following the construction of the Maryland experiment, many other Weber bars were built around the world. These included ALLEGRO at Louisiana State University; EXPLORER at CERN; NAUTILUS in Frascati, Italy; AURIGA at the INFN in Legnaro, Italy; and Niobe in Perth, Australia.  While most of the classic bars have gone offline, new efforts in bar detection technology have turned to spherical detectors, of which MiniGRAIL at Leiden University is the archetype. But still, no gravitational wave signal has been confirmed by any bar.

Given the steadfast absence of confirmed signals in our detectors, why are physicists so confident in the existence of gravitational waves? The answer lies in traditional, telescopic observations of the Cosmos.

The Hulse-Taylor pulsar is located just off the wing of Aquila.

The Hulse-Taylor pulsar is located just off the wing of Aquila.

In 1974, radio astronomers Joseph Taylor and Russel Hulse were observing on the 305 meter diameter Arecibo Radio Telescope in Puerto Rico. They were looking for new pulsars, and discovered one in the constellation of Aquila. Pulsing every 59 milliseconds, the pulsar rotates at a staggering 17 times per second. After studying it for some time, Hulse and Taylor noticed that the pulses varied regularly every 7.75 hours. The explanation? The pulsar was orbiting another neutron star (that was not pulsing)!  Masquerading under the scientific name PSR B1913+16, this remarkable system is more readily known by its common name: the Hulse-Taylor binary pulsar, or usually “THE Binary Pulsar.” We can track the arrival time of the pulses from the pulsar in the system, and precisely determine the size and shape of the orbit over time. After 40 years of observations, it is clear that the orbit of the binary pulsar is shrinking, by an amount of roughly 3.5 meters per year. This is exactly the amount of orbital decay astronomers expect to see if gravitational waves were carrying energy away from the system, sucking the energy out of the orbit. If all goes according to Nature’s plan, the orbit will decay to the point of collision in 300 million years (mark your calendars!).

The system has a neutron star that orbits with a pulsar -- the pulsar is a neutron star that sweeps a strong radio beam toward the Earth as it rotates. As they orbit, they emit gravitational waves, causing the orbit to shrink.

The system has a neutron star that orbits with a pulsar — the pulsar is a neutron star that sweeps a strong radio beam toward the Earth as it rotates. As they orbit, they emit gravitational waves, causing the orbit to shrink.

We now know of many systems like the Hulse-Taylor binary pulsar, giving astronomers confidence that gravitational waves do, without question, exist. So why haven’t we seen them?  The problem with Weber bars is they are “narrow band” — they are most sensitive to gravitational waves that are close matches to the sound waves that are made in the bar (a condition physicists call “resonant” — the gravitational waves closely match the shape and vibration time of the sound waves, so they reinforce each other). Since it is  unlikely a gravitational wave source will exactly match your bar’s vibration frequency, and because many phenomena generate gravitational waves at all kinds of different frequencies, an ideal detector should be “broad band” — sensitive to a wide range of gravitational waves. One solution is to build a laser interferometer.

Michelson (T) and Morley (B) built one of the first interferometers to make precision measurements.

Michelson (T) and Morley (B) built one of the first interferometers to make precision measurements.

Interferometers have a storied history with relativity and astronomy. The earliest scientific interferometers were made in the 1880’s by Albert A. Michelson, and used by Michelson and his collaborator Edward Morley to examine the propagation of light. The results of their experiments demonstrated to the scientific community that light was not propagated by a “luminiferous aether,” and was in fact able to propagate in pure vacuum. Their conclusions also support the founding postulates of special relativity, namely that all observers measure the speed of light in vacuum to be a constant, irrespective of their state of motion.

In the decades that followed, interferometry became a recognized technique for making precise measurements that could not be obtained in any other way. By the time the first results from Weber bars were being reported, people were thinking about other ways to make precision distance measurements, and laser interferometry was a prime candidate technology. The first laser interferometer designed for gravitational wave detection was a table-top experiment built in 1971 at Hughes Aircraft by Robert Forward, who was a student of Weber’s.

(L) Bob Forward's first gravitational wave interferometer at Hughes Aircraft. (R) Rai Weiss' initial sketch of the components and operation of a laser interferometer like LIGO.

(L) Bob Forward’s first gravitational wave interferometer at Hughes Aircraft. (R) Rai Weiss’ initial sketch of the components and operation of a laser interferometer like LIGO.

A year later, Rai Weiss at MIT published a report outlining in great detail the basic considerations for building what would evolve into modern day gravitational wave interferometers. Those initial musings came to fruition in the 1990s, when kilometer scale interferometers began to be constructed around the world with one intention: to observe the Cosmos in gravitational waves.

In the United States, there are two observatories that are called LIGO: one is in Hanford, Washington and the other is in Livingston, Louisiana. In Europe, a 600 meter interferometer called GEO-600 was built outside Hannover, Germany, and a 3 kilometer interferometer called VIRGO was built outside of Pisa, Italy. The Japanese built a 300 meter prototype in Tokyo called TAMA, but have now embarked on a much more ambitious instrument built underground in the Kamioka Observatory called KAGRA. These instruments are enormous endeavours, on the scale of large particle accelerators in terms of their physical size and in terms of the number of people required to bring the project to fruition. All of them can be seen from space (just fire up Google Earth or Google Maps: LIGO-Hanford from space, LIGO-Livingston from space, VIRGO from space, and GEO-600 from space).

(Top L) LIGO-Hanford; (Top R) LIGO-Livingston; (Lower L) GEO-600; (Lower-R) VIRGO.

(Top L) LIGO-Hanford; (Top R) LIGO-Livingston; (Lower L) GEO-600; (Lower-R) VIRGO.

For the first time, these observatories will show us a view of the Cosmos seen not with light, but with the whisper of gravity. The bread-and-butter source, the thing we expect to detect most often, are the merger of two neutron stars. Viewed from the right seats, such collisions generate tremendous explosions known as gamma ray bursts, but we only see a small fraction of the gamma ray bursts in the Universe because they aren’t all pointing toward us. LIGO and its fellow observatories will have no such difficulties — gravitational waves are emitted in every direction from these cataclysmic mergers.

What will we learn from these events? We hope to learn what the skeletons of exploded stars are like — what is their size and what are they made of? What is the matter at their cores like, and what do they become when they merge? Every detected neutron star merger is a clue in the story of stellar lives, which of course, is part of our story too, because we are all of us descended from the exploded ashes of ancient stars.

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]

Where do all these stellar skeletons come from? It’s a curious thing, looking out at the sky. The thing we see the most of are stars, and over the course of a human life, they change little if at all. Night after night, the stars wheel overhead, distant points of light that no human has ever visited, and no human is likely to visit in my and your lifetimes. But over the last few centuries, through a careful application of technology smothered under an insatiable desire to know, we have figured out their story. Like shrewd protégés of Jane Marple, we have pieced together many parts of the the puzzle to discover how stars are born, how they live, and ultimately how they die. Gravitational wave astronomy investigates these final end-states of stellar life. But when we see the stars, we are seeing the snapshot of the stars alive today — where are all the stars that have gone before?

They litter the galaxy — the Milky Way is a vast graveyard of stellar remnants, the burned out stellar husks of those stars that came before. Since only the largest stars produce neutron stars and black holes, and most stars are lighter-weight, like the Sun, astronomers think most of that stellar graveyard is full of white dwarf stars — tens of millions of them.

LIGO can’t see white dwarf stars because they are too big — they never shrink to small enough orbits to make gravitational waves that LIGO can detect. If we want to study this part of the stellar life story, we have to build something new.

lisa_astriumIn the next decade, NASA and ESA hope to fly laser interferometers in space. The LISA gravitational wave observatory will consist of three free flying spacecraft 5 million kilometers apart, using lasers to measure the distance between the three spacecraft. The first step toward flying LISA is a mission called LISA-Pathfinder that will launch in October 2015.

LISA will listen in on the gentle gravitational whispers of tens of millions of white dwarf stars — so many whispers that the galaxy will actually sound like racous party. Like any rowdy party, there will be loud contributors that can always be heard above the noise, perhaps as many as 20,000 that shout out above the cacophony.  These systems are called “ultra-compact binaries”, and orbit each other on orbits so small they would fit between the Earth and the Moon. We think of LISA’s view of the Cosmos as being complementary to LIGO’s — with observations from both observatories, we will be able to construct our first complete picture of the “decomposition phase” of stellar evolution.

But perhaps the most interesting thing LISA will detect are the supermassive black holes at the centers of galaxies. Some of the most fantastic pictures we have taken of the Cosmos show galaxies in collision. Occurring over billions of years, the graceful and delicate spirals are shredded, giving birth to a new, transformed galaxy. How often does this happen? Do all galaxies experience this at some point in their lives, or is it rare? How does it change the kinds of galaxies we see? Does it change the shapes of galaxies irrevocably, or do they return to their whirling spirals of arms?  And perhaps most interesting, what happens to the black holes that once lurked in their cores?

Examples of colliding galaxies. (T) NGC 4676 [the Mice], and (B) NGC 6621

Examples of colliding galaxies. (T) NGC 4676 [the Mice], and (B) NGC 6621

If astronomers are correct, those black holes will sink to the core of the new galaxy that forms, and eventually merge together. When they do, they will emit a wailing burst of gravitational waves that will be visible to LISA all the way to the edge of the Observable Universe. Encoded in that cry will be the birth announcement of a new, bigger black hole, as well as the threads of the story that led to its birth — where they were born, when they were born, and what the Cosmos was like at that time.

These stories and more are contained in the faint whispers of gravity that even now are washing across the shores of Earth. As you are reading this, astronomers and physicists are tuning up our technology to listen closely to those faint messages, and when we finally hear them, they will transform the way we think about the Cosmos.

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This post is part of an ongoing series written for the General Relativity Centennial, celebrating 100 years of gravity (1915-2015).  You can find the first post in the series, with links to the successive posts in this series here: http://wp.me/p19G0g-ru.

Behind the Curtains of the Cosmos 2: Gravitational Waves

by Shane L. Larson

Nearly every picture you have ever seen of galaxies and nebulae and stars, and virtually everything that we know about the Cosmos, has been obtained with light.  Light is plentiful and easily created, so it is natural to use it as a probe of the Universe. Vision is, for most of us, the primary sense by which we interact with the world, so again light is an obvious probe for us to lean on.

Three dimensional gravitational wave emission from colliding black holes (NASA-Goddard).

Three dimensional gravitational wave emission from colliding black holes (NASA-Goddard).

But there are other physical phenomena in Nature, and technology enables us to probe those phenomena, whether we can personally sense them or not. One such phenomena, first predicted by Albert Einstein in 1916, are gravitational waves. Gravitational waves are propagating disturbances of gravity — in the modern language of gravity, they are ripples in the fabric of space and time.  If a massive object moves, it takes time for the gravitational field to respond because nothing can travel faster than light.  Heuristically then, gravitational waves are the shifting of gravitational fields, in response to dynamical motion of mass.

When two massive objects (like stars or black holes) orbit one another, the information about how the gravitational field is changing moves outward at the speed of light. These ripples spiraling outward are gravitational waves, and carry detailed information about the source. [Image by S. Larson]

When two massive objects (like stars or black holes) orbit one another, the information about how the gravitational field is changing moves outward at the speed of light. These ripples spiraling outward are gravitational waves, and carry detailed information about the source. Click to animate. [Image by S. Larson]

Most of us are familiar with the idea of a spectrum from light, produced by a prism or seen as a rainbow after a storm passes. The “electromagnetic spectrum” encompasses more than just those few colors we can see with our eyes; it also includes kinds of light our eyes can’t see, like gamma rays, ultra-violet light, radio waves, and microwaves.

The electromagnetic spectrum --- light in all its varieties, illustrated in the many different ways that scientists describe the properties of a specific kind (or "color") of light.

The electromagnetic spectrum — light in all its varieties, illustrated in the many different ways that scientists describe the properties of a specific kind (or “color”) of light.

Gravitational waves also form a complete spectrum, independent of the electromagnetic spectrum. Since we can’t sense gravitational waves with our bodies, we have no sensory experience to describe them, and have no names for the different parts of the spectrum, so we simply identify the waves by their wavelength (the distance between peaks of the wave) or alternatively their frequency (how often a peak passes by you, if you just stand still and let the waves wash by). As astrophysicists, we have thought hard about the Universe, and can easily imagine Nature creating “high frequency waves” (short wavelength), where thousands of wave peaks pass by you every second, all the way down to “very low frequency waves” (long wavelength), where a wave peak may only pass by once every 30 million years.

There are many different sources of gravitational waves that we expect to see. These include (clockwise from upper left): Colliding neutron stars, merging supermassive black holes, white dwarf binaries, supernova explosions, the Big Bang, and the capture of compact stars by supermassive black holes.

There are many different sources of gravitational waves that we expect to see. These include (clockwise from upper left): Colliding neutron stars, merging supermassive black holes, white dwarf binaries, supernova explosions, the Big Bang, and the capture of compact stars by supermassive black holes.

Gravitational waves are generated by all kinds of different astrophysical systems, and as such the waves themselves are as varied as the phenomena that created them. From the simplest viewpoint, the astrophysical system making the waves defines how big they are. Systems that have large spatial extent and large masses tend to make much longer wavelength waves than small, super-compact systems. To develop some intuition about this, consider one of the bread-and-butter source for modern gravitational wave observatories, a binary black hole — two black holes, locked in a mutual gravitational dance, orbiting one another the way the planets orbit the Sun. When the black holes are far apart (larger orbit) they complete their orbits more slowly. As a result the gravitational waves emitted have low frequencies. Suppose the orbit shrinks. What happens? The black holes orbit each other more quickly, so the frequency of the gravitational waves increases.  What we see here is that the size of the orbit influences the size of the gravitational waves. Similar arguments can be made for other physical properties that influence the shape and form of the gravitational waves, which makes them useful for doing astronomy.

A simple example of how the properties of the source change the structure of the gravitational waves. If we can detect and measure the waves, that will tell us something about the sources. [Image by S. Larson]

A simple example of how the properties of the source change the structure of the gravitational waves. If we can detect and measure the waves, that will tell us something about the sources. [Image by S. Larson]

Astronomy is a game of detection — as I like to say, “astronomy, unlike life, IS a spectator sport!”  Our job as astronomers is to watch the Cosmos and record what we see. We build instruments to help us accomplish that mission, instruments with cool names that you can use to impress your mother or a date: telescope, observatory, detector, bolometer, radiometer, and so on. If you want to build an instrument to look for some form of putative radiation, then you need to know how it interacts with your detector. The basic behaviour we exploit with light is that it bounces off of appropriately designed surfaces, whether the light is radio waves, optical light, or x-rays, it bounces off of surfaces, a fact that humans have exploited to gather light for more in depth experiments.

The effect of a PLUS (+) polarized gravitational wave on a grid of particles. [S. Larson]

The effect of a PLUS (+) polarized gravitational wave on a grid of particles. Click to animate. [S. Larson]

The effect of a CROSS (X) polarized gravitational wave on a grid of particles. [S. Larson]

The effect of a CROSS (X) polarized gravitational wave on a grid of particles. Click to animate. [S. Larson]

So what do gravitational waves do? Gravitational waves change the proper distance between particles. There is a very nice way to visualize this. Imagine a grid of particles — little marbles or marshmallows, arranged on this screen.  Now imagine a gravitational wave shooting straight through the screen: from your eyes, through the middle of your grid, and out the back.  What happens? The distances between all the marshmallows changes in a very specific way, illustrated in the animated images here.  At the start, in one direction all the marshmallows are stretched farther away from each other, while at the same time marshmallows in the perpendicular direction are pulled closer together. If you wait a while, the gravitational waves move a bit farther on in its cycle and it returns the grid to its original appearance. But it doesn’t stop there! On the “downside” of the wave cycle, the stretching and pulling swap directions!  This pattern repeats itself for as long as the gravitational wave is flying through the grid of marshmallows. There are two different “flavors” of gravitational waves (what astronomers call “polarizations”) that both distort our grid of marshmallows, just in different directions.  One flavor is called “PLUS” because the pattern of deformation looks like a + sign.  The other flavor is called “CROSS” because the pattern of deformation looks like a X sign.

So if we want to detect gravitational waves, we need a way to see this distortion. Like other radiation we encounter in astronomy (particularly electromagnetic radiation — light), gravitational waves carry energy which can affect other objects in the Cosmos. A “detector” is a device which extracts some of that energy to let us know the wave is passing by. But gravitational waves have one big problem —- they interact very WEAKLY with matter! That means it is hard to get them to deposit energy in a detector, and that means they are hard to detect.  This is a fact that Einstein appreciated full-well — he knew if we were ever going to see gravitational waves, technology was going to have to get better — much better. It would not be until the 1960s that any serious effort to detect gravitational waves would begin.

The gravitational wave interferometer concept is to measure the changing proper distance between points using lasers.

The gravitational wave interferometer concept is to measure the changing proper distance between points using lasers.

If you look at our particle grids, you see the effect is more pronounced for particles that are farther apart, so the bigger the instrument is the better.  Today, the premiere technology for gravitational wave searches is a classic physics instrument called a “laser interferometer” —  a device that times how long it takes a laser to fly in two different directions and very precisely compares the results to tell you if the two directions have different lengths! This is perfect for gravitational wave detection! Laser interferometers are easy to build in the lab (or at home, if you can acquire a few parts — try these instructions), but it is easier to detect gravitational waves over larger distances, because the stretching effect is larger.  So we’ve built HUGE laser interferometers, that stretch 4 kilometers (2.5 miles) from one end to the other! We’ve built two of them here in the United States — they are called LIGO.  The Pictionary style picture of LIGO is three mirrors — one at a corner, and two at the far ends of two long arms. Think of any three marshmallows in our grid — those are the functional locations of the mirrors in LIGO that the lasers are measuring distances between. When gravitational waves change the distances between the mirrors, we can measure those changes with our lasers.

(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].

Just as in traditional photon astronomy, we have to build different kinds of detectors to detect different kinds of gravitational waves. As a general rule, when the wavelengths of the gravitational waves we want to see are longer, we have to build bigger detectors. A laser interferometer could be built in space by imagining the mirror locations being free-flying spacecraft. Such a concept exists, known as LISA, where the spacecraft are 5 million kilometers apart (that’s 13 times the distance from the Earth to the Moon; it will take the lasers 16.6 seconds to fly between the spacecraft!).

Spacecraft concept for a gravitational wave detector in space [eLISA Consortium]

Spacecraft concept for a gravitational wave detector in space [eLISA Consortium]

Eventually it becomes impractical to imagine building bigger and bigger interferometers. Does that mean there are parts of the gravitational wave spectrum that we simply will never be able to observe? No; astronomers are clever folks and have devised ways to use astrophysical systems as gravitational wave detectors by measuring the imprints the gravitational waves leave behind them.

There are two primary ways we monitor astrophysical systems to detect gravitational waves. One way is we watch pulsars — the spinning dead hulks of massive stars that died in supernova explosions, and now spin relentlessly, periodically flashing the Earth with a bright beam of electromagnetic radiation.  Many pulsars spin in a very stable manner, shining their spinning light on us at very precise intervals, which we can time and write down.  If a gravitational wave passes between us and a pulsar, then it stretches the space between us, and it takes the pulses longer or shorter amounts of time to reach us! We can detect gravitational waves by watching for changes in the arrival time of pulsar pulses!  This is called pulsar timing.

We can detect gravitational waves by monitoring the arrival times of pulses from many pulsars in different directions on the sky.

We can detect gravitational waves by monitoring the arrival times of pulses from many pulsars in different directions on the sky.

The second astrophysical method for gravitational wave detection is to look very closely at the Cosmic Microwave Background, and see if gravitational waves left very tiny distortions in the microwaves. There are many different distortions that can and do appear in the Microwave Background radiation, but the signature of gravitational waves is unlike just about every other kind of known distortion. They are of intense interest because if that signature could be detected, it would mean the gravitational waves came from the other side of the Curtain! This is exactly what the BICEP2 experiment was all about, and will be the theme of our discussion in the next and final post of this series.

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This is the second installment in a 3-part series about the March 2014 BICEP2 announcement about the detection of a putative signal from inflation in the Cosmic Microwave Background.  Part 1 can be found here.

Gravity does the talking

by Shane L. Larson

Obi-wan Kenobi, in perhaps one of the most famous utterances in cinematic history, claimed that the Force “is an energy field, created by all living things. It surrounds us, it penetrates us, it binds the galaxy together.” This propagated rapidly through popular culture when it was realized that Obi-wan must have been talking about duct tape, which after all has a light side, a dark side, and also binds our world together.

The famous utterance of Ben Kenobi's description of the Force (from "Star Wars").

The famous utterance of Ben Kenobi’s description of the Force (from “Star Wars”).

But an astute citizen of the Cosmos may grow curious at Kenobi’s observation, and ask “what does bind the galaxy together?” As it turns out there is a force that penetrates the fabric of the Universe, in a way it is the fabric of the Universe. We call it gravity.

Many of us have heard the idea that there are four fundamental forces in Nature: gravity, the electromagnetic force, the weak nuclear force, and the color force (the “strong nuclear force” is a faint bit of the color force that “leaks” out of atomic nuclei to be detectable by our experiments). Why is gravity The Force? Why not the others?

The four fundamental forces of Nature emerged after the Big Bang, as the Universe cooled and expanded.

The four fundamental forces of Nature emerged after the Big Bang, as the Universe cooled and expanded.

In order to fill the Cosmos, a force must be a long range force — the Cosmos is a BIG place!  The weak nuclear force and the color force are short range — they act very strongly over very tiny distances, in atomic nuclei and in the nuclear particles that comprise nuclei. The electromagnetic force is a long range force, but it acts in the presence of electrically charged particles, which come in two flavors — positive (+) and negative (-). It is easy to make separate positive and negative charges and to locally generate strong electromagnetic forces (lightning is a prime example from Nature), but by and large the Cosmos is electrically neutral — opposite charges are attracted to each other, and they quickly neutralize and cancel each other out, leaving no free charge behind.  Gravity is also a long range force, but it has only one kind of “charge,” which we call “mass.” There is no negative mass, so gravity cannot be shielded or canceled, and it acts over vast distances.

Gravity is the only game in town when it comes to forces acting on cosmic scales, despite being so incredibly weak.  I can see the skepticism on your face!  I said gravity binds the Cosmos together, and in the same breath said it was incredibly weak!  Whatever do I mean?

I tried as hard as I could to break the apple in two!

I tried as hard as I could to break the apple in two!

I mean that gravity is weak compared to the other forces of Nature, a fact you can easily demonstrate in your kitchen. Pick up an apple.  What is holding an apple together?  It is mostly intermolecular forces between the molecules that make the apple up, and those forces are electromagnetic in nature.  Now,using your bare hands, try to break the apple half.  Not so easy, is it?

Using the chemical energy from some Dr. Pepper, I can overcome the gravitational pull of the entire planet.

Using the chemical energy from some Dr. Pepper, I can overcome the gravitational pull of the entire planet.

Now, stand up and jump up in the air. How high did you get? Even if it was just a couple of inches consider this fact: you were able to momentarily over come gravity.  Using a little bit of chemical energy, gleaned from that rabbit food you ate at lunch (perhaps an apple you ate), you were able to overcome the gravitational pull of the ENTIRE EARTH!  Gravity is weak (and you are strong).

While these kinds of deep machinations are fascinating questions into the deep nature of Nature, you might still be scratching your head wondering what good is this knowledge? The first widely understood law of Gravity was Newtonian gravity, described by Isaac Newton in 1687.  It was used almost immediately to begin describing the motion of heavenly bodies, but by and large the world went about its business more or less oblivious to this stunning achievement of the human intellect.  The practical application of Newtonian gravity, using it for something that humans build or use, was not for almost 270 years: in 1957, the Soviet Union launched Sputnik, requiring a detailed understanding of orbital dynamics, which is derived from Newtonian gravity.  By a similar token, Albert Einstein wrote down the modern description of gravity, general relativity, in 1915. There were immediate applications of general relativity to astrophysics (a trend that has only grown since), but practical applications to human affairs did not seriously arise until the late Twentieth Century.  Let me tell you some stories about how gravity, general relativity, is changing our world.

GRACE.  Our society is engaged in much teeth-gnashing about the nature of the Earth’s changing climate, but most scientists are doing what scientists do best — they put their heads down, they collect data, then they figure out what the data is telling them.  Of particular importance to climate studies is the hydrological cycle on Earth.  Gram for gram, water is a bigger player in thermodynamics than any other substance on Earth. It is extremely effective at cooling and heating, which is why you use it to cool off in the summer and warm up in the winter!  The movement of water on Earth, in the oceans, the clouds, the rivers, and the atmosphere has enormous impacts on climate worldwide.  But the hydrosphere is HUGE! We can’t possibly hope to monitor water levels and water flow in lakes and rivers and oceans worldwide by placing individual sensors.  So how are we to learn about the water on Earth and how it moves and changes?  The answer is we use gravity.

(L) Satellite geodesey monitors the orbit of a satellite to understand the underlying source of gravity. (R) The GRACE geodesey system uses two satellites keeping track of each other using a microwave link.

(L) Satellite geodesey monitors the orbit of a satellite to understand the underlying source of gravity. (R) The GRACE geodesey system uses two satellites keeping track of each other using a microwave link.

Satellite geodesey can make precision measurements of the Earth’s gravitational field. As a satellite flies over the Earth, the changing mass below the satellite changes the strength of gravity, which alters the satellite’s trajectory in its orbit.  We monitor the orbit to know how the gravity (and the mass creating the gravity) is changing!  In 2002, NASA launched a mission called GRACE (Gravity Recovery and Climate Experiment), consisting of two satellites flying about 220 km apart, monitoring each others’ orbit using a microwave signal.  For over 5 years, GRACE monitored the Earth’s gravitational field and was able to see how it changes as water and ice move around our planet.  Just one example is shown below, illustrating how the gravity in the Amazon basin goes up and down with the coming and going of the rainy season.  Similar results illustrate the changing ice around the planet, particularly in the Arctic and Antarctic.

GRACE geodesey is sensitive enough to detect the change in gravity over the Amazon basin as the rainy season comes and goes.

GRACE geodesey is sensitive enough to detect the change in gravity over the Amazon basin as the rainy season comes and goes.

GPS.  phoneGPSPerhaps the most ubiquitous use of gravity in your everyday life is the global positioning system. Once relegated to navigation on planes and automobiles, the advent of GPS built into smartphones has enabled an explosion of location services that allows you to find friends, local restaurants, comic book stores, and concert venues in unfamiliar cities.

Fundamentally, GPS works by triangulation.  Satellites send out timing signals that are received by your smartphone or GPS navigator. The signals are broadcast in synch with one another. This means that if you are an equal, fixed distance from two satellites, you’ll get the same time from both (this is like using headphones — the sound from the L and R side are synchronized so you hear all the right parts of the song and the same time!).  If you are closer to one satellite, then you receive a time from that satellite sooner than a distant satellite (this is like watching a track meet from the stadium — runners hear the starting gun before you do, because they are closer).  Your navigator compares your local time to the time received from the satellites, allowing the determination of distance to each satellite. Since the position of each satellite is known, your location can be computed.

GPS triangulates your location by comparing the received time from multiple satellites.

GPS triangulates your location by comparing the received time from multiple satellites.

The satellite timing signals must be modified, using general relativity.  Why?  The satellites are much higher in the Earth’s gravitational field than you are, and general relativity tells us their clocks tick at a different speed. How much different? Over the course of a day, the general relativity correction to the clock times is about 38 microseconds — 38 millionths of a second!  You may be thinking “But that is so tiny!”  Yes it is tiny, but GPS works based on how far light travels in a given time.  In 38 microseconds, light travels 11.4 kilometers (7 miles)!  When you are trying to find a sushi restaurant, or the soccer field for your kids next game, 11 kilometers is a long way off!

Gravitational waves. ein_1920Let me tell you one last story, not about the practical uses of gravity, but about our dream of using gravity to reveal the secrets of the Cosmos.  In 1918, while exploring the implications of general relativity, Einstein discovered that there exists a kind of gravitational radiation, where the gravity from an astrophysical system carries energy away and into the far reaches of the Universe.  He calculated the strength of this radiation, and very quickly decided that it would be exceedingly difficult (if not impossible) to experimentally measure.

But fast-forward the blu-ray to today, and we have technology at our disposal that Einstein could never have imagined — high precision, high power lasers; GPS positioning systems to accurately locate anything anywhere on the planet; high performance computers capable of performing billions of computations per second; a globe girdling network that passes information from one continent to another as easily as one might shout down the hallway to a colleague; and most importantly, a vast community of scientists well-trained and well-versed in wresting secrets from Nature, the best minds our planet has to offer. You add that all together, and we find ourselves in the land of Einstein’s dreams, poised to measure the faint echoes of gravity bathing the Earth from distant corners of the Cosmos.

Nearly a century of thinking on the matter of gravitational radiation has coalesced around a magnificent machine called LIGO — the Laser Interferometer Gravitational-wave Observatory.  Using lasers shining up and down 4 kilometer long beam arms, a new generation of astronomers — gravitational wave astronomers — hope to detect the dance of neutron stars and black holes spiralling toward collision, the constant drone of young pulsars spinning down into their final rest in the stellar graveyard, and maybe (if we are lucky) the cataclysmic supernova explosion of a star dying, a process that synthesizes most of the atoms that comprise what we are all made of.

The LIGO Observatory at Livingston, LA. There is a companion observatory in Hanford, WA.

The LIGO Observatory at Livingston, LA. There is a companion observatory in Hanford, WA.

Gravitational wave astronomy is a way of asking anew the questions about who we are and what our place in the Cosmos is; it is a way of once again indulging in the unique gift to our species, an insatiable sense of curiosity and wonder.  But are there practical outcomes from this remarkable feat of human imagination? Perhaps not obvious ones, because the practical outcomes were not the driving force in the creation of the experiment. But as with all great feats of science and engineering, from the Manhattan Project to Apollo to LIGO, there are always beneficial outcomes.  Already LIGO’s technology is pushing the frontiers of optics and laser technology, environmental monitoring, and computer network capabilities.  But changes you see in your living room may be 7 or 70 or 270 years away.

This has always been the case for gravity; the timescale is simply a matter of how creative our engineers and scientists get!

Knowing something about everything

by Shane L. Larson

During the early 1970’s, a yellow cab crawled up Park Avenue in New York City. By all accounts, this was an innocuous happenstance, repeated thousands of times a day before and since.  But this cab ride was special, because it gave rise to one of the greatest treaties in human history, the so-called “Park Avenue Treaty.” The signatories were Isaac Asimov and Arthur C. Clarke, who agreed that Asimov was required to insist that Clarke was the best science fiction writer in the world (reserving second best for himself), while Clarke was required to insist that Asimov was the best science writer in the world (reserving second best for himself).  The treaty was famously referred to by Clarke in the dedication to his novel, Report on Planet Three, which read “In accordance with the terms of the Clarke-Asimov treaty, the second-best science writer dedicates this book to the second-best science-fiction writer”.

Arthur C. Clarke (left) and Isaac Asimov (right), the signatories of the Park Avenue Treaty.

The treaty is indicative of one of lost truths of those by-gone days — Asimov was widely regarded as one of the finest communicators of science, though he is most often remembered for his science fiction (if you haven’t read the original Foundation Trilogy, stop reading this now and go find a copy; this blog post will be here when you get back).  He became a proficient and popular science writer in the years after the Soviet Union launched Sputnik, when there was widespread concern about the “science gap” between Americans and the rest of the world (an earlier incarnation of the current growing science gap in our country).  Asimov’s writings were wide ranging, accessible to broad audiences, and enormously popular. Kurt Vonnegut once famously asked Asimov how it felt to know everything.  Asimov replied that he was uneasy with his reputation for omniscience.

Despite his play at modesty, Asimov’s reputation was not ill-deserved.  He was, by all accounts, a polymath — a person whose intellect and expertise span a vast number of areas in the entire body of human knowledge. There have been many polymaths throughout history, many of their names are well known in our popular culture.  Perhaps the most famous, was Leonardo da Vinci, widely regarded as one of the finest mechanical geniuses and artists who has ever lived. Apprenticed as a young boy to the artist Verrocchio in Firenze, Leonardo was immersed and trained in artistic and technical skills of the day: drafting, metalwork, drawing, sculpting, and painting.  Leonardo’s skill manifested itself even at this early age.  Anecdotal stories tell that when he began painting under the tutelage of Verrocchio, the young Leonardo’s skill was so great that Verrocchio swore to never paint again.  In his life, Leonardo produced stunning works of art that have survived and are revered today — the Mona Lisa, The Last Supper, and the Vitruvian Man.  One of my favorites works is the first sketch that we are certain is a work of Leonardo, of the Arno Valley from 1473. It is a simple line sketch that somehow captures the effervescent beauty of that far away Italian countryside, though I have never been there.

“Study of a Tuscan Landscape.” This sketch of the Arno Valley is the oldest known work of art by Leonardo da Vinci.

In my mind’s eye, I imagine the young Leonardo sitting on a grassy hillside, his pen and paper in hand, recording the image of his home in quick lines and shades. As the shape of the Arno Valley emerged and the walls of the Castle Montelupo sprang up on the page, his mind must have wandered in the fertile ground of imagination, exploring new seeds and thoughts planted by the sun and the landscape. Leonardo was not one to let seeds go untended. His genius and creativity are well known, spawning not only some of the most famous works of art in western culture, but also straying to ideas about flight and helicopters, harnessing the Sun’s energy by concentrating it, and the possibility that the Earth’s surface moved (something geologists today call plate-tectonics). No topic was too mundane, nor of little interest to Leonardo. He was a true polymath.

It is a funny fact of human nature that we discourage the behaviour that we so often value.  Polymaths dominate the ranks of the most revered scientists of all time: Leonardo, Galileo, Newton, Huygens, Feynman, Dyson. But in academic circles, polymathism is discouraged. University professors are often encouraged to be narrow minded, to focus their attention and efforts in narrow back-waters of science so they are the world’s single expert in very rigidly defined and narrow boxes of knowledge.  Somewhat surprisingly then, the most awesome applications of human imagination to science are efforts that are highly interdisciplinary, requiring expertise from hundreds of scientists in an astonishing variety of fields.

Approximately a hour to the west of Vinci, on the outskirts of Pisa, one of the greatest miracles of the modern age is taking shape.  Astronomers and physicists, in collaboration with computer scientists and engineers and laser technologists, are constructing an enormous, multi-kilometer long laser interferometer called Virgo (http://goo.gl/maps/CYzrE).  A similar, but smaller observatory called Geo has been constructed in the farmlands outside of Hannover, Germany (http://goo.gl/maps/Ozlco).  The Japanese are constructing another facility called Karga underground at the famed Kamioka Observatory in western Japan.  Two larger observatories have also been built in the United States, called LIGO — one in the high desert of eastern Washington near the Hanford Reservation (http://goo.gl/maps/C1QEj), and one in the verdant cypress forests of Louisiana near Livingston (http://goo.gl/maps/pifQn).

These massive scientific instruments are the cousins of interferometers that have been used in physics laboratories for the past century, simply enlarged by a factor of 4000 and instrumented with state of the art lasers, seismic isolation systems, the world’s largest vacuum system, 30,000 environmental sensors and one of the most powerful linked computer networks ever created for scientific analysis.  The goal is to detect one of the holy grails of physics: gravitational waves.

Gravitational waves are a completely new way of looking at the Universe, not with light, but with gravity.  Virtually everything you know about the Cosmos — everything you’ve ever been taught, everything you’ve ever read in a textbook or seen on the news, has been discovered with light using telescopes.

The Hubble Space Telescope (left) extends our vision deep into the Cosmos, providing views like this one of the Carina Nebula (right), showing us a secret birthplace of stars.

It is a time honored tradition that has passed down to us from another great polymath, Galileo Galilei who built the first telescope in 1609 and wrote about his experiences the following year in the celebrated Sidereus Nuncius (”The Starry Messenger”).  The descendants of that first modest spyglass are simple telescopes you might use in your backyard, as well as the Hubble Space Telescope.  The telescope has taught us much about the Cosmos and our place in it.  But there are new frontiers to be explored by changing our perspective.  The detection of gravitational waves will revolutionize our understanding of compact astrophysical systems. We will be able to directly probe the interior structure of neutron stars (the densest objects known) as they tear themselves apart in titanic collisions; we will watch black holes merge and ringdown, revealing their size and spin; we will see stars plummeting in chaotic spiraling orbits around black holes that will map out the gravitational field to reveal the structure and shape of the hole.  And, if we are lucky, we may even detect the faint echoes of gravitational waves from the Big Bang, whispers across time from an era 400,000 years earlier than any ordinary telescope will ever be able to see.

It was Einstein himself who discovered the idea of gravitational waves in 1916, but he almost immediately discarded the notion of detecting them because the physical effect that has to be measured was, in his estimation, beyond our abilities. Fast-forward to the modern era, and technology has changed.  Not just a single technology, but many technologies.  The instruments we build to detect gravitational waves are a complex synthesis of ideas requiring people of broad mind and discipline.

The enormous arms of these interferometers had to be laid out by our best construction contractors, because the arms are long enough that the curvature of the Earth matters!  The 1 meter diameter vacuum pipes had to be manufactured then spiral welded without any leaks or cracks over the entire 4 kilometers of the instrument arm.  Thermal engineers had to design expansion baffles on the beamtubes that contract and expand with the heating and cooling of the arms with the rising and setting of the Sun. Seismologists and meteorologists and electrical engineers had to create a network of some 30,000 environmental sensors that monitor and report on the health and environment of the observatory.  Exquisite isolation engineers had to build suspension systems capable of filtering out vibrations from everything — people walking down the hall, the echoing tremors of ground motion on the other side of the world, and the rumble of car tires on a highway ten miles away.  Computer scientists and network engineers have designed a computing and data acquisition system that has thousands of individual links, stores and processes data, and delivers that data to a collaboration of nearly 1000 scientists spread around the world.  Master optical engineers and laser physicists have built a laser injection and control system that takes as input a single infrared laser beam, circulates it over 1600 kilometers during 400 trips up and down the vacuum beam line, and brings the laser light all back together to measure minuscule changes in distances that herald the arrival of gravitational wave signals from remote corners of the Cosmos.

LIGO is an awesome machine, whether you are looking down one of the 4 km arms (left), or staring into the guts of the computer system interlinking the instrument and all of its vast sensor network (right).

Standing at the vertex of one of these great instruments, staring down the arm to the distant end stations 4 kilometers away, it is easy to be amazed by the ingenuity of our scientists and engineers — large teams who have butted heads, argued, designed, tested, and ultimately built the most sensitive scientific instruments our species has ever created.  A pool of talented people who had the where-with-all to imagine every possible problem that might be encountered along the way and design a solution.  Talented people who encountered unforeseen problems, ferreted out the cause of the trouble, then built a solution that allowed us to continue down the long road toward discovery.  These great machines, and ultimately the discoveries we make with them, are a testament to their dedication and perseverance, a legacy as great as that of Newton, and Huygens, and Leonardo.  We polymathed our way to these instruments, not through the intellect of a single person, but through the linked abilities of a vast team of people spanning multiple decades of work.  As a result of those efforts, we find ourselves poised on the brink of discovery: breathless with anticipation, and rightfully proud of our accomplishment.

The LIGO-Hanford interferometer, seen from the air.

Standing at the vertex of LIGO, one can’t help but be overwhelmed by two things. The first is the awe-inspiring example of what we can engineer through sheer ingenuity and perseverance. Instruments like LIGO will fundamentally change the way we view the Cosmos, pushing us to look beyond the simple prejudices imposed by the limitations of our physical senses and listen to the grandeur of a Universal symphony we’ve never been able to hear before. The second is that this machine is only the beginning of so much more than just astrophysics. New technology and new insights always flow back to society and are used in startling and unexpected ways, propelling our young species forward. This was true with Apollo, and as many others have pointed out, is true for LIGO.  The LIGO laser technology is already making its way into the carbon composites industry where it is being used to test aircraft parts. Einstein@Home (like it’s big sister, Seti@Home) was one of the first projects to use your home computer to do scientific crowd-computing while your computer was sitting idle during Monday Night Football, turning the world into a vast supercomputer. LIGO’s advanced laser control systems are demonstrating the precise methods needed to shape and control lasers in applications ranging from laser welding, to high precision laser cutting systems, to advanced laser weapon systems.  None of this was intended, but it all sprang from the same fertile ground — the seeds of ideas planted and nurtured from an exquisite mix of ideas stirred together with reckless abandon.  Polymathism in the large.

Standing at the vertex of LIGO, staring down the arm, the joy in our accomplishment is pierced by an unerring certainty that we should be doing more of this.  We need more polymathism in the world, on scales both large and small.  We should unfetter our young scientists, and let their minds stray to the far reaches of wonderfully crazy ideas and fantastic imaginings about what our future could be.  It is hard to imagine that good things can and will result from allowing such freedom, particularly in trying times of economic woe and political discord.  It is even harder for the vanguard of scientific leaders (the “greybeards”, as I call them) to encourage big expansive thinking among our young scientists when the great discoveries could easily overshadow our own seemingly meager contributions to the state of human knowledge; the egos of scientists (despite their outward bravado) are fragile. But that doesn’t change the fact that we need more polymaths, not just to inspire us by charging down the frontiers of discovery, but to address serious problems with new and creative connections and solutions that narrow box thinking will never discover.  The world has serious problems, and we need creative thinking to address those problems.

Standing at the vertex of LIGO, staring down the arm, I wonder what Leonardo would have thought if he was right here with me?  I can imagine him sitting here next to me, with a parchment and a pen in hand, sketching the long lines of LIGO’s arm, the scrub desert of eastern Washington and the distant shadow of Rattlesnake Mountain, and my mind strays into imagination, wondering all the things that could be.