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!

———————————

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:

 

26 responses to “The Harmonies of Spacetime — GW150914

  1. A very minor comment: You say near the end, “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.” I suppose you mean “electromagnetic radiation” or something of the kind—we haven’t been limited to light observations for some time.

    Thank you for the summary, and congratulations!

    • Well, as a physicist I usually call all electromagnetic radiation”light.” If I mean what our eyes can see, I usually say “visible light.”

      • That occurred to me later; I may even have seen the term used that way before and forgotten it. Some members of the public may not think of gamma rays or X-rays as light, but hey all they have to do is read the comments and see it explained. Thanks again.

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  3. Reblogged this on thflg.wordpress.com and commented:
    “…one of the most momentous discoveries in the history of astronomy.”

  4. What CAN’T be seen from satellite photos?

  5. I am just an ignorant non-astrophysicist. I was just wondering: how fast do gravitational waves propagate? Same speed as light? Why or why not?
    Another thing I was wondering is: how do you filter out local temperature fluctuations or perhaps geological events that I suppose could affect the observatories mechanically? Is that by their lack of correlation between the two observatories?

    • Hi Thomas!
      These are excellent questions!
      (1) General relativity predicts they move at the speed of light, and something that can be tested in gravitational wave data.
      (2) Having two observatories helps us not be fooled by exactly this sort of thing — gravitational waves are going to influence them both in the same way, only separated by a maximum amount of time (about 10milliseconds )
      — shane

  6. I like your article, very inspiring, and thank you for yout post

  7. The audio is worth listening to. Speeded up a little to make use of our hearing frequency range, it’s originally in the range of 30 Hz to 250 Hz.

    Audio ought to be used more often. Although as a species we process information best in visual form, our hearing is also a great way to understand wave-type data. Speeded up seismometer signals let us “hear” the earth ringing in an earthquake, or even the wobbles and oscillations of a distant star.

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  10. I had the opportunity of seeing the brilliant play Silent Sky (playwright Lauren Gunderson) at Taproot Theatre here in Seattle a few nights ago.
    I learned of Henrietta Leavitt’s discoveries and that because of them astronomers could then map the universe. That inspired me to Google and I found your site. In your own words, “What Leavitt discovered was if you measure how long it takes a Cepheid to change its brightness, then you know its absolute brightness. Comparing that to what you see in the telescope then let’s you calculate the distance to the star! This discovery was a watershed, arguably the most important discovery in modern astronomy: Leavitt showed us how to use telescopes and clocks to lay a ruler down on the Universe. Leavitt died of cancer at the age of 53, in 1921.”
    “If I have seen further than others, it is by standing upon the shoulders of giants.” – Isaac Newton
    Thanks for communicating scientific ideas so well.

  11. Question: About how long ago did this cosmic event actually take place? At what speed did the wave travel?

    • The black holes are 1.3 billion lightyears from Earth, so about 1.3 billion years ago. It gives new meaning to “a long time ago, in a galaxy far, far away!”🙂

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  13. Reblogged this on ashokbhatia and commented:
    A historic discovery announced recently confirms what Albert Einstein predicted almost a century back – the existence of gravitational waves.

    Here is an interesting post that you might enjoy!

  14. I feel fortunate to have found a blog this close to the work and so well written. Please continue to blog as often as your time and inclination allows.

    It is wonderful to see ‘behind the curtain’, and your writing and work is interesting, inspiring and exciting (at least from the armchair perspective). Your notebook was a beautiful sight to behold, a perfect blend of science and humour.

    I can’t wait to share your perspective on this discovery with my daughters (12 and 8) who have a keen interest in science and have been following this topic and LIGO since before the announcement (we like science:).

    They always want to know more about how we know things, and what did scientists do to figure it out. It is great for them to hear directly from the people involved.

    As to why this discovery is important, and what it means for science, I think I will close with my daughters answer when I posed her that exact question at dinner recently.

    “It’s the first time we sort of saw gravity, so that’s cool, but I don’t think we know yet why it’s important. I mean, we might be able to imagine why it’s important, but imagine all of the reasons that we can’t imagine. That’s probably why it’s important”. – TAPPpup

    Ahh science. You can still bring a tear to my eye.

    –TAPP

  15. Pingback: Congratulations Einstein – The Harmonies of Spacetime — GW150914 | The Amateur Philosophysicist

  16. Thank you for this blog post, it really has cleared up a lot about this discovery for me. I knew it was a pretty big deal, but wow, even better than I thought. Excellent job communicating the science and the passion behind your work!

  17. Pingback: The Harmonies of Spacetime — GW150914 — Write Science | triple206

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