LIGO Scientists who Detected Gravitational Waves Awarded Nobel Prize in Physics

In February of 2016, scientists working for the Laser Interferometer Gravitational-Wave Observatory (LIGO) made history when they announced the first-ever detection of gravitational waves. Since that time, multiple detections have taken place and scientific collaborations between observatories  – like Advanced LIGO and Advanced Virgo – are allowing for unprecedented levels of sensitivity and data sharing.

Not only was the first-time detection of gravity waves an historic accomplishment, it ushered in a new era of astrophysics. It is little wonder then why the three researchers who were central to the first detection have been awarded the 2017 Nobel Prize in Physics. The prize was awarded jointly to Caltech professors emeritus Kip S. Thorne and Barry C. Barish, along with MIT professor emeritus Rainer Weiss.

To put it simply, gravitational waves are ripples in space-time that are formed by major astronomical events – such as the merger of a binary black hole pair. They were first predicted over a century ago by Einstein’s Theory of General Relativity, which indicated that massive perturbations would alter the structure of space-time. However, it was not until recent years that evidence of these waves was observed for the first time.

The first signal was detected by LIGO’s twin observatories – in Hanford, Washington, and Livingston, Louisiana, respectively – and traced to a black mole merger 1.3 billion light-years away. To date, four detections have been, all of which were due to the mergers of black-hole pairs. These took place on December 26, 2015, January 4, 2017, and August 14, 2017, the last being detected by LIGO and the European Virgo gravitational-wave detector.

For the role they played in this accomplishment, one half of the prize was awarded jointly to Caltech’s Barry C. Barish – the Ronald and Maxine Linde Professor of Physics, Emeritus – and Kip S. Thorne, the Richard P. Feynman Professor of Theoretical Physics, Emeritus. The other half was awarded to Rainer Weiss, Professor of Physics, Emeritus, at the Massachusetts Institute of Technology (MIT).

As Caltech president Thomas F. Rosenbaum – the Sonja and William Davidow Presidential Chair and Professor of Physics – said in a recent Caltech press statement:

“I am delighted and honored to congratulate Kip and Barry, as well as Rai Weiss of MIT, on the award this morning of the 2017 Nobel Prize in Physics. The first direct observation of gravitational waves by LIGO is an extraordinary demonstration of scientific vision and persistence. Through four decades of development of exquisitely sensitive instrumentation—pushing the capacity of our imaginations—we are now able to glimpse cosmic processes that were previously undetectable. It is truly the start of a new era in astrophysics.”

This accomplishment was all the more impressive considering that Albert Einstein, who first predicted their existence, believed gravitational waves would be too weak to study. However, by the 1960s, advances in laser technology and new insights into possible astrophysical sources led scientists to conclude that these waves might actually be detectable.

The first gravity wave detectors were built by Joseph Weber, an astrophysics from the University of Maryland. His detectors, which were built in the 1960s, consisted of large aluminum cylinders  that would be driven to vibrate by passing gravitational waves. Other attempts followed, but all proved unsuccessful; prompting a shift towards a new type of detector involving interferometry.

One such instrument was developed by Weiss at MIT, which relied on the technique known as laser interferometry. In this kind of instrument, gravitational waves are measured using widely spaced and separated mirrors that reflect lasers over long distances. When gravitational waves cause space to stretch and squeeze by infinitesimal amounts, it causes the reflected light inside the detector to shift minutely.

At the same time, Thorne – along with his students and postdocs at Caltech – began working to improve the theory of gravitational waves. This included new estimates on the strength and frequency of waves produced by objects like black holes, neutron stars and supernovae. This culminated in a 1972 paper which Throne co-published with his student, Bill Press, which summarized their vision of how gravitational waves could be studied.

That same year, Weiss also published a detailed analysis of interferometers and their potential for astrophysical research. In this paper, he stated that larger-scale operations – measuring several km or more in size – might have a shot at detecting gravitational waves. He also identified the major challenges to detection (such as vibrations from the Earth) and proposed possible solutions for countering them.

Barry C. Barish and Kip S. Thorne, two of three recipients of the 2017 Nobel Prize in Physics. Credit: Caltech

In 1975, Weiss invited Thorne to speak at a NASA committee meeting in Washington, D.C., and the two spent an entire night talking about gravitational experiments. As a result of their conversation, Thorne went back to Calteh and proposed creating a experimental gravity group, which would work on interferometers in parallel with researchers at MIT, the University of Glasgow and the University of Garching (where similar experiments were being conducted).

Development on the first interferometer began shortly thereafter at Caltech, which led to the creation of a 40-meter (130-foot) prototype to test Weiss’ theories about gravitational waves. In 1984, all of the work being conducted by these respective institutions came together. Caltech and MIT, with the support of the National Science Foundation (NSF) formed the LIGO collaboration and began work on its two interferometers in Hanford and Livingston.

The construction of LIGO was a major challenge, both logistically and technically. However, things were helped immensely when Barry Barish (then a Caltech particle physicist) became the Principal Investigator (PI) of LIGO in 1994. After a decade of stalled attempts, he was also made the director of LIGO and put its construction back on track. He also expanded the research team and developed a detailed work plan for the NSF.

As Barish indicated, the work he did with LIGO was something of a dream come true:

“I always wanted to be an experimental physicist and was attracted to the idea of using continuing advances in technology to carry out fundamental science experiments that could not be done otherwise. LIGO is a prime example of what couldn’t be done before. Although it was a very large-scale project, the challenges were very different from the way we build a bridge or carry out other large engineering projects. For LIGO, the challenge was and is how to develop and design advanced instrumentation on a large scale, even as the project evolves.”

LIGO’s two facilities, located in Livingston, Louisiana, and Hanford, Washington. Credit: ligo.caltech.edu

By 1999, construction had wrapped up on the LIGO observatories and by 2002, LIGO began to obtain data. In 2008, work began on improving its original detectors, known as the Advanced LIGO Project. The process of converting the 40-m prototype to LIGO’s current 4-km (2.5 mi) interferometers was a massive undertaking, and therefore needed to be broken down into steps.

The first step took place between 2002 and 2010, when the team built and tested the initial interferometers. While this did not result in any detections, it did demonstrate the observatory’s basic concepts and solved many of the technical obstacles. The next phase – called Advanced LIGO, which took placed between 2010 and 2015 – allowed the detectors to achieve new levels of sensitivity.

These upgrades, which also happened under Barish’s leadership, allowed for the development of several key technologies which ultimately made the first detection possible. As Barish explained:

“In the initial phase of LIGO, in order to isolate the detectors from the earth’s motion, we used a suspension system that consisted of test-mass mirrors hung by piano wire and used a multiple-stage set of passive shock absorbers, similar to those in your car. We knew this probably would not be good enough to detect gravitational waves, so we, in the LIGO Laboratory, developed an ambitious program for Advanced LIGO that incorporated a new suspension system to stabilize the mirrors and an active seismic isolation system to sense and correct for ground motions.”

Rainer Weiss, famed MIT physicist and partial winner of the 2017 Nobel Prize in Physics. Credit: MIT/Bryce Vickmark

Given how central Thorne, Weiss and Barish were to the study of gravitational waves, all three were rightly-recognized as this year’s recipients of the Nobel Prize in Physics. Both Thorne and Barish were notified that they had won in the early morning hours on October 3rd, 2017. In response to the news, both scientists were sure to acknowledge the ongoing efforts of LIGO, the science teams that have contributed to it, and the efforts of Caltech and MIT in creating and maintaining the observatories.

“The prize rightfully belongs to the hundreds of LIGO scientists and engineers who built and perfected our complex gravitational-wave interferometers, and the hundreds of LIGO and Virgo scientists who found the gravitational-wave signals in LIGO’s noisy data and extracted the waves’ information,” said Thorne. “It is unfortunate that, due to the statutes of the Nobel Foundation, the prize has to go to no more than three people, when our marvelous discovery is the work of more than a thousand.”

“I am humbled and honored to receive this award,” said Barish. “The detection of gravitational waves is truly a triumph of modern large-scale experimental physics. Over several decades, our teams at Caltech and MIT developed LIGO into the incredibly sensitive device that made the discovery. When the signal reached LIGO from a collision of two stellar black holes that occurred 1.3 billion years ago, the 1,000-scientist-strong LIGO Scientific Collaboration was able to both identify the candidate event within minutes and perform the detailed analysis that convincingly demonstrated that gravitational waves exist.”

Looking ahead, it is also pretty clear that Advanved LIGO, Advanced Virgo and other gravitational wave observatories around the world are just getting started. In addition to having detected four separate events, recent studies have indicated that gravitational wave detection could also open up new frontiers for astronomical and cosmological research.

For instance, a recent study by a team of researchers from the Monash Center for Astrophysics proposed a theoretical concept known as ‘orphan memory’. According to their research, gravitational waves not only cause waves in space-time, but leave permanent ripples in its structure. By studying the “orphans” of past events, gravitational waves can be studied both as they reach Earth and long after they pass.

In addition, a study was released in August by a team of astronomers from the Center of Cosmology at the University of California Irvine that indicated that black hole mergers are far more common than we thought. After conducting a survey of the cosmos intended to calculate and categorize black holes, the UCI team determined that there could be as many as 100 million black holes in the galaxy.

Another recent study indicated that the Advanced LIGO, GEO 600, and Virgo gravitational-wave detector network could also be used to detect the gravitational waves created by supernovae. By detecting the waves created by star that explode near the end of their lifespans, astronomers could be able to see inside the hearts of collapsing stars for the first time and probe the mechanics of black hole formation.

The Nobel Prize in Physics is one of the highest honors that can be bestowed upon a scientist. But even greater than that is the knowledge that great things resulted from one’s own work. Decades after Thorne, Weiss and Barish began proposing gravitational wave studies and working towards the creation of detectors, scientists from all over the world are making profound discoveries that are revolutionizing the way we think of the Universe.

And as these scientists will surely attest, what we’ve seen so far is just the tip of the iceberg. One can imagine that somewhere, Einstein is also beaming with pride. As with other research pertaining to his theory of General Relativity, the study of gravitational waves is demonstrating that even after a century, his predictions were still bang on!

And be sure to check out this video of the Caltech Press Conference where Barish and Thorn were honored for their accomplishments:

Further Reading: NASA, Caltech

Sources of Gravitational Waves: The Most Violent Events in the Universe

One of the most promising gravitational wave sources: Bodies orbiting each other under their own gravity

Soon, very soon, Thursday, February 11, at 10:30 Eastern time, we are likely to learn at any one of several press conferences – at the National Press Club in Washington, D.C., in Hannover, Germany, near Pisa in Italy and elswhere – that gravitational waves have been measured directly, for the first time. This would mean the first direct detection of minute distortions of spacetime, travelling at the speed of light, first postulated by Albert Einstein almost exactly 100 years ago.

Time to brush up on your gravitational wave basics: In Gravitational waves and how they distort space, we had a look at what gravitational waves do. In Gravitational wave detectors: How they work we saw how you can measure gravitational waves. Third and final step: What are typical gravitational wave sources? How are these waves produced?

Objects in orbit

The simplest situation that produces gravitational waves in the cosmos is almost ubiquitous: two or more objects orbiting around each other under their own gravity. The waves they generate are reminiscent to a very slow mixer in the middle of a pool of water: One of the most promising gravitational wave sources: Objects in orbit around each other. By Sascha Husa, Universitat de les Illes Balears This is not something you would see, of course. The wave that is pictured here represents the strength of the minute changes in distance that would be caused by the gravitational wave, just as we’ve seen in Gravitational waves and how they distort space. The animation is courtesy of Sascha Husa of the Universitat de les Illes Balears.

Indirect evidence

Gravitational waves emitted by orbiting objects carry away energy. Elementary physics tells you that if you remove energy from an orbiting system, the distance between the orbiting objects will shrink, and they will orbit each other faster than before.

In fact, gravitational waves making a binary system of neutron stars speed up was the first evidence for the existence of gravitational waves. The binary neutron star was discovered by Hulse and Taylor in 1974, and the speed-up caused by gravitational waves published by Taylor and Weisberg in 1984, after a careful analysis of seven years’ worth of data. Hulse and Taylor were awarded the Nobel prize in physics in 1993 for their discovery.

Here, in an image from an article by Weisberg 2010, is the match between general relativistic prediction and observation in all its glory (or at least in all its glory up to 2005): weisberg2010As the two neutron stars speed up, they will reach the point of closest approach within their orbit earlier and earlier. How much earlier, in seconds, is plotted on the vertical axis, year of measurement on the horizontal axis.

A matter of frequency

Today’s ground-based detectors cannot detect gravitational waves from all kinds of bodies in mutual orbit. The bodies need to be massive, compact and, crucially, orbit each other quickly enough. For bodies orbiting each other less than a few times per second (very quick, if you are talking about astronomical bodies!), the frequency of the resulting gravitational wave will be too low for ground-based detectors to measure reliably. In the low-frequency regime, below 10–100 Hertz, disturbances caused by undulating motions of the Earth’s surface (“seismic noise”) are dominant, and drown out the minute effects of gravitational waves.

When it comes to gravitational waves from supermassive black holes, or from white dwarfs, we will have to wait for future space-based gravitational wave detectors.

The most promising gravitational wave sources go “chirp”

When an orbiting system emits gravitational waves, orbital motion speeds up. And when orbital motion speeds up, the system emits even more energy in form of gravitational wave. This runaway process ends only when the orbiting objects collide and merge.

The final phase is marked by a quick increase in orbital speed, corresponding to ever higher gravitational wave frequency, and ever higher intensity. Here’s what such a signal looks like (image and audio from “Chirping Neutron Stars” on Einstein Online): chirp-enYou can see how the frequency and intensity increase right up to time 0, when the two neutron stars collide and merge.

For stellar black holes (with masses between a few and a few dozen solar masses) and neutron stars, in any combination, the frequencies of these gravitational waves are the same as the frequencies of audible sound waves. One can actually represent these changes in frequency as an audible tone, as in this example of two neutron stars merging (Audio © B. Owen, Penn State University):

Here is the same kind of audible representation for the merger of a black hole and a neutron star (© AEI/GEO600):

Sadly, what a gravitational wave detector registers is the combination of this sound plus assorted noise, which sounds like this (© AEI/GEO600):

Colleagues at Cardiff University have made this into a nice online game: Black Hole Hunter. Head over there and see if you can hear the signal beneath the noise!

(And you can hear live chirps by various astrophysicists (and others) under the hashtag #chirpForLIGO on Twitter.)

This kind of signal, from merging stellar black holes or neutron stars (in any combination) is the most promising candidate signal for today’s detectors – and going by the rumors, that is indeed what LIGO appears to have found.

The final part of the signal is interesting for a particular reason: It doesn’t follow from any simple formulae, and can only be modelled with complex computer simulations of such situations known as numerical relativity. If the detectors get a good detection of this very last bit, that will be a good test for current numerical simulations of general relativity!

Other gravitational wave sources

Chirps are comparatively simple, and likely the first signals to be found.

Another kind of signal that could be found is periodic (or nearly so), and would be produced e.g. if rapidly rotating neutron stars are less than perfectly smooth. No such luck as of yet, though.

Next would come the gravitational wave sources that are somewhat less understood, such as the processes in the interior of supernova explosions. And finally, once numerous signals have been detected, showing the scientists that their detectors are indeed working as they should, there might be the detection of completely unexpected signals. Whenever astronomers have opened a new window to the cosmos – the radio window, infrared window, x-ray window – they have found something new and unexpected. Who can tell what opening the Einstein window, the window of gravitational waves, will teach us about the universe?

Update: Gravitational Waves Discovered