Chasing Gravitational Waves

Chasing Gravitational Waves

We’ll watch them all starting with this one. Data is a hero in this one!

Chasing Gravitational Waves

Watch out! It’s the gravitational waves amplified by the Enterprise shields!

Chasing Gravitational Waves

Data saves the day. And the Enterprise! 🙂

Captain, drop the shields.
Captain, drop the shields.

To find out more about gravitational waves, Puffles and Jay went to a presentation by David Blair, Winthrop Professor of Physics at the University of Western Australia.

Chasing Gravitational Waves

About 1.3 billion years ago two black holes swirled closer and closer together until they crashed in a furious bang. Each black hole packed roughly 30 times the mass of our sun into a minute volume, and their head-on impact came as the two were approaching the speed of light. The staggering strength of the merger gave rise to a new black hole and created a gravitational field so strong that it distorted spacetime in waves that spread throughout space with a power about 50 times stronger than that of all the shining stars and galaxies in the observable universe. Such events are, incredibly, thought to be common in space, but this collision was the first of its kind ever detected and its waves the first ever ‘seen’. Scientists with the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced on February 11, 2016 at a much-anticipated press conference in Washington, D.C. (one of at least five simultaneous events held in the U.S. and Europe) that the more than half-century search for gravitational waves had finally succeeded.

There are people who’ve put their entire life into this search, and there are people who died before having a chance to see anything. One of the people who has put his entire life into this search is Prof Blair, who has been working towards the discovery of gravitational waves since 1972, along the way adjusting his expectations about when the waves will actually be discovered.

In March 1972, as an enthusiastic young postdoc, Prof Blair arrived in Livingston, Louisiana to help build one the first high sensitivity gravitational wave detectors. The basement corridor — where the huge detector lab was located — was adorned with signs: Gravitational wave detection for Christmas! ‘Wow,’ he thought. ‘Nine months to detection!’

As Christmas approached, he started to worry: how would they ever possibly finish in time? He approached his boss, Bill Hamilton. ‘Oh, don’t worry about that! Those signs were put up for last Christmas,’ he said. Later, Bill explained the law of time for physics experiments: to determine the date of completion, take the experimenter’s estimate, multiply by pi, and change to the next highest unit of time. That law of time could apply to so many other projects!

In the case of gravitational waves, one-and-three-quarter years became 50 years. As estimates go, it was quite a good one!

Since Einstein first described them a century ago, gravitational waves have been the subject of more sustained controversy than perhaps any other phenomenon in physics. Albert Einstein first predicted gravitational waves in 1916 based on his general theory of relativity, but even he waffled about whether or not they truly existed.

“There are no gravitational waves … ” … “Plane gravitational waves, traveling along the positive X-axis, can therefore be found … ” … “ … gravitational waves do not exist … ” … “Do gravitational waves exist?” … “It turns out that rigorous solutions exist … ”

This is the waffle of Albert Einstein. For 20 years he equivocated about gravitational waves, unsure whether these undulations in the fabric of space and time were predicted or ruled out by his revolutionary 1915 theory of general relativity. For all the theory’s conceptual elegance — it revealed gravity to be the effect of curves in “space-time” — its mathematics was enormously complex.

In 1915, the year that Einstein first introduced general relativity, Karl Schwarzschild, a German physicist and astronomer, provided the first exact solution to the Einstein field equations of general relativity, for the limited case of a single spherical non-rotating mass. Einstein himself had produced only an approximate solution, so he was pleasantly surprised to learn that the field equations admitted exact solutions. Still, in June of 1916, Einstein stated that gravitational waves were of academic interest only, while Arthur Eddington said that gravitational waves travelled at the speed of thought. Scientists talked about the idea of black holes and gravitational waves, but nobody believed it.

Chasing Gravitational Waves

Progress was only made in the 1960s when the more exact tools of differential geometry entered the field of general relativity. Scientists began seeking these ripples in spacetime but none succeeded in measuring their effects on Earth until September 2015. LIGO’s discovery not only provides the first direct evidence for gravitational waves but also opens the door to using them to study the powerful cosmic events that create them.

More than 1,000 scientists work on the $1-billion LIGO experiment, which is funded by the National Science Foundation. The project uses two detectors, one located in Hanford, Washington and the other in Livingston, Louisiana, to sense the distortions in space that occur when a gravitational wave passes through Earth. Each detector is shaped like a giant L, with legs four kilometres long. Laser light bounces back and forth through the legs, reflecting off mirrors, and amazingly precise atomic clocks measure how long it takes to make the journey. Normally, the two legs are exactly the same length, and so the light takes exactly the same amount of time to traverse each. If a gravitational wave passes through, however, the detector and the ground beneath it will expand and contract infinitesimally in one direction, and the two perpendicular legs will no longer be the same size. One of the lasers will arrive a fraction of a second later than the other.

LIGO must be unbelievably sensitive to measure this change in the length of the legs, which is smaller than one ten-thousandth the diameter of a proton, or less than the size of a soccer ball compared with the span of the Milky Way. The experiment is so delicate that unrelated events such as an airplane flying overhead, wind buffeting the building or tiny seismic shifts in the ground beneath the detector can disturb the lasers in ways that mimic gravitational signals. The researchers have to carefully screen out contaminating signals and also take advantage of the fact that the detectors in Washington and Louisiana are highly unlikely to be affected by the same contamination at the same time. By comparing the two detectors, they can be even more certain that what they are seeing is something that’s coming from outside the Earth.

LIGO began its first run in 2002, and hunted through 2010 without finding any gravitational waves. The scientists then shut down the experiment and upgraded nearly every aspect of the detectors, including boosting the power of the lasers and replacing the mirrors, for a subsequent run, called Advanced LIGO, that began officially on September 18, 2015. Yet even before then the experiment was up and running: the signal arrived on September 14 at 5:51am US Eastern time, reaching the detector in Louisiana seven milliseconds before it got to the detector in Washington. The small teams operating the detectors at the time included Eleanor King from the University of Adelaide and Carl Blair (Prof Blair’s son) from the University of Western Australia.

The signal at first seemed too good to be true and for many months mundane explanations were sought. Could it be hacking? Could it be lightning? Could it be a computer glitch or accidental vibrations? Eventually, all the mundane explanations were ruled out, and the chirping from binary black holes was confirmed.

Gravitational waves from binary black holes are predominantly emitted at twice the orbital frequency and carry away the binary’s energy and angular momentum. Since the system loses its rotational energy, the two black holes gradually inspiral towards each other. Black holes that are closer together emit more radiation, thereby accelerating the inspiral. This produces a characteristic chirp waveform whose amplitude and frequency both increase with time until eventually the two bodies merge together. Prior to merger, the two black holes approach each other at speeds very close to that of light; their collision will be astounding.

The merger will result in a highly deformed single black hole which settles into a perfectly spherical shape by ridding itself of its deformity through the emission of gravitational radiation that is characteristic of the mass and spin of the final black hole. This is called the quasi-normal mode or the ringdown signal.

Gravitational Waves, As Einstein Predicted Image Credit: Caltech/MIT/LIGO Lab
Gravitational Waves, As Einstein Predicted
Image Credit: Caltech/MIT/LIGO Lab

A black-hole merger is the most energetic event known; the power of the gravitational waves that it emits can briefly rival that of all the stars in the observable Universe combined. Black-hole mergers are also among the cleanest gravitational-wave signals to interpret. The signal size tells the distance of the source, the frequencies tell the masses of the two black holes and the final ringing tells the spin and mass of the final black hole.

It was Professor Hyung Mok Lee, from Seoul National University, the second speaker for the night, who predicted that the first gravitational waves detected will come from binary black holes and not from binary neutron stars, as everyone was expecting.

One of the most important scientific consequences of detecting a black-hole merger is confirmation that black holes really do exist — at least as the perfectly round objects made of pure, empty, warped space-time that are predicted by general relativity. Another is that mergers proceed as predicted. Astronomers already have plenty of circumstantial evidence for these phenomena, but so far that evidence has come from observations of the stars and super-heated gas that orbit black holes, not of black holes themselves.

Advanced LIGO is already about three times more sensitive than the initial LIGO, and is designed to become about 10 times more sensitive than the first iteration in the next few years. To build the gravitational wave detectors for advanced LIGO, scientists had to learn how to make quantum measurements on masses ranging in size from tonnes to micrograms; to make mirrors precise to atomic dimensions, and able to reflect light with unsurpassed perfection; how to suppress the natural vibrations of atoms caused by heat, earthquakes, cars or people; how to screen out vibrations one billion times bigger than the signals they were aiming to measure; how to program supercomputers to mimic the human ability of picking complex sounds out of background noise; and how to prevent the detectors from creating spurious noises from the enormous power of the laser light that drives them.

In 2005, the UWA team predicted that in Advanced LIGO, the gravitational wave detectors would suffer from parametric instabilities. That made them really popular! Parametric instabilities had never been observed in a kilometer-scale interferometer. Undeterred, the UWA embarked on a decade of theoretical calculation, numerical modeling, and laboratory-scale experimentation. It was a grand collaboration: at Gingin Observatory in WA, innovative vibration isolation systems gave the world’s best performance. Australian National University provided a length stabilisation system, and developed technology that uses quantum entanglement of the photons to reduce the noise in the laser light. At the University of Adelaide, sensors were created to allow errors in laser light beams to be corrected at the level of 1/20,000th of the wavelength. The team went on to develop methods for controlling the instabilities once they occurred.

In April 2015, LIGO reported the first observation of a self-sustaining parametric instability in a gravitational wave detector. The observation confirmed that the models which had been built to understand the phenomenon in the complex setting of a gravitational wave interferometer were substantially correct. The mitigation techniques do not eliminate the problem and are not trivial, but they allow Advanced LIGO to operate stably at full power. Since the detection of the parametric instabilities, the UWA team has been involved in stabilising the new detectors.

LIGO’s ability to study the characteristics of gravitational waves will allow scientists to study black holes in a whole new way. The observatory should also be able to see gravitational waves created by other cataclysmic events, such as exploding supernovae and collisions of two neutron stars.

LIGO and future gravitational wave experiments will also allow physicists to put general relativity to the test. The 100-year-old theory has stood the test of time but it still conflicts with the theory of quantum mechanics that rules over the subatomic realm.

Gravitational-Wave Observatories Across the Globe Image Credit: Caltech/MIT/LIGO Lab
Gravitational-Wave Observatories Across the Globe
Image Credit: Caltech/MIT/LIGO Lab

LIGO is the first of many observatories that will join this new era of gravitational astronomy. A similar project called Virgo will come online this year in Italy, and later this decade the Kamioka Gravitational Wave Detector (KAGRA) in Japan will begin observations. Ground-based telescope projects called pulsar timing arrays aim to study gravitational waves by noting delays in light from pulsars arriving on Earth after traveling through wave-stretched space. And a spacecraft called Lisa Pathfinder launched last December to test technology for a proposed space-based observatory that will be sensitive to longer-wavelength gravitational waves from supermassive black hole collisions.

As for practical applications, stay tuned. Kip Thorne, Caltech general-relativity guru and LIGO’s most prominent supporter, has explained many times over the years why a discovery like this would be exciting, and in 300 Years of Gravitation (1987) he said: “If cosmic gravitational waves can be detected and studied, they will create a revolution of our view of the universe comparable to or greater than that which resulted from the discovery of radio waves.”

It was 1865 when James Clerk Maxwell, an English scientist, published his first paper on the theory of the electromagnetism. The most dramatic prediction of the theory was the existence of electromagnetic waves moving at the speed of light, and the conclusion that light itself was just such a wave. This challenged experimentalists to generate and detect electromagnetic radiation using some form of electrical apparatus.

It took 21 years, and it was Heinrich Hertz, in 1886, who first conclusively proved the existence of electromagnetic waves theorized by James Clerk Maxwell’s electromagnetic theory of light. Hertz proved the theory by engineering instruments to transmit and receive radio pulses using experimental procedures that ruled out all other known wireless phenomena. The unit of frequency – cycle per second – was named the “hertz” in his honor.

However, when asked by a journalist what is the possible use of the waves he had detected, Hertz answered ‘of no possible use whatsoever. I am just trying to prove Maestro Maxwell was correct.’

Good thing Guglielmo Marconi came along and developed, demonstrated and marketed the first successful long-distance wireless telegraph by the end of the century and in 1901 broadcast the first transatlantic radio signal, a feat which earned him the Nobel Prize for physics in 1909.

The work of Maxwell and Hertz was foundational to the harnessing of radio waves for human use. The practical application of radio waves had its beginnings with Marconi.

Today we have pretty much harnessed the full electromagnetic spectrum:
Radio waves – broadcasting; communications; satellite transmissions
Microwaves – cooking; communications; satellite transmissions
Infrared radiation – cooking (toasters); thermal imaging (detecting tumours in medicine); short-range communications; optical fibres; television remote controls; security systems
Visible light – vision; photography; illumination
Ultraviolet – security marking; fluorescent lamps; detecting forged bank notes; disinfecting water, medical instruments and pretty much anything else
X-rays – observing the internal structure of objects; airport security scanners; medical X-rays
Gamma rays – sterilizing food and medical equipment; detection of cancer and its treatment

The LIGO scientists have proven Maestro Einstein right. That was no small feat. While the merging of the two black holes was the largest astronomical burst of energy every observed, it was also the smallest amount of energy every measured. A feat which has already been repeated. On December 26, 2015 LIGO measured its second set of spacetime ripples, in this case coming from colliding black holes 14 and 8 times the mass of the sun.

Advanced LIGO has made two confirmed gravitational wave detection and seen one candidate event during its initial run from September 2015 to January 2016. A second run is due to start later this year. Credit: LIGO
Advanced LIGO has made two confirmed gravitational wave detections and seen one candidate event during its initial run from September 2015 to January 2016. A second run is due to start later this year. Credit: LIGO

The opening of the radio sky led to the discovery of radio galaxies, quasars and astrophysical black holes. The detection of gravitational waves could be similarly transformative because it would mark the beginning of an era in which scientists can use gravitational waves just as they use electromagnetic radiation — as a means of observing the cosmos and even probe the Big Bang.

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