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Saturday, May 7, 2016

Reading List: Black Hole Blues

Levin, Janna. Black Hole Blues. New York: Alfred A. Knopf, 2016. ISBN 978-0-307-95819-8.
In Albert Einstein's 1915 general theory of relativity, gravitation does not propagate instantaneously as it did in Newton's theory, but at the speed of light. According to relativity, nothing can propagate faster than light. This has a consequence which was not originally appreciated when the theory was published: if you move an object here, its gravitational influence upon an object there cannot arrive any faster than a pulse of light travelling between the two objects. But how is that change in the gravitational field transmitted? For light, it is via the electromagnetic field, which is described by Maxwell's equations and implies the existence of excitations of the field which, according to their wavelength, we call radio, light, and gamma rays. Are there, then, equivalent excitations of the gravitational field (which, according to general relativity, can be thought of as curvature of spacetime), which transmit the changes due to motion of objects to distant objects affected by their gravity and, if so, can we detect them? By analogy to electromagnetism, where we speak of electromagnetic waves or electromagnetic radiation, these would be gravitational waves or gravitational radiation.

Einstein first predicted the existence of gravitational waves in a 1916 paper, but he made a mathematical error in the nature of sources and the magnitude of the effect. This was corrected in a paper he published in 1918 which describes gravitational radiation as we understand it today. According to Einstein's calculations, gravitational waves were real, but interacted so weakly that any practical experiment would never be able to detect them. If gravitation is thought of as the bending of spacetime, the equations tell us that spacetime is extraordinarily stiff: when you encounter an equation with the speed of light, c, raised to the fourth power in the denominator, you know you're in trouble trying to detect the effect.

That's where the matter rested for almost forty years. Some theorists believed that gravitational waves existed but, given the potential sources we knew about (planets orbiting stars, double and multiple star systems), the energy emitted was so small (the Earth orbiting the Sun emits a grand total of 200 watts of energy in gravitational waves, which is absolutely impossible to detect with any plausible apparatus), we would never be able to detect it. Other physicists doubted the effect was real, and that gravitational waves actually carried energy which could, even in principle, produce effects which could be detected. This dispute was settled to the satisfaction of most theorists by the sticky bead argument, proposed in 1957 by Richard Feynman and Hermann Bondi. Although a few dissenters remained, most of the small community interested in general relativity agreed that gravitational waves existed and could carry energy, but continued to believe we'd probably never detect them.

This outlook changed in the 1960s. Radio astronomers, along with optical astronomers, began to discover objects in the sky which seemed to indicate the universe was a much more violent and dynamic place than had been previously imagined. Words like “quasar”, “neutron star”, “pulsar”, and “black hole” entered the vocabulary, and suggested there were objects in the universe where gravity might be so strong and motion so fast that gravitational waves could be produced which might be detected by instruments on Earth.

Joseph Weber, an experimental physicist at the University of Maryland, was the first to attempt to detect gravitational radiation. He used large bars, now called Weber bars, of aluminium, usually cylinders two metres long and one metre in diameter, instrumented with piezoelectric sensors. The bars were, based upon their material and dimensions, resonant at a particular frequency, and could detect a change in length of the cylinder of around 10−16 metres. Weber was a pioneer in reducing noise of his detectors, and operated two detectors at different locations so that signals would only be considered valid if observed nearly simultaneously by both.

What nobody knew was how “noisy” the sky was in gravitational radiation: how many sources there were and how strong they might be. Theorists could offer little guidance: ultimately, you just had to listen. Weber listened, and reported signals he believed consistent with gravitational waves. But others who built comparable apparatus found nothing but noise and theorists objected that if objects in the universe emitted as much gravitational radiation as Weber's detections implied, it would convert all of its mass into gravitational radiation in just fifty million years. Weber's claims of having detected gravitational radiation are now considered to have been discredited, but there are those who dispute this assessment. Still, he was the first to try, and made breakthroughs which informed subsequent work.

Might there be a better way, which could detect even smaller signals than Weber's bars, and over a wider frequency range? (Since the frequency range of potential sources was unknown, casting the net as widely as possible made more potential candidate sources accessible to the experiment.) Independently, groups at MIT, the University of Glasgow in Scotland, and the Max Planck Institute in Germany began to investigate interferometers as a means of detecting gravitational waves. An interferometer had already played a part in confirming Einstein's special theory of relativity: could it also provide evidence for an elusive prediction of the general theory?

An interferometer is essentially an absurdly precise ruler where the markings on the scale are waves of light. You send beams of light down two paths, and adjust them so that the light waves cancel (interfere) when they're combined after bouncing back from mirrors at the end of the two paths. If there's any change in the lengths of the two paths, the light won't interfere precisely, and its intensity will increase depending upon the difference. But when a gravitational wave passes, that's precisely what happens! Lengths in one direction will be squeezed while those orthogonal (at a right angle) will be stretched. In principle, an interferometer can be an exquisitely sensitive detector of gravitational waves. The gap between principle and practice required decades of diligent toil and hundreds of millions of dollars to bridge.

From the beginning, it was clear it would not be easy. The field of general relativity (gravitation) had been called “a theorist's dream, an experimenter's nightmare”, and almost everybody working in the area were theorists: all they needed were blackboards, paper, pencils, and lots of erasers. This was “little science”. As the pioneers began to explore interferometric gravitational wave detectors, it became clear what was needed was “big science”: on the order of large particle accelerators or space missions, with budgets, schedules, staffing, and management comparable to such projects. This was a culture shock to the general relativity community as violent as the astrophysical sources they sought to detect. Between 1971 and 1989, theorists and experimentalists explored detector technologies and built prototypes to demonstrate feasibility. In 1989, a proposal was submitted to the National Science Foundation to build two interferometers, widely separated geographically, with an initial implementation to prove the concept and a subsequent upgrade intended to permit detection of gravitational radiation from anticipated sources. After political battles, in 1995 construction of LIGO, the Laser Interferometer Gravitational-Wave Observatory, began at the two sites located in Livingston, Louisiana and Hanford, Washington, and in 2001, commissioning of the initial detectors was begun; this would take four years. Between 2005 and 2007 science runs were made with the initial detectors; much was learned about sources of noise and the behaviour of the instrument, but no gravitational waves were detected.

Starting in 2007, based upon what had been learned so far, construction of the advanced interferometer began. This took three years. Between 2010 and 2012, the advanced components were installed, and another three years were spent commissioning them: discovering their quirks, fixing problems, and increasing sensitivity. Finally, in 2015, observations with the advanced detectors began. The sensitivity which had been achieved was astonishing: the interferometers could detect a change in the length of their four kilometre arms which was one ten-thousandth the diameter of a proton (the nucleus of a hydrogen atom). In order to accomplish this, they had to overcome noise which ranged from distant earthquakes, traffic on nearby highways, tides raised in the Earth by the Sun and Moon, and a multitude of other sources, via a tower of technology which made the machine, so simple in concept, forbiddingly complex.

September 14, 2015, 09:51 UTC: Chirp!

A hundred years after the theory that predicted it, 44 years after physicists imagined such an instrument, 26 years after it was formally proposed, 20 years after it was initially funded, a gravitational wave had been detected, and it was right out of the textbook: the merger of two black holes with masses around 29 and 36 times that of the Sun, at a distance of 1.3 billion light years. A total of three solar masses were converted into gravitational radiation: at the moment of the merger, the gravitational radiation emitted was 50 times greater than the light from all of the stars in the universe combined. Despite the stupendous energy released by the source, when it arrived at Earth it could only have been detected by the advanced interferometer which had just been put into service: it would have been missed by the initial instrument and was orders of magnitude below the noise floor of Weber's bar detectors.

For only the third time since proto-humans turned their eyes to the sky a new channel of information about the universe we inhabit was opened. Most of what we know comes from electromagnetic radiation: light, radio, microwaves, gamma rays, etc. In the 20th century, a second channel opened: particles. Cosmic rays and neutrinos allow exploring energetic processes we cannot observe in any other way. In a real sense, neutrinos let us look inside the Sun and into the heart of supernovæ and see what's happening there. And just last year the third channel opened: gravitational radiation. The universe is almost entirely transparent to gravitational waves: that's why they're so difficult to detect. But that means they allow us to explore the universe at its most violent: collisions and mergers of neutron stars and black holes—objects where gravity dominates the forces of the placid universe we observe through telescopes. What will we see? What will we learn? Who knows? If experience is any guide, we'll see things we never imagined and learn things even the theorists didn't anticipate. The game is afoot! It will be a fine adventure.

Black Hole Blues is the story of gravitational wave detection, largely focusing upon LIGO and told through the eyes of Rainer Weiss and Kip Thorne, two of the principals in its conception and development. It is an account of the transition of a field of research from a theorist's toy to Big Science, and the cultural, management, and political problems that involves. There are few examples in experimental science where so long an interval has elapsed, and so much funding expended, between the start of a project and its detecting the phenomenon it was built to observe. The road was bumpy, and that is documented here.

I found the author's tone off-putting. She, a theoretical cosmologist at Barnard College, dismisses scientists with achievements which dwarf her own and ideas which differ from hers in the way one expects from Social Justice Warriors in the squishier disciplines at the Seven Sisters: “the notorious Edward Teller”, “Although Kip [Thorne] outgrew the tedious moralizing, the sexism, and the religiosity of his Mormon roots”, (about Joseph Weber) “an insane, doomed, impossible bar detector designed by the old mad guy, crude laboratory-scale slabs of metal that inspired and encouraged his anguished claims of discovery”, “[Stephen] Hawking made his oddest wager about killer aliens or robots or something, which will not likely ever be resolved, so that might turn out to be his best bet yet”, (about Richard Garwin) “He played a role in halting the Star Wars insanity as well as potentially disastrous industrial escalations, like the plans for supersonic airplanes…”, and “[John Archibald] Wheeler also was not entirely against the House Un-American Activities Committee. He was not entirely against the anticommunist fervor that purged academics from their ivory-tower ranks for crimes of silence, either.” … “I remember seeing him at the notorious Princeton lunches, where visitors are expected to present their research to the table. Wheeler was royalty, in his eighties by then, straining to hear with the help of an ear trumpet. (Did I imagine the ear trumpet?)”. There are also a number of factual errors (for example, a breach in the LIGO beam tube sucking out all of the air from its enclosure and suffocating anybody inside), which a moment's calculation would have shown was absurd.

The book was clearly written with the intention of being published before the first detection of a gravitational wave by LIGO. The entire story of the detection, its validation, and public announcement is jammed into a seven page epilogue tacked onto the end. This epochal discovery deserves being treated at much greater length.

Posted at May 7, 2016 14:55