Advancing Basic Science for Humanity
2016 Astrophysics Prize Explanatory Notes
The signal picked up by the Laser Interferometer Gravitational-wave Observatory (LIGO) in the US on September 14 last year lasted just a fifth of a second but brought to an end a decades-long hunt to directly detect the ripples in space-time known as gravitational waves. It also opened up a completely new way of doing astronomy, which uses gravitational rather than electromagnetic radiation to study some of the most extreme and violent phenomena in the universe.
Gravitational waves were predicted by Albert Einstein in 1916. A year earlier, Einstein had formulated his general theory of relativity, which describes gravity as warping four-dimensional space-time. Using his theory, he found that accelerated masses would create distortions in space-time rather like the ripples created when a stone is thrown into a pond. These “gravitational waves”, radiating at the speed of light, would carry information about the objects that had produced them.
The LIGO Laboratory operates two detector sites, one near Hanford in eastern Washington, and another near Livingston, Louisiana. This photo shows the Livingston detector site. (Credit: Caltech/MIT/LIGO Lab)
Gravity is by far the weakest force of nature, and its effects are generally only visible when produced by extremely large masses. To look for gravitational waves, therefore, scientists turned to the heavens. The first evidence for such waves actually came in 1982, but it was indirect. Several years earlier, the American physicists Joseph Taylor and Russell Hulse had discovered a pulsar orbiting a neutron star. By carefully monitoring the pulsar’s radio emissions, Taylor and another colleague, Joel Weisberg, found that the object’s orbit was shrinking at just the rate that would be expected if it were radiating gravitational waves.
What scientists really wanted, however, was a direct detection - to observe the distorting effect of a gravitational wave emitted by a celestial object that has travelled across the universe and then passed through the Earth. Unfortunately, because the waves are expected to originate from very far away, their distortions will be extraordinarily small once they reach Earth - LIGO’s dimensions being changed by about a thousandth of the width of an atomic nucleus. The challenge is being able to detect such minute variations while screening out far larger sources of background noise, such as vibrations caused by earthquakes or the thermal jiggling of atoms.
In fact, until the 1950s, physicists were unsure whether gravitational waves were real physical entities, as opposed to being purely mathematical, and whether they carried energy. Efforts to detect them began with the American electrical engineer Joseph Weber, who in 1969 reported having observed that a pair of large, suspended aluminium cylinders he had set up for the purpose had been made to “ring” by a passing wave. However, other groups using their own “bar detectors” failed to reproduce the result and by the mid-1970s Weber’s claim had been largely discredited.
It was at that point that scientists started working with interferometers. The basic idea is to divide a laser beam in two using a device known as a beam splitter, and send the resulting beams down a pair of hollow tubes arranged at right angles to one another. Each beam bounces off a mirror at the end of its respective tube and then recombines with the other beam at the beam splitter. The apparatus is set up so that normally the peaks of one beam line up with the troughs of the other and the two beams cancel one another out. A light sensitive detector placed behind the beam splitter therefore registers no signal.
Gravitational Waves, As Einstein Predicted. These plots show the signals of gravitational waves detected by the twin LIGO observatories at Livingston, Louisiana, and Hanford, Washington. The signals came from two merging black holes, each about 30 times the mass of our sun, lying 1.3 billion light-years away. (Credit: Caltech/MIT/LIGO Lab)
A passing gravitational wave, however, changes the length of the arms. First it stretches one arm and simultaneously squeezes the other, before squeezing the stretched one and vice versa. With the peaks and troughs of the two beams no longer perfectly aligned, the detector registers a signal. More precisely, it registers a characteristic brightening and dimming as the gravitational wave propagates.
The first paper describing the principles of such detectors was published in 1962 by a pair of Soviet physicists, Mikhail Gertsenshtein and Vladislav Pustovoit, who argued that interferometers could be far more sensitive than bar detectors because they could be made much longer. Longer devices are better because a given fractional change in distance caused by a passing gravitational wave will translate into a larger absolute change.
It was not until almost a decade later, however, that physicist Rainer Weiss of the Massachusetts Institute of Technology calculated in detail how interferometer-based detectors would perform, given all of the various noise sources they would have to overcome. Weiss started thinking about interferometers after teaching a course on general relativity and finding he was unable to explain to his students how Weber’s bar worked. His interest was further piqued following the null results from other bar experiments, and he then spent an entire summer in a little room in a temporary building on the MIT campus working through his interferometer calculations. His results, which he published informally in 1972, would form the basis of LIGO.
LIGO has turned out to be the largest ever facility funded by the US National Science Foundation (NSF), costing many hundreds of millions of dollars and involving over 1000 scientists from across the globe. But its founders, Weiss and Kip Thorne, a theoretical physicist at the California Institute of Technology (Caltech), had no idea of such grandeur when they first met in a Washington, D.C. hotel room in the summer of 1975. The two were in town for a NASA meeting, and Thorne took the opportunity to discuss how Caltech might set up a new group in experimental gravitation. Weiss described his work on interferometers and Thorne was sold; the latter went away and established an interferometer group at Caltech that would later build LIGO together with Weiss and colleagues at MIT.
Thorne worked on the scientific aspects of the project, such as the computations needed to predict the gravitational-wave signals from different kinds of astrophysical object. But he also played a vital role in the project management. In particular, he brought to Caltech the third of the LIGO “troika” - Ron Drever.
LIGO Hanford Control Room. Desks full of computers, and walls covered with projection screens and large monitors keep LIGO’s interferometer operators busy as they monitor the instrument’s status 24 hours a day, 7 days a week. (Credit: Kim Fetrow)
Drever, a Scottish physicist who had been conducting his own research on interferometers at the University of Glasgow, was recruited by Thorne in 1979. Renowned as something of a technical genius, he devised many improvements to the basic design put forward by Weiss; these have enabled LIGO to become the ultra-sensitive device it is today. For example, he showed how it was possible to greatly increase the effective length of the interferometer arms by creating what is known as a Fabry-Pérot cavity, in which the laser beams bounce back and forth many times between mirrors at either ends of the arms before their recombination at the beam splitter.
LIGO, approved in 1990, actually consists of two interferometers, each having arms 4 kilometres long, located on opposite sides of the US - one in Washington state and the other in Louisiana. It initially operated between 2002 and 2010 but saw no gravitational waves during that time (as was largely expected). Over the next few years it was upgraded in order to boost its sensitivity - its noise being reduced, in part, thanks to more powerful lasers, better isolation of the mirrors, as well as mirrors that are both bigger and more highly reflective. It then started operating again last September.
The erroneous claims of Weber from 45 years ago have induced immense caution in gravitational-wave physicists ever since. So when LIGO researchers saw the now-famous signal just a few days after switching their machine back on, they carried out a series of painstaking checks to make sure it was real - even though they could see it with the naked eye, unaided by statistical analysis. They compared it to waveforms predicted by general relativity, analysed every conceivable source of noise, and even considered the possibility of an elaborate hoax. In the end, about three months later, they were convinced that they had seen a gravitational wave.
The discovery provides the first confirmation of general relativity in very strong gravitational fields (as opposed to the weak fields of Earth and other planets). But what most excites scientists about the find are the prospects it opens up for astronomy. The shape of LIGO’s signal showed it was generated by two black holes in a distant galaxy that spiralled in on one another and then coalesced about 1.3 billion years ago - the time it has taken the wave to reach Earth. The fact that those black holes were more massive than was thought possible - weighing in at about 29 and 36 times the mass of the Sun - is intriguing in itself. But astronomers are looking forward to many more breakthroughs in the future. One tantalizing possibility is being able to observe merging neutron stars and then to follow up those observations with electromagnetic spectroscopy, since it is possible that such mergers are where gold and platinum are made.
The potential for such observations will be enhanced when new and existing interferometers start up alongside LIGO, allowing improved sensitivity and better pinpointing of gravitational-wave sources. Virgo in Italy, currently being upgraded, is due to be turned back on by the end of the year, while KAGRA in Japan should be ready to join in the hunt towards the end of this decade. Looking further ahead, scientists are planning a ground-based interferometer with 10km long arms, as well as a space-based observatory with virtual arms millions of kilometres in length.
To get as far as they have, and detect gravitational waves directly for the first time, scientists have had to take a long and, at times, difficult road. The Norwegian Academy of Science and Letters acknowledges the vital contributions made by hundreds of individuals around the world, whose technical innovations have beaten down the many sources of noise that would otherwise plague observations.
However, for the Academy, the contributions of Ronald Drever, Kip Thorne and Rainer Weiss stand out. Their “ingenuity, inspiration, intellectual leadership and tenacity”, it says, were the “driving force” behind the discovery of gravitational waves.
—Edwin Cartlidge, science writer