The whisper of a black hole

Building a laser interferometer with the enormous sensitivity required to "hear" faraway black holes colliding was the result of a continuous process that goes back more than half a century.

Inspecting LIGO's optics for contaminants. Before sealing up the chamber and pumping the vacuum system down, a LIGO optics technician clad in special clean room garb inspects one of LIGO's core optics (mirrors) by illuminating its surface with light at a glancing angle. It is critical to LIGO's operation that there is no contamination on any of its optical surfaces.

THE recent detection of gravitational-waves (GWs) by the Laser Interferometer Gravitational-wave Observatory (LIGO), nearly 100 years after they were first recognised by Einstein as a consequence of his general theory of relativity, is the result of one of modern science’s longest campaigns. For more than 40 years, the theoretical world debated back and forth about whether what was predicted was a real phenomenon or just a mathematical artefact with no physical significance.

This debate was finally settled in favour of their physical reality in a scientific workshop held in Chapel Hill, North Carolina, in the United States in 1957. At that point, experimental scientists, most notably Joseph Weber, began to search for ways to detect them.

The challenge for GW detection is the sensitivity that must be achieved. Scientists often speak casually of GWs as being extremely weak, but that is not quite correct.

For this event, during the tenth of a second just before the two black holes collided, the energy emitted in GWs exceeded the combined electromagnetic output (light, radio waves, X-rays, etc.) of the entire visible universe by a factor of 50.

Waves carrying such energy certainly cannot be called “weak”. What is true is that the effects of these waves on the earth are very small.

A GW is a distortion of space itself, and creating even a tiny distortion of space requires huge amounts of energy. GWs are created by very rapid acceleration of large masses and they travel away from its source at the speed of light, literally stretching space along one direction perpendicular to the direction of travel and squeezing space along the other direction (Figure 1). For the burst of GWs detected by LIGO, the degree of spatial distortion was incredibly small, 1 part in 10 ²¹ or one part in one thousand billion billion. For two points one kilometre apart (LIGO is four kilometres long), this corresponds to a change in their separation of 10 ^-18 metre, or about 1/1000 the diameter of an atomic nucleus .

By the 1970s, several scientists, most notably Professor Rainer Weiss of Massachusetts Institute of Technology (MIT), recognised that the venerable Michelson interferometer would be ideally suited to measure the distortion pattern of a GW. This interferometer configuration was invented by the Nobel laureate Albert Michelson and used by him and Edward Morley in the early 1880s to try to measure the motion of the earth through the “ether”, a hypothetical medium in which light travelled. The null result in this experiment, the fact that they could find no evidence for the existence of the “ether”, became one of the key experimental underpinnings of the theory of special relativity, namely, that the speed of light is constant for all observers, regardless of their frame of reference.

In a basic Michelson interferometer (Figure 2), a beam of light falls on a half-reflecting “beam splitter” oriented at 45 degrees. Half of the light is transmitted, while the other half is directed perpendicular to the original direction. After letting the light travel some distance, a mirror on each arm reflects the light back to the beam splitter, where the two beams overlap and recombine.

The pattern of light that results (how much is directed back toward the light source and how much is sent in the perpendicular direction) depends critically on how equal the lengths of the two perpendicular arms are. Small changes in the length of the arms, such as might be caused by a GW, cause changes in the pattern of the recombined light, which can be recorded with a photodetector.

At its core, the concept of a Michelson interferometer is so simple that it is studied in virtually every first year university optics course. However, when a Michelson interferometer was first proposed as a means of detecting GWs, the typical sensitivity of practical devices was on the order of 1 nanometre (about 10 times the diameter of an atom), and the typical size was a few metres. To meet the requirements for GW detection, the sensitivity would have to be improved by a factor of a billion, and the size increased by a factor of one thousand. These have been the major experimental challenges for LIGO over the past 40 years.

To achieve the sensitivity, a number of technologies needed to be developed, tested and refined. First, the arc lamps used by Michelson as the light source for his interferometer had to be replaced by lasers, but the lasers available in the 1970s and 1980s were far from adequate. The purity and stability of their light had to be improved by a factor of roughly one million. Professor Ron Drever of Caltech proposed a new technique for stabilising lasers, and LIGO scientists worked with the laser industry over decades to gain the needed improvements. To go with the improved laser, the configuration of the Michelson interferometer was altered by adding new mirrors which send the light back and forth along each arm to increase its sensitivity (Figure 3). In turn, these new configurations required mirrors that were well beyond the state of the art in 1980. Again, LIGO scientists and optical engineers worked with industry to improve the accuracy of the mirror shapes, to reduce the losses due to scattering and absorption, and to improve the materials used. These technical developments were made for the esoteric purpose of GW detection, but have expanded into many other applications in space, in semiconductor manufacturing and in timekeeping standards, among others.

Another challenge to reach the required sensitivity was to protect the interferometer from sources of external influence. Such external stimuli include acoustical noise, natural ground vibrations, and man-made disturbances. Site selection is an important component. The two U.S. LIGO sites were selected in the early 1990s, balancing remoteness (avoiding potential disturbance by local man-made sources such as mines or manufacturing facilities) and accessibility (since the detectors require skilled staff to install, maintain and operate them). Similar considerations will come into play in the selection of the site for LIGO-India. Once a site is selected, the facility design must ensure that it does not itself create noise and other disturbances through its mechanical systems (air-conditioning, electrical distribution, plumbing, and so on).

The interferometer components themselves are placed in a high vacuum system to prevent any disturbances from surrounding gas. Carefully designed vibration-isolation systems form the final protection from any outside disturbances. The LIGO detectors use a multilayered approach to vibration isolation. In the most extreme cases, the vibration-isolation system consists of six layers of increasingly effective fine isolation. The last stage suspends the critical interferometer mirrors on fused silica (glass) fibres which are stretched to approximately one-half of their breaking strength. An accompanying photograph shows one of the LIGO mirrors in its final installation, during the final inspection prior to sealing the vacuum system and evacuating the air.

Engineering challenges

Scaling the interferometer to multi-kilometre arm length includes engineering challenges as well. To achieve the required sensitivity, the laser beams must travel in ultra-high vacuum pipes, aligned to millimetre accuracy. While the techniques for building such a large vacuum system were available when LIGO was first proposed, they would have been too expensive for such an uncertain venture. LIGO scientists and engineers worked with commercial companies to develop lower-cost ways to fabricate and install the high vacuum tubes that carry the laser beams. This led to new fabrication techniques for high-vacuum systems, new treatments for the stainless steel used to fabricate them, and even one of the first demonstrations of the use of GPS for precision alignment over long baselines.

From the early days of LIGO, it was structured as a two-step programme: first, an initial set of LIGO detectors which would use the best technologies available in the late 1990s, to be followed by a set of advanced detectors incorporating technologies that needed more time for development. The initial detectors were to be sensitive enough to detect GWs if the optimistic (but very uncertain) predictions for their strength proved correct, but might well not detect any if more pessimistic estimates were right.

Either way, the experience with the first-generation detectors would be extremely valuable in refining the developments needed for the advanced detectors. The initial detectors were completed in 2002. They looked for GWs until 2010 and served as a test bed for some of the advanced detector technologies. That no GWs were found with the initial LIGO was disappointing, but not surprising.

From 2010 to 2015, the initial LIGO detectors were replaced with entirely new, advanced ones. By mid-2015, the two advanced LIGO detectors were fully installed and commissioned to the point where a first observational run could be scheduled for mid-September. The new detectors are designed to observe GWs 10 times fainter than with the initial LIGO and over a broader range of frequencies, although in this initial observational run they were not operating at full capability and were “only” better than the initial LIGO by a factor of 3 to 5. However, even in that state, they would reproduce the entire scientific output of the initial LIGO within their first two weeks of operation, so the anticipation was high.

Culmination of a quest

On September 14, 2015, the two LIGO detectors were undergoing their final tests in the week before the designated start of the first observation run when a strong signal was seen. Coming so early, the LIGO scientists had to perform many checks to be sure that the detectors were behaving properly and that this was not some unexpected “misbehaviour” on the part of the new instruments. Because of the significance of this discovery, every possible alternative explanation was thoroughly and completely explored. In the end, the LIGO scientists became convinced that there was no other possible explanation and that their decades-long quest had come to a successful conclusion.

The amazing detection of GWs and the discovery of a binary black hole system are the culmination of a century of effort. We are now poised at the beginning of a new era of using GWs to explore an unknown side of our universe. To truly gain the most from this new way of exploring the universe, a global network is required, and the announcement of this discovery was followed closely by the announcement that India would join in this effort (see article by Bala Iyer and Taurun Souradeep).

The international community looks forward to the experimental contributions that India will make to this endeavour.

Prof. Stanley Whitcomb is currently the Chief Scientist of the Laser Interferometer Gravitational-wave Observatory (LIGO), California Institute of Technology, U.S.