100 Years of Relativity Space-Time Structure: Einstein and Beyond ...

Receiving Gravitational Waves 247a simple linear exposition. But perhaps a few principal themes of theirwork can illustrate how this phase fits into the arc of the story that we aretelling. After the theory, the thought experiments, the conceptual designsof the interferometers and the LIGO observatory system overall, this wasthe period in which the final choices of materials, dimensions, constructionmethods, and assembly plans had to be made, after which parts were builtor bought, and the whole interferometer assembled. The stakes were high:real money was being spent, and choices needed to be right (or to be fixedif they weren’t.)One interesting document from this phase of work is the Detector SubsystemRequirements, written in 1996. 23 In the early part of this work, thedivision of the interferometer into subsystems is outlined. (See the explanatorydiagram, Figure 3.) Each major subsystem was assigned an officialTLA (three letter acronym.) The subsystems were:• PSL, the pre-stabilized laser,• IOO, input optics components,• COC, core optics components (the main interferometer mirrors),• COS, core optics support components,• LSC, length sensing and control system,• ASC, angular sensing and control system,• SUS, mirror suspension system, and• SEI, seismic isolation system.Also in LIGO were the physical environment monitoring system (PEM, notshown in the diagram) and the control and data system (CDS, shown butnot with its proper TLA.)This diagram was constructed to show the interfaces (mechanical, optical,and electrical) of the various subsystems as they related to one anotherto form a whole LIGO interferometer. The fact that such a diagram wasneeded testifies to both the complexity of the whole system, and to the consequentneed to parcel out design work to different teams, who neverthelessneeded to ensure that all of the parts would work together.The main function of the Detector Subsystems Requirements documentwas to deal with the various sources of “technical noise”, noise processesthat were considered amenable to careful engineering. Beyond a simple listingof known noise processes, the document assigned budgeted amounts ofnoise that were allowed from each process. The goal was to ensure that noneof the technical noise sources would make a significant contribution to thefinal noise spectrum of the interferometers. Only seismic, pendulum ther-