Advanced LIGO (the paper)

Continuing with my New Year’s resolution to write a post on every published paper, the start of March see another full author list LIGO publication. Appearing in Classical & Quantum Gravity, the minimalistically titled Advanced LIGO is an instrumental paper. It appears a part of a special focus issue on advanced gravitational-wave detectors, and is happily free to read (good work there). This is The Paper™ for describing how the advanced detectors operate. I think it’s fair to say that my contribution to this paper is 0%.

LIGO stands for Laser Interferometer Gravitational-wave Observatory. As you might imagine, LIGO tries to observe gravitational waves by measuring them with a laser interferometer. (It won’t protect your fencing). Gravitational waves are tiny, tiny stretches and squeezes of space. To detect them we need to measure changes in length extremely accurately. I had assumed that Advanced LIGO will achieve this supreme sensitivity through some dark magic invoked by sacrificing the blood, sweat, tears and even coffee of many hundreds of PhD students upon the altar of science. However, this paper actually shows it’s just really, really, REALLY careful engineering. And giant frickin’ laser beams.

The paper goes through each aspect of the design of the LIGO detectors. It starts with details of the interferometer. LIGO uses giant lasers to measure distances extremely accurately. Lasers are bounced along two 3994.5 m arms and interfered to measure a change in length between the two. In spirit, it is a giant Michelson interferometer, but it has some cunning extra features. Each arm is a Fabry–Pérot etalon, which means that the laser is bounced up and down the arms many times to build up extra sensitivity to any change in length. There are various extra components to make sure that the laser beam is as stable as possible, all in all, there are rather a lot of mirrors, each of which is specially tweaked to make sure that some acronym is absolutely perfect.

Advanced LIGO optical configuration. IT's a bit more complicated than a basic Michelson interferometer.

Fig. 1 from Aasi et al. (2015), the Advanced LIGO optical configuration. All the acronyms have to be carefully placed in order for things to work. The laser beam starts from the left, passing through subsystems to make sure it’s stable. It is split in two to pass into the interferometer arms at the top and right of the diagram. The laser is bounced many times between the mirrors to build up sensitivity. The interference pattern is read out at the bottom. Normally, the light should interfere destructively, so the output is dark. A change to this indicates a change in length between the arms. That could be because of a passing gravitational wave.

The next section deals with all the various types of noise that affect the detector. It’s this noise that makes it such fun to look for the signals. To be honest, pretty much everything I know about the different types of noise I learnt from Space-Time Quest. This is a lovely educational game developed by people here at the University of Birmingham. In the game, you have to design the best gravitational-wave detector that you can for a given budget. There’s a lot of science that goes into working out how sensitive the detector is. It takes a bit of practice to get into it (remember to switch on the laser first), but it’s very easy to get competitive. We often use the game as part of outreach workshops, and we’ve had some school groups get quite invested in the high-score tables. My tip is that going underground doesn’t seem to be worth the money. Of course, if you happen to be reviewing the proposal to build the Einstein Telescope, you should completely ignore that, and just concentrate how cool the digging machine looks. Space-Time Quest shows how difficult it can be optimising sensitivity. There are trade-offs between different types of noise, and these have been carefully studied. What Space-Time Quest doesn’t show, is just how much work it takes to engineer a detector.

The fourth section is a massive shopping list of components needed to build Advanced LIGO. There are rather more options than in Space-Time Quest, but many are familiar, even if given less friendly names. If this section were the list of contents for some Ikea furniture, you would know that you’ve made a terrible life-choice; there’s no way you’re going to assemble this before Monday. Highlights include the 40 kg mirrors. I’m sure breaking one of those would incur more than seven years bad luck. For those of you playing along with Space-Time Quest at home, the mirrors are fused silica. Section 4.8.4 describes how to get the arms to lock, one of the key steps in commissioning the detectors. The section concludes with details of how to control such a complicated instrument, the key seems to be to have so many acronyms that there’s no space for any component to move in an unwanted way.

The paper closes with on outlook for the detector sensitivity. With such a complicated instrument it is impossible to be certain how things will go. However, things seem to have been going smoothly so far, so let’s hope that this continues. The current plan is:

  • 2015 3 months observing at a binary neutron star (BNS) range of 40–80 Mpc.
  • 2016–2017 6 months observing at a BNS range of 80–120 Mpc.
  • 2017–2018 9 months observing at a BNS range of 120–170 Mpc.
  • 2019 Achieve full sensitivity of a BNS range of 200 Mpc.

The BNS range is the distance at which a typical binary made up of two 1.4 solar mass neutrons stars could be detected when averaging over all orientations. If you have a perfectly aligned binary, you can detect it out to a further distance, the BNS horizon, which is about 2.26 times the BNS range. There are a couple of things to note from the plan. First, the initial observing run (O1 to the cool kids) is this year! The second is how much the range will extend before hitting design sensitivity. This should significantly increase the number of possible detections, as each doubling of the range corresponds to a volume change of a factor of eight. Coupling this with the increasing length of the observing runs should mean that the chance of a detection increases every year. It will be an exciting few years for Advanced LIGO.

arXiv: 1411.4547 [gr-qc]
Journal: Classical & Quantum Gravity; 32(7):074001(41); 2015
Science summary: Introduction to LIGO & Gravitational Waves
Space-Time Quest high score: 34.859 Mpc


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