LIGO Magazine: Issue 7

It is an exciting time time in LIGO. The start of the first observing run (O1) is imminent. I think they just need to sort out a button that is big enough and red enough (or maybe gather a little more calibration data… ), and then it’s all systems go. Making the first direct detection of gravitational waves with LIGO would be an enormous accomplishment, but that’s not all we can hope to achieve: what I’m really interested in is what we can learn from these gravitational waves.

The LIGO Magazine gives a glimpse inside the workings of the LIGO Scientific Collaboration, covering everything from the science of the detector to what collaboration members like to get up to in their spare time. The most recent issue was themed around how gravitational-wave science links in with the rest of astronomy. I enjoyed it, as I’ve been recently working on how to help astronomers look for electromagnetic counterparts to gravitational-wave signals. It also features a great interview with Joseph Taylor Jr., one of the discoverers of the famous Hulse–Taylor binary pulsar. The back cover features an article I wrote about parameter estimation: an expanded version is below.

How does parameter estimation work?

Detecting gravitational waves is one of the great challenges in experimental physics. A detection would be hugely exciting, but it is not the end of the story. Having observed a signal, we need to work out where it came from. This is a job for parameter estimation!

How we analyse the data depends upon the type of signal and what information we want to extract. I’ll use the example of a compact binary coalescence, that is the inspiral (and merger) of two compact objects—neutron stars or black holes (not marshmallows). Parameters that we are interested in measuring are things like the mass and spin of the binary’s components, its orientation, and its position.

For a particular set of parameters, we can calculate what the waveform should look like. This is actually rather tricky; including all the relevant physics, like precession of the binary, can make for some complicated and expensive-to-calculate waveforms. The first part of the video below shows a simulation of the coalescence of a black-hole binary, you can see the gravitational waveform (with characteristic chirp) at the bottom.

We can compare our calculated waveform with what we measured to work out how well they fit together. If we take away the wave from what we measured with the interferometer, we should be left with just noise. We understand how our detectors work, so we can model how the noise should behave; this allows us to work out how likely it would be to get the precise noise we need to make everything match up.

To work out the probability that the system has a given parameter, we take the likelihood for our left-over noise and fold in what we already knew about the values of the parameters—for example, that any location on the sky is equally possible, that neutron-star masses are around 1.4 solar masses, or that the total mass must be larger than that of a marshmallow. For those who like details, this is done using Bayes’ theorem.

We now want to map out this probability distribution, to find the peaks of the distribution corresponding to the most probable parameter values and also chart how broad these peaks are (to indicate our uncertainty). Since we can have many parameters, the space is too big to cover with a grid: we can’t just systematically chart parameter space. Instead, we randomly sample the space and construct a map of its valleys, ridges and peaks. Doing this efficiently requires cunning tricks for picking how to jump between spots: exploring the landscape can take some time, we may need to calculate millions of different waveforms!

Having computed the probability distribution for our parameters, we can now tell an astronomer how much of the sky they need to observe to have a 90% chance of looking at the source, give the best estimate for the mass (plus uncertainty), or even figure something out about what neutron stars are made of (probably not marshmallow). This is the beginning of gravitational-wave astronomy!

Monty and Carla map parameter space

Monty, Carla and the other samplers explore the probability landscape. Nutsinee Kijbunchoo drew the version for the LIGO Magazine.

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

White lab coats, pink tutus and camouflage fatigues

In this post I contemplate the effects of stereotypes and biases. I hope that this will encourage you to examine these ideas too. I promise I’ll get back to more science soon.

Just over a week ago, I helped with an outreach event for year nine students. Some of the astrophysics PhD students and I ran an interactive lecture on gravity and its importance in astrophysics. These type of events are fun: you get to teach some physics to a (usually) enthusiastic audience, and hopefully inspire them to consider studying the subject. I also get to play with our Lycra Universe. I think it’s especially important to show students what a university environment is like and have them interact with real scientists. It is important to counter the stereotype that studying science means that you’ll spend all day in a lab wearing a white lab coat. (Although that would be cool. I’d want goggles too, and maybe a doomsday device).

This event was to promote the studying of STEM subjects. That’s science, technology, engineering and mathematics, because there’s nothing like an acronym to make things accessible. It is often argued that we need more people trained in STEM subjects for the economy, industry, or just so we can finally get pizza over the Internet. I like to encourage people to study these areas as I think it’s good to have a scientifically-literate population. Also, because science is awesome! The event was aimed specifically at encouraging a group who are under-represented at university-level STEM, namely girls.

There has been much written on gender and subject choice. I would recommend the Closing Doors report by the Institute of Physics. I will not attempt to unravel this subject. In all my experience, I have never noticed any difference in aptitude between genders. I don’t believe that the ability to pee standing up gives any advantage when studying physics—one could argue for a better understanding of parabolic motion, but anyone who has paid attention to the floor in the gents (I advise against this), knows this is demonstrably not the case. I assume the dominant factors are social pressures: a vicious circle of a subject becoming more associated with one gender, which makes people feel self-conscious or out of place studying it. Also: there are always bigots. It’s a real shame to be potentially missing out on capable scientists. There have been many attempts to try to counter this trend, to break the cycle—some of them truly awful.

Good arguments have been made that the gender segregation of toys pushes girls away from science and technology from an early age. (For some reason, there seems to be a ridiculous idea that women can only relate to things that are pink). It makes sense to me that if only boys get the chemistry sets and construction toys, then they are going to be more numerous in the STEM subjects. The fact that a few female LEGO scientists merits coverage in nation newspapers, the BBC, etc. shows something isn’t quite right.

We are all influenced by our childhoods, and this got me thinking: I know of negative impacts for women from these gender biases, what are they for men? If women are under-represented in engineering, maths and physics, then men must be under-represented somewhere else to balance things: namely English, biology (conspicuous amongst the STEM subjects) and languages. We are short of male teachers and nurses. It seems that men are pushed away from caring careers or those with emphasis on communication.

The lack of men in certain professions is a problem, although I would say less so than the continued under-representation of women at senior positions (say as professors, CEOs or members of government). I was about to relax, since I hadn’t uncovered yet another unconscious bias to add to the list. Then I checked the news. I don’t know what’s in the news when you’re reading this, but at the time it was conflict in Ukraine, Iraq and Israel–Palestine—I assume things are much better in the future? One thing that struck me was that the combatants in the photos were almost exclusively men. It then occurred to me that for every girl who plays with a ballerina doll, there is a boy who plays with an action figure with a weapon. I’m not as naive as to suggest it’s a simple as growing up to be exactly like your toys (I, regrettably, am neither a dinosaur nor a cuddly elephant), but perhaps it is worth keeping in the front of our mind what identities we associate with each gender and how we project these onto children. I don’t want to say that being a ballerina isn’t a good vocation or hobby, or that being a soldier is a bad career. (Curiously, I believe that some of the requirements to be a good ballet dancer or soldier overlap, say discipline, determination, physical fitness and, perhaps, empathy). However, I think it is dangerous if we raise girls who primarily aspire to be pretty, and boys who resolve conflict through violence (men are both more likely to be victims of homicide and suicide).

In conclusion, stereotypes can be damaging, be it that scientists are all socially-awkward comic-book geeks as in The Big Bang Theory, that men can’t talk about their feelings, or that women must be mothers. There is a balance between the genders: by assigning one quality to a particular gender, you can push the other away. Mathematical ability shouldn’t be masculine and compassion shouldn’t be feminine. This is not a new idea, but conveniently coincides with Emma Watson’s wonderful speech for the UN as part of the HeForShe campaign. Cultural biases might be more significant than you think, so give them some extra attention. Sexism hurts everyone, so let’s cut it out and all go play with some LEGO.

The Big Bang Theory

The Big Bang Theory‘s popularity has been credited with encouraging more students to take physics. The cast reflects traditional stereotypes: the men are physicists, an astronomer and an engineer, the women are two biologists and Penny.

How sport is like science

Athene Donald, Professor of Experimental Physics and soon-to-be Master of my old college, Churchill, recently blogged about how athletics resembles academia. She argued that both are hard careers: they require many years of training, and even then success is not guaranteed—not everyone will reach the top to become an Olympian or a Professor—there is a big element of luck too—a career can stall because of an injury or because of time invested in a study that eventually yields null results, and, conversely, a single big championship win or serendipitous discovery can land a comfortable position. These factors can make these career paths unappealing, but still most people who enter them do so because they love the area, and have a real talent for the field.

The Breakfast Club

As The Breakfast Club taught us, being into physics or sports can have similar pressures.

I find this analogy extremely appealing. There are many parallels. Both sports and academic careers are meritocratic and competitive. Most who enter them will not become rich—those who do, usually manage it by making use of their profile, either through product endorsement or through writing a book, say Stephen Hawking, or Michael Jordan (although he was still extremely well paid). Both fields have undisputed heavy-weights like Einstein or Muhammad Ali, and media superstars like Neil deGrasse Tyson or Anna Kournikova; both have inspirational figures who have overcome adversity, be they Jesse Owens or Emmy Noether, and idols whose personal lives you probably shouldn’t emulate, say Tiger Woods or Richard Feynman. However, I think the similarity can stretch beyond career paths.

Athene says that although she doesn’t participate in athletics, she does enjoy watching the sport. I’m sure many can empathise with that position. I think that this is similarly the case for research: many enjoy finding out about new discoveries or ideas, even though they don’t want to invest the time studying themselves. There are many excellent books and documentaries, many excellent communicators of research. (I shall be helping out at this year’s British Science Festival, which I’m sure will be packed with people keen to find out about current research.) However, there is undoubtedly more that could be done, both in terms of growing the market and improving the quality—reporting of science is notoriously bad. If you were to go into any pub in the country, I’d expect you’d be able to find someone to have an in-depth conversation with about how best to manage the national football team, despite them not being a professional footballer. Why not someone with similar opinions about research council funding? Can we make research as popular as sport?

Increasing engagement with and awareness of research is a popular subject, most research grants with have some mention of wider impact; however, I don’t think that this is the only goal. According to UK government research, many young students do enjoy science, they just don’t feel it is for them. The problem is that people think that science is too difficult. Given my previous ramblings, that’s perhaps understandable. However, that was for academic research; science is far broader than that! There are many careers outside the lab, and understanding science is useful even if that’s not your job, for example when discussing subjects like global warming or vaccination that affect us all. Coming back to our sports analogy, the situation is like children not wanting to play football because they won’t be a professional. It’s true that most people aren’t good enough to play for England (potentially including members of the current squad, depending upon who you ask in that pub), but that doesn’t mean you can’t enjoy a kick around, perhaps play for a local team at weekend, or even coach others. Playing sports regular keeps you physically fit, which is a good thing™; taking an interest in science (or language or literature or etceteras) keeps you mentally fit, also a good thing™.

Chocolate models

Chocolate is also a good thing™. However, neither Nobel Prizes nor Olympic Medals are made of chocolate, something I’m not sure that everyone appreciates. I’d make the gold Olympic models out of milk chocolate, silver out of white and bronze out of dark. The Nobel Prize for Medicine should contain nuts as an incentive to cure allergies; the Prize for Economics should be mint(ed) chocolate, the Peace Prize Swiss chocolate, the Chemistry Prize should contain popping candy, and the Physics Prize should be orange chocolate (that’s my favourite).

How to encourage more people to engage in science is a complicated problem. There’s no single solution, but it is something to work on. I would definitely prefer to live in a science-literate society. Stressing applications of science beyond pure research might be one avenue. I would also like to emphasis that it’s OK to find science (and maths) hard. Problem solving is difficult, like long-distance running, but if you practise, it does get easier. I can only vouch for one side of that simile from personal experience, but since I’m a theoretician, I’m happy enough to state that without direct experimental confirmation. I guess that means I should take my own advice and participate more myself: spend a little more time being physically active? Motivating myself is also a difficult problem.