KAGRA

Prospects for observing and localizing gravitational-wave transients with Advanced LIGO, Advanced Virgo and KAGRA

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This paper, known as the Observing Scenarios Document with the Collaboration, outlines the observing plans of the ground-based detectors over the coming decade. If you want to search for electromagnetic or neutrino signals from our gravitational-wave sources, this is the paper for you. It is a living review—a document that is continuously updated.

This is the second published version, the big changes since the last version are

  1. We have now detected gravitational waves
  2. We have observed our first gravitational wave with a mulitmessenger counterpart [bonus note]
  3. We now include KAGRA, along with LIGO and Virgo

As you might imagine, these are quite significant updates! The first showed that we can do gravitational-wave astronomy. The second showed that we can do exactly the science this paper is about. The third makes this the first joint publication of the LIGO Scientific, Virgo and KAGRA Collaborations—hopefully the first of many to come.

I lead both this and the previous version. In my blog on the previous version, I explained how I got involved, and the long road that a collaboration must follow to get published. In this post, I’ll give an overview of the key details from the new version together with some behind-the-scenes background (working as part of a large scientific collaboration allows you to do amazing science, but it can also be exhausting). If you’d like a digest of this paper’s science, check out the LIGO science summary.

Commissioning and observing phases

The first section of the paper outlines the progression of detector sensitivities. The instruments are incredibly sensitive—we’ve never made machines to make these types of measurements before, so it takes a lot of work to get them to run smoothly. We can’t just switch them on and have them work at design sensitivity [bonus note].

Possible advanced detector sensitivity

Target evolution of the Advanced LIGO and Advanced Virgo detectors with time. The lower the sensitivity curve, the further away we can detect sources. The distances quoted are binary neutron star (BNS) ranges, the average distance we could detect a binary neutron star system. The BNS-optimized curve is a proposal to tweak the detectors for finding BNSs. Figure 1 of the Observing Scenarios Document.

The plots above show the planned progression of the different detectors. We had to get these agreed before we could write the later parts of the paper because the sensitivity of the detectors determines how many sources we will see and how well we will be able to localize them. I had anticipated that KAGRA would be the most challenging here, as we had not previously put together this sequence of curves. However, this was not the case, instead it was Virgo which was tricky. They had a problem with the silica fibres which suspended their mirrors (they snapped, which is definitely not what you want). The silica fibres were replaced with steel ones, but it wasn’t immediately clear what sensitivity they’d achieve and when. The final word was they’d observe in August 2017 and that their projections were unchanged. I was sceptical, but they did pull it out of the bag! We had our first clear three-detector observation of a gravitational wave 14 August 2017. Bravo Virgo!

LIGO, Virgo and KAGRA observing runs

Plausible time line of observing runs with Advanced LIGO (Hanford and Livingston), advanced Virgo and KAGRA. It is too early to give a timeline for LIGO India. The numbers above the bars give binary neutron star ranges (italic for achieved, roman for target); the colours match those in the plot above. Currently our third observing run (O3) looks like it will start in early 2019; KAGRA might join with an early sensitivity run at the end of it. Figure 2 of the Observing Scenarios Document.

Searches for gravitational-wave transients

The second section explain our data analysis techniques: how we find signals in the data, how we work out probable source locations, and how we communicate these results with the broader astronomical community—from the start of our third observing run (O3), information will be shared publicly!

The information in this section hasn’t changed much [bonus note]. There is a nice collection of references on the follow-up of different events, including GW170817 (I’d recommend my blog for more on the electromagnetic story). The main update I wanted to include was information on the detection of our first gravitational waves. It turned out to be more difficult than I imagined to come up with a plot which showed results from the five different search algorithms (two which used templates, and three which did not) which found GW150914, and harder still to make a plot which everyone liked. This plot become somewhat infamous for the amount of discussion it generated. I think we ended up with something which was a good compromise and clearly shows our detections sticking out above the background of noise.

CBC and burst search results

Offline transient search results from our first observing run (O1). The plot shows the number of events found verses false alarm rate: if there were no gravitational waves we would expect the points to follow the dashed line. The left panel shows the results of the templated search for compact binary coalescences (binary black holes, binary neutron stars and neutron star–black hole binaries), the right panel shows the unmodelled burst search. GW150914, GW151226 and LVT151012 are found by the templated search; GW150914 is also seen in the burst search. Arrows indicate bounds on the significance. Figure 3 of the Observing Scenarios Document.

Observing scenarios

The third section brings everything together and looks at what the prospects are for (gravitational-wave) multimessenger astronomy during each observing run. It’s really all about the big table.

Ranges, binary neutron star detections, and localization precesion

Summary of different observing scenarios with the advanced detectors. We assume a 70–75% duty factor for each instrument (including Virgo for the second scenario’s sky localization, even though it only joined our second observing run for the final month). Table 3 from the Observing Scenarios Document.

I think there are three really awesome take-aways from this

  1. Actual binary neutron stars detected = 1. We did it!
  2. Using the rates inferred using our observations so far (including GW170817), once we have the full five detector network of LIGO-Hanford, LIGO-Livingston, Virgo, KAGRA and LIGO-India, we could be detected 11–180 binary neutron stars a year. That something like between one a month to one every other day! I’m kind of scared…
  3. With the five detector network the sky localization is really good. The median localization is about 9–12 square degrees, about the area the LSST could cover in a single pointing! This really shows the benefit of adding more detector to the network. The improvement comes not because a source is much better localized with five detectors than four, but because when you have five detectors you almost always have at least three detectors(the number needed to get a good triangulation) online at any moment, so you get a nice localization for pretty much everything.

In summary, the prospects for observing and localizing gravitational-wave transients are pretty great. If you are an astronomer, make the most of the quiet before O3 begins next year.

arXiv: 1304.0670 [gr-qc]
Journal: Living Reviews In Relativity; 21:3(57); 2018
Science summary: A Bright today and brighter tomorrow: Prospects for gravitational-wave astronomy With Advanced LIGO, Advanced Virgo, and KAGRA
Prospects for the next update: After two updates, I’ve stepped down from preparing the next one. Wooh!

Bonus notes

GW170817 announcement

The announcement of our first multimessenger detection came between us submitting this update and us getting referee reports. We wanted an updated version of this paper, with the current details of our observing plans, to be available for our astronomer partners to be able to cite when writing their papers on GW170817.

Predictably, when the referee reports came back, we were told we really should include reference to GW170817. This type of discovery is exactly what this paper is about! There was avalanche of results surrounding GW170817, so I had to read through a lot of papers. The reference list swelled from 8 to 13 pages, but this effort was handy for my blog writing. After including all these new results, it really felt like this was version 2.5 of the Observing Scenarios, rather than version 2.

Design sensitivity

We use the term design sensitivity to indicate the performance the current detectors were designed to achieve. They are the targets we aim to achieve with Advanced LIGO, Advance Virgo and KAGRA. One thing I’ve had to try to train myself not to say is that design sensitivity is the final sensitivity of our detectors. Teams are currently working on plans for how we can upgrade our detectors beyond design sensitivity. Reaching design sensitivity will not be the end of our journey.

Binary black holes vs binary neutron stars

Our first gravitational-wave detections were from binary black holes. Therefore, when we were starting on this update there was a push to switch from focusing on binary neutron stars to binary black holes. I resisted on this, partially because I’m lazy, but mostly because I still thought that binary neutron stars were our best bet for multimessenger astronomy. This worked out nicely.

All-sky search for long-duration gravitational wave transients with LIGO

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It’s now about 7 weeks since the announcement, and the madness is starting to subside. Although, that doesn’t mean things aren’t busy—we’re now enjoying completely new forms of craziness. In mid March we had our LIGO–Virgo Collaboration Meeting. This was part celebration, part talking about finishing our O1 analysis and part thinking ahead to O2, which is shockingly close. It was fun, there was cake.

Gravitational wave detection cake

Celebratory cake from the March LIGO–Virgo Meeting. It was delicious and had a fruity (strawberry?) filling. The image is February 11th’s Astronomy Picture of the Day. There was a second cake without a picture, that was equally delicious, but the queue was shorter.

All the business means that I’ve fallen behind with my posts, and I’ve rather neglected the final paper published the week starting 8 February. This is perhaps rather apt as this paper has the misfortune to be the first non-detection published in the post-detection world. It is also about a neglected class of signals.

Long-duration transients

We look for several types of signals with LIGO (and hopefully soon Virgo and KAGRA):

  • Compact binary coalescences (like two merging black holes), for which we have templates for the signal. High mass systems might only last a fraction of a second within the detector’s frequency range, but low mass systems could last for a minute (which is a huge pain for us to analyse).
  • Continuous waves from rotating neutron stars which are almost constant throughout our observations.
  • Bursts, which are transient signals where we don’t have a good model. The classic burst source is from a supernova explosion.

We have some effective search pipelines for finding short bursts—signals of about a second or less. Coherent Waveburst, which was the first code to spot GW150914 is perhaps the best known example. This paper looks at finding longer burst signals, a few seconds to a few hundred seconds in length.

There aren’t too many well studied models for these long bursts. Most of the potential sources are related to the collapse of massive stars. There can be a large amount of matter moving around quickly in these situations, which is what you want for gravitational waves.

Massive stars may end their life in a core collapse supernova. Having used up its nuclear fuel, the star no longer has the energy to keep itself fluffy, and its core collapses under its own gravity. The collapse leads to an explosion as material condenses to form a neutron star, blasting off the outer layers of the star. Gravitational waves could be generated by the sloshing of the outer layers as some is shot outwards and some falls back, hitting the surface of the new neutron star. The new neutron star itself will start life puffed up and perhaps rapidly spinning, and can generate gravitational waves at it settles down to a stable state—a similar thing could happen if an older neutron star is disturbed by a glitch (where we think the crust readjusts itself in something like an earthquake, but more cataclysmic), or if a neutron star accretes a large blob of material.

For the most massive stars, the core continues to collapse through being a neutron star to become a black hole. The collapse would just produce a short burst, so it’s not what we’re looking for here. However, once we have a black hole, we might build a disc out of material swirling into the black hole (perhaps remnants of the outer parts of the star, or maybe from a companion star). The disc may be clumpy, perhaps because of eddies or magnetic fields (the usual suspects when astrophysicists don’t know exactly what’s going on), and they rapidly inspiralling blobs could emit a gravitational wave signal.

The potential sources don’t involve as much mass as a compact binary coalescence, so these signals wouldn’t be as loud. Therefore we couldn’t see them quite as far way, but they could give us some insight into these messy processes.

The search

The paper looks at results using old LIGO data from the fifth and sixth science runs (S5 and S6). Virgo was running at this time, but the data wasn’t included as it vastly increases the computational cost while only increasing the search sensitivity by a few percent (although it would have helped with locating a source if there were one). The data is analysed with the Stochastic Transient Analysis Multi-detector Pipeline (STAMP); we’ll be doing a similar thing with O1 data too.

STAMP searches for signals by building a spectrogram: a plot of how much power there is at a particular gravitational wave frequency at a particular time. If there is just noise, you wouldn’t expect the power at one frequency and time to be correlated with that at another frequency and time. Therefore, the search looks for clusters, grouping together times or frequencies closer to one another where there is more power then you might expect.

The analysis is cunning, as it coherently analysis data from both detectors together when constructing the spectrogram, folding in the extra distance a gravitational wave must travel between the detectors for a given sky position.

The significance of events is calculated is a similar way to how we search for binary black holes. The pipeline ranks candidates using a detection statistic, a signal-to-noise ratio for the cluster of interesting time–frequency pixels \mathrm{SNR}_\Gamma (something like the amount of power measured divided by the amount you’d expect randomly). We work out how frequently you’d expect a particular value of \mathrm{SNR}_\Gamma by analysing time-shifted data: where we’ve shifted the data from one of the detectors in time relative to data from the other so that we know there can’t be the same signal found in both.

The distribution of \mathrm{SNR}_\Gamma is shown below from the search (dots) and from the noise background (lines). You can see that things are entirely consistent with our expectations for just noise. The most significant event has a false alarm probability of 54%, so you’re better off betting it’s just noise. There are no detections here.

False alarm rate distribution

False alarm rate (FAR) distribution of triggers from S5 (black circles) and S6 (red triangles) as a function of the
signal-to-noise ratio. The background S5 and S6 noise distributions are shown by the solid black and dashed red lines respectively. An idealised Gaussian noise background is shown in cyan. There are no triggers significantly above the expected background level. Fig. 5 from Abbott et al. (2016).

Since the detectors are now much more sensitive, perhaps there’s something lurking in our new data. I still think this in unlikely since we can’t see sources from a significant distance, but I guess we’ll have to wait for the results of the analysis.

arXiv: 1511.04398 [gr-qc]
Journal: Physical Review D; 93(4):042005(19); 2016
Science summary: Stuck in the middle: an all-sky search for gravitational waves of intermediate duration
Favourite (neglected) middle child: Lisa Simpson

View from Guano Point

Sunset over the Grand Canyon. One of the perks of academia is the travel. A group of us from Birmingham went on a small adventure after the LIGO–Virgo Meeting. This is another reason why I’ve not been updating my blog.

Neutrino oscillations and Nobel Prizes

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This year’s Nobel Prize in Physics was awarded to Takaaki Kajita and Arthur McDonald for the discovery of neutrino oscillations. This is some really awesome physics which required some careful experimentation and some interesting new theory; it is also one of the things that got me interested in astrophysics.

Neutrinos

Neutrinos are an elusive type of subatomic particle. They are sometimes represented by the Greek letter nu \nu, and their antiparticle equivalents (antineutrinos) are denoted by \bar{\nu}. We’ll not worry about the difference between the two. Neutrinos are rather shy. They are quite happy doing their own thing, and don’t interact much with other particles. They don’t have an electric charge (they are neutral), so they don’t play with the electromagnetic force (and photons), they also don’t do anything with the strong force (and gluons). They only get involved with the weak force (W and Z bosons). As you might expect from the name, the weak force doesn’t do much (it only operates over short distances), so spotting a neutrino is a rare occurrence.

Particle Zoo

The charming bestiary of subatomic particles made by Particle Zoo.

There is a large family of subatomic particles. The electron is one of the most familiar, being a component of atoms (and hence you, me, cake and even marshmallows). The electron has two cousins: the muon (not to be confused with the moo-on) and the tau particle. All three have similar characteristics, with the only real difference being their mass. Electrons are the lightest, muons are about 207 times heavier, and tau are about 17 times heavier still (3477 times the mass of the electron). Each member of the electron family has a neutrino counterpart: there’s the electron-neutrino \nu_e, the muon-neutrino \nu_\mu (\mu is the Greek letter mu) and the tau-neutrino \nu_\tau (\tau is the Greek letter tau).

Neutrinos are created and destroyed in in certain types of nuclear reactions. Each flavour of neutrino is only involved in reactions that involve their partner from the electron family. If an electron-neutrino is destroyed in a reaction, an electron is created; if a muon is destroyed, a muon-neutrino is created, and so on.

Solar neutrinos

Every second, around sixty billion neutrinos pass through every square centimetre on the Earth. Since neutrinos so rarely interact, you would never notice them. The source of these neutrinos is the Sun. The Sun is powered by nuclear fusion. Hydrogen is squeezed into helium through a series of nuclear reactions. As well as producing the energy that keeps the Sun going, these create lots of neutrinos.

The pp chain

The nuclear reactions that power the Sun. Protons (p), which are the nuclei of hydrogen, are converted to Helium nuclei after a sequence of steps. Electron neutrinos \nu_e are produced along the way. This diagram is adapted from Giunti & Kim. The traditional names of the produced neutrinos are given in bold and the branch names are given in parentheses and percentages indicate branching fractions.

The neutrinos produced in the Sun are all electron-neutrinos. Once made in the core of the Sun, they are free to travel the 700,000 km to the surface of the Sun and then out into space (including to us on Earth). Detecting these neutrinos therefore lets you see into the very heart of the Sun!

Solar neutrinos were first detected by the Homestake experiment. This looked for the end results of nuclear reactions caused when an electron-neutrino is absorbed. Basically, it was a giant tub of dry-cleaning fluid. This contains chlorine, which turns to argon when a neutrino is absorbed. The experiment had to count how many atoms of argon where produced. In 1968, the detection was announced. However, we could only say that there were neutrinos around, not that they were coming from the Sun…

To pin down where the neutrinos were coming from required a new experiment. Deep in the Kamioka Mine, Kamiokande looked for interactions between neutrinos and electrons. Very rarely a neutrino will bump into an electron. This can give the electron a big kick (since the neutrino has a lot of momentum). Kamiokande had a large tank of water (and so lots of electrons to hit). If one got a big enough kick, it could travel faster than the speed of light in water (about 2/3 of the speed of light in vacuum). It then emits a flash of light called Cherenkov radiation, which is the equivalent of the sonic boom created when a plane travels faster than the speed of sound. Looking where the light comes from tells you where the electron was coming from and so where the neutrino came from. Tracing things back, it was confirmed that the neutrinos were coming from the Sun!

This discovery confirmed that the Sun was powered by fusion. I find it remarkable that it was only in the late 1980s that we had hard evidence for what was powering the Sun (that’s within my own lifetime). This was a big achievement, and Raymond Davies Jr., the leader of the Homestake experiment, and Masatoshi Koshiba, the leader of the Kamiokande experiment, were awarded the 2002 Nobel Prize in Physics for pioneering neutrino astrophysics. This also led to one of my all-time favourite pictures: the Sun at night.

The Sun at night!

The Sun at night. Solar neutrinos as detected by Super-Kamioknade looking through the Earth. I think this is the astronomical equivalent of checking if the fridge light does go off when you close the door. Credit: Marcus Chown & Super-Kamiokande.

The mystery of the missing neutrinos

Detecting solar neutrinos was a big success, but there was a problem. There were only a fraction of the predicted number. This became known as the solar neutrino problem. There were two possibilities, either solar physicists had got their models wrong, or particle physicists were missing a part of the Standard Model.

The solar models were recalculated and tweaked, with much work done by John Bahcall and collaborators. More sophisticated calculations were performed, even folding in new data from helioseismology, the study of waves in the Sun, but the difference could not be resolved.

However, there was an idea in particle physics by Bruno Pontecorvo and Vladimir Gribov: that neutrinos could change flavour, a phenomena known as neutrino oscillations. This was actually first suggested before the first Homestake results were announced, perhaps it deserved further attention?

The first evidence in favour of neutrino oscillations comes from Super-Kamiokande, the successor to the original Kamiokande. This evidence came from looking at neutrinos produced by cosmic rays. Cosmic rays are highly energetic particles that come from space. As they slam into the atmosphere, and collide with molecules in the air, they produce a shower of particles. These include muons and muon-neutrinos. Super-Kamiokande could detect muon-neutrinos from cosmic rays. Cosmic rays come from all directions, so Super-Kamiokande should see muon-neutrinos from all directions too. Just like we can see the solar neutrinos through the Earth, we should see muon-neutrinos both from above and below. However, more were detected from above than below.

Something must happen to muon-neutrinos during their journey through the Earth. Super-Kamiokande could detect them as electron-neutrinos or muon-neutrinos, but is not sensitive to tau-neutrinos. This is evidence that muon-neutrinos were changing flavour to tau-neutrinos.

Sudbury Neutrino Observatory

The Sudbury Neutrino Observatory detector, a 12-metre sphere containing 1000 tonnes of heavy water which is two kilometres underground. Credit: SNOLAB.

The solar neutrino problem was finally solved in 2001 through measurements of the Sudbury Neutrino Observatory (SNO). SNO is another Cherenkov detector like (Super-)Kamiokande, but it used heavy water instead of regular water. (High-purity heavy water is extremely expensive, it would have cost hundreds of millions of dollars for SNO to buy the 1000 tonnes it used, so it managed to secure it on loan from Atomic Energy of Canada Limited). Using heavy water meant that SNO was sensitive to all flavours of neutrinos. Like previous experiments, SNO found that there were not as many electron-neutrinos from the Sun as expected. However, there were also muon-neutrinos and tau-neutrinos, and when these were added, the total worked out!

The solar astrophysicists had been right all along, what was missing was that neutrinos oscillate between flavours. Studying the Sun had led to a discovery about some of the smallest particles in Nature.

Neutrino oscillations

Experiments have shown that neutrino oscillations occur, but how does this work? We need to delve into quantum mechanics.

The theory of neutrino oscillations say that each of the neutrino flavours corresponds to a different combination of neutrino mass states. This is weird, it means that if you were to somehow weight an electron-, muon- or tau-neutrino, you would get one of three values, but which one is random (although on average, each flavour would have a particular mass). By rearranging the mass states into a different combination you can get a different neutrino flavour. While neutrinos are created as a particular flavour, when they travel, the mass states rearrange relative to each other, so when they arrive at their destination, they could have changed flavour (or even changed flavour and then changed back again).

To get a more detailed idea of what’s going on, we’ll imagine the simpler case of there being only two neutrino flavours (and two neutrino mass states). We can picture a neutrino as a clock face with an hour hand and a minute hand. These represent the two mass states. Which neutrino flavour we have depends upon their relative positions. If they point in the same direction, we have one flavour (let’s say mint) and if they point in opposite directions, we have the other (orange). We’ll create a mint neutrino at 12 noon and watch it evolve. The hands more at different speeds, so at ~12:30 pm, they are pointing opposite ways, and our neutrino has oscillated into an orange neutrino. At ~1:05 pm, the hands are aligned again, and we’re back to mint. Which neutrino you have depends when you look. At 3:30 am, you’ll have a roughly even chance of finding either flavour and at 6:01 pm, you’ll be almost certain to have orange neutrino, but there’s still a tiny chance of finding an mint one. As time goes on, the neutrino oscillates back and forth.

With three neutrinos flavours, things are more complicated, but the idea is similar. You can imagine throwing in a second hand and making different flavours based upon the relative positions of all three hands.

We can now explain why Super-Kamiokande saw different numbers of muon-neutrinos from different sides of the Earth. Those coming from above only travel a short distance, there’s little time between when they were created and when they are detected, so there’s not much chance they’ll change flavour. Those coming through the Earth have had enough time to switch flavour.

A similar thing happens as neutrinos travel from the core of the Sun out to the surface. (There’s some interesting extra physics that happens here too. A side effect of there being so much matter at the centre of the Sun, the combination of mass states that makes up the different flavours is different than at the outside. This means that even without the hands on the clock going round, we can get a change in flavour).

Neutrino oscillations happen because neutrino mass states are not the same as the flavour states. This requires that neutrinos have mass. In the Standard Model, neutrinos are massless, so the Standard Model had to be extended.

2015 Physics Nobel laureates

2015 Physics Nobel laureates, Takaaki Kajita and Arthur B. McDonald. Credit: Nobel Foundation.

Happy ending

For confirming that neutrinos have mass, Takaaki Kajita of Super-Kamiokande and Arthur McDonald of SNO won this year’s Nobel Prize. It is amazing how much physics has been discovered from trying to answer as simple a question as how does the Sun shine?

Even though neutrinos are shy, they are really interesting characters when you get to know them.

Now that the mystery of the missing neutrinos is solved, what is next? Takaaki Kajita is now involved in another project in the Kamioka Mine, the construction of KAGRA, a gravitational-wave detector.

KAGRA control room

The control room of KAGRA, the gravitational-wave detector in the Hida Mountains, Japan. I visited June 2015. Could a third Nobel come out of the Kamioka Mine?

Perks and perils of a PhD

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Nijō Castle garden.

Pond in the gardens of Nijō Castle, Kyoto. A good spot for pondering. The castle has whistling floorboards, to warn you if an assassin is sneaking up on you. Modern buildings don’t do enough to warn you of assassins.

This blog has been neglected recently as I have been busy travelling, with conferences and meetings (with a little holiday in between) in Japan, Korea and Germany. I am now back in Birmingham where we have a veritable army of summer students. They are all enthusiastic, and seem to be doing well at both their projects and joining in lunchtime conversations. One asked whether it was a good idea to do a PhD? Travelling to interesting places in one of the perks of being an academic, but does it compensate all the hard work? Here are my thoughts on doing a PhD now mine is safely done but still fresh in my memory.

The third degree

A PhD is not a simple continuation of your studies. One of the things that surprised me was how different research is from study (although they may share many of the same skills). At school and undergraduate you learn: you pay attention in class, you do assignments and projects, you revise and you take assessments. If you work hard at these, you pick up new knowledge and skills, and end up doing well. (Wooh!) In research, you have to solve problems, to figure out how to do things that have never been done before (which may require picking up new knowledge and skills). This can be extremely exciting: you could be the only person in the world to know how to do something, but since you are trying something new, it could also turn out not to work… You can work hard in a particular area for days, weeks or even years, and it all come to nothing.

Research projects at an undergraduate level are different from those at postgraduate. The former are usually designed to be safely solvable, and even if things don’t work out, you come to the end of your time and be given marks for effort. It’s much harder to put together a PhD dissertation without results, and a lack of progress (perhaps especially if through no fault of your own) can be especially demotivating.

When asked about doing a PhD, the current PhDs showed varying levels of enthusiasm. This is usually correlated with how things are going and how close they are to finishing. Maggie, who is always keen on encouraging people to learn about science, has put together a list of 5 reasons why you should do a PhD. I think these neatly sum up some of main motivations for doing a PhD.

1. Freedom and flexibility

Academia enjoys a lot of freedom. And I don’t just mean that you don’t have to wear a tie…

You don’t have to work standard office hours, but can often schedule things around you. This can be especially handy if you have family commitments, you don’t function well in the morning, or just fancy an afternoon off. It can also have the downside of blurring the work/life divide. Working from home gives you the flexibility to work in your pyjamas, but it can also makes it easy to work evenings and weekends—perhaps the weekends are the best time to come in to the lab because there are fewer people trying to use the most shiny equipment. It can be difficult to maintain a healthy work/life balance, and it can also lead to ridiculous expectations that you can work all the time (or should feel guilty if you’re not). Of course, sometimes you have to visit the lab every two hours to look after your experiment, and there’s no flexibility at all.

As well of freedom in when you schedule work, there is also freedom in what you do. It’s difficult to predict where a PhD will go, but you can focus in on what you are interested in and what you enjoy. Your supervisor, future examiners and potential employers may disagree with you about what’s worthwhile researching, so you do have some constraints; however, as long as solve an interesting problem, it doesn’t matter as much as in industry if it’s a different one to the one you started with. Some of the best PhD projects I have seen (or been involved with) come about because the student came across a new technique they wanted to play with, read up on a different area out or just wanted to help answer someone else’s question. Procrastination can have some useful side-effects.

2. The title

Being a doctor is pretty cool. Not as cool as being the Doctor, but still, it can command some respect. However, that doesn’t mean you receive (or deserve) any special treatment. Contrary to popular opinion, your title doesn’t go on your passport. It does indicate that you are an expert in one particular area; however, this might be an obscure and unhelpful one.  If you are ever on a flight and the attendant asks “Is there a doctor of astrophysics on board?” you are probably sufficiently doomed that you might as well just stay seated and try to finish up your peanuts in whatever time remains.

In the end, it is having completed the difficult research and produced a quality thesis that is worth the respect, and not the extra letters with your name. If you are not interested in the the former, the latter will not give you the motivation to put in the time and effort to complete it.

3. To prove you’re smart

Leading on from the last point, a doctorate is a seal of academic quality. However, I really wouldn’t suggest doing a PhD because you need to validate your intelligence. You are intelligent whether or not you decide to go to graduate school, and one should never assume that someone is less smart because they lack a PhD—first, because you do not know what opportunities they may or may not have had in life, and second, intelligence is about more than academic achievements. If you’re a lazy writer, giving a character a PhD is an easy way to establish they are clever without having to think of way for them to show it. In real life, people will soon figure out how smart you are by interacting with you (if they are only interested in titles, find someone else).

Getting a PhD isn’t just a case of being smart, it’s not a prize on a game show. As much as intelligence, a doctorate requires determination. Undergraduate is like a sprint, you can work really hard for a short stretch (around exams) and then collapse. The quickest people will come out on top, but you could still take it at a jog and make it to the end provided you don’t mind being second. A doctorate is more like a marathon, its not enough to be fast, you need to be able to keep going, to pace yourself and to pick yourself up if you trip. Both can be exhausting and painful, but it’s much more important to figure out if you can really go the distance before starting on the 26 miles.

Perhaps you are unsure if you’re up-to-scratch and want to try a PhD to see? Finding out that you can do it may be a huge confidence boost! However, academia can also batter your ego as you’ll be surrounded by other equally intelligent people. I guess you just need to be happy with who you are.

Finally, parents may like to show off the achievements of their children, and you may make your friends proud, but it’s not them who have to spend the time in the library. Making people you love proud is wonderful, but so is spending time with them. A PhD can consume huge amounts of time, energy and attention (especially while writing up). It should be something that you want to do, not something other people want you to do.

4. Broadening your horizons

Academia does give you the chance to visit new places and work with people all over the world. I really enjoyed my summer travels and Maggie is currently observing at a telescope in Chile. Of course you don’t always have that luxury: sometimes conferences aren’t in interesting places or funding could be running short. My first two conferences were in Glasgow and Cardiff. Both are lovely cities to visit, but neither was a once-in-a-lifetime opportunity. If you are really keen on travel, then there are other careers that give you better excuses to travel. Or you could take a better paid job and just pay for yourself to go on holiday. Travel, like free coffee, is a perk, but it’s not enough to justify doing a PhD.

The KAGRA detector

I visited the in-construction Kamioka Gravitational Wave Detector (KAGRA) in Japan. It is being built underground, in an old mine in the Hida Mountains. You can see part of the vacuum tubing for one of the laser-interferometer arms in the foreground, and where they’re going to suspend a mirror from the room above in the background. It’s amazing engineering and the views outside are impressive too! They’re on a tight schedule, aiming for a first run (albeit at terrible sensitivity) this year.

More importantly, a PhD gives you the opportunity to come across new ideas and ways of looking at a problem; to work with interesting, intelligent people from a range of backgrounds, and time to examine the world (or Universe) in detail in many different ways. That might all be from your cluttered desk, but it can be really exciting.

5. For knowledge

Over the course of a doctorate you will learn many things: the best seminars for free food, how to manage you supervisor and lots of transferable skills. However, the big thing is your thesis. Through your research, you will contribute something to the sum of human knowledge. It may be revolutionary, it’s more likely to be something that will go towards a bigger question (with help from lots of others), but it could also be the discovery that this particular thing doesn’t work. You research will push back the boundary of the unknown. You will become a world expert in your area: no-one will know your research as well as you do. If there is a topic that really interests you, if there is something that you want to know more about, then a PhD gives you the chance to explore this.

In my opinion, this is the only reason to do a PhD. There are other benefits and perks, but this should be your motivation. A PhD is not just a training course, but is another step towards understanding everything. I think that is amazing.

The forbidden motivation

Having been through the list, you may think there is something missing. What about doing a PhD to get a job? There are few careers that require a PhD, and it may not serve any more advantage than a Masters. Doing a PhD probably won’t make you rich. It may make you more attractive to some employers, but maybe spending the same amount of time working your way up from a lower rung would be just as effective? Extremely few employers have any kind of hiring scheme for PhDs, so in many cases you would start at a similar level as someone with an undergraduate degree. Some areas, of course, have strong industrial links, so it’s easy to move across. In this case, doing a PhD can be a great option: you can even get to work with potential future employers during your study (and possibly get some extra funding). The usefulness of a PhD strongly depends on the area.

There is one domain where a PhD is the well-established first step. Academia. Many think of academia as the logical progression, but it is not. You are not guaranteed an academic job with a PhD. In the sciences, most PhDs will not continue in academia. According to a report from the Royal Society, only 3.5% of science PhDs in the UK end up with a permanent academic position, and only 0.45% become professors. Competition is extremely tough: the number of PhDs awarded is increasing rapidly, but the number of faculty positions is remaining constant. I do not think the situation is better in the arts. A PhD student should not expect to get a job in academia.

The answer

A PhD is a big commitment, and requires careful contemplation. There are many reasons why you might be considering doing one; however, I think that if you’re going to enjoy the experience, the motivation you need is the desire to spend several years getting to know one particular area really well. You must be happy to invest years of your life without any guarantee of returns. You’ll pick up many useful skills, but that will not make you irresistibly desirable to employers—some will regard you as overqualified, and the prospects of an academic career are slim. You will receive opportunities you wouldn’t otherwise, in particular, to meet some awesome people. A PhD is a challenge, research can be both deeply rewarding and excruciatingly frustrating (sometimes in the same afternoon). On balance, if it is the right thing for you is deeply personal. As is common in research, to answer this question, we need further data.

Should you decide to go for it, the next thing to think about is the area and the location. You should make sure you get adequate funding, and take care in picking a supervisor—always talk to their current students. Perhaps we’ll come back to these points later. Good luck!

Lanterns at the Jogyesa temple, Seoul.

Panorama of lanterns at the Jogyesa temple, Seoul. Beautiful, and you can keep following it around in circles, just like a PhD…