by Joel Beckles

On a sunny summer afternoon in England, we have a chat with Sarah Johnston, a former Astrophysics MPhys student at St Andrews, now doing her PhD at Durham University. Sarah was School president for Physics and Astronomy in the academic year 2020/21, and was the overall faculty president for Science and Medicine in 2021/22. She also (somehow) was involved in dance, musical theatre and charity events during her time at St Andrews. Among many things, we talk to her about her interests in Physics, her experience with student representation and what exactly makes fuzzy dark matter “fuzzy”.

So, where are you from, and what got you into Physics?

Okay, I’m originally from Glasgow. My parents very much tried to keep science open and accessible to me. I really liked a lot of different things when I was a teenager and for a long time, I did not think physics was the thing I was going to go into. I had a lot of other hobbies and interests but it was something that I kept going back to because it was always interesting, I think, because I found it difficult. I was never someone who found Physics super easy. It’s always been a challenging subject, but I like that.I liked the challenge and I like not knowing what’s going on. In high school, when we started looking at subjects, I was like ”Physics! This is great. I want to do this.”

I applied to university, really just hoping wildly that somewhere was going to take me, and then got my offer from St Andrews. I went to visit and knew this was the place I where wanted to be. I loved doing my MPhys and had a great time.I learned so much and discovered so much more about what I thought physics was than just what I had in my head as a teenager. I got to the end of my MPhys and was like, “Oh, I don’t know what I’m going to do with my life.” I applied for a bunch of PhDs just to see what was out there and ended up getting one I’m really passionate about. Now, that’s what I do.

Did you always know that Astrophysics is the branch you wanted to do?

I think I always knew I wanted to do astro. Space was just one of those things that always caught my attention. It always caught my interest from high school onwards basically. I knew I wanted to do astro, but I really thought that astro was just you having to sit and look at things through a telescope. I think I really firmed up that Astro is what I wanted to do when I realised that I didn’t just have to sit and look at things through a telescope, because I’m quite notoriously bad at using telescopes. When it got to second year and I realised that a lot of astro wasn’t actually just going and sitting in a field, I was like, “This is this is ideal for me.”. I actually do just want to be behind the computer all the time.

So yeah, it’s always something I’ve known I was going to do, but I think experiencing it first-hand really locked in that it was exactly what I wanted to do. There was a while where I was thinking about switching to maths and physics because I quite liked maths, but Astro cemented it for me that that was where I should be.

What got you interested in student representation?

I think I was passionate about speaking up where I thought things were wrong or being done badly or people weren’t being represented. So I really started from a disability standpoint, I guess. In my first couple years, I noticed that, while they were trying really hard, the department didn’t exactly have great disability support in place for students. I was quite passionate about this. So I start to being a class Rep in my second year and I just wanted to make sure that everyone feels included and happy and appropriately supported. I realised, through doing that, that appropriate support isn’t just disability support. That was the one that affected me the most so that was the one that I was most passionate about, but it was also career support and progression and social activities and stuff like that.

I did two years as a class Rep then realised that if I wanted to keep going, the next step where I wanted to do useful things was to be school president.  I was really unsure that I was actually going to be any good at it. I thought I was quite under-qualified, and that people weren’t going to like me very much but I spoke to lots of my friends and they were like “No, you’d be great, you should do this!” and I got elected. Then COVID happened, which was not something that any of us thought was going to happen. But yeah, I saw that there was a need somewhere for improvement to be happening and I felt like I had the skills, or at least I had the passion to gain the skills to make the improvement work.

How would you say you benefited from being involved in student representation?

I think for me it was kind of two-fold thing. One is the obvious stuff, which is the skills that you gain. You gain a lot of skills in public relations, in speaking to people, in diplomacy, and things like that – how you handle issues. I think everyone in the physics department as a whole is treated as an adult from the second they step through the doors but as a Rep, you are casually talking to staff about things that are really quite important. They never talk down to you. They speak to you like you’re on the same level as them. So gaining that kind of diplomatic reasoning skill, and also obviously organisational skills for organising events, leading teams and managing people – all those kind of things were huge benefits. The other side that I think I didn’t really appreciate until after I had done the role was just the amount of confidence and soft skills it gave me. I’ve always been a bit of a socially awkward person, so I was really concerned when I went into it that people aren’t going to want me as a president or aren’t going to want to be led by me. And it [the benefit] was actually just realising that I was capable of doing those things.

The things that I never thought were going to be part of the school president role are actually the things that are most beneficial in the long run. I was a bit sceptical about, you know, organising events and how useful that would sort of be. It’s great at the time when you’re president, but it has actually translated really well. The same skills that I was using when I was school president to organise socials for a year group have become the skills that are helping me organise conferences now that I’m a PhD student. It’s one of those things where you don’t even realise how much you gain from it until you look back and you’re like, “Where did I get this skillset from?”

What challenges did you have to face at the time?

Yes, I mean COVID was the largest one just in terms of it being physically present all year. It was the first time we’d ever had a “COVID freshers” happening and nobody really knew what to expect. No one quite knew how teaching was going to be. So I felt that was the obvious large challenge. I think other challenges that always came up are, you know, where the students and the staff sit in relation to one another and how they work with one another.

One of my big things was that I wanted the students to feel like they are coming to the staff and they’re working with the staff, not working for the staff because you can feel a bit like a little worker drone as a class rep sometimes, just doing all the mid-semester surveys and stuff. But it needs to be a functional two-way thing. Obviously, people are used to certain traditional things there was a certain element of having to go, “Okay, we’ve done it like this up until now. What if we just tried the other way, and if it doesn’t work next year, we’ll put it back”. I think a lot of it for me was me coming up with slightly outlandish ideas and going, “But can we just try it? What if we just try it once and see if it works?”. Some of them worked really well and some of them probably didn’t work so well and we learnt lessons from all of them.

What challenges did you have to face at the time?

I think the biggest difference for me was just how much hands-on time I had. With Physics, it was very much dealing with my peers and working with the class reps that they had. It was very much hands on: I was organising events, I was doing things. Faculty president was a lot more of a people-management role. So, it was line managing all of the school presidents, which was great because I think I got especially lucky that the school presidents I had were so insanely competent and just fantastic people. It’s line-managing them and their concerns, and dealing a lot more with the higher-ups in the university – looking at policies and long term things like that. I liked school president because it was so hands on. If I wanted to do an event, I just went “I’m organising an event” and then would go and find somewhere to get money from, book a room and sort out all the stuff for it. For faculty president, I needed to convince the University that they need to run an event because they’re the ones now who sort of organise everything. It was definitely a big change and it was also representing people whose Schools I hadn’t experienced personally. With Physics, you’re a Physics student so you know how the department’s running – you know the staff really well. I was suddenly dealing with problems from medicine and Biology and Computer Science and all these places that I had never touched before, and I had to learn how to cooperate with those members of staff and those student bodies as well, because every department has a personality and they’re all a little bit different. It was definitely a good role. It was just differently challenging.

So, how did you get time to breathe? How how did you manage it all?

Haha, looking back, I actually don’t know how I managed to do everything I did. I think honestly I just found lots of things that I was really passionate about that were quite different from one another. So when I wasn’t doing physics, I was maybe doing musical theatre stuff. And then when the musical theatre stuff would get stressful, I would switch over and do the charities campaign stuff. And when the charities campaign stuff was stressful, switch over and do the dance stuff. I sort of rotated my way through the different committees and groups that I worked for.

There were some times where maybe I didn’t breathe enough. There were definitely times where I was very under-slept and, you know, decisions that looking back were questionable about when pieces of work were being done. But overall, it was a balancing act of knowing that if I treated my degree like a job in terms of time scale, I was okay. I need to be doing my degree 9 to 5, five days a week. That still left me evenings and weekends to do with as I please. If I chose to fill those up with stuff, then that was on me. If I wanted to have free time, then I would, you know, pull out of doing certain things. I kind of used that [method] to balance it.

I think, honestly, that I just got very lucky that I found a lot of things that I cared about and I found a lot of groups of people I worked with who were so supportive of what I did, including staff and the School. They knew how busy I was and they were still very much like “When you need a breather, let us know. We can chip in.” Again, when I was faculty President, staff in the Union were very understanding of the amount of stuff that I did and that that really helped. I think it was just the “finding things you love so that none of your work actually feels like work” thing. I was very lucky that I managed to find so many things that that applied to.

What might you give as a piece of advice to someone who wants to manage a lot of stuff?

I think it’s follow where your energy comes from, because activities you love give you joy. They give you happiness, they give you energy to do them, and so you can keep doing them. If you let yourself get too bogged down in things that don’t give you that energy (just because you feel like, “Oh, I should do this role, or I should be in this sports team”), then you lose that kind of trail of energy. You just have to follow where your joy comes from.

And all-nighters are not okay in the long run, but once in a while they’re all right.

I like that advice. Getting on to slightly more recent things, how has your PhD been going? What are you getting up to?

It’s going great. It’s wildly different from anything I’ve ever done before, in a really good way. That keeps it fresh every day. I have a bit of a two-pronged PhD project. My official PhD is working on how we transfer astronomy codes that are written in C to Graphics Processing Units (GPUs), which are the devices that make graphics on screens, but can also be used in computing for doing lots of simultaneous calculations basically. But they don’t use the language C, so you have to work out a way to transport either your whole code or sections of your code over onto them. I work on an astronomy code called SWIFT, which is a cosmological simulation. I’m attempting to do the transfer of the code onto GPUs in hopes that we can speed up the code, make it a bit more energy efficient, etc. So I have a hardware/software programming/computer science sort of half of the PhD, and then my other half is completely unrelated.

I work on alternative dark matter models. Specifically, I work on one called fuzzy dark matter¹, which asks “What if we took quantum effects and put them into place in dark matter?”It’s not an alternative for cold dark matter, which is the most widely accepted [model], but what if we did it alongside? So a mixed fuzzy-cold dark matter scenario. I’m basically working out: parameters on how much fuzzy dark matter we could have in the universe, if it’s even possible to have it and whether we would be able to identify by observation.

I have a very sort of split PhD now, not related to anything I basically ever looked at in St Andrews, maybe minus the dark matter. Other than that, all completely new for the PhD, which is really cool because it was sort of like starting completely fresh again, but with more knowledge than when I started university the first time around. Starting over and getting to just try something completely different, and getting money to do it for four years, is just a bit nuts.

For the layperson, what makes fuzzy dark matter “fuzzy”?

Totally valid question. So basically, when we think of classical cold dark matter, which is sort of what you get taught in an astro module in University at the minute, you think about this dark matter that sort of interacts in a certain way with things. We know we can’t see it, it doesn’t touch light, it doesn’t interact with materials in the same way. But one thing we do know about it is that it doesn’t really interact with itself or large-scale interactions in the universe so much.

With fuzzy dark matter, we imagine it’s made out of an axion-like particle, so an incredibly light, very small particle. Because it’s so small and because of the range of masses and electron volts that it can occupy, the de Broglie wavelength of the particle becomes really long, so long that it’s sort of on the scale of a galaxy. What you get is when you look at the structure of the universe, rather than seeing these very distinct clumpy dark matter halos that you see in cold dark matter, it literally just makes everything look a bit more fuzzy. It kind of smooths out all those little clumps and you end up with the same overall structure, but not as much fine-grained detail. It [fuzzy dark matter] has a couple of physical properties inside the dark matter halos. One is that it gives us what’s called a soliton core. A soliton is a self-reinforcing wave packet – imagine a standing wave that’s bouncing back and forward and always has the same shape. That’s what the core of a dark matter halo becomes: essentially this soliton packet. So it has this really interesting behaviour in terms of how the profile of the dark matter halo looks in the galaxy. The kind of halo shape that we would recognise from measurements now have some supposed problems of core versus cusp halos, which are about how the dark matter is distributed.

This [fuzzy dark matter] gives [the halos] a sort of in-between structure, which makes it a lot more amenable to different measurements. Obviously, your structure depends on how much of this fuzzy dark matter you have. If 100% of your halo is made of this stuff, it gives it a really wacky-looking profile that we know can’t be correct. If you add just a little sprinkle of it in the mix, it can give you something that can quite well replicate what we see, with obviously a whole bunch of other caveats and problems. But it’s basically asking “What if when we add in quantum effects, we smooth out some of the problems that we get with regular cold dark matter?”.

That’s cool. Is your work related at all to Modified Newtonian Dynamics?

It’s not something I personally work on but there’s another PhD student in my year group who works on modified cosmology, rather than just specifically gravity. I don’t know his work well enough to tell you exactly what he would say, but I think he would say that they have a different set of problems going on. On the fuzzy dark matter side, I feel like there’s so many different models and I don’t honestly know if the one that I’m working on is the way to go. It may well be that it’s completely wrong as a model, but if we can prove that it’s impossible or we can prove that it’s really unlikely, that’s still just as important a scientific discovery. The more things we can rule out as being potential options, the closer we get to honing in on something that could be happening.

How would you say the stuff you learned from St Andrews helped? I know you said on the computing side, you’ve done a lot of new stuff?

I have to say, I think the computing at Saint Andrews was incredibly useful. At the time, I will admit I loathed it a bit. I had a difficult relationship with the second year Comp. Astro module. It really tested me in ways I’d never been tested by a module before.
I remember, I think it was Ian Bonnell saying these exact words to me at one point: once you learn to code in one language, regardless of what the language is, you pick up the mindset of how to code in others.
It’s an incredibly useful skill because across maths and physics, I learned Python and Mathematica and a bit of Maple and there were some opportunities to dabble in bits of other stuff like Fortran. I did quite quickly gain a really good coding mindset of rather than saying “I need to use a for-loop to do this thing”, I’d go “Here is my problem. Here is what I want to do with the code. How do I find the syntax, or a language that works?”

I work basically explicitly in C and C++ now and I never touched C before I started my PhD. I started my PhD absolutely blind and within nine months I was pretty proficient at using C, never having done an actual course or a module or really been taught it, just from sort of playing around. I got that good attitude from the way they taught the courses at St Andrews – of thinking “I need to think of this as being just a physics problem rather than a problem in Python or a problem in C”.

In terms of general problem-solving skills, the sheer amount of different stuff I got to cover in various modules was really beneficial to me, especially because I didn’t quite know where I wanted to go with Astro other than knowing that I wanted to do it. It was really great for me to have that very broad knowledge base to sit on because even though I work my day-to-day on the coding stuff and dark matter stuff, a lot of my department is based around galaxies, AGN [active galactic nuclei] or black hole stuff. Because I did a research studentship with AGN, I’m still quite up to speed on how their stuff works so I can follow along with their science pretty easily.

Do you have any general PhD advice for students maybe looking into applying?

Yeah, there’s a few things that I wish like people told me before I applied. One is don’t under-sell yourself because I think you get so self-conscious when you apply for PhDs or you look at doing anything after university where you’re like, oh, but I’m not good enough for it. You probably are. You’re probably exactly who they’re searching for, actually.

The second is not be afraid to start over and try something new. When I applied for the PhD project that I have now, I went into the interview thinking there was absolutely no way that I was going to get the project because I had none of the relevant skills that were written on the little description with desired qualities for applicants. I was just applying for it because it sounded really cool and I think I literally went into the interview and told them that I don’t know much about it, but that I’m willing to learn and I thought it sounded really good. I gave some examples of not even related things I’ve done before, but just times that I’d proved that I was nothing if not persistent with stuff. I was still their top choice candidate even after admitting I didn’t have the right skills, just because I was willing to learn. I’m willing to try.

Also, applying for PhDs is exhausting. From what I’ve heard, applying for postdocs and positions after PhDs is also exhausting but I think I did quite a lot of applications. In the end, I think I submitted 13 PhD applications. It’s more than you ever need to as a regular, normal person because I applied to some in the United States, Europe and the UK. I knew I wanted to do something related to coding, but other than that, lots of things were interesting to me. Even just writing a few can be really, really tiring, and it’s okay to take your time. Most of them come out in a block, but some of them come out afterwards and it’s not the end of the world if you don’t get one the first time around. Plenty of people get late offers and plenty of people take a year out and get a position afterwards. It’s not a make or break, and you are in the middle of your 4th or usually 5th year while you’re applying for them. It’s a hard year anyway. It was definitely the time that I needed most to take breaks from doing stuff and rest up because you write all these applications then you have like a month of break before all your exams are happening. Then you come back and you’re getting interviews done while you’re trying to do your project. It all just becomes a little bit overwhelming, but taking breaks is great. PhDs are great, but they are hard work to apply for and to get to do. Taking breaks is really important across the board.

What extracurriculars do you do now?

I’m a really bad example of PhD student because I still do a lot. Generally, the advice is that once you have a PhD, you should reduce the amount of extracurriculars you do. I do musical theatre and dance. I do technical stuff for theatre, so I set build and do programming and stuff like that. I do various voluntary sort of groups, especially kids groups. I work with some special education needs kids through the church that I go to. This year, I was a postgrad rep in Durham, and I do student welfare, and I think that’s it for the minute. Is that it? Yeah, I think that’s it:  arts, dance, music, student representation and welfare – my usual standard that it’s always been. But yeah, I would say most PhDs don’t do that much stuff. Again, I just do my “follow where the good energy comes from and pick up the things that you enjoy” attitude.

It’s great to hear that you haven’t slowed down. I don’t think she’ll mind me saying, but I interviewed the most recent physics president and she said that you were an inspiration.

Yeah, this is Anna, right? She was always a legend. She was nuts. She’s one of those people that I met where I was like, “You’ve got big things in your future.”. There were a couple people that I met like that, where I was like “I’m going to keep an eye on you”. The amount of people that I’ve come back across – the ones who are younger who just in general have been school presidents or reps or got into the Union or whatever, and the ones above me who I’m now meeting as these really prolific researchers! St Andrews has got a really good trend of these really efficient students who just go out into the world and do insane stuff.

Great! So any final words? Anything you want to say to St Andrews students, or say in general?

Honestly, I’m just really glad you’ve got the magazine going again. That makes me very happy. I saw when you guys’ first issue came out and I was like, “Oh, it’s still running! It still exists!” That’s so exciting because COVID again did a number on most things and we were just trying to get by. The magazine was always one of the ones that I was really sad about because an issue got released in February of 2020 – right before COVID. Then it was suddenly just nothing now – no magazine. So I’m really glad that it’s coming back.

It was great interviewing Sarah, and we would like to thank her for taking the time to chat with us. We wish her the best of luck with her PhD and many (many) other endeavours!

References

1. Dome, T., May, S., Laguë, A., Marsh, J.D.E., Johnston, S., Bose, S., Tocher, A., Fialkov, A. Improved halo model calibrations for mixed dark matter models of ultralight axions, Monthly Notices of the Royal Astronomical Society, Volume 537, Issue 1, February 2025, Pages 252–271 ↩︎