# A day in the life of a Theoretical Physicist

Following up on trends I see continuously on YouTube and other platforms, I thought I might talk a little bit about what a day in the life of a theoretical physicist looks like based on my own experience, given that not many people might know much about this. Firstly, I must point out that I am not technically a “theoretical physicist” yet, since I am still in the process of completing my PhD in the next couple of months. There are a lot of people who prefer not to call themselves “physicists” until they have acquired the title of “Dr.” next to their name, and there are others who like to use the term as soon as they complete a Bachelor’s degree in the discipline. In my case, as long as someone is doing physics related research at a university or company, they can call themselves a physicist. In this story, I will answer some questions about what it is like to work in theoretical physics research, from the point of view of a PhD student who has been doing this for many years.

# What does a theoretical physicist do?

So let’s start with the first thing: what does a theoretical physicist do? The short answer is: a lot of maths and a lot of computer simulations. Long gone are the days in which theorists would do all their work on pen and paper. Since the advent of computers, theoretical physicists have made ample use of numerical programming to solve complicated mathematical problems that are unsolvable by hand. Therefore, a theoretical physicist is someone who can work with mathematical problems not only on giant whiteboards and paper, but also in the context of computer programs, often doing most of the coding themselves. This is essentially what my entire PhD has been like: manipulating and re-arranging equations by hand on a piece of paper to get them into the perfect format before coding them on a program like Python or Matlab. Most of the equations I deal with (and every other physicist does in general) are what we call “differential equations”, particularly “partial differential equations”. These equations involve some unknown function(s) of one or many variables (like space, time, etc.) and its derivatives (in the context of calculus, a derivative tells you the rate of change of a function with respect to some other variable, for instance velocity is the time derivative of displacement). These equations are often extremely hard to solve, so I use numerical methods to approximate solutions to a certain degree of accuracy, and test them against “special cases” for which I may be able to find an analytic (or exact) solution by hand.

In fact, I think I have spend more time trying to find analytic solutions by hand than actually coding up numerical solutions to such equations, so I guess most of my work as a physicist is still very “pen and paper”-ish. When analytic solutions are not possible at all, I try to use what I already know about physics to answer fundamental questions, such as “does my solution obey the laws of conservation of energy and momentum?”, “does the system behave as I would expect under these specific conditions?”, “does the numerical solution make sense in terms of both mathematical properties (e.g. continuity, differentiability) as well as physical properties (e.g. changes to the system over time)?”, and so on. Physical intuition is extremely important to my work, as without it I would not know how to interpret the results I am producing or explain them in a coherent, sensible manner.

In a way, I would summarise the work of a theoretical physicist as follows:

• Theoretical physicists seek to develop mathematical models and general theories for explaining physical phenomena that we can observe (directly or indirectly) in nature. They need to have a very strong understanding of mathematical methods as well as physical principles.
• These theories and models can often be extremely abstract and complicated, such as in the case of particle physics, so it is important to keep in mind that whatever theorists come up with must be measurable or testable in some way. If experiments do not agree with theoretical predictions, then theorists need to revisit their assumptions and change them accordingly, perhaps by adding or removing mechanisms in them, or starting from scratch.
• Theoretical physicists also validate their assumptions using computer simulations, which make it a lot more efficient and cost-effective to run experiments once they know what to look for or what to measure. Computer simulations also help with discovering new physics.

# What is the daily work like?

Another important question to ask. The work itself is a bit weird I must admit. I don’t work the usual 9–5 job, but I do have to work a lot of weekends (most of them anyway) and public holidays. Sometimes my work might start early morning (e.g. 7am) and go until perhaps 1pm, but other times it could easily go until midnight. I testify that I have had to pull many all-nighters in the past 3 years of my degree. I have also worked during every single one of my birthdays in the past 4 years (which is not too bothersome for me), but it comes to show that the work is extremely involved and unstructured.

The amount of work dedicated to each day depends on the short term goals I am trying to achieve, which also relies on what the other people in my research group want. If we are working on a research paper for example, we set timelines for each week, and work step by step on different aspects of the paper, starting with the results (because no results = no paper), and then analysing those results to the death (when I say this, I am not exaggerating: every single detail must be accounted for, as if counting subatomic particles in an olympic swimming pool). This also means that I have multiple research meetings per week (often 2–3 times a week), where we review any progress made so far and come up with new tasks to work on for the next few days. It is a tedious process, but it helps with getting papers done quicker.

Most of my work these days consists of simulations in Matlab, which I code from scratch using a bit of background knowledge in numerical methods as well as some wizardry. The coding is very exhausting, writing up 1,000+ lines of code in one day is not at all uncommon, and requires a lot of troubleshooting (troubleshooting in fact makes up 95% of the time spent coding in my case). The codes must be rigorously tested against known exact solutions, and we must also investigate how the simulations behave under different conditions in order to identify possible weaknesses or errors. I also have to do a lot of convergence testing, which essentially means you run a simulation with different step-sizes (e.g. in space or time) and see whether the solution approaches some constant value as the step-sizes decrease (which is what we call “numerical convergence”).

Prior to all the coding I do now, most of my work was centered around deriving solutions to equations by hand (if possible), and trying to gain physical insight into the nature of those solutions by using physics principles. I would say my entire PhD has thus far been around 70% pen and paper and 30% programming, which is fine by me since I always enjoyed working problems out by hand more than coding.

# What is the main goal of physics research?

This is a bit of a complex question because physics has changed so much from what it was in the 17th-20th centuries. Physics research today serves two main purposes: to search for “new physics”, and to enhance our knowledge on existing physics so that we can come up with better engineering applications in the real world. Physics in the 21st century is therefore a cross-over between pure physics (knowledge for the sake of knowledge) and engineering. Many people will dispute whether the engineering aspects of physics can truly be called “physics research” since there is very little distinction between “applied physics” and “engineering”, but I don’t see why this is so problematic, I mean: most of my research is “physics-based” in the sense that I am looking at fundamental behaviours and mechanisms, but at the same time the work I do is tailored specifically to a sub-set of engineering applications, so I need to keep my theories and models practical and realistic at the same time.

Nevertheless, I do envision physics and engineering merging more and more in the near future, because I have seen many engineers doing physics work, and I have also seen many physicists doing engineering work. And there is a lot of collaboration between the two when it comes to large research projects (e.g. the large Hadron collider). So perhaps in 100 years there won’t be much of a distinction between the two and we will just be called “phyngineers”.

# Is theoretical physics work satisfying?

It depends on the person and how much they enjoy this kind of lifestyle. I would say the work is definitely interesting if you are curious about the inner workings of nature and you absolutely love solving mathematical problems, and coming up with your own abstract theories about how “things work”. At the same time, I can’t say that the work is easy or well-balanced. Throughout my PhD I have worked on average around 60 hours a week all-year round. Needless to say: it gets very tiring. Even when I am not “technically doing the work” I am at least “thinking about the work very seriously”, which basically means my brain is never fully switched off. So I would say as a long-term career (e.g. 20+ years), theoretical physics is probably not the most sustainable profession for a human.

Another aspect which is a little bit underwhelming about theoretical physics in the 21st century is that the research you do will probably not be ground-breaking on any fronts. It was way easier in past centuries to do ground-breaking work, even solving a single equation by hand was enough to grant you a Nobel Prize. These days, out of the millions of physicists doing research around the world, you might have a better chance at winning the lottery than getting a Nobel Prize for your work, especially as a theorist. Some people may find this a bit too pessimistic, but it is good to maintain realistic expectations in order to say sane. After all, you shouldn’t become a theoretical physicist if you are expecting a lot of prestige or awards from it. The most you will probably get in your career is a few handshakes from other physicists.

# Final thoughts

In conclusion, theoretical physics is a difficult profession. The work entails a lot of mathematics, problem solving, abstract thinking, and computer programming. The work is interesting but also very long and tedious, and at times it feels like it is leading nowhere. But despite all the challenges it presents, I think it is important work, especially if we want to develop a deeper understanding of nature and its inner workings.

Oscar is a physicist, educator and STEM enthusiast. He is currently finishing a PhD in Theoretical Physics with a focus on photonics and stochastic dynamics.

## More from Oscar Nieves

Oscar is a physicist, educator and STEM enthusiast. He is currently finishing a PhD in Theoretical Physics with a focus on photonics and stochastic dynamics.

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