What directions can you take in theoretical physics?

I mentioned in my previous post that many students go to uni with the perception that all theoretical physics lies in the realm of particle physics. This is not unexpected, popular science books that talk about theory often focus on these areas or the mysteries of quantum mechanics from a purely particle perspective. TV programmes also don't focus on other areas of theoretical physics (maybe cosmology), or at least don't portray these fields as theoretical. So lets talk about some different fields and where you can go in theoretical physics, as well as a couple of opportunities to cross over into different disciplines!

Disclaimer: These 'fields' are not concrete or a dictionary definition, some would argue that what I put under a certain discipline is not correct, but I think they're reasonable. I also won't go too heavily into the techniques used in these areas, they're just general descriptions designed to convey a flavour of the field. I have more detail in some areas than others, there is no specific reason for this, I've added details where I think its appropriate but tried to keep each section brief(ish).

Condensed Matter Theory:
Condensed matter physics is the study of 'matter', generally the microscopic and macroscopic properties of material and larger scale systems of atoms. It is also interested in how and why such systems can change state.  Theorists in this area are concerned with a wide array of activities: for instance developing simulations of interesting systems like superconductors, as well as developing more analytical theories of such systems.

(Image Credit: Nobelprize.org)

Super-conductors and semi-conductors are two of the systems studied by hard condensed matter theorists, hard being literally hard materials. They seek to accurately describe the properties of these systems and predict new phenomena that could arise from interesting states of matter. They also contextualise these predictions and descriptions more generally in electromagnetism and circuit theory.

Soft condensed theorists can be concerned with gases and how they behave, whilst also using quantum mechanics they use classical gas mechanics and modify current 'laws' with the aim of describing the physical properties of gases more accurately. This can sometimes cross over into fluid dynamics which I'll talk about later. They can focus on super-fluids like Bose-Einstein condensates and their strange properties.

Theoretical Astrophysics:
This covers a wide range of subjects. Galactic dynamics is often concerned with predicting the motions of complex gravitational systems, utilising numerical general relativity. Galactic dynamics researchers also have to account for phenomena like solar physics to model the behaviour of these systems. Researchers can also be involved in modelling the development of structures in the universe, like galaxies.

(Image Credit: LIGO Caltech)

Speaking of stars some theorists focus on solar physics. This involves developing numerical models of the many complex aspects of stars, nuclear physics, thermodynamics, statistical physics and more advanced topics like magnetospheres.

Plasma physics links to this but is seen as a separate field. In the way that soft condensed theorists can study gases plasma physicists try and model the behaviour of plasma in relevant physical situations which are commonly astrophysics related such as stars. Plasma physics is also relevant to nuclear physics.

Cosmology:
In some areas strongly linked to theoretical astrophysics, cosmology is primarily concerned with the history and development of the universe and the structures within it, going from the Big Bang to predicting the future development of the universe. This involves thermodynamics, statistical physics, general relativity and quantum mechanics.

(Image Credit: Mapping Ignorance)

Cosmologists can focus on objects like black holes and supernovae, understanding their history and relevance to the development of the universe. Alternatively they may concern themselves with more theoretical topics like inflation, dark energy, space-time geometries or cosmic strings.

Theoretical Particle Physics/Particle Astrophysics:
This often publicised field covers an array of topics. Particle Astrophysics is the study of particles that arrive from space whereas Particle Physics focuses its work around accelerators and labs here on Earth. Theorists can be a blend of both so I feel a generalised description is valid.

(Image Credit: inspirehep.net)

Despite the publicised view a large number of theorists in this area are focused on improving approximate methods that are used accelerator science to measure rates of particle physics processes or predict what these should be. The other side can involve a number of activities, the main one is building new models of particle physics which aim to fix current unexplained problems or phenomena. Such models predict new particles which experimentalists hope to find. Outside of models some physicists are interested in (arguably) more fundamental problems. This can involve topics like string theory, or quantum field theories.

Theoretical particle astrophysics can also link to cosmology and as you might be guessing by now, theoretical physics can be quite the melting pot. Theorists in this area might focus on subjects like dark matter or neutrino physics.

Mathematical Physics:
Despite the fact I have said I'm pursuing particle theory, this is really the direction I'm heading in. It's not often mentioned as a sub field of theory though, perhaps because it's a different style of thinking and often seen as separate from theoretical physics.

(Image Credit: lorrainewise.wordpress.com)

The aim of mathematical physics is to bring the rigour of mathematical proof to theoretical physics, it is often not linked with experiments. Mathematical physics topics can include string theory, quantum gravity, quantum field theories but also fluid dynamics and many body systems. They can also focus on mathematics topics like differential/algebraic geometry or differential equations. Further discussion of this field is a more involved subject.

Theoretical Nuclear Physics:
Nuclear physics is still a very active field, especially regarding nuclear fusion. Theorists in this field often focus on improvement of numerical methods and study theoretical heavy element nuclear processes or study the aspects of fusion processes including plasma physics. My 2nd year course in nuclear physics taught me that this field is VERY hard, there are loads of quantum effects to take into account and this is certainly a field with lots still to do.

Quantum Information Science:
A reasonably new field, but one that is quickly expanding, this focuses on studying information and its relevance to physics and potentially computer science. This is often linked to condensed matter theory as the mathematical language they both speak is incredibly similar. Much of the current research also contextualises how to use the theoretical ideas in the real world for technological development i.e. quantum computers.

(Image Credit: sureshemre.wordpress.com)

Applied Mathematics (Cross-over):
An opportunity to cross over into a different field, theoretical physicists can often transfer to applied mathematics. Mathematical physics is often seen to lie within applied mathematics, but there are other fields of study that are available. These could include complexity science, mathematical biology, differential equations, fluid dynamics, financial mathematics etc.

Computer Science (Cross-over):
This can link to quantum information science but also classical information theory. Because physicists these days often do so much programming it is not unheard of for a physicist to transition over to computer science. This really isn't my field so all I can't comment hugely, but I know that topics like machine learning,  applied combinatoric and cryptography are certainly accessible to theoretical physicists. This can also have links with electronic engineering, developing theories which utilise new electronic systems being developed in condensed matter physics.

(Image Credit: Domain of Science)

So in conclusion, there's loads of places you can go in theoretical physics! The great thing about doing theory is that you don't have to spend a life time working in one of these areas, some researchers change their angle completely more than once during their careers. The important thing is to be excited by these areas, there's plenty of opportunity to mix them too!

One for all the Freshers! What if you didn't get into the 'perfect' uni?

If you’ve just got a place on a theoretical physics course well done! If you’ve got on a non-specific or other specialist physics course also well done and maybe I can convince you to pick theory options in future posts!

I’d like to address something that was on my mind for ages as an undergrad. I imagine it will be on some other’s minds at this moment as well.

You are not doomed to a non-theory career if you didn’t get into a ‘top’ university for undergraduate. Oxbridge, Imperial, Durham etc. I'm sure most want to be at a university like that for their degree but for various reasons not all us can be.

Myself, I went to a good (in my opinion) uni for undergrad, but it wasn’t renowned for particle theory (which is where I want to be heading) and it wasn’t a top 20 for physics or Russel Group. But now I’m doing a master’s at Imperial and hopefully my chances have improved.

So if you’re at a uni which for you personally causes concern about your future in theory, hopefully I can convince you why you shouldn’t worry so much.

Number 1: There are LOADS of different areas of theory, some crossing into applied mathematics. It’s not uncommon for students to start university thinking that all theoretical physics is to do with particle physics. That couldn’t be further from the truth. There’s a whole bunch of fields that attract great minds and also a lot more funding than particle theory does. The good thing about these is that because of the greater funding universities can take on more PhDs in these fields and keep more postdocs on as well. More on theory fields in another blog.

(Image Credit: The Map of Physics by Domain of Science)

Now you might find that one of these areas sparks your interest more than particle physics! In that case the threshold for university/course reputation is (I think) lowered to an extent. 

Number 2: A lot of uni’s are pushing the MSci as the best route for students to take. MSci’s are often great courses and there’s no escaping the fact that you get full government funding for the four years. But if you are concerned about what your uni can offer you, bare in mind you could do a three year BSc and then do an MSc at another uni. There are loads of great MSc courses out there which could provide a real boost to your odds if you go to the right place. In fact if you are considering very pure particle theory, leaning to or in the realm of mathematical physics then I would suggest doing a very specific MSc if you can. More on this another time.

Number 3: The ranking/reputation of a uni does not mean it is good at teaching theory specifically.  As I said earlier my uni was not tip top but it was good in the ranking/reputation department. But over the course of my studies it offered me a number of things supposedly ‘better’ uni’s did not. 

In Quantum Mechanics we were taught Dirac Notation in our 2nd Year (thank you Dr Sordi), a number of higher ranked uni’s did not introduce this key aspect of modern quantum mechanics language until 3rd year. Another example, in my Particle Physics module we covered basic Feynman diagrams (though from a qualitative stand point), again I have heard a top uni that doesn’t go into anywhere near this detail (thank you Prof. Cowan and Dr. Boisvert).

(Image Credit: Wikimedia Commons)

This is not to say that my course was objectively better, but that it offered me skills that other uni’s didn’t. In the same way, other uni’s will offer their students something that mine didn’t give me! This means that even if you are sat in a room with someone who studies at Oxbridge or a similar calibre of institution, bare in mind there’s a fair chance you BOTH know things the other doesn’t.

Number 4: Postgraduate study is WAY more important. This could mean a specific high level master’s but ultimately we’re talking PhD. I was constantly told when I asked for advice about doing theory that getting into a good PhD was the key. Because ultimately, undergraduate is a base knowledge, master’s level study is to transition you into current research knowledge and PhD is where you actually do something. Produce some good research for your PhD and hopefully you’ll be able to carry on in academia. The reputation of the institution you do your PhD at is FAR more important than your undergraduate university.

Undergrad is the first stepping stone, an important one but only the first. This might seem cruel to say if you’ve just finished A levels but trust me, you’ll forget about A levels after one term of a physics degree.

Now if you got into a fantastic institution well done! But equivalent to the above you will know very well it does not guarantee you success. This is none more true in physics as admission for PhDs are often heavily marks based. If you get a 2:2 from Oxford and someone from UCL gets a 2:1, unless you have specific circumstances or can demonstrate skills in other ways, that someone is likely going to beat you to a PhD place. 

So regardless of where you are heading, study hard! And most importantly, work with your circumstances. Not every academic started at Oxbridge, you don’t have to either to get where you want to be!