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.
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!
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