This blog is dedicated to a question a lot of undergraduates applying to graduate school in physics ask me before they have obtained any offers (typically during the application process): what should I choose to study in graduate school? They appear to be confused by the question, and agonize over it quite a bit.
For some thoughts on this issue, read on:
i) It may be too early to decide. Some students might not have had exposure to research in any field. Some might have had some exposure. Even those with substantial exposure - say a first author paper - may be advised, in my opinion, against becoming too set on pursuing that same field in graduate school. I think it is fine to say that's what you are interested in, in your application, and relate it to your experiences to build your case for being admitted, but you should keep an open mind when you reach the PhD program.
In my own case I spent a summer as a senior undergraduate working in an experimental group handling a medium energy particle accelerator (at TIFR). I applied to graduate school with the stated intention of becoming an experimental high energy physicist.
A week after my arrival (at the University of Rochester), late in August 1995, I met a new-ish professor walking along the hallway with Science magazine in his hand. On the cover were images of a Bose-Einstein condensate; this new state of matter had just been made, was generating buzz, and soon would be recognized with a Nobel prize (in 2001 - that's very soon, for a Nobel).
The professor was working on similar things and took me downstairs to his basement lab where lasers were being used to cool atoms. This changed my mind, and I chose to do my thesis on optics instead of high energy physics.
In this fashion (sic) you may change your mind as related/new fields come into being while you are getting in to grad school or taking courses there: laser cooling was an example from my grad school days; a more recent example is the explosion in gravitational wave astronomy.
ii) Sometimes, it may not matter. I obtained a PhD in experimental cold atom physics and now work in theoretical quantum optics. I have many colleagues who have made similar or more dramatic switches (string theory to theoretical solid state physics; condensed matter theory to experimental atomic physics; astrophysics to solid state physics, etc).
There is plenty of distinguished company here: for example, Max Delbruck and Venki Ramakrishnan won Nobel prizes in Medicine and Chemistry, respectively, although they had PhDs in physics. Pierre Gilles de Gennes worked on neutron scattering and magnetism for his PhD, then switched to the study of liquid crystals, for which he received the Nobel prize.
Depending on the problem you become interested in, with some effort, you can change your field even after your PhD. Thus there is - perhaps - no wrong choice (of subject in graduate school).
iii) Switching fields might even be beneficial for your career. In a previous post I described how Paul Corkum was able to make pioneering contributions to attosecond physics by applying his PhD training in plasma physics to the problems he faced in optics.
Indeed if we go back a bit and look at the great theoretical physicists, for example, many of them have contributed in diverse fields: Landau (solid state physics, quantum electrodynamics); Onsager (electrolytic solutions, magnetism, superfluidity, thermodynamics); Feynman (quantum electrodynamics, superfluidity, particle physics), C. N. Yang (particle physics, condensed matter physics, statistical mechanics - he's still going strong at 101!). More recently, Anton Zeilinger (from neutron scattering to optics - I'm not saying anything about neutron scattering here!).
I think the physics PhD not only trains us in a specific discipline, it trains us how to learn, and therefore become able to switch fields.
iv) The decision might be made for extra-scientific reasons. You may find a well-funded, charismatic mentor in one of the two fields you are interested in, and not in the other. You may get along better with the students in one group than the other. And so on.
Conclusion
Many graduate schools now have students rotate between various groups so they can get a taste of everything available before they make a choice. This should help remove much of the selection anxiety that entering-level students experience, and lead to better-informed decisions both in the part of the student as well as the advisor.
This is also the reason I advise students to apply to schools where they will have plenty of choices for research disciplines, rather than that one university which has a world class research program in the single discipline that they are currently interested in. What are you going to do, I ask them, in case you change your mind?
Of course there are many more challenges waiting along the way until students finish their PhD, but once they are in graduate school there may not be as many wrong choices as they fear (as far as subject choice is concerned).
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