Practical aspects -- how to change subfields between PhD and postdoc?

Question

For the context of the question, I'm interested in switching between different areas of a field -- e.g. I did my PhD in biological physics mainly involving simulations on a specific biological system, and I am now interested in pursuing more theoretical work (e.g. applied mathematics) still in biological physics but potentially on a different system.

So theoretically I know there should be no problem and my PhD should have prepared me for any new learning I need to do to catch up, but practically I'm wondering if there are 'better' ways of addressing some issues:

1. Background knowledge: I have some theoretical background, but not the familiarity of having done theoretical work daily in the past years. I would also have some catching up to do (probably several books to get through before I would be comfortable). When would be the best time to do this catching up? If I do it after beginning a postdoc, my postdoc work will be delayed during this (probably months) of catch-up time. Should I take time off between my PhD and postdoc? However then I have to explain an awkward unemployed period on my CV, and additionally not having an institution affiliation would take away my access to journals/libraries.
2. Competitiveness as a candidate: If I were a PI, I would favour candidates who had demonstrated a capability in the field already. How can I demonstrate an ability to do theoretical work if it was not a large component of my PhD? One thought I had was to look within my own institution for a postdoc advisor since it might be easier to convince them, but I'm also aware that staying in the same institution for my postdoc could be a red flag later in my career.

Also welcome tips on how to prepare for such an anticipated switch, if I still have a few years left of my PhD.

Answer 1

When would be the best time to do this catching up? If I do it after beginning a postdoc, my postdoc work will be delayed during this (probably months) of catch-up time.

A piece of post-doc advice that has worked for me is to have your fingers in several pies. Build up some new expertise while working on some low-hanging fruits from your existing background that will lead to immediate results.

If I were a PI, I would favour candidates who had demonstrated a capability in the field already. How can I demonstrate an ability to do theoretical work if it was not a large component of my PhD?

The qualities that advisors are looking for may vary significantly. If they are looking specifically for a candidate with a strong theoretical background, there's only so much you can do. Others will prioritize broader research skills, like

• a strong ability to finish projects (which you can show by building an appropriate track record; so be sure to have your pending papers out before you apply), and
• a strong ability to tell a good story (which you can show by being outstanding in your application materials and communication to them; basically, explain how you will help their team succeed based on your expertise).

Answer 2

An established method of making the transition is to take a 1 year Masters in Theoretical Physics or Mathematics after completing your PhD, or to attend a Summer School. This will probably take you out of your current university and could give you contacts to apply for a PostDoc position elsewhere.

While working on your PhD could take an undergraduate course in theoretical physics or mathematics at your own university.