What is the eligibility criteria for doing research in physics? I will be completing my MSc in mathematics, in India; and have before completed my PUC (11 &amp; 12th classes) in PCMC (Physics, Chemistry, Mathematics, Computer Science), and BSc in PCM (Physics, Chemistry, Mathematics). In the MSc Math here in India, the courses are pre-fixed, and involves uniform concentration of all the math.

If I want to switch to doing research in physics, how difficult would it be? Is it harder to change fields at the stage of a PhD or a postdoc (or later)?
What is the eligibility criteria for doing research in physics?


There are none.  

If you want to get paid, again, there are no fixed criteria but you had best be more qualified than the other applicants for the job.


  If I want to switch to doing research in physics, how difficult would it be?


You already studied a lot of physics plus some extra math.  It should not be so hard.  If you apply for PhD programs, you should clearly articulate why you are switching.  Many people do PhDs in areas that include both physics and math - if you go that route it will be easier than if you try to so something purely experimental.