Is it possible to do a Masters in Condensed Matter theory and then a PhD in high energy physics?
Question like this is highly irrelevant. That's like asking if it is possible if your child can grow up and become a multi-billionaire. OF COURSE it is possible! The statistical phase space for it is not zero!
If that is the kind of answer you are looking for, then you just got it. But how does that help you?
People who keep asking question like this seem to forget that there is a difference between "is it possible" and "is it very likely". Secondly, why would someone want to take a circuitous route to doing a PhD in HEP by first doing a MSc in another area of physics? Thirdly, what is prompting someone to switch in mid-stream?
There are a whole bunch of unanswered questions with something as terse and vague as this. So the best that one can do is offer an answer that is equally vague, which is "Yes, of course!"
Edit: this topic also belongs in Academic Guidance. This is not a question on "careers".
If you are going straight to a PhD program (like in the US), no one cares about your masters. If you take a certain number of classes, you'll get a masters, if you don't then you won't. So you should take classes relevant to what you will be doing your PhD in.
However, as a condensed matter theorist, you would be taking some of the same classes. You need two semesters of QFT, and if you want to do holography, which people do from both HET and CMT backgrounds nowadays, you will need GR. The difference is in the special topics classes. The ones I have taken have been advanced condensed matter courses focused on research topics. My friends in het on the other have taken electives in strings and black holes for example. However, the classes I have taken in advanced cmt are pretty mathematical and use a lot of QFT, but coming from a different standpoint. For example, in CMT, if you are on the more field theoretic side, you will be doing a lot of RG, maybe Feynman diagrams, writing down effective actions, generating functionality, etc. the difference is you may or may not have Lorentz invariance, and if you decide to analytically continue to finite T, you get these nasty Matsubara sums.
The other thing to note is that a lot of people in HET have started to become interested in CMT over the past decade. For example, there is a professor famous in string theory who just wrote a paper on the fractional quantum Hall effect. Additionally, many people who are/where string theorists or in quantum gravity have been studying CM systems using the holographic correspondence. There are a lot of holography postdocs I know who did their PhD in HET but their postdoc working more with CMT people.
First of all, let me point out that I am talking about masters programs in Canada.
As an international student, I might find it difficult to find a high energy theory professor willing to supervise me, because the competition for these spots is very intense, I presume. Condensed matter theory might be an easier route to break into high energy theory, by first doing a Masters in condensed matter theory while at the same time self-studying all the prerequisites courses in high energy theory.
My problem is in finding a professor who would want to supervise me in high energy theory. I'm thinking of self-studying the high energy theory courses by myself and become familiar with the landscape of high energy theory research by reading journal articles.
If I want to do holography, must I not have my supervisor already working in holography?
I presume you are referring to this paper by Cumrun Vafa: http://arxiv.org/abs/1511.03372 ?
Does it work the other way around - CMT people working in HEP? Or does it depend on research trends?
There are professors in holography who come from the CMT side, but not as many of them. However, a lot of the people who came from high energy now do most if not all of their work in CMT and are also studying things like topological QFTs, emergent gauge theories, etc. Apparently FQHE is becoming quite a hot topic in HET. I attended a seminar on composite fermions given by a professor who also came from a HET background. This was actually given as a strings seminar, not a CMT seminar and lots of HET people came. That is the paper I referred to. I haven't read it, but the people I've talked to said it's pretty difficult to understand.
Something a lot of people don't realize is that field theory is the natural language of CMT. You actually have a lot more variety since these are low energy effective theories talking about emergent phenomena, not a standard model. At critical points you can have things like emergent Lorentz symmetry (z=1) and even SUSY (usually happens in Dirac/weyl semimetals or on the surface of a TI). All these CM systems can be described in field theoretic language in the continuum. You can even use a Wick rotation and describe the systems at finite T. Quantum phase transitions are studied using this machinery. People use RG with some effective theory of an order parameter to find fixed points and scaling properties in phase transitions. There are also a lot of areas of CMT involving topology which are incredibly mathematical.
An interesting fact is that the Higgs mechanism people talk about is actually the Anderson-Higgs mechanism. Anderson discovered the general idea first as an explanation for the Meissner effect.
It seems to be a trend these days that people in HET are becoming interested in CMT. The reason is because a lot of the tools used in either field are applicable to the other and CM systems are a good place to study applications of string theory. It seems that a lot of string theorists are now more interested in applying their ideas to model systems rather than calculating things like string scattering amplitudes. CM systems make quite nice "playgrounds" to use ideas from HET.
I've noticed that a lot of people who want to go into HET, see CMT as somehow easier and less fundamental. That's not true at all. It would argue that it is just as fundamental as HET. When describe CM systems, we are not really describing electrons, we are studying emergent phenomena due to collective excitations in a system. We are not talking about individual particles, sometimes there isn't even a quasiparticle picture available.
In terms of taking classes, I would not recommend self-studying core areas in HET, like string theory without taking a class. My friends have taken two semesters of it and are still reading Polchinski. Once you start a new chapter of a book like Polchinski, you are really on all the previous chapters as well since it takes a while to understand everything at a very deep level.
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