# What's the big difference between high energy physics and condensed matter physics?

Can anyone tell me the difference between high energy physics and condensed matter physics?

I am confused about how these concepts, such as second quantisation, Green's function and many others, in high energy physics are introduced into condensed matter physics?

• The concepts are the same in both fields. The difference resides in the systems to which they are applied. Condensed matter physics used to be called solid state physics. High energy physics Is the physics of high temperature systems. – Lewis Miller Nov 25 '16 at 13:51

Also, for a few years (I would say in the 70's-80's but I could say something stupid), condensed matter physicists were interested in many-body problems at a given temperature, so they were using the real partition function (i.e. defined by statistical physics) and Matsubara frequency technique (in terms of the Green's functions it is a simple way to implement temperature in time-independent many body problems), whereas high-energy physicists were mainly concern by single particle processes, so their $Z$ function was mainly a tool to simplify calculations and was just marginally connected to a partition function.