When considering how to describe an interacting system, we should be very clear what it is we require. First of all, it will not be useful to consider the ground state; it will almost certainly be a highly complicated object and, apart perhaps from its formation energy from some other state, of very little interest. What we are normally interested in is what happens to the system when it is disturbed, by some experimental probe for instance. What is required then, are expectation values of operators, transitions between states, time development in particular circumstances, etc. In the sense of the previous chapter, it is the group of quasiparticles and their properties that will be of prime importance, since these relate to the excited states of the system.
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