Elements of Classical Field Theory C6, HT 2016 Uli Haischa aRudolf Peierls Centre for Theoretical Physics University of Oxford OX1 3PN Oxford, United Kingdom Please send corrections to
[email protected]. 1 Classical Field Theory In this part of the lecture we will discuss various aspects of classical fields. We will cover only the bare minimum ground necessary before turning to the quantum theory, and will return to classical field theory at several later stages in the course when we need to introduce new concepts or ideas. 1.1 Lorentz Group The Lorentz group L is of fundamental importance for the construction of relativistic field theories, since L is associated to the symmetry of 4-dimensional space-time. Using the Minkowski metric η = diag (1; 1; 1; 1), the Lorentz group L consists of the real 4 4 matrices Λ that satisfy − − − × η = ΛT ηΛ ; (1.1) which written in components (µ, ν; ρ, σ = 0; 1; 2; 3) takes the form µν σρ µ ν η = η Λ ρ Λ σ : (1.2) It is readily seen that the transformations µ µ ν x Λ ν x ; (1.3) ! with Λ satisfying (1.1) leave the distance ds2 invariant. Setting for simplicity the speed of light c to 1, one has 2 µ ν µ ρ ν σ ρ σ 2 ds = ηµν dx dx ηµν Λ ρ dx Λ σ dx = ηρσ dx dx = ds : (1.4) ! The Lorentz transformations (LTs) (1.3) are therefore consistent with the postulate of special relativity that tells us that the speed of light is the same in all inertial frames.