- #1
Higgsono
- 93
- 4
We make an infinitesimal Lorentz transformation of the Lagrangian and require it to be invariant. We then arrive at the following expression.
$$\epsilon^{\mu\nu}j_{\mu\nu} = P_{\mu}\epsilon^{\mu\nu}X_{\nu}$$ which can be written as
$$\epsilon^{\mu\nu}j_{\mu\nu} = -\frac{1}{2}\epsilon^{\mu\nu}(X_{\mu}P_{\nu}-P_{\nu}X_{\mu})$$
The left hand side is anti symmetric since ##\epsilon^{\mu\nu}## is anti symmetric matrix. Next in the derivation the book says that we can cancel ##\epsilon^{\mu\nu}## on both sides of the equation. And we get
$$j_{\mu\nu} = -\frac{1}{2}(X_{\mu}P_{\nu}-P_{\nu}X_{\mu})$$
But I don't understand why. How can we cancel ##\epsilon^{\mu\nu}## when we are summing over it's indices on both sides?
$$\epsilon^{\mu\nu}j_{\mu\nu} = P_{\mu}\epsilon^{\mu\nu}X_{\nu}$$ which can be written as
$$\epsilon^{\mu\nu}j_{\mu\nu} = -\frac{1}{2}\epsilon^{\mu\nu}(X_{\mu}P_{\nu}-P_{\nu}X_{\mu})$$
The left hand side is anti symmetric since ##\epsilon^{\mu\nu}## is anti symmetric matrix. Next in the derivation the book says that we can cancel ##\epsilon^{\mu\nu}## on both sides of the equation. And we get
$$j_{\mu\nu} = -\frac{1}{2}(X_{\mu}P_{\nu}-P_{\nu}X_{\mu})$$
But I don't understand why. How can we cancel ##\epsilon^{\mu\nu}## when we are summing over it's indices on both sides?