I have a confusion regarding the notation that is used for infintesimal Lorentz transformations and the parameters that define the Lorentz transformation (used in various books such as Srednicki's and Weinberg's).

Both of the above sources first define an infinitesimal Lorentz transformation as $\Lambda^\mu_\nu=\delta^\mu_\nu+\epsilon^\mu_\nu$ which leads to the condition $\epsilon^{\mu\nu}=-\epsilon^{\nu\mu}$.

They then go on define a finite Lorentz transformation as $$\Lambda=\exp(\frac{i}{2}\epsilon_{\mu\nu}M^{\mu\nu})$$ where $M^{\mu\nu}$ are the generators of the Lorentz group. This makes it seem like $\epsilon_{\mu\nu}$ are the parameters of the transformation.

Based on the first definition, what I understand is that a general Lorentz transformation is got by exponentiating $\epsilon_{\mu\nu}=(\frac{i}{2}\Omega_{\rho\sigma}M^{\rho\sigma})_{\mu\nu}$ where $\epsilon_{\mu\nu}$ is the whole Lorentz transformation and $\Omega_{\rho\sigma}$ defines the parameters of the transformation.

It would be great if someone could clarify what is going on here.

  • $\begingroup$ What exactly do you need explaining? $\endgroup$ – gented Jun 27 at 7:53
  • $\begingroup$ I’m confused as to whether epsilon is the infinitesimal transformation or just the parameters of the transformation as it seems like it’s being used both ways. $\endgroup$ – adithya Jun 27 at 7:55
  • $\begingroup$ $\epsilon$ is indeed the parameter, the "infinitesimal angle" or however else people want to call it; from your notation it seems it is indeed used for both and you can just correct the latter notation with a new symbol. $\endgroup$ – gented Jun 27 at 7:58
  • $\begingroup$ @adithya they're the same thing $\endgroup$ – Avantgarde Jun 27 at 8:08
  • $\begingroup$ @Avantgarde I don’t understand. Can you please explain how they’re the same? Because as far I can see, in one case, epsilon would be a part of the Lie algebra while in the other it’s not. $\endgroup$ – adithya Jun 27 at 8:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.