Internal and spacetime symmetries

physics.stackexchange.com/questions/106392/internal-and-spacetime-symmetries

The different only shows up for field, not with particles. For fields, there are two types of changes that we can make that can keep the Lagrangian unchanged as mentioned at Physics from Symmetry by Jakob Schwichtenberg (2015) chapter "4.5.2 Noether's Theorem for Field Theories - Spacetime":

spacetime symmetry: act with the Poincaré group on the Four-vector spacetime inputs of the field itself, i.e. transforming $L (Φ (x), \partial Φ (x), d x)$ into $L (Φ^{'} (x^{'}), \partial Φ^{'} (x^{'}), x^{'})$
internal symmetry: act on the output of the field, i.e.: $L (Φ (x) + δ Φ (x), \partial (Φ (x) + δ Φ (x)), x)$

From defining properties of elementary particles:

spacetime:
- mass
- spin
internal

From the spacetime theory alone, we can derive the Lagrangian for the free theories for each spin:

Then the internal symmetries are what add the interaction part of the Lagrangian, which then completes the Standard Model Lagrangian.

Table of contents 126 2
- Internal symmetry Internal and spacetime symmetries 2
- Spacetime symmetry Internal and spacetime symmetries 2

Internal and spacetime symmetries

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