Why is it obvious that this Lagrangian is Lorentz invariant?

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SUMMARY

The Lagrangian density defined as ##\mathcal{L} = \frac{1}{2} (\partial ^{\mu}) (\partial_{\mu}) -\frac{1}{2} m^2\phi^2## is Lorentz invariant due to its scalar nature. The scalar field ##\phi## and its derivative ##\partial_\mu\phi## transform appropriately under Lorentz transformations, ensuring the invariance of the Lagrangian. This property is fundamental in the formulation of relativistic field theories, confirming that physical laws remain consistent across different inertial frames.

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Kara386
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We've just been introduced to Langrangians, and my lecturer has told us that the Lagrangian density ##\mathcal{L} = \frac{1}{2} (\partial ^{\mu}) (\partial_{\mu}) -\frac{1}{2} m^2\phi^2## is obviously Lorentz invariant. Why? Yes it's a scalar, but I can't see why it obviously has to be a Lorentz invariant one. Thanks for any help! :smile:
 
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Did you try performing a Lorentz transformation to see how it transforms?

##\phi## is a scalar field and its derivative ##\partial_\mu\phi## therefore transforms as a dual vector.
 

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