Thursday, 12 November 2020

general relativity - Deriving the 4-momentum of a free particle moving in curved spacetime


Consider a free particle with rest mass m moving along a geodesic in some curved spacetime with metric gμν:


S=mdτ=m(dτdλ)dλ=L dλ


L=mdτdλ=m(gμνdxμdλdxνdλ)1/2


The canonical 4-momentum Pα can be derived from the Lagrangian L using the following calculation: Pα=L(dxα/dλ)=m2dλdτ(gανdxνdλ+gμαdxμdλ)=m gανdxνdτ=m dxαdτ

where we have used the fact that the metric gμν is symmetric.



Thus, expressed in contravariant form, we have derived an expression for the 4-momentum Pα given by
Pα=m dxαdτ

using a completely general metric gμν.


Is it correct to interpret the components of Pα in the following manner:


P0 is the energy of the particle,


Pi is the 3-momentum of the particle in the i direction?


In other words is Pα the energy-momentum vector with respect to a local orthonormal basis?




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