- #1
Tertius
- 52
- 8
- TL;DR Summary
- I want to obtain the SEM tensor a from a scalar field that has been integrated over all space.
It seems the field φ(t, xi) could be integrated over all space to form a single temporal variable (which isn't a field anymore, but is just a function of time) as follows:
Φ(t) = ∫φ(t, xi)dxi
Suppose we then assume a Lagrangian from this temporal variable to be: L1 = -1/2 Φ'(t)2 + 1/2 b2 Φ(t)2
From this we can turn to finding the SEM tensor. From Carroll pg. 164, the SEM tensor for a scalar field with a Lagrangian density of the form
L2 = -1/2 gμν(∇μΦ)(∇νΦ) - V(Φ)
is given by:
Tμν = ∇μΦ∇νΦ - 1/2 gμν gρσ∇ρΦ ∇σΦ - gμνV(Φ)
The only difference between the two Lagrangians is the sign and form of the "potential" term. So, substituting in the second term of the first Lagrangian in for the second term of the second Lagrangian, since neither depends on the variation of the inverse metric, we can obtain the following as the SEM tensor for L1is given by:
Tμν = ∇μΦ∇νΦ - 1/2 gμν gρσ∇ρΦ ∇σΦ - gμνV(Φ)
Tμν = ∇μΦ∇νΦ - 1/2 gμν gρσ∇ρΦ ∇σΦ + gμνb2 Φ(t)2
That is at least what we get if we change the sign of the last term. However, when I leave it a negative, i.e. (- gμνb2 Φ(t)2), I get an answer that makes more sense for the T00 component (it looks like the hamiltonian total energy when I leave it negative, and has a subtracted term when positive). Am I making a mistake in doing this substitution?