- #1
binbagsss
- 1,254
- 11
## L(x) = L(\phi(x), \partial_{u} \phi (x) ) = -1/2 (m^{2} \phi ^{2}(x) + \partial_{u} \phi(x) \partial^{u} \phi (x))## , the Lagrange density for a real scalar field in 4-d, ##u=0,1,2,3 = t,x,y,z##, below ##i = 1,2,3 =x,y,z##
In order to compute the Hamiltonian I first of all need to compute the conjugate momentum:
## \Pi (t, x) = \frac{\partial L}{\partial \dot{\phi (x)}} ##
I can see it's coming from the second term : ##- \partial_{u} \phi(x) \partial^{u} \phi (x)) = - \partial_{0} \phi(x) \partial^{0} \phi (x)) - \partial_{i} \phi(x) \partial^{i} \phi (x)) ##, where I'm only interested in ##\partial_{0} \phi(x) \partial^{0} \phi (x)) ##,
But I'm unsure how to deal with the one upper and one lower index.
The Hamiltonian is ## H = \int d^{3} x \dot{\phi} \Pi - L(t, {x}) = \int d^{3} x (1/2m^{2}\phi^{2} + 1/2( \partial_{i} \phi )^{2} + 1/2\dot{\phi^{2}}) ##
I can see I clearly need to lower an index. So if do ##g_{\alpha u} \partial ^{\alpha} \phi \partial_{u} \phi = (\partial _{u} \phi )^{2} = (\partial_{0} \phi(x))^{2} - (\partial_{i} \phi(x))^{2} ## , I get the correct answer that ##\Pi = \dot{\phi} ##
however then surely I have found ##g_{\alpha u} \Pi ## and subbed in ## g_{\alpha u} L ## in H, as a pose to ##L##
Many thanks in advance.
In order to compute the Hamiltonian I first of all need to compute the conjugate momentum:
## \Pi (t, x) = \frac{\partial L}{\partial \dot{\phi (x)}} ##
I can see it's coming from the second term : ##- \partial_{u} \phi(x) \partial^{u} \phi (x)) = - \partial_{0} \phi(x) \partial^{0} \phi (x)) - \partial_{i} \phi(x) \partial^{i} \phi (x)) ##, where I'm only interested in ##\partial_{0} \phi(x) \partial^{0} \phi (x)) ##,
But I'm unsure how to deal with the one upper and one lower index.
The Hamiltonian is ## H = \int d^{3} x \dot{\phi} \Pi - L(t, {x}) = \int d^{3} x (1/2m^{2}\phi^{2} + 1/2( \partial_{i} \phi )^{2} + 1/2\dot{\phi^{2}}) ##
I can see I clearly need to lower an index. So if do ##g_{\alpha u} \partial ^{\alpha} \phi \partial_{u} \phi = (\partial _{u} \phi )^{2} = (\partial_{0} \phi(x))^{2} - (\partial_{i} \phi(x))^{2} ## , I get the correct answer that ##\Pi = \dot{\phi} ##
however then surely I have found ##g_{\alpha u} \Pi ## and subbed in ## g_{\alpha u} L ## in H, as a pose to ##L##
Many thanks in advance.
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