- #1
Plaetean
- 36
- 0
I'm working through some intro QFT using Peskin accompanied by David Tong's notes, and have a question over notation. From Peskin I have:
[tex]
x^\mu=x^0+x^1+x^2+x^3=(t,\mathbf{x})
[/tex]
and
[tex]
x_\mu=g^{\mu\nu}x^\nu=x^0-x^1-x^2-x^3=(t,-\mathbf{x})
[/tex]
so
[tex]
p_\mu p^\mu=g^{\mu\nu}p^\mu p^\nu=E^2-|\mathbf{p}|^2
[/tex] with
[tex]
\partial_\mu=\frac{\partial}{\partial x^\mu}=\bigg(\frac{\partial}{\partial x^0},\mathbf{\nabla}\bigg)
[/tex]
Does this mean that:
[tex]
\partial^\mu=\frac{\partial}{\partial x_\mu}=\bigg(\frac{\partial}{\partial x^0},-\mathbf{\nabla}\bigg)
[/tex]
and if so, is there any reason why the upper/lower index flips when expressing a derivative compared with when writing just a normal vector? It's a bit of a pain when you're starting out so I'm guessing there must be a good reason for it that emerges later.
I'd also like someone to just confirm that I've taken this derivative properly (might seem a bit laboured but I want to make triple sure I've got the notation correct right away):
If we have a Lagrangian density of
[tex]
L=\frac{1}{2}\partial^\mu\phi\partial_\mu\phi
[/tex]the derivative with respect to dphi is:[tex]
\frac{\partial L}{\partial(\partial_\mu\phi)}=\frac{\partial}{\partial_\mu}\bigg(\frac{1}{2}\partial^\mu\phi\partial_\mu\phi\bigg)=\frac{\partial}{\partial_\mu}\bigg(\frac{1}{2}g^{\mu\nu}\partial_\mu\phi\partial_\mu\phi\bigg)=\frac{1}{2}g^{\mu\nu}\frac{\partial}{\partial_\mu}\bigg(\partial_\mu\phi\partial_\mu\phi\bigg)=\frac{1}{2}g^{\mu\nu}\frac{\partial}{\partial_\mu}(\partial_\mu\phi)^2
[/tex]
[tex]
=g^{\mu\nu}\partial_\mu\phi=\partial^\mu\phi
[/tex]
Thanks as always to you good folk.
[tex]
x^\mu=x^0+x^1+x^2+x^3=(t,\mathbf{x})
[/tex]
and
[tex]
x_\mu=g^{\mu\nu}x^\nu=x^0-x^1-x^2-x^3=(t,-\mathbf{x})
[/tex]
so
[tex]
p_\mu p^\mu=g^{\mu\nu}p^\mu p^\nu=E^2-|\mathbf{p}|^2
[/tex] with
[tex]
\partial_\mu=\frac{\partial}{\partial x^\mu}=\bigg(\frac{\partial}{\partial x^0},\mathbf{\nabla}\bigg)
[/tex]
Does this mean that:
[tex]
\partial^\mu=\frac{\partial}{\partial x_\mu}=\bigg(\frac{\partial}{\partial x^0},-\mathbf{\nabla}\bigg)
[/tex]
and if so, is there any reason why the upper/lower index flips when expressing a derivative compared with when writing just a normal vector? It's a bit of a pain when you're starting out so I'm guessing there must be a good reason for it that emerges later.
I'd also like someone to just confirm that I've taken this derivative properly (might seem a bit laboured but I want to make triple sure I've got the notation correct right away):
If we have a Lagrangian density of
[tex]
L=\frac{1}{2}\partial^\mu\phi\partial_\mu\phi
[/tex]the derivative with respect to dphi is:[tex]
\frac{\partial L}{\partial(\partial_\mu\phi)}=\frac{\partial}{\partial_\mu}\bigg(\frac{1}{2}\partial^\mu\phi\partial_\mu\phi\bigg)=\frac{\partial}{\partial_\mu}\bigg(\frac{1}{2}g^{\mu\nu}\partial_\mu\phi\partial_\mu\phi\bigg)=\frac{1}{2}g^{\mu\nu}\frac{\partial}{\partial_\mu}\bigg(\partial_\mu\phi\partial_\mu\phi\bigg)=\frac{1}{2}g^{\mu\nu}\frac{\partial}{\partial_\mu}(\partial_\mu\phi)^2
[/tex]
[tex]
=g^{\mu\nu}\partial_\mu\phi=\partial^\mu\phi
[/tex]
Thanks as always to you good folk.