# Energy of Scalar Field

## Homework Statement

My question is just about a small mathematical detail, but I'll give some context anyways.
(From Rubakov Sec. 2.2)
An expression for energy is given by
$$E= \int{}d^3x\frac{\delta{}L}{\delta{}\dot{\phi}(\vec{x})}\dot{\phi}(\vec{x}) - L,$$
where L is the Lagrangian,
$$L=\int{}d^3{}x(\frac{1}{2}\dot{\phi}^2-\frac{1}{2}\partial_i\phi\partial_i\phi-\frac{m^2}{2}\phi^2).$$
To derive the expression for energy, Rubakov says that
$$\frac{\delta{}L}{\delta{}\dot{\phi}(\vec{x})}=\dot{\phi}(\vec{x}).$$
What I want to know is, simply, how does he get this expression for
$$\frac{\delta{}L}{\delta{}\dot{\phi}(\vec{x})}$$?

## The Attempt at a Solution

If I evaluate the expression, I just get
$$\delta{}L=\int{}d^3x(\dot{\phi}).$$

Where'd the integral go in Rubakov's expression?

## Answers and Replies

Matterwave
Science Advisor
Gold Member
I may be wrong, but I believe what is happening is a confusion between the Lagrangian and the Lagrangian density. Look at the expression for the energy, it has an integral in it, so probably the ##L## which appears in there should actually be the Lagrangian density ##\mathcal{L}## defined by ##L=\int d^3x \mathcal{L}##

Ah, that would make a lot of sense (and fix the weird unit problem). Maybe there's a tiny typo in the text.

Matterwave
Science Advisor
Gold Member
A lot of field theory texts refer to the Lagrangian density as simply the "Lagrangian", so the language might be confusing. Usually the notation is used so that the Lagrangian density is in a calligraphic font though.