• Register
PhysicsOverflow is a next-generation academic platform for physicists and astronomers, including a community peer review system and a postgraduate-level discussion forum analogous to MathOverflow.

Welcome to PhysicsOverflow! PhysicsOverflow is an open platform for community peer review and graduate-level Physics discussion.

Please help promote PhysicsOverflow ads elsewhere if you like it.


PO is now at the Physics Department of Bielefeld University!

New printer friendly PO pages!

Migration to Bielefeld University was successful!

Please vote for this year's PhysicsOverflow ads!

Please do help out in categorising submissions. Submit a paper to PhysicsOverflow!

... see more

Tools for paper authors

Submit paper
Claim Paper Authorship

Tools for SE users

Search User
Reclaim SE Account
Request Account Merger
Nativise imported posts
Claim post (deleted users)
Import SE post

Users whose questions have been imported from Physics Stack Exchange, Theoretical Physics Stack Exchange, or any other Stack Exchange site are kindly requested to reclaim their account and not to register as a new user.

Public \(\beta\) tools

Report a bug with a feature
Request a new functionality
404 page design
Send feedback


(propose a free ad)

Site Statistics

205 submissions , 163 unreviewed
5,064 questions , 2,215 unanswered
5,347 answers , 22,734 comments
1,470 users with positive rep
818 active unimported users
More ...

  Why does the free hermitian scalar field Lagrangian need a factor of 1/2?

+ 0 like - 0 dislike

Why does the free hermitian scalar field Lagrangian need a factor of 1/2 even though the equation of motion is invariant if the Lagrangian is multiplied by any number, as in: \[L=\alpha \frac12(\partial_\mu\phi\partial^\mu\phi-m^2\phi^2)\]

asked Dec 29, 2015 in Theoretical Physics by ConservedCharge (30 points) [ no revision ]

This is for convenience as otherwise a factor of 2 would appear in the Euler-lagrange equations, though this doesn't change the final field equations. The factor can be changed to any positive number by rescaling the field; hence there is no loss of generality assuming it to be $1/2$.

Note also that the scale matters in the quantum case, as the path integral depends on $e^{i S/\hbar}$, where the action $S$ is obtained by integrating the Lagrangian density.

I wasn't sure whether the factor of half had significance. Does such a scale, which shows up in the action, have any non-trivial consequences in QFT?

@conservedcharge: As Arnold has explained, yes, it matters because we normalize fields for them to have some physical meaning and the normalization conditions are not contained in the field equations. When you calculate the transition probability amplitude, say from a free quantum motion to the same free quantum motion, you have to have this $1/2$ in $S$ in order to obtain unity.

Your answer

Please use answers only to (at least partly) answer questions. To comment, discuss, or ask for clarification, leave a comment instead.
To mask links under text, please type your text, highlight it, and click the "link" button. You can then enter your link URL.
Please consult the FAQ for as to how to format your post.
This is the answer box; if you want to write a comment instead, please use the 'add comment' button.
Live preview (may slow down editor)   Preview
Your name to display (optional):
Privacy: Your email address will only be used for sending these notifications.
Anti-spam verification:
If you are a human please identify the position of the character covered by the symbol $\varnothing$ in the following word:
Then drag the red bullet below over the corresponding character of our banner. When you drop it there, the bullet changes to green (on slow internet connections after a few seconds).
Please complete the anti-spam verification

user contributions licensed under cc by-sa 3.0 with attribution required

Your rights