# How can the D'Alembertian of a field be interpreted intuitively?

+ 2 like - 0 dislike
177 views

The D'Alembertian operator is defined as $$\Box = g^{\nu\mu}\nabla_\nu\nabla_\mu$$ For the Minkowski metric in Cartesian coordinates that is $$\Box=\frac{1}{c^2}\frac{\partial^2}{\partial t^2} - \frac{\partial^2}{\partial x^2} - \frac{\partial^2}{\partial y^2} - \frac{\partial^2}{\partial z^2}$$

Can it be intuitively described, just as a gradient or divergence, curl or Laplacian may be?

I'm looking for something similar to the interpretation of a Laplacian given in this question and answer.

This post imported from StackExchange Physics at 2015-08-29 05:13 (UTC), posted by SE-user MycrofD
I think it was Rudy Rucker who wrote that in 15 years trying, he "saw 4 d" spacetime for a total of 15 mins, and he didn't like the experience. Maybe the scalar properties are easier for some people to imagine, personally I can't do it.

This post imported from StackExchange Physics at 2015-08-29 05:13 (UTC), posted by SE-user Acid Jazz

+ 2 like - 0 dislike

The operator is just $\partial_t^2-\nabla^2$. So it is the difference between a "temporal laplacian" and a "spatial laplacian". Since laplacian measures curvature, this is basically telling you the difference in curvature between the spatial and temporal variation of the field.

One reason this comes up in physics is in describing elastic sheets under tension. In an elastic sheet, if there is (spatial) curvature at a point, the tension in the sheet will pull the point in order to flatten out the curvature. Thus the point feels a force in the same direction as the curvature. So by newtons second law, the point on the sheet will accelerate, that is, have a second time derivative, in the direction of the curvature. This is why you would expect the difference in the "temporal laplacian" and "spatial laplacian" to be zero.

If this operator is non-zero, then it means the temporal and spatial variations are inconsistent with each other, and it looks like there is an external force acting on the point in your elastic sheet.

This post imported from StackExchange Physics at 2015-08-29 05:13 (UTC), posted by SE-user NowIGetToLearnWhatAHeadIs
answered Jun 2, 2015 by (80 points)
Is it anything like picking up a rubber mat and giving it a shake?

This post imported from StackExchange Physics at 2015-08-29 05:13 (UTC), posted by SE-user John Duffield
@JohnDuffield Yes, I think that would be a good concrete example of the "elastic sheet" I was referring to in my answer.

This post imported from StackExchange Physics at 2015-08-29 05:13 (UTC), posted by SE-user NowIGetToLearnWhatAHeadIs
Thanks. Have you read work by Percy Hammond concerning electromagnetic geometry? "We conclude that the field describes the curvature that characterizes the electromagnetic interaction." I say this because curved spacetime is where space is inhomogeneous rather than curved. See Baez and Einstein, but people seem to think curved spacetime is curved space.

This post imported from StackExchange Physics at 2015-08-29 05:13 (UTC), posted by SE-user John Duffield

 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): Email me at this address if my answer is selected or commented on: 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:p$\varnothing$ysicsOverflowThen 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). To avoid this verification in future, please log in or register.