Intuition behind eigenfunctions of the Laplacian operator I'm reading about the notion of spectral dimension which is a measure of how particles diffuse in some space at different scales. An important aspect of spectral dimension is the eigenvalues/eigenfunctions of the Laplacian operator which in some sense determine what scale the diffusion process is probing. I was wondering if there was some geometrical interpretation of the eigenfunctions of the Laplacian that can give some how they relate to the notion of scale.
Pages 3-5 of this paper explain the context of my question.
 A: There are two scales at play here.
Let $M$ be a closed Riemannian manifold and let $\Delta$ be the Laplace-Beltrami operator on it.
There is an orthonormal eigenbasis $\{\phi_k\}_{k=1}^\infty$ of $L^2(M)$ and an increasing sequence of eigenvalues $\lambda_k\geq0$ so that $\Delta\phi_k=-\lambda_k\phi_k$.
The piece of an article you linked to discusses the heat equation (and the related heat trace).
For any $k$, the function $u_k(x,t)=e^{-\lambda_kt}\phi_k(x)$ solves the heat equation $\partial_tu=\Delta u$.
(You can expand a more general initial condition in terms of the eigenfunctions and write a solution to the heat equation as a series of $u_k$s.)
The eigenvalue $\lambda_k$ determines the time scale of the decay of the eigenfunction $\phi_k$ under the heat equation; if $\lambda_k$ is large, then $u_k\to0$ quickly as $t\to\infty$.
On the other hand, $\lambda_k$ is related to the scale of oscillations of $\phi_k$.
This is most transparent in the one dimensional case $M=\mathbb R/2\pi\mathbb Z$ (the unit circle).
The eigenfunctions are $\phi_m(x)=e^{imx}$ with eigenvalue $\lambda_m=m^2$.
(Here $m$ ranges over $\mathbb Z$ for convenience. Other eigenvalues than zero are degenerate.)
The number $m^2$ describes how rapidly the function $\phi_m$ oscillates, right?
The picture is similar in any dimension, but it is generally difficult to find explicit eigenfunctions.
That is, short time scales correspond to long distance scales (and both correspond to low eigenvalues) and vice versa.
Does this interplay between the two scales and their relation to the Laplacian answer your question?
