  We study the pairs of projections $$ P_If=\chi_If ,\ \ Q_Jf= \left(\chi_J
\hat{f}\right)\check{\ } , \ \ f\in L^2(\mathbb{R}^n), $$ where $I, J\subset
\mathbb{R}^n$ are sets of finite Lebesgue measure, $\chi_I, \chi_J$ denote the
corresponding characteristic functions and $\hat{\ } , \check{\ }$ denote the
Fourier-Plancherel transformation $L^2(\mathbb{R}^n)\to L^2(\mathbb{R}^n)$ and
its inverse. These pairs of projections have been widely studied by several
authors in connection with the mathematical formulation of Heisenberg's
uncertainty principle. Our study is done from a differential geometric point of
view. We apply known results on the Finsler geometry of the Grassmann manifold
${\cal P}({\cal H})$ of a Hilbert space ${\cal H}$ to establish that there
exists a unique minimal geodesic of ${\cal P}({\cal H})$, which is a curve of
the form $$ \delta(t)=e^{itX_{I,J}}P_Ie^{-itX_{I,J}} $$ which joins $P_I$ and
$Q_J$ and has length $\pi/2$. As a consequence we obtain that if $H$ is the
logarithm of the Fourier-Plancherel map, then $$ \|[H,P_I]\|\ge \pi/2. $$ The
spectrum of $X_{I,J}$ is denumerable and symmetric with respect to the origin,
it has a smallest positive eigenvalue $\gamma(X_{I,J})$ which satisfies $$
\cos(\gamma(X_{I,J}))=\|P_IQ_J\|. $$
