  Black holes play an important role in linking microphysics with macrophysics,
with those of the Planck mass ($M_P \sim10^{-5}$g) featuring in any theory of
quantum gravity. In particular, the Compton-Schwarzschild correspondence posits
a smooth transition between the Compton wavelength ($R_C \propto 1/M$) below
the Planck mass and the Schwarzschild radius ($R_{\rm S} \propto M$) above it.
The duality between $R_{\rm C}$ and $R_{\rm S}$ implies a form of the
Generalized Uncertainty Principle (GUP) and suggests that elementary particles
may be sub-Planckian black holes. The simplest possibility is that the ADM mass
has the form $M + \beta M_P^2/M$ for some constant $\beta$ and this model can
be extended to charged and rotating black holes, clearly relevant to elementary
particles. Another possibility is that sub-Planckian black holes may arise in
loop quantum gravity and this explicitly links black holes and elementary
particles. Higher dimensions may modify both proposals. If there are $n$ extra
dimensions, all with the same compactification scale, one expects $R_{\rm S}
\propto M^{1/(1+n)}$ below this scale but $R_{\rm C}$ depends on the form of
the higher-dimensional wave-function. If it is spherically symmetric, then
$R_{\rm C} \propto M^{-1}$, so duality is broken and the Planck mass is
reduced, allowing the possibility of TeV quantum gravity. If the wave-function
is pancaked in the extra dimensions, $R_{\rm C} \propto M^{-1/(1+n)}$ and so
duality is preserved but the Planck mass is unchanged.
