  Conjectures play a central role in theoretical physics, especially those that
assert an upper bound to some dimensionless ratio of physical quantities. In
this paper we introduce a new such conjecture bounding the ratio of the
magnetic moment to angular momentum in nature. We also discuss the current
status of some old bounds on dimensionless and dimensional quantities in
arbitrary spatial dimension. Our new conjecture is that the dimensionless
Schuster-Wilson-Blackett number, c{\mu}/JG^{(1/2)}, where {\mu} is the magnetic
moment and J is the angular momentum, is bounded above by a number of order
unity. We verify that such a bound holds for charged rotating black holes in
those theories for which exact solutions are available, including the
Einstein-Maxwell theory, Kaluza-Klein theory, the Kerr-Sen black hole, and the
so-called STU family of charged rotating supergravity black holes. We also
discuss the current status of the Maximum Tension Conjecture, the Dyson
Luminosity Bound, and Thorne's Hoop Conjecture.
