  Why does the physical 4-dimensional space have a 3 + 1 signature rather than
a 4 + 0 or a 2 + 2 for its metric? We give a simple explanation based largely
on a group-theoretic argument a la Wigner. Applied to flat spaces of higher
dimensions the same approach indicates that metrics with more than one time
dimension are physically unacceptable because the corresponding irreducible
unitary representations are infinite dimensional (besides the trivial
representation).
