  We first argue that the collective behaviour of the Cooper pairs created by
thermal fluctuations well above the superconducting transition temperature, Tc,
is dominated by the uncertainty principle which, in particular, leads to a
well-defined temperature, T^C, above which the superconducting coherence
vanishes. On the grounds of the BCS approach, the corresponding
reduced-temperature, ln(T^C/Tc), is estimated to be around 0.55, i.e., above
T^C \approx 1.7Tc coherent Cooper pairs cannot exist. The implications of these
proposals on the superfluid density are then examined using the
Gaussian-Ginzburg-Landau approximation. Then we present new measurements of the
thermal fluctuation effects on the electrical conductivity and on the
magnetization in different low- and high-Tc superconductors with different
dopings which are in excellent agreement with these proposals and that
demonstrate the universality of ln(T^C/Tc).
