  Recently, the room-temperature superconductor (RTSC) was discovered as a
two-dimensional (2D) square lattice made of a metal wherein positive charges,
i.e. holes, were heavily concentrated. The experimental result for the critical
magnetic field $H_{c}$ was fully consistent with the view on the RTSC that its
lattice unit -- a metal island -- is filled with the Slater's atoms. Each
Slater's atom has the expanded diameter of 14.5 nm in order to have perfect
diamagnetism with the magnitude corresponding to a single flux quantum
$\phi_{0}$. Its expanded orbit is associated with the fine structure constant
$\alpha \approx \frac{1}{137}$. In this paper, another important critical value
-- the critical current $I_{c}$ -- is reported. It was found that the
supercurrent has achieved the ultimate speed of matter, i.e., the speed of
light, $c$. Beginning with a warm-up exercise for the Bohr's atom, how the
Slater's atom is formed and why the $c$ appears are shown. These considerations
lead to a simple view on the pairing mechanism of superconductivity, which also
gives an ample indication of the most mysterious physical number $\alpha$.
Finally, it is shown that the proposed pairing mechanism in terms of London's
canonical momentum naturally generates the perfect diamagnetic $\phi_{0}$ of
the Slater's atom, and the superconducting energy gap $\Delta$ is predicted.
