  The cosmic microwave background covers the entire universe, which suggests
the absence of ay closed system, except the universe itself. In this paper, I
consider the effect of the cosmic microwave background on the hydrogen atom,
which must be very small, otherwise, changes in energy levels would have been
measurable. But how small is small? This I compute by considering a system in
an environment or bath. I derived the bath's, (in this case the CMB) effect on
the hydrogen atom in the Feynman-Vernon approach to an open system. The effect
is small and quantified in terms of a correction to the hydrogen atom that
breaks time-reversal symmetry, as expected of memory effects. This is
significant. There are imperceptible changes in the state of the hydrogen atom,
which means that the pervasive CMB must have similar small effects on other
atoms, thus breaking time-reversal symmetry in all physical systems.
