The important factor is not the absolute speed of the atoms but
the speed relative to each other. I would guess the article on
the rubidium atoms is related to making a Bose-Einstein
condensate, and slow relative speed is needed otherwise your
cloud of atoms just disperses instead of making a condensate.
Increasing the speed of an isolated atom makes absolutely no
difference to it, but if you increase the relative speed of
atoms in some assemblage the atoms will collide with each other
at that speed. As you increase the relative speed from the low
levels of the rubidium atoms the collision energy will grow to
the point where it ionises the atoms, so the atoms form a
plasma. If you carry on increasing the speed up to near light
speed the collisions between the nuclei will be so violent that
they completely destroy the atomic nuclei and break them into
showers of hadrons. We can be confident about what happens in
near light speed collisions, because this is exactly what the
RHIC does. The LHC has also done heav