Today, I learned that using rubidium atoms at very low temperature in a Magneto-optical trap, one can experimentally show that the Lorentz classical derivation of absorption using dipole is valid. The atoms were laser cooled to show this. Why is this valid only at low temperature and not at room temperature?
This is because at cold temperatures their wavelengths expand and overlap, forming Bose-Einstein Condensates. Basically, they all act like one big rubidium atom.