Do the conduction electrons in a copper wire act quantum mechanically or not? roughly speaking, quantum mechanics comes in when the de broglie wavelength of an electron is comparable to or longer than the spacing between atoms. the lattice constant (roughly the spacing between atoms in a crystal) for copper is about 4 x 10-10 m. the drift velocity of electrons is typically less than 102 m/s under household conditions (b) what about atoms in the interstellar medium? here we want to compare the average spacing between atoms (they move! ) to the de broglie wavelength of an atom. the interstellar medium is mostly hydrogen at an average density of order 1/cm3 and temperature about 3 k. in this case you' need the thermody- namic resul p/(2m)(3/2)kt, where k is boltzmann's constant (c) now let's switch gears to talk abou the uncertainty principle when ligo detects gravitational waves, it is measuring displacements of its test masses of order 10-18 m. the test masses are about 40 kg. what is the minimum uncertainty induced in the velocities of the test masses?