But even a temporary or transient dipole moment can induce a (temporary) dipole moment in any nearby molecules (picture 3) causing them to be attracted to the first molecule. However at any instant the electron distribution around an atom or molecule will likely produce a dipole moment (figure 2) which will average out to zero over a period of time.
Figure 1 on the left depicts the average electron clouds of two nonpolar molecules. The average shape and electron distribution for non-polar molecules have a certain minimum symmetry or eveness. Dispersion forces (or London forces, after the scientist Fritz London) arise from the temporary variations in electron density around atoms and molecules.
