In 1927 the German-born Werner Heisenberg found out that the more we know about an electron¹s position, the less we know about its velocity and vice versa. This is called the uncertainty principle. It is caused by the fact that you can only determine the position of an electron to within a margin of error equal to that of the wavelength (frequency) of the light you use. At higher frequencies (smaller wavelengths) there is more energy which disturbs the velocity of the electron more. At lower frequencies and therefore lower energies, the electron's velocity is disturbed less but the wavelength is longer so there is greater uncertainty about the particle's position.

Einstein claimed that electrons have an exact position and velocity even though we cannot measure both exactly. Evidence from the last few decades suggest that he was incorrect about this. If we could capture an electron in a box and collapse the box so it became smaller and smaller and, consequently, know the electron¹s position more accurately, the electron would start moving frantically around and bounce off the walls unpredictably in a kind of quantum claustrophobia. This adds to the reality that we will never be able to know the exact velocity or position of an electron.