Harry Potter can do a lot of things that are impossible for mere mortals - he can even walk through walls: To reach platform 9 3/4, at which the train to Hogwarts School of Witchcraft and Wizardry stops, he and his classmates slip through a wall between platforms nine and ten. This impossible feat in real life is normal in the weird world of quantum physics. Particles such as electrons are actually able to cross insuperable energy barriers. Physicists refer to this effect as quantum-mechanical tunnelling. Now, researchers at the Max Planck Institute for Nuclear Physics in Heidelberg have been able to show, for the first time, that it takes electrons a finite amount of time to tunnel. Although the phenomenon has been known for nearly a century, it had been unclear whether the process of electron tunnelling takes time or is instantaneous.