The positively charged atomic nucleus is the source of intense electric forces, which are felt throughout the atom. On the one hand, these hold the negatively charged electrons in place, in the periphery of the atom; on the other, they repel positively charged invaders, such as alpha particles, violently ejecting them from whence they came. It had been this dramatic repulsion of the alpha particles that had first hinted at the existence of the massive, compact positive centre in the atom. The problem now was to see inside the nucleus, or even to get a direct look at it, as these same electric forces protected it from easy entry. So although the existence of a massive compact heart to the atom was established, its nature remained a mystery for nearly 20 years.