I have asked this question before and I was unclear about the answer that I recieved. The way that it was explained before is that when an electron comes incontact with a gamma ray it goes backwards in time. The moving backwards in time of the electron is what we call the positron. Thus, this allows the electron to be in more than one place at the same time, and leading to the untimate conclusion that there may only be one electron in the universe. Does anyone have anything that they could add to this? Or do they have another solution. Also, do you know of any books or sites that would explain this in more detail.