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It all began with E=mc 2 . Einstein's original equation relating mass and energy looked like this: E 2= m 2c 4+ p 2 c 2 To get to the famous reduction, the square root of the above equation must be taken, which results in both a positive and a negative energy value (corresponding to +mc 2 and -mc 2). For years, this negative root was discarded, because physicists did not know what to make of a negative energy value. The first person to recognize the implications of this was Paul Dirac, who puzzled over the apparent existence of negative energy levels. Electrons tend to occupy the lowest unoccupied energy state, and as even the highest negative energy state will still be lower than the lowest postive energy state, electrons should all fall into negative energy states and disappear, which they obviously do not. Because electrons are fermions (which means that only one electon can go into each possible state- or two per energy level, one with each spin), Dirac reasoned that electrons don't fall into these states because they are already filled- which implies that we are surrounded by invisible, 'non-existent' negative energy electrons. Given enough of an energy boost, any of these negative energy electrons should burst into existence and be observable as an ordinary electron, seemingly coming from nothing but energy- but it would leave a hole behind in this 'sea' of negative energy electrons, identical to the electron we can now observe except that it would be opposite in charge: a positive electron. And so, the idea of the anti-particle burst into existence.
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