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For every fundamental particle, there is an euivalent particle of antimetter known as its antiparticle. Why do we think this?
The existence of antimatter was predicted by the Cambridge physicist Paul Dirac in the late 1920s. He developed an equation that could account for the effects of Special Relativity in the (then new) theories of Quantum Mechanics. When he solved the equation, it predicted the behaviour of the electron. This showed that it was probably a successful analysis. However, his solution also predicted the existence of another particle with similar but completely opposite properties to the electron. It had the same mass, a positive charge and negative energy states. He interpreted the solution to mean that, as well as the electron, there must be another particle that we don't normally come across.
He suggested that this was a new particle of antimatter. It was an anti-electron and he called it the positron. Soon afterwards, in 1932, Carl Anderson observed positrons in cloud chamber events that were triggered by cosmic radiation. He photographed a positively charged particle being deflected in a magnetic field. He showed that its mass was equivalent to that of the electron. Since then, the existence of antimatter has been verified many times. Indeed, there are particle accelerators that fire beams of positrons at electrons to create spectacular collisions. In 1995, scientists at CERN (the European particle accelerator near Geneva) managed to combine anti-electrons with anti-protons to make tiny amount of anti-hydrogen. In 2003, they were able to make it in larger quantities.