Antimatter
In modern physics, antimatter is defined as matter composed of the antiparticles of the corresponding particles in "ordinary" matter, and can be thought of as matter with reversed charges and parity, or going backward in time. Antimatter occurs in natural processes like cosmic ray collisions and some types of radioactive decay. Antiparticles can also be generated at particle accelerators, but only a tiny fraction of these have successfully been bound together in experiments to form antiatoms. Total artificial production has been only a few nanograms. No macroscopic amount of antimatter has ever been assembled due to the extreme cost and difficulty of production and handling. Nonetheless, antimatter is an essential component of widely available applications related to beta decay, such as positron emission tomography, radiation therapy, and industrial imaging.
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