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Nuclear power stations exploit this idea inside their reactors where subatomic particles, called neutrons, are fired at the nuclei of uranium atoms, which causes the uranium to split into smaller atoms. Mass is just a super-concentrated form of energy and, moreover, these things can turn from one form to the other and back again.
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In Einstein's new world, mass became a way to measure the total energy present in an object, even when it was not being heated, moved or irradiated or whatever else. Beforehand, the mass of something in kilograms was just a measure of how much stuff was present and how resistant it was to being moved around. In relativity theory, Einstein introduced mass as a new type of energy to the mix. All these types of energy can be transformed from one to another, but nothing can ever be created or destroyed. Before Einstein, scientists defined energy as the stuff that allows objects and fields to interact or move in some way – kinetic energy is associated with movement, thermal energy involves heating and electromagnetic fields contain energy that is transmitted as waves.
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Like all good equations, though, its simplicity is a rabbit-hole into something profound about nature: energy and mass are not just mathematically related, they are different ways to measure the same thing. It says that the energy (E) in a system (an atom, a person, the solar system) is equal to its total mass (m) multiplied by the square of the speed of light (c, equal to 186,000 miles per second). At one level, the equation is devastatingly simple.