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The universe is made of varieties of elementary particles. Physicists classify these into fermions and bosons. Modern cosmology can even give the ratios of number of particles such as the ratio of number of protons and photons in the universe.

Quantum mechanics tells us that an electron in the universe is identical to every other electron. They have exactly the same electric chage, same rest-mass, exactly the same responses to the interaction of the weak and strong force, and the same total spin. All other particles are also identical to these distinctive properties.

But what can distinguish two electrons is their quamtum states. The quantum state is a probability. And there are many kinds of probability. The probability of being in a particular location of spacetime. The probability of the spin pointing in a particular direction. And the probability of having a particular velocity and energy.

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# What is Identity in Physics ?

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