Quantum tunneling

Quantum tunneling is the quantum-mechanical effect of transitioning through a classically-forbidden energy state. It can be generalized to other types of classically-forbidden transitions as well.

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Consider rolling a basket ball up a hill. If the ball is not given enough push, then the ball will not make it to the other side of the hill. In this case the ball does not have enough energy to roll over the hill. But in quantum mechanics, things are not inherently classical particles (balls). In quantum mechanics things are fundamentally probability waves of finite extent. The implication is that in the analagous quantum situation of a quantum particle moving against a potential hill, some of the probability wave can extend all the way through to the other side of the potential hill. Having some of the wave on the other side of the hill means that there is a probability the quantum particle can be on the other side of the hill. The quantum particle can not travel over the hill, but it can possibly tunnel through the hill.

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As this is a quantum and non-classical effect, it can generally only be seen in microscopic phenomena where the wave nature of particles is more pronounced.

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