The Uncertainty Principle was first identified by a physicist called Heisenberg in the 1920s. It is a fundamental consequence of quantum theory which means that a particle's physical properties are never all fixed at the same time. If you measure one of them precisely you will just make another of them more uncertain.

To understand this, remember that pure energy waves, which have an exact energy associated with them, are very spread out in space. This means that when a particle has an exact energy, its wave is one of the spread-out pure energy waves, which means that the particle could be anywhere within the spread-out wave.

Since a particle can be anywhere within its wave, for a particle to have a clear position, its wave must be a tall narrow ripple. The taller and narrower the single ripple becomes, the more precise is the particle's position.

You can make a tall narrow wave by combining portions of pure energy waves with different energies, but you need lots of them. In fact the more energy waves you add to the recipe, the narrower the ripple you can make with them. But that means you have no idea what energy the particle has when it has a narrow wave, because its narrow wave is made up of lots and lots and lots of small parts of different energy waves, each with a different energy.

The example we've just worked through explains why a particle can't have a well-defined energy and a well-defined position - it can have one or the other. The uncertainty principle applies to other combinations of physical properties too. In the language of quantum theory, when two properties can't be precisely held by a particle at the same time, they are said to 'not commute'.

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