The Uncertainty Principle was formulated by Heisenberg in 1927. It asserts that there is a fundamental limit to the precision with which certain pairs of physical properties, called conjugate variables, can be simultaneously known. The most famous pair of these variables are position and momentum. The more precisely we know the position of a particle, the less precisely we can know its momentum, and vice versa. This is not due to technological limitations. It is a fundamental feature of the quantum world.
The Uncertainty Principle can be thought of as a result of the wave-particle duality described above. A particle's position corresponds to a localised state, while its momentum corresponds to a wavelength (from its wave-like behaviour). Localising the particle more precisely in space makes its momentum (or wavelength) less defined. This principle upends classical mechanics, where, at least in theory, you can measure a particle's position and momentum as precisely as you want. It introduces the idea that at a quantum level, reality isn't fully deterministic. The future states of a system can't be predicted with absolute certainty but only in terms of probabilities.
There is also an uncertainty relation between energy and time that implies that during very short time intervals, large fluctuations in energy can occur, which is often invoked in phenomena like quantum tunnelling or the existence of virtual particles in quantum field theory. (Unlike position and momentum, time is not an operator in quantum mechanics, so the energy–time relation is more subtle, but the physical intuition is correct.)
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