There are two basic ideas covered by the uncertainty principle:

• It is not possible to measure both the position and the momentum of a particle with infinite precision.
  
  
• The more accurately you measure a particle's position, the less accurately you're able to measure its momentum, and vice versa.

For objects we're used to dealing with (e.g., apples, pens, cars, etc.) the uncertainty principle is irrelevant. It becomes very important for tiny particles, like electrons, however.

A good way to find an object in a dark room is to shine a flashlight on it. The light reflecting from the object tells you where the object is, but the object is not really affected by that light.

If you try to locate an electron using the same method, the act of measuring the electron's position (bouncing photons off it) has a significant effect on the electron. Each photon that bounces off it gives the electron some momentum. The more photons you use, the more precisely you can measure the electron's position. However, the more photons you use the more momentum is transferred and the more uncertainty you're introducing in the measurement of the electron's momentum.

One way to look at it: the more accurately you know where a particle is now the less accurately you know where the particle will be later.

In 1927 Werner Heisenberg showed that there is a limit to the accuracy you can measure things:

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The uncertainty can also be stated in terms of the energy of a particle in a particular state, and the time in which the particle is in that state:

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