In Quantum Mechanics, the most basic and indivisible thing is not simply one thing.

Rather, it always comes as a pair of complementary attributes.

If you’ve heard about Heisenberg’s Uncertainty Principle, you already know that we can measure the momentum (or speed) of a particle or what its position is, at the same time, with only so much certainty:

For instance…  Say you want to measure the speed and location of an object.  You can only do this as well as your instruments will allow.

Assuming your stopwatch and ruler are only accurate up to a second and up to a millimeter, respectively, you know you can only get so accurate in your measurements with those devices.

If you want to reduce the uncertainty of a value and obtain a better, more precise measurement, you will need to buy a more expensive and more accurate apparatus.

On a limited budget, however, buying a more accurate ruler means you will have to contend with a cheaper stopwatch, or vice versa.

The uncertainty is the accuracy of the measurement.

It is a boundary to our knowledge – a boundary beyond which science cannot go.