The Elements of the Uncertainty Principle

Physicists have defined many quantities with which to describe properties of an object. Two of these are important to the Heisenberg uncertainty principle. The first of these is the position of an object. This is fairly straightforward. Just pick a point, and measure how far left or right of that point the object is. That number is your position. In three dimensions, it takes three numbers to describe the position of an object: how far east or west, how far north or south, and how far above or below a point the object is.

The other quantity is called momentum. Momentum is a measure of how "hard" an object is moving. It's the product of the "mass" (think "weight") of an object and its velocity. (If an object is moving close to the speed of light, the formula is a bit different.) Think of it this way: if a baseball is lobbed gently at you, it doesn't hurt as much as it does if it strikes you at 90 mph. And a ping-pong ball that hits you at 20 mph doesn't hurt nearly as much as a bowling ball that hits you at the same speed. So how "hard" an object is moving depends on both the velocity of the object and its mass. This is reflected in the definition of momentum.

These two quantities, position and momentum, are the subject of the Heisenberg uncertainty principle. The principle describes the precision with which these two quantities can be known for any object.

Wave-particle duality: light
The Page of Uncertainty