In 1927, Werner Heisenberg determined that it is impossible to
measure both a particle's position *and* its momentum exactly.
The more precisely we determine one, the less we know about the other.
This is called the **Heisenberg Uncertainty Principle,** and it is a fundamental
property of quantum mechanics.

The precise relation is:

This constant is Planck's constant divided by two; Planck's constant is represented by the
symbol , or "h-bar," and equals 1.05 x 10^{-34}
joule-seconds, or 6.58 x 10^{-22} MeV-seconds.

The act of measuring a particle's position will affect your knowledge of its momentum, and vice-versa.

We can also express this principle in terms of energy and time:

This means that if a particle exists for a very brief time, you cannot precisely determine
its energy. A short-lived particle could have a tremendously uncertain energy, which leads
to the idea of **virtual particles**.