Uncertainty Principle
Year of Discovery: 1927
What Is It? It is impossible to know the position and motion of an elementary
particle (e.g., an electron) at the same time.
Who Discovered It? Werner Heisenberg
Why Is This One of the 100 Greatest?
Werner Heisenberg is famed worldwide for discovering the Uncertainty Principle,
which states that it is impossible to determine both the position and momentum (motion) of
an elementary particle at the same time since the effort to determine either would change the
other in unpredictable ways. This pivotal theorem marked a fundamental turning point in
science. For the first time it was no longer possible to precisely and completely measure or
observe the world. At a certain point, Heisenberg showed, scientists had to step back and
take the mathematical equations describing the world on faith.
The Heisenberg Uncertainty Principle also undermined the position of cause and effect as a most basic and unassailable foundation block of scientific research, a position it
had enjoyed for over 2,500 years. Aat an elementary particle level, every cause had only a
fixed probability of creating an anticipated effect.
How Was It Discovered?
Opening the mail in his Helgoland, Germany, home, in the fall of 1926, Werner
Heisenberg found a letter from famed physicist Max Planck. The letter glowed with praise
for Heisenberg’s paper presenting the “matrix mechanics” Heisenberg had developed. It
was Heisenberg’s fifth congratulatory letter from a famed physicist that week.
Every letter hailed Heisenberg’s matrix mechanics and talked about its “vast potential.” They called it “new and exciting” and “extremely valuable.”
But Werner Heisenberg’s deep sense of unease was not relieved by these letters. Buried in his matrix equations, Heisenberg had detected what he thought was a hard limit to science. If true, it would be the first time science had been told it was impossible to be more
precise. A deep dread rumbled the foundations of Heisenberg’s scientific beliefs. Yet there
it was in black and white. If he was right, science had reached an unscalable wall.
The great physics debate at that time centered on the image of an atom. Was it a ball of
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