Heisenberg · Uncertainty
Can everything be known precisely at once?
That time, that place
In the spring of 1925, his face swollen with hay fever, Heisenberg went to recover on Helgoland, a nearly treeless rock in the North Sea. There, he recalled, he calculated through the night and caught the thread.
Why this question
His principle was severe: keep out of the theory anything you cannot observe.
No one has seen an electron's orbit. What is actually observed are the frequencies and intensities of the light an atom emits. So the theory should be built from those alone.
What was found
Building the calculation that way, something odd appeared: multiply the quantity for position by the quantity for momentum and the answer depends on the order.
With numbers, order does not matter. At first he thought he had made a mistake.
It was the point. The more precisely position is known, the more momentum blurs, and the reverse. Knowing both exactly at once is impossible.
Not because measurement is crude. Better instruments do not help. It means the electron does not possess definite values of both to begin with.
Measure precisely enough and anything can be known exactly
A calculus in which the answer depends on the order of multiplication
Einstein objected that God does not play dice, and his argument with Bohr ran for decades. Heisenberg later led Nazi Germany's nuclear program, and whether he deliberately slowed it is disputed to this day