Heisenberg states the uncertainty principle
In 1927, Werner Heisenberg was in Denmark working at Niels Bohr's research institute in Copenhagen. The two scientists worked closely on theoretical investigations into quantum theory and the nature of physics. Bohr was away on a skiing holiday, and Heisenberg was left to mull things over himself. He had a shocking but clear realization about the limits of physical knowledge: the act of observing alters the reality being observed. At least at the subatomic level. To measure the properties of a particle such as an electron, one needs to use a measuring device, usually light or radiation. But the energy in this radiation affects the particle being observed. If you adjust the light beam to accurately measure position, you need a short-wavelength, high-energy beam. It would tell you position, but its energy would throw off the momentum of the particle. Then, if you adjust the beam to a longer wavelength and lower energy, you could more closely measure momentum, but position would be inaccurate.
This principle punctured the centuries-old, firmly held belief that the universe and everything in it operates like clockwork. To predict the workings of the "clock," one needs to measure its qualities and parts at a specific point in time. Classical physics assumed that the precision of measuring is theoretically unlimited. But Heisenberg stated that since you could never with great certainty measure more than one property of a particle, you could only work with probability and mathematical formulations. (Heisenberg called this matrix mechanics, soon shown to be equivalent to Erwin Schrödinger's more visualizable wave theory.)
The uncertainty principle was hard even for scientists to accept at first. After struggling with it, however, Bohr developed complementarity theory. This stated that there was a dual nature to things -- an electron was a wave and a particle, for example -- but we could only perceive one side of that dual nature. A sphere, for instance, has a convex and concave aspect. We can sense the convex from outside the sphere, but from inside it appears completely concave. This theory would affect much more than physics, but other fields of science, as well as art and philosophy.
Heisenberg and Bohr's theories were compatible and became known together as the Copenhagen interpretation and accepted as the foundation for quantum theory.
From the New York Times, September 2, 1927
DETAILS CONCEPTS OF QUANTUM THEORY
By Waldemar Kaemppfert
Copyright 1927, by The New York Times Company. By Wireless to The New York Times.
LEEDS, England, Sept. 1. -- Of thirty addresses delivered today before the various sections of the British Association for the Advancement of Science, one of the most important was that of a young German, Dr. W. Heisenberg. Fully 200 mathematical physicists listened to his brief exposition of a conception which will make it necessary to modify belief in what we are pleased to call "common sense" and "reality."
The layman without knowledge of higher mathematics, listening to Dr. Heisenberg and those who discussed his conclusions, would have decided that this particular section of the British Association is composed of quiet and polite but determined lunatics, who have created a wholly illusory mathematical world of their own. ...
To explain the quantum theory and its modification by Dr. Heisenberg and others is even more difficult than explaining relativity. It is much like trying to tell an Eskimo what the French language is like without talking French. In other words, the theory cannot be expressed pictorially and mere words mean nothing. One is dealing with something that can be expressed only mathematically.
The consequences, however, are startling. Electrons and atoms cease to have any reality as things that can be detected by the senses directly or indirectly. Yet we are convinced the world is composed of them.
In the new mathematical universe events are more important than substances, and energy more important than matter. All mental pictures we have formed of bodies moving through space are thrown into confusion. So simple a conception as a baseball flying from the pitcher to the batter turns out to be obscure, doubtful and even ridiculous.
Planck, the originator of the quantum theory, Heisenberg, Schrödinger, and De Broglie have shown that the whole science of mechanics must be rewritten. And when it is rewritten, no one but a mathematician will be able to understand it. The sicentific world is faced with an upheaval as great as that brought about by Einstein.