Quantum theory, although of tremendous scientific value, has nevertheless prompted debate among physicists. The debate arose because quantum theory addresses the peculiar properties of minute objects such as photons and electrons. While one type of experiment shows that these objects behave like particles, with well-defined trajectories through space, another demonstrates that, on the contrary, they behave waves, their peaks and troughs producing characteristic "interference" effects. However, scientists have failed to devise an experiment to demonstrate both behaviors simultaneously.
In the 1920s, two alternate interpretations of quantum theory attempted to resolve this apparently contradictory wave-particle duality. Physicist Niels Bohr argued that wave-particle properties are not contradictory, but complementary. Contrary to our intuition that an object continues to exist in some determined form even though we cannot perceive it, he concluded that the physical of a quantum object is actually undetermined before the object is observed via experiment.
Physicist Werner Heisenberg's "uncertainty principle," by contrast, postulated that we cannot precisely determine two complementary properties, such as position and momentum, of a quantum object simultaneously: if we measure an object's position with absolute certainty, then there is an infinite uncertainty in its momentum, and vice versa. He concluded that although we are limited in our ability to measure objects at the atomic and subatomic levels, their position and momentum are nonetheless defined all along.