“I remember discussions with Bohr which went through many hours till very late at night and ended almost in despair; and when at the end of the discussion I went alone for a walk in the neighboring park I repeated to myself again and again the question: Can nature possibly be so absurd as it seemed to us in these atomic experiments?” – Werner Heisenberg, Physics and Philosophy (1958)
In 1905, Albert Einstein argued that light behaves not only as a continuous wave but sometimes as an individual particle. This led to the development of Quantum Mechanics in the mid-1920s by Werner Heisenberg, Niels Bohr, Erwin Schrödinger, and others. Einstein questioned parts of the theory with his “God does not play dice with the universe” quote. Even 100 years later, Quantum Mechanics still troubles us with its strange phenomena.
Heisenberg (12/5/1901 – 2/1/1976) was a key pioneer in quantum mechanics. He published a breakthrough paper in 1925, and additionally with Max Born and Pascual Jordan described the matrix formulation of quantum mechanics. Heisenberg won the Nobel Prize in 1932 “for the creation of quantum mechanics.” In 1927, Heisenberg stated his famous Uncertainty Principle as:
The more precise the measurement of position, the more imprecise the measurement of momentum, and vice versa.
This is a fairly easy idea to understand. To measure the object’s location, some energy (usually light) must impact the object. For big objects, this effect is trivial. But when an object is extremely small (like an electron) the light imparts energy to the particle. Thus the momentum (mass times velocity) changes. Einstein agreed with this theory, as it can be shown within the structure of classical physics.
But other parts of Quantum Theory are difficult, such as the dual (particle and wave) nature of light. In the famous two slit experiment, a light (or electron) source is effectively split into two separate waves that later combine, resulting in the well-known interference pattern of classical mechanics. Since Einstein showed that light can sometimes behave like a particle, how does a particle exhibit special wave properties going through either slit? You can send only one electron at a time and the effect is still valid. Even today, people still struggle with this phenomenon:
Could it be that each electrons somehow splits, passes through both slits at once, interferes with itself, and then recombines to meet the second screen as a single, localized particle?
To find out, you might place a detector by the slits, to see which slit an electron passes through… If you do that, then the pattern on the detector screen turns into the particle pattern of two strips… The interference pattern disappears. Somehow, the very act of looking makes sure that the electrons travel like well-behaved little tennis balls.
This is similar to what’s involved in the Uncertainty Principle but very weird! No wonder Heisenberg remarked:
Quantum theory provides us with a striking illustration of the fact that we can fully understand a connection though we can only speak of it in images and parables.