The Basic Features

We described quantum physics with the dominant interpretation, called the “Copenhagen interpretation,” in mind. This interpretation primarily reflects the views of Niels Bohr and Werner Heisenberg, but many other thinkers have contributed various parts over the years. It is named after the capital of Denmark because that is where Heisenberg and Bohr did their seminal work together around 1927. Here are some of its main features:

• Neither matter nor electromagnetic radiation can be completely described without reference to both wave-like and particle-like properties.
• Wave and particle aspects are never seen in the same measurement.
• Heisenberg’s uncertainty relationships are intrinsic and cannot be violated.
• A subatomic entity (like an electron) is described by the mathematical construct called a wave function.
• Wave functions cannot be directly observed or measured.
• The square of a wave function gives information on the probabilities of future measurements.
• Wave functions change smoothly in time, and as time goes on a wave function typically takes on characteristics of more than one possible quantum state simultaneously.
• When a measurement is made, the wave function instantaneously changes to one singular state as a direct result of the measurement.
• The devices we use to make experimental measurements are themselves macroscopic, so they naturally measure classical things like position and velocity.
• Starting from the microscopic, as we gradually consider larger and larger systems the quantum mechanical description must approach the classical description.
• There is only one universe, and this is it.

The Copenhagen interpretation is a loose combination of all of these different ideas. We’ve seen a number of them before, though some are shown here for the first time. Some of them are easy to understand and accept, but others are problematic for anyone seeking a deeper understanding. To see how these various elements start to combine into a common basis for interpreting quantum physics, we’ll now consider a few of the major linkages and themes in a little more detail.