The Copenhagen interpretation is composed of three related arguments.
1. The wave function is a complete description of a wave/particle. Any information that cannot be derived from the wave function does not exist. For example, a wave is spread over a broad region, therefore does not have a specific location.
2. When a measurement of the wave/particle is made, its wave function collapses. In the case of momentum, a wave packet is made of many waves each with its own momentum value. Measurement reduced the wave packet to a single wave and a single momentum.
and 3. If two properties are related by an uncertainty relation, no measurement can simultaneously determine both properties to a precision greater than the uncertainty relation allows. So, if we measure a wave/particles position, its momentum becomes uncertain.
Copenhagen Interpretation - Philosophy for Heroes
January 21, 2019