Short answers, pulled from the story.
Max Planck introduced the concept of quanta in the year 1900 to solve the black-body radiation problem. He proposed that energy is radiated and absorbed in discrete packets rather than continuously. This hypothesis initially served as a mathematical trick rather than a description of physical reality.
The uncertainty principle states that it is impossible to simultaneously know both the precise position and momentum of a particle. This is a fundamental property of nature arising from the non-commuting nature of position and momentum operators. It forces physicists to abandon the idea of a deterministic universe where the future is perfectly predictable from the past.
The double-slit experiment shows that quantum entities behave as both waves and particles depending on how they are observed. When light passes through two parallel slits, it creates an interference pattern expected of waves, yet individual photons build up this pattern over time. If a detector determines which path the photon takes, the interference pattern disappears and the photons behave like classical particles.
Max Born introduced the Born rule to calculate the probability of finding a particle in a specific location. This rule takes the square of the absolute value of a complex number known as the probability amplitude. The probabilistic nature of quantum mechanics is a final renunciation of classical causality rather than a temporary feature.
The Copenhagen interpretation, championed by Niels Bohr and Werner Heisenberg, posits that the probabilistic nature of quantum mechanics is final. It suggests that wavefunction collapse is a real physical event where a system in a superposition of states reduces to a single definite state upon measurement. This interpretation stands in contrast to Albert Einstein's concerns about determinism and locality.