A Brief History Of Quantum Mechanics
The history of quantum mechanics began essentially with the 1838 discovery of cathode rays by Michael Faraday, the 1859 statement of the black body radiation problem by Gustav Kirchhoff, the 1877 suggestion by Ludwig Boltzmann that the energy states of a physical system could be discrete, and the 1900 quantum hypothesis by Max Planck that any energy is radiated and absorbed in quantities divisible by discrete energy elements, E, such that each of these energy elements is proportional to the frequency ν with which they each individually radiate energy.
Planck insisted that this was simply an aspect of the processes of absorption and emission of radiation and had nothing to do with the physical reality of the radiation itself.
However, at that time, this appeared not to explain the photoelectric effect (1839), i.e. that shining light on certain materials can function to eject electrons from the material.
In 1905, basing his work on Plancks quantum hypothesis, Albert Einstein postulated that light itself consists of individual quanta. These later came to be called photons (1926). From Einstein’s simple postulation was born a flurry of debating, theorizing and testing, and thus, the entire field of quantum physics.
Quantum mechanics (QM) is a set of principles describing the physical reality at the atomic level of matter (molecules and atoms) and the subatomic (electrons, protons, and even smaller particles). These descriptions include the simultaneous wave-like and particle-like behavior of both matter and radiation (“waveparticle duality”).
Quantum Mechanics is a mathematical description of reality, like any scientific model. Some of its predictions and implications go against the “common sense” of how humans see a set of bodies (a system) behave. This isn’t necessarily a failure of Quantum mechanics - it’s more of a reflection of how humans understand space and time on larger scales (e.g., centimetres, seconds) rather than much smaller.
Quantum mechanics says that the most complete description of a system is its wavefunction, which is just a number varying between time and place. One can derive things from the wavefunction, such as the position of a particle, or its momentum. Yet the wavefunction describes probabilities, and some physical quantities which classical physics would assume are both fully defined together simultaneously for a system are not simultaneously given definite values in Quantum mechanics.
It is not that the experimental equipment is not precise enough - the two quantities in question just are not defined at the same time by the Universe. For instance, location and velocity do not exist simultaneously for a body (this is called the Heisenberg uncertainty principle)
Certain systems, however, do exhibit quantum mechanical effects on a larger scale; superfluidity (the frictionless flow of a liquid at temperatures near absolute zero) is one well-known example. Quantum theory also provides accurate descriptions for many previously unexplained phenomena such as black body radiation and the stability of electron orbitals. It has also given insight into the workings of many different biological systems, including smell receptors and protein structures.
Even so, classical physics often can be a good approximation to results otherwise obtained by quantum physics, typically in circumstances with large numbers of particles or large quantum numbers. (However, some open questions remain in the field of quantum chaos.)