Two operators that are also observables, which means that they correspond to some physically measurable quantity. Compatible observables are two or more such operators that can be measured at the same time. Position and momentum are an example of Incompatible observables, since one can only know either the position or the momentum of an object to 100% accuracy.
An observable is an expectation value of an operator.
The mixed state in quantum mechanics is the statistical ensemble of the pure states.
Classical Mechanics and Wave Theory.
There is no reasonable alternative to quantum mechanics, at least not something that can even compare with the predictive power and experimental accuracy as quantum theory. If you want to make predictions about things happening at small scales you cannot do without quantum mechanics. Also note that certain models which are now considered as possible theories of everything (e.g. string theory) all expand upon quantum mechanics, they do not make quantum mechanics invalid or unnecessary.
This is the title of a book that teaches how to perform the calculations of Quantum Mechanics, in very simple easy to follow terms.
Because light waves and radioactive decay are some of the key factors that lead to the development of Quantum Mechanics. Quantum mechanics is also our best apparatus for describing and predicting those phenomena.
Principles of Quantum Mechanics was created in 1930.
The distinction is sometimes made to distinguish normal quantum mechanics (which does not incorporate special relativity) and quantum field theory (relativistic quantum mechanics). Since we know special relativity is correct it is the relativistic form of quantum mechanics which is true, but non-relativistic quantum mechanics is still used, because it is a good approximation at low energies and it is much simpler. Physics students typically study regular quantum mechanics before moving on to quantum field theory.
The concepts of quantum mechanics were not explored until the 20th century. Newton only lived into the 18th century, so Newton did no work on quantum mechanics.
The mixed state in quantum mechanics is the statistical ensemble of the pure states.
Quantum Mechanics "replaced" Classical Mechanics in particle physics in mid-1930s.
It is also called wave mechanics because quantum mechanics governed by Schrodinger's wave equation in it's wave-formulation.
Classical Mechanics and Wave Theory.
Quantum mechanics is a separate branch of physics. It is a general term given to all quantum physics. There are many subbranches, for example Quantum chronodynamics which describes the strong nuclear interaction.
There is no reasonable alternative to quantum mechanics, at least not something that can even compare with the predictive power and experimental accuracy as quantum theory. If you want to make predictions about things happening at small scales you cannot do without quantum mechanics. Also note that certain models which are now considered as possible theories of everything (e.g. string theory) all expand upon quantum mechanics, they do not make quantum mechanics invalid or unnecessary.
I am not aware of it "not being explained". I would guess that you can explain the relevant aspects with quantum mechanics.
Quantum mechanics
Also referred to as the 'uncertainty' principle, it is a principle in quantum mechanics holding that increasing the accuracy of measurement of one observable quantity increases the uncertainty with which another conjugate quantity may be known.