Because there are no definite positions. There are only probability distributions.
The German scientist Werner Heisenberg developed his uncertainty principle, a major concept in quantum mechanics, in 1927.
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 there are no definite positions. There are only probability distributions.
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 German scientist Werner Heisenberg developed his uncertainty principle, a major concept in quantum mechanics, in 1927.
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.
Basically, quantum mechanics provides the answer for some questions which are previously unanswered like why the energies of each emited electron increases (but the number of electrons not increased) when the frequency of light waves striking the metal surface increased in photoelectric effect? This can only be explained when we regard light waves as particles (photons). So, it is the birth of particle-wave duality concept and quantum mechanics.
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.
I am not aware of it "not being explained". I would guess that you can explain the relevant aspects with quantum mechanics.