Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

 providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 and matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

. It departs from classical mechanics
Classical mechanics
In physics, classical mechanics is one of the two major sub-fields of mechanics, which is concerned with the set of physical laws describing the motion of bodies under the action of a system of forces...

 primarily at the atomic and subatomic scales, the so-called quantum realm
Quantum realm
Quantum realm is a term of art in physics referring to scales where quantum mechanical effects become important . Typically, this means distances of 100 nanometers or less. Not coincidentally, this is the same scale as nanotechnology....

. In advanced topics of quantum mechanics, some of these behaviors are macroscopic and only emerge at very low or very high energies or temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...


If the price of avoiding non-locality is to make an intuitive explanation impossible, one has to ask whether the cost is too great.

David Bohm et al. Physc. Rep. 144, 321 (1987)

For those who are not shocked when they first come across quantum theory cannot possibly have understood it.

Niels Bohr, quoted in

...the "paradox" is only a conflict between reality and your feeling of what reality "ought to be."

Richard Feynman, in The Feynman Lectures on Physics, vol III, p. 18-9 (1965)

I think I can safely say that nobody understands quantum mechanics.

Richard Feynman, in The Character of Physical Law (1965)

Quantum theory was split up into dialects. Different people describe the same experiences in remarkably different languages. This is confusing even to physicists.

David Finkelstein, in Physical Process and Physical Law, in an edition by