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1.0 Definition, Scope, and History
1.1 Quantum physics is a branch of physics which is the fundamental theory of nature at small scales and low energies of atoms and subatomic particles. An atom is defined as the smallest unit of matter that has the properties of a chemical unit. They consist of a nucleus of protons and neutrons (making up around 99.94% of the mass) and one or more electrons (hydrogen ion excepted). Protons, neutrons, and electrons are fermions, contrasted with bosons. Fermions obey the Pauli exclusion principle and includes all quarks and leptons (electrons, muons, tau, and neutrinos). Bosons include photons, gluons, guage bosons, and the Higgs boson.
1.2 Quarks are elementary subatomic particles for protons and neutrons, both of which have an internal structure. Electrons are elementary particles in their own right with no internal structure. There are six types of quarks, known as flavors: up, down, strange, charm, top, and bottom. Up and down quarks, found in protons and neutrons, have the lowest masses of all quarks. The heavier quarks rapidly change into up and down quarks through a process of particle decay. Gluons "glue" quarks together.
1.3 Quantum mechanics gradually arose from the wave nature of light began in the 17th and 18th centuries, when several scientists proposed a wave theory of light; in 1838, Michael Faraday discovered cathode rays, and in 1859 the statement of the black-body (thermal) radiation problem by Gustav Kirchhoff. Max Planck's provided a solution in 1900 to the black-body radiation problem, Albert Einstein in 1905 offered a quantum-based theory to explain the photoelectric effect, and Niels Bohr's a new model of the atom included quantized electron orbits in 1913.
2.0 Differences from Classical Physics
2.1 Quantum mechanics differs from classical mechanics in that energy, momentum, etc are often restricted to discrete values (Quantization), objects have characteristics of both particles and waves (wave-particle duality), and there are limits to the precision with which quantities can be known (Uncertainty principle).
2.2 Quantization transitions from a classical physical phenomena to quantum mechanics, involving a procedure for constructing a quantum field theory starting from a classical field theory. Quantization is based on elementary particles (for example, a photon, because "half a photon" is never observed). Examples include canonical quantization, covariant canonical quantization, deformation quantization, geometric quantization etc. Quantum mechanics shows that many quantities (e.g., angular momentum), that appears continuous in classical mechanics, turn out to be quantized in discrete steps.
2.3 The wave-particle duality states that elementary particles or quantic entities are partly described in terms of particles and waves, as their behaviour cannot be encapsulated by either term by themselves. This phenomenon has been verified not only for elementary particles, but also for compound particles like atoms and even molecules (although for larger particles have extremely short wavelengths).
2.4 In classic physics, conjugate and complementary variables can be defined so they become Fourier transform duals of one another. In quantum mechanics, the Heisenberg uncertainty principle (1927) applies; the more precisely the position of some particle is determined, the less precisely its momentum can be known, and vice versa. The uncertainty principle is inherent in the properties of all wave-like systems (e.g.. Doppler effect), and that it arises in quantum mechanics due to the matter wave nature of all quantum objects. Thus, the uncertainty principle actually is a fundamental property of quantum systems.
3.0 Spookiness in Quantum Physics
"We often discussed his notions on objective reality. I recall that during one walk Einstein suddenly stopped, turned to me and asked whether I really believed that the moon exists only when I look at it. "
(Abraham Pais, Einstein and the quantum theory, Reviews of Modern Physics 51, 863–914 (1979), p. 907.)
3.1 There are several aspects of quantum physics which seem - from our human perspective - to be counter-intuitive. According to Richard Feynman, quantum mechanics deals with "nature as She is - absurd". Albert Einstein, expressing an opposition to uncertainty said: "God doesn't play dice with the world." (Bohr, in response, said, "Einstein, don't tell God what to do."). Addressing the counter-intuitive components, Dyson responded: ".. the important thing about quantum mechanics is the equations, the mathematics. If you want to understand quantum mechanics, just do the math". Some aspects of quantum physics that are worth considering in this context include: the observer effect, quantum entanglement.
3.2 The observer effect refers to changes that the act of observation will make on a phenomenon being observed. Note that 'observation' mean instrument recording; there is a ridiculous popular culture view that it is conscious observation that causes the change. In actual fact, wave function collapse - which is initially is mathematically expressed a superposition of several eigenstates - appears to reduce to a single eigenstate. Measurement in quantum mechanics and connects the wave function with variables such as position and momentum. It was popularised with the thought experiment (1935) of Erwin Schrödinger of a "half-alive half-dead" cat in a state of quantum superposition. Another common misconception is that observation is required for existence; Einstein challenged Abraham Pais on the existence of objective reality asking him whether the moon existed only when under observation.
3.3 Described by Einstein as "spooky action at a distance" (as a criticism), quantum mechanics predicts that particles can become "entangled"; pairs or groups of particles are generated or interact in ways such that the quantum state of each particle cannot be described independently of the others. When an observation is performed on one particle (position, momentum, spin), the other particle responds *immediately*. How this is happens is *unknown*.
4.0 Alternative Interpretations
"I think I can safely say that nobody understands quantum mechanics." - Richard Feynman
4.1 Of special interests to philosophers (and especially philosophers of physics), is the multiple interpretations of the meaning of quantum meachancs and their mathematical formulations with respect to observed phenomen - thus crossing the boundaries of formalism and phenomenology. The debates occur on epistemological and ontological matters (for example, Schrodinger's cat - is it really both alive and dead prior to measurement - an ontological claim, or is this an indication of the limitations of our knowledge - and epistemological claim. The major interpretations include The Copenhagen interpretation (first proposed by Max Born, 1927)., the Many worlds interpretation (Hugh Everett in 1957), minimalist statistical models (e.g., Ensemble interpretation proposed by Born, 1926, supported by Einstein).
4.2 The Copenhagen interpretation is the most common; it claims that physical systems generally do not have definite properties prior to measurement, and quantum mechanics can only predict the probabilities. The act of measurement affects the system, causing the set of probabilities to reduce to only one of the possible values immediately after the measurement (wave function collapse). The many-worlds interpretation claims the objective reality of the wavefunctions and denies wavefunction collapse. Instead, Many-Worlds suggests all possible alternate histories and futures are real. The ensemble interpretation of quantum mechanics considers the quantum state description to apply only to an ensemble. Advocates claim that it is minimalist, making the fewest physical assumptions about the meaning of the standard mathematical formalism.
4.3 There is a vast number of popular culture interpretations of quantum mechanics which often do not engage in the mathematical formalism or even understand the physical phenomenon. Collectively they are disparagingly called "Quantum Mysticism" and are considered to be a form of pseudoscience - however some scientists have argued that consciousness plays a role in quantum events, including Eugene Wigner (1961, Remarks on the Mind-Body Question), Fritjof Capra (The Tao of Physics, 1975), and Brian Josephson (The Paranormal and the Platonic Worlds 1997). Others use the term "quantum" to represent things that have nothing to do with quantum mechanics e.g., (Deepak Chopra, Quantum Healing, 1989). In 1998 Chopra was awarded the parody Ig Nobel Prize in the physics category for "his unique interpretation of quantum physics as it applies to life, liberty, and the pursuit of economic happiness".