"Classical physics explains the world quite
well. It's just the details it can't handle. Quantum
physics handles the details quite well. It's just the world it can't
explain." from __Quantum Enigma__, Bruce Rosenblum and Fred Kuttner.

DETAILS:

- Planck's quantization of blackbody radiation, early 20th century. To explain how the "Ultra-violet Catastrophe" could be avoided.
- Einstein, the photo-electric effect, 1905. To explain the anomalous behaviour of electrons ejected from a metal by light shone on it; light as particles, "photons", instead of waves. This work originated the relation between change of energy, delta E, of a system and the frequency of the absorbed or emitted radiation, f. The relation is delta E = hf (h is Planck's constant and f is the frequency of the radiation, more usually denoted as the Greek letter "nu").
- Bohr, atomic structure of hydrogen atoms, 1913. To explain why electrons circling the small positively charged nucleus didn't radiate away energy and fall into the nucleus, Bohr quantized angular momentum (circular motion) into discrete integral amounts, which quantization yielded thereby only certain allowed orbits and energies; this also explain the hitherto mysterious discrete line spectra of hydrogen atoms.
- DeBroglie, wavelike character of particles: wavelength = h/ momentum (h is Planck's constant), 1923. Justified Bohr's quantization of angular momentum; experimentally proved by the Davisson-Germer effect.
- Heisenberg, matrix mechanics and the Uncertainty Principle, 1925. Gave a means of calculating probabilities for transitions between quantum states; showed that you could not measure exactly, the simultaneous values for two "conjugate" dynamic variables (e.g. energy and time, position and velocity)
- Schrodinger, the Schrodinger equation, 1925. Gave a wave-like differential equation for systems to satisfy, such that the "wave-function" represented a amplitude which gave, on proper mathematical treatment, the probability for an experimental value to be measured.
- Dirac, proposed a relativistic equation for the electron, unified the Heisenberg and Schrodinger QM picture, 1928-1930.

- Overview--The "mysteries" and principles of quantum theory can be exemplified in the two slit experiment, which exhibits the superposition principle and the wave-particle duality.
- Here is a web site that discusses in a simple way the properties of waves:
- Here are three web sites that explain the double-slit experiment:1) Felder and Felder, NCSU ; 2) The Feynmann Double-Slit Experiment, Univ. of Toronto ; 3) "Bottom Layer"(which also gives a simple picture of wave motion.)
- Superposition: the true state (before detection) of the particle--photon or electron--is both through upper slit and through lower slit; this is shown by the diffraction pattern (even for one particle at a time through the slits). When one attempts to measure which slit the particle came from (before detection) the superposed state collapses into one or the other, state depending on whether the upper or lower slit position was measured. See also this web site. See also these websites on the thought experiment, "Schrodinger's Cat", as state collapse.
- Wave-Particle Duality: whether the object behaves as a wave or particle depends on the type of measurement that one chooses to do--if you let the objects pass through both slits until they hit the screen, they will behave as waves, if you choose to measure the object's position (whether upper or lower slit), it will behave as a particle.

- Two particles, for example, two photons arising from a single event, have their properties correlated--e.g. polarization, direction of electric field of the light. This correlation persists, even though particle may be separated by galactic distances (experiments have been done to show entanglement even over a distance of 100 meters). This correlation is in the absence of any known force--Einstein termed it "spooky". It's like the apochryphal stories of identical twins, separated at birth, who marry similar partners on the same day, fall ill to the same disease at the same time, etc. Such entanglement violates a philosophical principle known as "local realism": realism, what we observe, corresponds to a real "thing" and isn't just a construct of our minds; locality, the "thing" we observed is at some definite time and place, and isn't extended over an indeterminate space/time locality.
- The Irish physicist/philosopher, John Bell, derived an inequality that had to be satisfied about properties of two linked objects if the principle of "local realism" was to hold--Bell's Inequality, and also showed that this inequality would be violated if quantum theory principles were to obtain. Experiments (Clauser, Aspect) showed that Bell's Inequality was indeed violated and quantum theory, in all its spookiness, was right.
- The violation of Bell's Inequality shows that "local realism" is not valid, and we can choose which of the two assumptions don't hold--realism or locality--or both. D'Espagnat (see below) a French physicist/philosopher, introduced a third condition that has to hold: free will choice for the observer in choosing what observations--experiments--to make.

- This was a "thought" experiment originally proposed by the American theoretical physicist, John Wheeler, to get rid of the possibility that particles passing through a double-slit somehow "knew" whether both slits were open, or one was closed. The gimmick was to remove the detecting screen AFTER the particle had passed through the double-slit arrangement and then use very distant telescopes (in Wheeler's original thought-experiment--at astronomical distances from the double slit) so that they would focus on the slit of origin. The thought experiment was later realized in real experiments, delayed choice "quantum eraser" experiments.
- Raymond Chiao, an American Physicist, interpreted the results of
experiments showing non-locality and delayed-choice from a philosophical
and theological perspective: "Chiao concludes with some
additional philosophical and theological reflections in light of these results
and his Christian faith. He supports a point of view in which
the free choices of observers lead to nonlocal correlations of properties of
quantum systems in time as well as in space, giving Berkeley's dictum,
*esse est percipi*, temporal as well as spatial significance. Theologically he uses this generalized Berkeleyan point of view to depict God as the Observer of the universe. Here God creates the universe as a whole (*ex nihilo*) and every event in time (*creatio continua*). The quantum nonseparability of the universe is suggestive of the New Testament's view of the unity of creation. In the process Chiao discusses such ideas as the quantum entanglement of all events in the universe given their common origin in the Big Bang." (taken from a summary of the CTNS Vatican sponsored series on what science shows about Divine intervention.) - I'll quote here two limericks taken from
__Quantum Enigma__(Rosenblum and Kuttner): "There was a young fellow named Todd, who said 'it is extremely odd, to think that this tree continues to be, when no one is about in the Quad' ". The reply: "There is nothing especially odd; I am always about in the Quad. And that's why this tree continues to be. Yours faithfully, God."

- Von Neumann (the mathematician who invented game theory) very early (1932) in his mathematical formulation of quantum theory proposed a sharp demarcation between the system being observed and the observer in order to account for the wavefunction "collapse" upon measurement. Measurement and wavefunction collapse is outside the theory of standard quantum mechanics (although attempts have been made to incorporate this into the theory) and this externality to the theory is what requires the presence of an observer, and thereby, implicitly, consciousness.
- Eugene Wigner was a theoretician who at the beginning
stages of quantum theory proposed the involvement of consciousness:
"When the province of physical theory was extended to encompass
microscopic phenomena through the creation of quantum mechanics, the
concept of consciousness came to the fore again. It was not possible
to formulate the laws of quantum mechanics in a fully consistent way
without reference to the consciousness." (as quoted in
__Quantum Enigma__). Wigner proposed a paradoxical thought experiment, known as the "Wigner's Friend" paradox. Instead of the cat inside Schrodinger's box, a friend of Wigner would observe whether a radioactive decay had occured. Wigner would then note when his friend had marked down a decay. The question then is, who is the observer--Wigner or his friend? Suppose Wigner and his friend had both been in the box and noted down the instant of decay, and they had noted down different times; what would this mean? Stephen Barr says that the wavefunction represents potential knowledge, so that the act of measurement reduces ("collapses") the wave-function from a probability (before measurement) to a certainty after measurement. According to Barr the Wigner's friend situation presents no paradox. The friend and Wigner could have different knowledge of what occured; in general, however, there will be an observer consensus of what happens.

- The Copenhagen: it works, so don't bother about "reality"; things are what you measure, so just calculate.
- The Many Worlds Theory (Everett): measurement doesn't collapse the wave-function, it just gives rise to an alternative branch of reality.
- Consistent Histories and Decoherence (Zurek, Gell-Mann, Hartle): interaction of microscopic systems with their macroscopic environment gives rise to lots of phase shifts (many, many superpositions centered on a classical situation); however consciousness is still implicit.
- Pilot-Wave Theory (de Broglie, Bohm): hidden variables (a pilot wave) guide the microscopic entities so what appears to be probabilistic is actually deterministic; however, there is still a connectivity--entanglement--of everything.

- Quantum gravity and microtubules (Penrose and Hameroff).The link is to a site of the "Center for Consciousness" studies at the University of Arizon.
- Quantum Zeno effect (Stapp): consciousness determined by repeated actions of entangled neurons.
- For more general comments, and other approaches, see the web article from the Stanford Encyclopedia of Philosophy.