Newtonian mechanics is replaced by the Schrodinger equation in non-relativistic quantum mechanics. The classical idea of an actual point particle disappears entirely in the dominant Copenhagen interpretations and many-worlds interpretations of quantum measurement. All that remains of the classical particle in Bohr's Copenhagen theory is its quantum "smile" like Lewis Carroll's "Cheshire Cat"! Bohr's student, John Archibald Wheeler called this the "Great Smoky Dragon".
The Schrodinger equation is a deterministic equation. Indeed, given the initial Schrodinger wave function at initial time t = 0 and for all space positions (x,y,z) for a single particle, the wave function for all future time is completely determined. The quantum measurement problem is that this determined wave function solution does not describe what actually will happen in an individual detection event. It only describes what might happen in a statistical sense confirmed very precisely only in simple situations involving a relatively small number of coupled particles at a scale where the collective behavior of life is not directly observable.
Launched by the old Wheeler-Feynman electrodynamics, independent efforts by I. J. Good, Fred Hoyle, Yakir Aharonov and his students, Roger Penrose, John Archibald Wheeler, John Cramer, Oliver Costa-DeBeauregard, Charles Bennett, C. Woo, Fred Alan Wolf and this author are all coming to the same conclusion that influences from the future contribute essentially to what actually happens now in addition to influences from the past and from the absolute elsewhere outside the light cone. The indeterminism of quantum events is explained by the idea that an effect here and now at the quantum level has not only the common sense past "retarded" causes but counter-intuitive future "advanced" or what Aristotle called "final causes". This return of old teleological ideas of meaning, purpose and destiny into physics is also found in the "strong" and "final" versions of the "Anthropic Cosmological Principle" by John Barrow and Frank Tipler. Indeed, Frank Tipler has published a recent book, "The Physics of Immortality", which lends support to this author's peculiar view in which "God" is defined in physical terms as the ultimate Future and Final Cause literally creating the universe backward-in-time ( BIT ). This sheds new light on Wheeler's "IT from BIT" remark. I fancy that the same idea was expressed artistically in the painting "The Creation" by Michelangelo, an anatomy student of Samuel Sarfatti who was personal physician to Pope Julius II, on the ceiling of the Sistine Chapel in which God reaches back to Eden from the future to Adam's outstretched hand.
The modern technical idea of causality as incorporated into the global "black hole" singularity theorems of classical general relativity of Penrose and Hawking, and into special relativistic quantum field theory, depends on the field of light cones at each point event in spacetime as being an absolute barrier to effective influences leaking outside the cones. Special relativity shows that influences outside the light cone imply future causes of past effects in some Lorentz frames of reference. Therefore, if one wants to admit only "retarded" or past causes of present effects, one must exclude faster-than-light influences outside the light cones. If a new quantum phase of exotic matter can be found, it is possible to construct traversable wormhole time machines allowing travel to the past provided that particle pair creation does not prevent the travel as in the "chronology protection conjecture" mechanism proposed by Stephen Hawking. Kip Thorne, Igor Novikov, and this author independently, have a global self-consistency conjecture that permits time travel to the past in a benign violation of causality that does not permit paradox. The Feynman quantum probability amplitude for inconsistent histories for both closed timelike world lines and for retroactive information flows on spacelike worldlines vanishes exactly. This kind of global principle is already found in nature in the Pauli exclusion principle requiring antisymmetric multiparticle amplitudes for identical fermions. You lose your free will to create a paradox if you travel to the past. Things are as they will be because such travel from the future to the past did happen.
An even bigger fly in the causality ointment is that Bell's theorem shows that the statistical predictions of orthodox quantum mechanics require that such superluminal influences exist in any model of physical reality in which things actually happen in a unique universe. The traditional many-worlds models of reality in principle can be local. Gell-Mann argues for this kind of locality in his book, "The Quark and the Jaguar". However, Henry Stapp refutes Gell-Mann in a thoughtful analysis. The Feynman propagator used in quantum field theory does permit influences outside the light cone but they decay exponentially on a scale of the Compton wavelength of the subluminal particle. This wavelength is infinite for the massless photon. The Feynman propagator for a superluminal particle would oscillate rather than decay outside the light cone, and it would decay inside the light cone. However, the microcausality axiom for the photon field and subluminal particle creation and destruction operators rigs things in such a way that the commutator of two field operators spacelike separated from each other commute. This ensures that it is not possible to communicate directly faster-than-light by shifting a local statistical pattern by an action-at-a-distance outside the light cone. However, Nobel Prize Cambridge University physicist, Brian Josephson, has questioned the completeness of this statistical criterion for communication in the case of living organizations of matter.
Even apart from Josephson's new criterion that would permit superluminal communication on nonlocal quantum connections in living biological systems, there is a model by Henry Stapp based on a theory of nonlinear quantum mechanics created by and then, in a failure of nerve, rejected by Nobel Prize physicist, Steven Weinberg which also permits such communication. However, there is a very clear and simple case within orthodox quantum mechanics which shows, beyond a shadow of a doubt, that information can be communicated backward-in-time. The "Catch 22" that saves retarded causality is that, within orthodox quantum mechanics, we cannot access that information before it is sent even though we can, in hindsight, show unequivocally that the information was, in fact, sent backward-in-time. This means it is impossible to try to create a time travel paradox with this experiment.
The backward-in-time experiment is a simple "delayed choice" variation on the experiment performed by Aspect's group at the University of Paris in the early 1980's. An atom emits two photons in opposite directions. The relative orientations of two polarizers detecting each photon is changed very quickly in a time short compared to the flight time of the photons from atom to detector. In addition, the wavepackets of each photon are small compared to the distance each photon travels from atom to detector. Now the local probability to detect each photon individually is 1/2 no matter what the relative angular difference in orientation of the polarizers is as reckoned from the times each photon wave packet passes its polarizer. That is, each photon appears locally to be unpolarized under all conditions. This implies that the data stream from each separate detector is locally completely random with no hidden patterns of meaning to be decoded if we but had the cipher. Nevertheless, if the data from each end is compared by a correlation analysis, the coincidence rate is found to be proportional to the square of the cosine of the angle of relative orientation of the two polarizers. This agrees with what orthodox quantum mechanics predicts, but for certain relative angles it strongly disagrees with the predictions of Bell's locality inequality which is based upon the idea that there are no faster-than-light influences for individual quantum events.
The distances between atom and detectors were equal in the original experiment. The delayed choice variation makes these distances unequal and it systematically changes the orientation of the more distant polarizer while keeping the orientation of the closer polarizer fixed throughout the experiment. To make matters clearer we will arrange that the source atoms emit at a very low rate so that there is only one photon pair in the apparatus at a time. In fact the atom emits three photons, the third is detected at the source to identify relevant counts at each end in the correlation analysis. The observer S at the more distant polarizer is the "sender". The observer R at the closer polarizer is the "receiver". S freely chooses some message, say a melody line from Wagner's "Parsifal" for example, he uses a standard algorithm to code that melody message into the sequence of successive orientations of his polarizer for successive photon pair emissions. This algorithm is given to R before the experiment is performed. Although R knows the algorithm he does not know the message -- the free choice of S is kept secret. The important point is that for a given pair of photons emitted from a particular atom, S's photon is detected after R's twin photon is detected, and that S does not choose the orientation of her polarizer until after R's twin photon is irreversibly recorded. There is no question that S is the active cause of the message since it is S that changes the orientation of her polarizer while R does not touch his polarizer.
R is the passive receiver of his stream of seemingly random data. Similarly, S's data looks random to her. It is only the sequence of S's variations in the orientation of her polarizer that is non-random and meaningful encoding the message. After the experiment is completed, S sends her record of random data to R. The third observer O also sends his third photon data to R. R puts all of this into his computer and indeed out comes Wagner's "Parsifal". The technology of untappable "quantum cryptography" is based on this ability to decode messages stored in nonlocal quantum correlations even though these messages cannot apparently be locally decoded from one data stream alone. The key point is that R's datum is received pair by pair before S's datum and that S makes her choice after R's datum is recorded. Yet, R's datum obviously has been influenced by the future choices of S otherwise R could not decode the melody later on. The two seemingly random data streams are subtly interconnected backward-in-time. These are the rudiments of the quantum force of destiny found even in orthodox quantum mechanics. The effect gets more astonishing when we go beyond the orthodox quantum.
to be continued (under construction) Back