Quantum physics: Copenhagen vs decoherence interpretations

To further highlight the weirdness of the Copenhagen view, Einstein and two of his Princeton colleagues constructed the E.P.R. (Einstein-Podolsky-Rosen) thought experiment. Let an atom spit out two particles, which then fly apart for an enormous distance. The physics equations tell us that the two particles must have opposite spin: if one is ''spin up,'' the other must be ''spin down.'' But according to the Copenhagenians, the particles have no spin state at all until observed. So now we observe one particle and find, say, that it is spin up. We have thus ''resolved'' the spin state of a particle that allegedly had no such state until we made the observation. But that means that the other particle, millions of miles away, suddenly ''became'' spin down. How did it know that its state was now supposed to be spin down?Logic would seem to dictate that the particles actually had spin states all along. Einstein took that position, postulating that some underlying, causal agency must carry the spin. In other words, the spin state is not decided at the moment of observation after all, as Bohr claimed, but was in the particles to start with. But there is no evidence that any such mechanism exists. Theories like Einstein's are therefore known as ''hidden variables'' interpretations, since they propose the existence of something hidden in the quantum systems.Since the Copenhagen view rejected hidden variables, the question remained at an impasse for decades. Which interpretation a given physicist preferred mainly depended on which seemed less repugnant, the hidden variables theory or spooky action at a distance.Then the Irish physicist John Stewart Bell thought up an experiment that would decide whether hidden variables exist, as Einstein believed. The experiment, which involved obtaining the statistics of large numbers of photon interactions, was not technologically feasible when he proposed it in 1964. But in the 1970's Bell's experiment was performed, first by John Clauser and Stuart J. Freedman at Berkeley and later by Alain Aspect and his colleagues at the University of Paris. The verdict was clear: there are no hidden variables of the sort Einstein envisioned. Quantum physics really does exhibit nonlocality......Despite Bohr's posthumous defeat of Einstein, the Copenhagen interpretation is on the wane.One reason is that theorists have begun to apply quantum physics to the study of the origin and early evolution of the universe -- and unless you believe in some Robert Service-like devil amid the flames, it's very difficult to imagine that observers existed in the fireball of the Big Bang, as the Copenhagen interpretation requires. A popular approach nowadays is to replace the concept of observation with that of ''decoherence,'' an interference among particles that resolves quantum systems into one or the other of their complementary states. The jury is still out on decoherence, but neither its success nor its failure will do much to rescue the Copenhagen interpretation from its wider difficulties.Scientists, curious souls that they are, are growing tired of being told by the Copenhagenians, Don't ask. ''We've always had people who say you can't ask this or that question,'' remarks Marlan Scully, whose experiments on quantum weirdness at the University of Arizona and Germany's Max Planck Institute have led to prototypes of a new, more efficient form of laser. ''And yet these questions are so natural that we are led to ask them. As Eugene Wigner used to say: Well, why can't I ask? What will happen to me if I do?''

The distinction is demonstrated by a simple test that the physicist Richard Feynman called ''the experiment with the two holes.'' To run the experiment, punch two small holes in a sheet of steel, fire a stream of photons or other quanta at the sheet and record what comes through, using a detector of some sort on the far side of the steel sheet. (The detector can be something as simple as a sheet of photographic film.) When both holes are open, the detector records an interference pattern -- the signature of interacting waves. Drop two stones in a pond and an interference pattern appears where the waves intersect. Wave peaks reinforce each other where they coincide, as do valleys, and where a wave peak intersects with a valley, the two cancel each other out. In the two-holes experiment, the interference pattern appears even if you send only a single photon through the apparatus: the photon finds its way through both holes and interferes with itself. Close one hole, however, and the photon's wavelike behavior disappears. Now it acts like a bullet: either it emerges from the single open hole to register a point impact on the detector, or it misses the hole and hits the steel sheet, and nothing comes through.The weird thing is that the photon does this -- responds to whether one or both holes are open -- instantly, even if you wait until the last moment, just before it reaches the steel sheet, before deciding to close one hole or leave both open. It is as if the particle (or wave, whichever you prefer) were everywhere at once, feeling out the entire setup and responding to it instantaneously, everywhere.Bohr explained the wave-particle duality by declaring that subatomic systems don't have either of their complementary states until they are observed. This view came to be known as the ''Copenhagen'' interpretation of quantum mechanics, named for the city where Bohr and his colleagues set up shop. It might be summarized as ''Don't ask, don't tell.'' Are photons particles or waves? Don't ask! They are neither -- or they are both. Their complementary states are only resolved, one way or the other, by their being observed.

In physics, nonlocality is a direct influence of one object on another, distant object, in violation of principle of locality. In classical physics, nonlocality in the form of action at a distance appeared in corpuscular theories and later disappeared in field theories. Action at a distance is incompatible with relativity. In quantum physics nonlocality re-appeared in the form of entanglement. Physical reality of entanglement has been demonstrated experimentally [1] leading to its application in quantum cryptography and quantum computing.

The early quantum physicists dealt with this unreality by saying that the "is"--the fundamental objects handled by the equations of quantum theory--were not actually particles that had an extrinsic reality but "probability waves" that merely had the capability of becoming "real" when an observer makes a measurement. This so-called Copenhagen Interpretation makes sense, if you're willing to accept that reality is probability waves and not solid objects. Even so, it still doesn't sufficiently explain another weirdness of quantum theory: nonlocality.In 1935, Einstein came up with a scenario that still defies common sense. In his thought experiment, two particles fly away from each other and wind up at opposite ends of the galaxy. But the two particles happen to be "entangled"--linked in a quantum-mechanical sense--so that one particle instantly "feels" what happens to its twin. Measure one, and the other is instantly transformed by that measurement as well; it's as if the twins mystically communicate, instantly, over vast regions of space. This "nonlocality" is a mathematical consequence of quantum theory and has been measured in the lab. The spooky action apparently ignores distance and the flow of time; in theory, particles can be entangled after their entanglement has already been measured.
On one level, the weirdness of quantum theory isn't a problem at all. The mathematical framework is sound and describes all these bizarre phenomena well. If we humans can't imagine a physical reality that corresponds to our equations, so what? That attitude has been called the "shut up and calculate" interpretation of quantum mechanics. But to others, our difficulties in wrapping our heads around quantum theory hint at greater truths yet to be understood.Some physicists in the second group are busy trying to design experiments that can get to the heart of the weirdness of quantum theory. They are slowly testing what causes quantum superpositions to "collapse"--research that may gain insight into the role of measurement in quantum theory as well as into why big objects behave so differently from small ones. Others are looking for ways to test various explanations for the weirdnesses of quantum theory, such as the "many worlds" interpretation, which explains superposition, entanglement, and other quantum phenomena by positing the existence of parallel universes. Through such efforts, scientists might hope to get beyond the discomfort that led Einstein to declare that "[God] does not play dice."

Experimental test of quantum nonlocality in three-photon Greenberger-Horne-Zeilinger entanglement

Yep, another 'ta-da' experiment showing that experimental quantum mechanics does indeed behave just like us theorists say it should. Great. Next?Cutting to the chase, the simple fact is that no form of 'non-locality ' exhibited by quantum mechanics allows for superluminal transfer of information, leaving causality intact. Or from your layman's position, are you suggesting otherwise?

It is clear that local realism is violated. It is not clear at all that causality is violated; quite the opposite. Why would you try to claim otherwise?

In that context, cavediver was discussing nonrelativistic quantum mechanics, which is certainly not causal because it doesn't take relativity into account. Nonrelativistic QM lives in Newton's space and time, not the spacetime of Einstein's special theory of relativity.
However quantum field theory, which is basically quantum mechanics made relativistic, does obey causality.