Decoherence is a broad explanation for the lack of uniquely quantum effects in macroscopic objects. Decoherence theorists say in particular that it explains the absence of superpositions of live and dead Schrödinger Cats. The "decoherence program" of H. Dieter Zeh, Wojciech Zurek, and their colleagues is an attempt to describe the "appearance" or "emergence" of a classical world from the microscopic quantum world. Decoherence theorists trace the emergence of classical properties to the interaction of quantum systems with the environment, generally the exchange of photons between quantum systems and the environment, but also the collisions of particles (which are mediated by virtual photons). No physical system is ever completely isolated from the environment. A perfectly isolated system is by definition unobservable and thus of no interest to an experimental physicist, though theoreticians often work with such ideas. Although isolation is one of the fundamental principles of all physics experiments, it is the case that it is practically impossible to prevent high-energy particles and photons from passing through any experiment.
Denials of Standard Quantum Physics
Most decoherence theorists subscribe to what they call a "universally valid quantum theory." Despite the name, this theory denies one of the basic hypotheses of standard quantum physics, namely the collapse of the quantum-mechanical wave function (Dirac's projection postulate), which is the ultimate source of chance, indeterminism, free will, and creativity. "Universally valid" refers to the Wheeler-Everett-DeWitt-Wigner view that replaces the wave-function collapse with a splitting of a universal wave function Ψ into separate branches or components, each of which contains all the material of the universe just before the quantum event, typically a quantum measurement. Where standard quantum theory predicts the probabilities of the measurement yielding two or more eigenvalues of the physical observable being measured, the "Many Worlds" theory assumes that new worlds are created with each world realizing one of those eigenvalues, all other things about the new worlds being the same as before the measurement. John Bell described the many-worlds theory as "extravagant." We find it extremely so, since it blatantly violates the most fundamental conservation laws of physics. In order to create another parallel universe, it must double the amount of energy, mass, charge, etc. And the new universe must be as large as our observable universe. All this because they find the idea of the collapse of the wave function non-intuitive, which in some respects it is. But in other respects it is simply the actualization of a single outcome from among many alternative possibilities. Some of the decoherence theorists appear to share a dislike of indeterminism, exemplified by Albert Einstein's famous dictum "God does not play dice." Einstein objected to determinism, even more strongly, he objected in his famous EPR paper to the non-local character of quantum reality, which suggested to him that "influences" are traveling faster than the speed of light, violating his theory of special relativity. And if Einstein disliked the measurement of one quantum particle instantly altering the properties of another particle a significant distance away (in this universe), what would he have thought about duplicating the entire observable universe contents in an unobservable parallel branch of the universal wave function Ψ?
Decoherence in Standard Quantum PhysicsDecoherence can be separated from the "many worlds" and "no-collapse" theories. The core idea is that classical macroscopic properties depend on decoherence of quantum properties, especially the interference of different components of a coherent quantum system. We can endorse that view and show that it is not classical properties that "emerge" under conditions of decoherence, but quantum properties that show up when we look at sufficiently isolated systems small enough to exhibit coherence. Paul Dirac described the breakdown of classical mechanics as "an inadequacy of its concepts to supply us with a description of atomic events." (Dirac, p.3) The early Greek philosophers Democritus, Leucippus, and Epicurus argued that large objects were made from smaller objects, but there comes a size when something is absolutely small. Quantum mechanics defines the absolutely small as objects that cannot be seen without disturbing them. Decoherence says that the very act of looking at a quantum system destroys the coherence that reflects its fundamental quantum nature. Dirac defined an object to be "big" when the disturbance accompanying our observation of it may be neglected, and "small" when the disturbance cannot be neglected. There comes a size when every attempt to minimize the disturbance fails. Dirac says "there is a limit to the fineness of our powers of observation and the smallness of the accompanying disturbance - a limit which is inherent in the nature of things and can never be surpassed by improved techniques or skill on the part of the observer.." An important consequence of absolute smallness is that we must revise our idea of causality. If a system is small, we cannot observe it without producing a serious disturbance and hence we cannot expect to find causal and deterministic connections connections between our observations. There is an unavoidable indeterminacy in measurement of quantum systems, so that we can only calculate the probability of various possible measurements. Under pressure from Niels Bohr, Werner Heisenberg modified his original idea that indeterminacy is always a result of observations (see Heisenberg's Microscope). Indeterminacy is an intrinsic property of quantum objects in space and time and does not depend on observations by conscious observers (with Ph.D.'s, as John Bell quipped). Note that quantum systems can make information-generating measurements on themselves, which the decoherence theorists accept.
A Two-State Example of Coherence and DecoherenceConsider the famous Two-Slit Experiment. We can label the probability-amplitude wave function passing through the left hand slit in the figure ψleft and the waves passing through the right-hand slit ψright. These are coherent and show the characteristic quantum interference fringes on the detector screen (a photographic plate or CCD array). This is the case even if the intensity of particles is so low that only one particle at a time arrives at the screen. In a dramatic experimental proof of decoherence, Gerhard Rempe sent matter waves of heavy Rubidium atoms through two slits. He then irradiated the left slit with microwaves that could excite the hyperfine structure in Rb atoms passing through that slit. As he turned up the intensity, the interference fringes diminished in proportion to the number of photons falling on the left slit. The photons decohere the otherwise coherent wave functions.
The Quantum-Classical "Boundary"