User:Patrick0Moran/Entanglement

Quantum entanglement occurs when some things (particles with mass such as electrons or even larger things such as "buckyballs," particles without mass such as photons, etc.) originally interact physically and then become separated in such a way that each resulting thing carries the same quantum mechanical description (state), a description that is indefinite in terms of important factors such as position, momentum, spin, polarization, etc. When a measurement is made and it causes one object to take on a definite value (e.g., clockwise spin), the other part of this pair of entangled objects will at any subsequent time be found to have taken the complementary value (e.g., counterclockwise spin). There is then a correlation between the states of entangled things even though quantum mechanics depicts the states of both as indefinite until measured. There is no question about this much of the picture, either in theory or in the laboratory. However, there is a profound dispute about whether the objects that are being described by quantum mechanics, in any case under discussion here, already had their real values (e.g., clockwise spin or counterclockwise spin) preset in some way at the instant they became separated, or whether the objects being described by the mathematically indeterminate equations were themselves as indeterminate as were their quantum mechanical descriptions. If the objects were indeterminate until one of them was measured, then the question becomes, "How can one account for something that was at one point indefinite with regard to its spin (or whatever is in this case the subject of investigation) suddenly becoming definite in that regard even though no physical interaction with the second object occurred, and, if the two objects are sufficiently far separated, could not even have had the time needed for such an interaction to proceed from the first to the second object?" The answer to the latter question involves the issue of locality, i.e., whether for a change to occur in something the agent of change has to be in physical contact (at least via some intermediary such as a field force) with the thing that changes. Study of entanglement brings into sharp focus the dilemma between locality and the completeness or lack of completeness of quantum mechanics.

In general, if a collection of things as described above, i.e., a system, is composed of multiple particles, one of the particles cannot be fully described without also considering the other(s), even if the particles are separated by some distance. In a system of entangled electrons, before a measurement is made it is impossible to describe their spins, and only the combined spin of the two-electron system is known. After the measurement of one of the electrons, the correlated spins of the two electrons become determinate. Measuring the value of the spin of one of them disentangles the particles, and forces the other to take on its own, separate spin value. This occurs even though the particles are now separated by arbitrarily large distances. Quantum entanglement is a consequence, in the mathematics and logic of quantum mechanics, that pertains to the quantum mechanical characterizations of objects (masses, photons, etc.) that are at one time in union and then later on become separated. The mathematical necessity of entanglement is undisputed. When one has any situation wherein two things are are brought together in such a way that they become indistinguishable in some respect or respects, then the quantum theoretical descriptions of the original two things have to be merged in such a way that the descriptions do not assert a now-untrue distinction between what are no longer two things. For instance, if two masses are brought together so closely that they remain in contact with each other for some time at three or more points, then the new thing has to be treated as having one position and one velocity. An everyday example of this kind of situation would be a deck chair bolted to a ship at sea. The deck chair goes where the ship goes, and vice versa. So the idea that the quantum theoretical descriptions of two objects brought together in this way must merge is not something that provokes astonishment.

Often, quantum entanglement is a consequence of a single operation that simultaneously produces two objects. When a quantum-scale object is in some way split so that two or more objects have separate positions in space, the consequences both in theory and in fact are not what people ordinarily expect based on their everyday experience—because the factors that were originally discovered as pertaining to a whole single object do not sort themselves out somehow into, e.g., the position of object one, the position of object two, the momentum of object one, and the momentum of object two.

Perhaps the easiest and most commonly used method of creating entanglement is by producing pairs of photons using spontaneous parametric down conversion, a procedure by which something such as a beta barium borate crystal is fed a photon of some frequency and subsequently emits, in one operation, two photons of half the original frequency. These photons will be described by identical sets of characteristics. These sets of characteristics are called quantum mechanical states. Certain characteristics are governed by physical laws that prevent each entangled object from having a value that is not complemented by the value of the other. The experimental demonstration of this requirement has been repeated innumerable times and is well attested—if one measures one of a pair of entangled photons and discovers it to have clockwise spin, then when the other photon is measured it will inevitably have counterclockwise spin.

All that quantum mechanics affirms is what will happen; it does not describe how it happens. Therefore it is possible that, without being mapped by the theory, something occurs at the instant of the production of the two entangled photons that determines that when photon A is measured it will be discovered to have clockwise spin, and when photon B is measured it will be discovered to have counterclockwise spin. But it is also possible that the quantum mechanical theory models what is really going on in the laboratory, and that each photon is in a superposition of physical states, i.e., that each photon is in an indeterminate state that will be ended only when taking a measure of that photon forces the issue and the photon shows up as having one spin or the other. On that understanding, when the other entangled photon is measured it will be precluded from showing up as having the same spin as its twin. It is not any clearer how this change in the status of the second entangled photon is effected than how any wave-function collapse results in a photon's showing up at one of the fringes in the double-slit experiment or results in a radium atom's emission of radioactivity. Nevertheless, events that can regularly be observed in the laboratory support the impression that the experimenter's making a measurement of photon A "does something to" photon B—even in cases where nothing traveling at or slower than the speed of light could have reached from A to B before B changed. This fact has led one camp of researchers to maintain that the idea of locality (i.e., the idea that things have to be in physical touch at least via some intermediary to change each other) is not valid on the quantum scale. The other camp of researchers maintains that there are characteristics set at the moment of creation of the entangled members that determine how they will later manifest themselves, and that the quantum mechanical description is only an incomplete map of reality, a map that leaves out these important hidden variables.

The quantum-theoretical state of entanglement is perhaps only to be regarded as a "convenient fiction" that helps us produce useful predictions about the behavior of things that have at some time been united in a single system and have later been separated, but it may also be a theoretical statement that maps deeper features of reality, i.e., some logical implications of this theoretical construct may be instructive and helpful in our understanding the world to which the theory called quantum mechanics relates. Ordinary experience rejects the idea that spatially separate objects can have states (systems of characteristics) that are dependent upon each other. Ordinary experience accepts only that objects have interacting states when in direct contact or when their actions are mediated by some physical process (field forces, electromagnetic signals, etc.). But if the real states of two spatially separate objects are independent of each other, then the quantum theoretical description of the two objects must be incomplete. On the other hand, if the prejudgments suggested by everyday experience are rejected and quantum mechanics is affirmed to give a complete description, then the idea of entangled objects that are spatially separate must include acknowledgement that these objects are not independent of each other.