Kvantfysikens vågfunktion

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A quantum system is represented mathematically by a wave function, which is derived from Schrodinger's equation.

Alias: a wave function, kvantfysikens vågfunktion och wave-function


Collapsing the Wave Function

A quantum system is represented mathematically by a wave function, which is derived from Schrodinger's equation. The wave function can be used to calculate the probability of finding a particle at any particular point in space. When a measurement is made, the particle is of course found in only one place, but if the wave function is assumed to provide a complete and literal description of the state of a quantum system_as it is in the conventional interpretation_it would mean that in between measurements the particle dissolves into a superposition of probability waves and is potentially present in many different places at once. Then, when the next measurement is made, this wave packet is supposed to instantaneously collapse, in some random and mysterious manner, into a localized particle again. This sudden and discontinuous collapse violates the Schrodinger equation, and is not further explained in the conventional interpretation.

Since the measuring device that is supposed to collapse a particle's wave function is itself made up of subatomic particles, it seems that its own wave function would have to be collapsed by another measuring device (which might be the eye and brain of a human observer), which would in turn need to be collapsed by a further measuring device, and so on, leading to an infinite regress. In fact. the standard interpretation of quantum theory implies that all the macroscopic objects we see around us exist in an objective. unambiguous state only when they are being measured or observed. Schrodinger devised a famous thought-experiment to expose the absurd implications of this interpretation. A cat is placed in a box containing a radioactive substance, so that there is a fifty-fifty chance of an atom decaying in one hour. If an atom decays, it triggers the release of a poison gas, which kills the cat. After one hour the cat is supposedly both dead and alive (and everything in between) until someone opens the box and instantly collapses its wave function into a dead or alive cat.

Various solutions to the measurement problem associated with wavefunction collapse have been proposed. Some physicists maintain that the classical or macro-world does not suffer from quantum ambiguity because it can store information and is subject to an arrow of time, whereas the quantum or micro-world is alleged to be unable to store information and time-reversible (Pagers, 1983). A more extravagant approach is the many-worlds hypothesis, which claims that the universe splits each time a measurement (or measurement-like interaction) takes place, so that all the possibilities represented by the wave function (e.g. a dead cat and a living cat) exist objectively but in different universes. Our own consciousness, too, is supposed to be constantly splitting into different selves, which inhabit these proliferating, non-communicating worlds.

Other theorists speculate that it is consciousness that collapses the wave function and thereby creates reality. In this view, a subatomic particle does not assume definite properties when it interacts with a measuring device, but only when the reading of the measuring device is registered in the mind of an observer (which may of course be long after the measurement has taken place). According to the most extreme, anthropocentric version of this theory, only self-conscious beings such as ourselves can collapse wave functions. This means that the whole universe must have existed originally as potentia in some transcendental realm of quantum probabilities until self-conscious beings evolved and collapsed themselves and the rest of their branch of reality into the material world, and that objects remain in a state of actuality only so long as they are being observed by humans (Goswami, 1993). Other theorists, however, believe that non-self-conscious entities, including cats and possibly even electrons, may be able to collapse their own wave functions (Herbert, 1 993).

The theory of wave-function collapse (or state-vector collapse, as it is sometimes called) raises the question of how the probability waves that the wave function is thought to represent can collapse into a particle if they are no more than abstract mathematical constructs. Since the very idea of wave packets spreading out and collapsing is not based on hard experimental evidence but only on a particular interpretation of the wave equation, it is worth taking a look at one of the main alternative interpretations, that of David Bohm and his associates. which provides an intelligible account of what may be taking place at the quantum level.

In Bohm's model, then, the quantum world exists even when it is not being observed and measured. He rejects the positivist view that something that cannot be measured or known precisely cannot be said to exist. In other words, he does not confuse epistemology with ontology, the map with the territory. For Bohm, the probabilities calculated from the wave function indicate the chances of a particle being at different positions regardless of whether a measurement is made, whereas in the conventional interpretation they indicate the chances of a particle coming into existence at different positions when a measurement is made. The universe is constantly defining itself through its ceaseless interactions_of which measurement is only a particular instance_and absurd situations such as dead-and- alive cats therefore cannot arise.

Thus, although Bohm rejects the view that human consciousness brings quantum systems into existence, and does not believe that our minds normally have a significant effect on the outcome of a measurement (except in the sense that we choose the experimental set-up), his interpretation opens the way for the operation of deeper, subtler, more mind-like levels of reality. He argues that consciousness is rooted deep in the implicate order. and is therefore present to some degree in all material forms. He suggests that there may be an infinite series of implicate orders, each having both a matter aspect and a consciousness aspect: everything material is also mental and everything mental is also material, but there are many more infinitely subtle levels of matter than we are aware of (Weber, 1990, p. 151). The concept of the implicate domain could be seen as an extended form of materialism, but, he says, it could equally well be called idealism, spirit, consciousness. The separation of the two_ matter and spirit_is an abstraction. The ground is always one. (Weber, 1990.p. 101)


Följande är citat ur Meddelande från SPF No12. Referenser återges sist i detta inlägg. Det gäller ett försök genomfört 1987 av Edwin May vid SRI.

För experimentet använde man en modifierad Michelson-Morley interferometer. I en standard interferometer använder man en ljus-stråle som delas upp i två strålar. Dessa reflekteras och förenas sedan vid detektorn. Interferensen mellan de två fas-relaterade strålarna skapar ljusa och mörka fransar vid detektorn. Det är alltså bara en foton i systemet vid en given tidpunkt, men ändå uppvisas ett kvant-beteende -- man ser lätt interferens-fransar. En tolknoing av detta är att det i en komplett beskrivning av systemet måste ingå bidrag från de två tillåtna 'eigenstates' (dvs de två vägarna fotonen kan ta).

Vår modifiering innebar att vi placerade en slutare, kontrollerad av radioaktivt sönderfall i de båda interferometer-strålarna. När båda slutarna är öppna skulle ett interferens-mönster observeras vid detektorn. Om båda slutarna är stängda når inget ljus detektorn. Vad händer om bara en slutare, vid något tillfälle är öppen, och det inte finns någon uppgift (vare sig i hårdvara eller i någons medvetande) om vilken av slutarna som då var öppen? Det förutspådda resultatet beror på var och när the state vector kollapsar, dvs när och var kvant mätningen äger rum. Äger kollapsen rum i systemet med elektronik och switchar, eller i systemet med ljusdetektor/film/mänsklig observatör?

Man antar automatiskt att kollapsen sker i systemet med elektronik, men i detta försök kan det lika gärna vara systemet med ljusdetektor. Om så är fallet, förutsäger kvantmekaniken att fransar skulle uppträda vid detektorn.


När modulatorerna slumpmässigt öppnades och stängdes kunde man inte se några fransar. Försöket visade alltså att medvetandet inte behövs för att avgöra kollapsen av the state vector, dvs fastställa verkligheten.