User:Elliott.Lawrenson/Observer effect

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The Heisenberg uncertainty principle is also frequently confused with the "observer effect". The uncertainty principle actually describes how precisely we may measure the position and momentum of a particle at the same time &mdash; if we increase the accuracy in measuring one quantity, we are forced to lose accuracy in measuring the other. The observer effect however, relates to the influence the observer has on a system. The superposition principle (ψ = Σanψn) of quantum physics says that for a wave function ψ, a measurement will give a state of the quantum system of one of the m possible eigenvalues fn, n=1,2...m, of the operator $$\hat{F}$$ which is part of the eigenfunctions ψn, n=1,2,...n. Once we have measured the system, we know its current state and this stops it from being in one of its other states. This means that the type of measurement that we do on the system affects the end state of the system.

This is demonstrated in a common thought experiment using the double slit setup. Imagine a double slit experiment where quantum particles are fired towards the two slits. The quantum particles pass through the slits and hit a momentum sensor a distance of D behind the slits. The momentum sensor has the ability to be turned off and on via a pin which stops the movement of the sensor when it is hit by a quantum particle. When the pin is in place, no measurement of the momentum can take place. When the pin is removed, the sensor can recoil when struck by a quantum particle and by measuring the recoil determine from which slit the quantum particle came. If the pin is removed and we can detect from which slit the particle came, then the wave-like passage through both slits cannot occur and no interference pattern will develop. However if we put the pin in place, and can no longer determine from which slit the particle passes through, then an interference pattern can develop. This can be taken a step further using the delayed choice experiment.

This thought experiment was proved correct experimentally. The people conducting the experiment found that when the sensor was turned off, an interference pattern developed, but when it was turned on, the interference pattern was destroyed. It was even found that the level of detection could affect the result.

The change of the wave function from ψ to ψn is called the collapse of the wave function and occurs when the measurement takes place. This collapse of the wave function is not explainable using the Copenhagen interpretation. Other explanations on why the wave function collapses have been developed such as that consciousness causes the collapse or hidden variable theory.

The many-worlds interpretation posits the existence of multiple universes in which an observed system displays all possible states to all possible observers. In this model, observation of a system does not change the behavior of the system—it simply answers the question of which universe the observer is located in. In some universes, the observer would observe one result from one state of the system, and in others the observer would observe a different result from a different state of the system.

Old version - what was on the site
The Heisenberg uncertainty principle is also frequently confused with the "observer effect". The uncertainty principle actually describes how precisely we may measure the position and momentum of a particle at the same time &mdash; if we increase the precision in measuring one quantity, we are forced to lose precision in measuring the other. Thus, the uncertainty principle deals with measurement, and not observation. The idea that the uncertainty principle is caused by disturbance (and hence by observation) is not considered to be valid by some, although it was extant in the early years of quantum mechanics, and is often repeated in popular treatments.

There is a related issue in quantum mechanics relating to whether systems have pre-existing (prior to measurement, that is) properties corresponding to all measurements that could possibly be made on them. The assumption that is often made is referred to as "realism" in literature, although it has been argued that the word "realism" is being used in a more restricted sense than philosophical realism. A recent experiment in the realm of quantum physics has been quoted as meaning that we have to "say goodbye" to realism, although the author of the paper states only that "we would [..] have to give up certain intuitive features of realism". These experiments demonstrate a puzzling relationship between the act of measurement and the system being measured, although it is clear from experiment that an "observer" consisting of a single electron is sufficient—the observer need not be a conscious observer. Also, note that Bell's Theorem strongly suggests that the idea that the state of a system exists independently of its observer may be false.

Note that the special role given to observation (the claim that it affects the system being observed, regardless of the specific method used for observation) is a defining feature of the Copenhagen interpretation of quantum mechanics. Other interpretations resolve the apparent paradoxes from experimental results in other ways. For instance, the many-worlds interpretation posits the existence of multiple universes in which an observed system displays all possible states to all possible observers. In this model, observation of a system does not change the behavior of the system—it simply answers the question of which universe the observer is located in. In some universes, the observer would observe one result from one state of the system, and in others the observer would observe a different result from a different state of the system.

The impact of observation on quantum systems has been demonstrated experimentally.