Observer effect (physics)

In physics, the term observer effect refers to changes that the act of observation will make on a phenomenon being observed. This is often the result of instruments that, by necessity, alter the state of what they measure in some manner. A commonplace example is checking the pressure in an automobile tire; this is difficult to do without letting out some of the air, thus changing the pressure. Furthermore, it is not possible to see any object without light hitting the object, and causing it to emit light; while this may seem negligible, the object still experiences a change. This effect can be observed in many domains of physics and can often be reduced to insignificance by using better instruments or observation techniques.

In quantum mechanics, there is a common misconception that it is the mind of a conscious observer that causes the observer effect in quantum processes. It is rooted in a misunderstanding of the quantum wave function ψ and the quantum measurement process.

For an electron to become detectable, a photon must first interact with it, and this interaction will inevitably change the path of that electron. It is also possible for other, less direct means of measurement to affect the electron. It is necessary to distinguish clearly between the measured value of a quantity and the value resulting from the measurement process. In particular, a measurement of momentum is non-repeatable in short intervals of time. A formula (one-dimensional for simplicity) relating involved quantities, due to Niels Bohr (1928) is given by

where

The measured momentum of the electron is then related to v_x, whereas its momentum after the measurement is related to v′_x. This is a best-case scenario.

In electronics, ammeters and voltmeters are usually wired in series or parallel to the circuit, and so by their very presence affect the current or the voltage they are measuring by way of presenting an additional real or complex load to the circuit, thus changing the transfer function and behavior of the circuit itself. Even a more passive device such as a current clamp, which measures the wire current without coming into physical contact with the wire, affects the current through the circuit being measured because the inductance is mutual.

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