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
- Δpx is uncertainty in measured value of momentum,
- Δt is duration of measurement,
- vx is velocity of particle before measurement,
- v '
x is velocity of particle after measurement,
- ħ is the reduced Planck constant.
The measured momentum of the electron is then related to vx, 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.
The theoretical foundation of the concept of measurement in quantum mechanics is a contentious issue deeply connected to the many interpretations of quantum mechanics. A key focus point is that of wave function collapse, for which several popular interpretations assert that measurement causes a discontinuous change into an eigenstate of the operator associated with the quantity that was measured, a change which is not time-reversible.
More explicitly, the superposition principle (ψ = Σanψn) of quantum physics dictates that for a wave function ψ, a measurement will result in a state of the quantum system of one of the m possible eigenvalues fn , n = 1, 2, ..., m, of the operator which in the space of the eigenfunctions ψn , n = 1, 2, ..., m.
Once one has measured the system, one knows its current state; and this prevents it from being in one of its other states−−it has apparently decohered from them without prospects of future strong quantum interference. This means that the type of measurement one performs on the system affects the end-state of the system.
An experimentally studied situation related to this is the quantum Zeno effect, in which a quantum state would decay if left alone, but does not decay because of its continuous observation. The dynamics of a quantum system under continuous observation is described by a quantum stochastic master equation known as the Belavkin equation. Further studies have shown that even observing the results after the photon is produced leads to collapsing the wave function and loading a back-history as shown by delayed choice quantum eraser.
When discussing the wave function ψ which describes the state of a system in quantum mechanics, one should be cautious of a common misconception that assumes that the wave function ψ amounts to the same thing as the physical object it describes. This flawed concept must then require existence of an external mechanism, such as the mind of a conscious observer, that lies outside the principles governing the time evolution of the wave function ψ, in order to account for the so-called "collapse of the wave function" after a measurement has been performed. But the wave function ψ is not a physical object like, for example, an atom, which has an observable mass, charge and spin, as well as internal degrees of freedom. Instead, ψ is an abstract mathematical function that contains all the statistical information that an observer can obtain from measurements of a given system. In this case, there is no real mystery that mathematical form of the wave function ψ must change abruptly after a measurement has been performed.
In the ambit of the so-called hidden-measurements interpretation of quantum mechanics, the observer-effect can be understood as an instrument effect which results from the combination of the following two aspects: (a) an invasiveness of the measurement process, intrinsically incorporated in its experimental protocol (which therefore cannot be eliminated); (b) the presence of a random mechanism (due to fluctuations in the experimental context) through which a specific measurement-interaction is each time actualized, in a non-predictable (non-controllable) way.
A consequence of Bell's theorem is that measurement on one of two entangled particles can appear to have a nonlocal effect on the other particle. Additional problems related to decoherence arise when the observer is modeled as a quantum system, as well.
The uncertainty principle has been frequently confused with the observer effect, evidently even by its originator, Werner Heisenberg. The uncertainty principle in its standard form describes how precisely we may measure the position and momentum of a particle at the same time — if we increase the precision in measuring one quantity, we are forced to lose precision in measuring the other. An alternative version of the uncertainty principle, more in the spirit of an observer effect, fully accounts for the disturbance the observer has on a system and the error incurred, although this is not how the term "uncertainty principle" is most commonly used in practice.
- "Of course the introduction of the observer must not be misunderstood to imply that some kind of subjective features are to be brought into the description of nature. The observer has, rather, only the function of registering decisions, i.e., processes in space and time, and it does not matter whether the observer is an apparatus or a human being; but the registration, i.e., the transition from the "possible" to the "actual," is absolutely necessary here and cannot be omitted from the interpretation of quantum theory." - Werner Heisenberg, Physics and Philosophy, p. 137
- "Was the wave function waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer for some highly qualified measurer - with a PhD?" -John Stewart Bell, 1981, Quantum Mechanics for Cosmologists. In C.J. Isham, R. Penrose and D.W. Sciama (eds.), Quantum Gravity 2: A second Oxford Symposium. Oxford: Clarendon Press, p.611.
- According to standard quantum mechanics, it is a matter of complete indifference whether the experimenters stay around to watch their experiment, or leave the room and delegate observing to an inanimate apparatus, instead, which amplifies the microscopic events to macroscopic measurements and records them by a time-irreversible process (Bell, John (2004). Speakable and Unspeakable in Quantum Mechanics: Collected Papers on Quantum Philosophy. Cambridge University Press. p. 170. ISBN 9780521523387.). The measured state is not interfering with the states excluded by the measurement. As Richard Feynman put it: "Nature does not know what you are looking at, and she behaves the way she is going to behave whether you bother to take down the data or not." (Feynman, Richard (2015). The Feynman Lectures on Physics, Vol. III. Ch 3.2: Basic Books. ISBN 9780465040834.).
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