In science, 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. This effect can be observed in many domains of physics.
The observer effect on a physical process can often be reduced to insignificance by using better instruments or observation techniques.
Historically, the observer effect has been confused with the uncertainty principle.^{[1]}^{[2]}
Particle physics
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.
Electronics
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 behaviour 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.
Thermodynamics
In thermodynamics, a standard mercuryinglass thermometer must absorb or give up some thermal energy to record a temperature, and therefore changes the temperature of the body which it is measuring.
Quantum mechanics
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 topic is that of wave function collapse, for which some interpretations assert that measurement causes a discontinuous change into an eigenstate of the operator associated with the quantity that was measured. More explicitly, the superposition principle (ψ = Σa_{n}ψ_{n}) 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 f_{n}, n=1,2...m, of the operator \hat{F} which is part of the eigenfunctions ψ_{n}, 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.^{[3]} This means that the type of measurement that we do 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.^{[4]}^{[5]}^{[6]}
An important aspect of the concept of measurement has been clarified in some QM experiments where a small, complex, and nonsentient sensor proved sufficient as an "observer"—there is no need for a conscious "observer".^{[7]}
In the ambit of socalled hiddenmeasurements interpretation of quantum mechanics, the observereffect can be understood as an instrument effect which results form 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 measurementinteraction is each time actualized, in a nonpredictable (noncontrollable) way.^{[8]}^{[9]}^{[10]}
A consequence of Bell's theorem is that measurement on one of two entangled particles can appear to have a nonlocal effect on the opposite particle. Additional problems related to decoherence arise when the observer too is modeled as a quantum system.
The uncertainty principle has been frequently confused with the observer effect, evidently even by its originator, Werner Heisenberg.^{[1]} The uncertainty principle in its standard form actually 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.^{[11]} An alternative version of the uncertainty principle,^{[12]} more in the spirit of an observer effect,^{[13]} 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.
References

^ ^{a} ^{b} Furuta, Aya (2012), "One Thing Is Certain: Heisenberg's Uncertainty Principle Is Not Dead", Scientific American

^ Ozawa, Masanao (2003), "Universally valid reformulation of the Heisenberg uncertainty principle on noise and disturbance in measurement", Physical Review A 67 (4),

^ B.D'Espagnat, P.Eberhard, W.Schommers, F.Selleri. Quantum Theory and Pictures of Reality. SpringerVerlag, 1989, ISBN 3540501525

^ V. P. Belavkin (1989). "A new wave equation for a continuous nondemolition measurement". Physics Letters A 140 (78): 355–358.

^ Howard J. Carmichael (1993). An Open Systems Approach to Quantum Optics. Berlin Heidelberg NewYork: SpringerVerlag.

^ Michel Bauer, Denis Bernard, Tristan Benoist. Iterated Stochastic Measurements (Technical report).

^ Science Daily

^ Sassoli de Bianchi, M. (2013). The Observer Effect. Foundations of Science 18, pp. 213243, arXiv:1109.3536.

^ Sassoli de Bianchi, M. (2013). God may not play dice, but human observers surely do. To be published in: Foundations of Science. arXiv:1208.0674.

^ Aerts, D. and Sassoli de Bianchi, M. (2014). The extended PoincareBloch representation of quantum mechanics and the hiddenmeasurement solution to the measurement problem. Accepted for publication in Annals of Physics, arXiv:1404.2429.

^ Heisenberg, W. (1930), Physikalische Prinzipien der Quantentheorie, Leipzig: Hirzel English translation The Physical Principles of Quantum Theory. Chicago: University of Chicago Press, 1930. reprinted Dover 1949

^ Ozawa, Masanao (2003), "Universally valid reformulation of the Heisenberg uncertainty principle on noise and disturbance in measurement", Physical Review A 67,

^ V. P. Belavkin (1992). "Quantum continual measurements and a posteriori collapse on CCR". Communications in Mathematical Physics 146 (3): 611–635.
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