Inphysics, theobserver effect is the disturbance of an observed system by the act of observation.[1][2] This is often the result of utilising instruments that, by necessity, alter the state of what they measure in some manner. A common example is checking the pressure in an automobile tire, which causes some of the air to escape, thereby changing the amount of pressure one observes. Similarly, seeing non-luminous objects requires light hitting the object to cause it to reflect that light. While the effects of observation are often negligible, the object still experiences a change (leading to theSchrödinger's cat thought experiment). This effect can be found in many domains of physics, but can usually be reduced to insignificance by using different instruments or observation techniques.
A notable example of the observer effect occurs inquantum mechanics, as demonstrated by thedouble-slit experiment. Physicists have found that observation of quantum phenomena by a detector or an instrument can change the measured results of this experiment. Despite the "observer effect" in the double-slit experiment being caused by the presence of an electronic detector, the experiment's results have been interpreted by some to suggest that a conscious mind can directly affect reality.[3] However, the need for the "observer" to be conscious is not supported by scientific research, and has been pointed out as a misconception rooted in a poor understanding of the quantum wave functionψ and the quantum measurement process.[4][5][6]
Anelectron is detected upon interaction with aphoton; this interaction will inevitably alter the velocity and momentum of that electron. It is possible for other, less direct means of measurement to affect the electron. It is also 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 toNiels Bohr (1928) is given bywhere
Themeasured momentum of the electron is then related tovx, whereas its momentumafter the measurement is related tov′x. This is a best-case scenario.[7]
Inelectronics,ammeters andvoltmeters 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 complexload to the circuit, thus changing thetransfer function and behavior of the circuit itself. Even a more passive device such as acurrent clamp, which measures the wire current without coming into physical contact with the wire, affects the current through the circuit being measured because theinductance is mutual.
Someinterpretations of quantum mechanics posit a central role for anobserver of a quantum phenomenon.[8] The quantum mechanical observer is tied to the issue ofobserver effect, where a measurement necessarily requires interacting with the physical object being measured, affecting its properties through the interaction. The term "observable" has gained a technical meaning, denoting aHermitian operator that represents a measurement.[9]: 55
^"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, andit 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 instead leave the room and delegate observing to an inanimate apparatus 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.ISBN9780521523387.). The measured state is not interfering with the states excluded by the measurement. AsRichard 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. Basic Books. Ch 3.2.ISBN9780465040834.).
