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Particle physics

An electron is detected upon interaction with a photon; 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 to Niels Bohr (1928) is given by | v x ′ − v x | Δ p x ≈ ℏ / Δ t , {\displaystyle |v'_{x}-v_{x}|\Delta p_{x}\approx \hbar /\Delta t,} where

• 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.⁷

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 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.

Thermodynamics

In thermodynamics, a standard mercury-in-glass 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

This section is an excerpt from Observer (quantum physics).[edit]

Some interpretations of quantum mechanics posit a central role for an observer of a quantum phenomenon.⁸ The quantum mechanical observer is tied to the issue of observer 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 a Hermitian operator that represents a measurement.⁹: 55 

See also

• Observer (special relativity)

References

1. Dirac, P.A.M. (1967). The Principles of Quantum Mechanics (4th ed.). Oxford University Press. p. 3. /wiki/The_Principles_of_Quantum_Mechanics ↩

2. Dent, Eric B. "The Observation, Inquiry, and Measurement Challenges Surfaced by Complexity Theory" (PDF). In Richardson, Kurt (ed.). Managing the Complex: Philosophy, Theory and Practice. Archived from the original (PDF) on 19 August 2019. Retrieved 23 April 2019. https://web.archive.org/web/20190819065402/http://faculty.uncfsu.edu/edent/Observation.pdf ↩

3. Squires, Euan J. (1994). "Does wavefunction reduction require conscious observers?". The Mystery of the Quantum World. Taylor & Francis Group. p. 62. ISBN 9781420050509. 9781420050509 ↩

4. "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 /wiki/Werner_Heisenberg ↩

5. "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. /wiki/John_Stewart_Bell ↩

6. 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. 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. Basic Books. Ch 3.2. ISBN 9780465040834.). 97805215233879780465040834 ↩

7. Landau, L.D.; Lifshitz, E. M. (1977). Quantum Mechanics: Non-Relativistic Theory. Vol. 3. Translated by Sykes, J. B.; Bell, J. S. (3rd ed.). Pergamon Press. §7, §44. ISBN 978-0-08-020940-1. 978-0-08-020940-1 ↩

8. Schlosshauer, Maximilian; Kofler, Johannes; Zeilinger, Anton (1 August 2013). "A snapshot of foundational attitudes toward quantum mechanics". Studies in History and Philosophy of Science Part B. 44 (3): 222–230. arXiv:1301.1069. Bibcode:2013SHPMP..44..222S. doi:10.1016/j.shpsb.2013.04.004. S2CID 55537196. /wiki/ArXiv_(identifier) ↩

9. Rieffel, Eleanor G.; Polak, Wolfgang H. (4 March 2011). Quantum Computing: A Gentle Introduction. MIT Press. ISBN 978-0-262-01506-6. 978-0-262-01506-6 ↩