Observer effect (physics)

Particle physics

To "observe a particle" means to gain information about one of its physical properties (such as its position, momentum, energy, or spin) by having it interact with a measuring device.

A free particle is a particle that experiences no external forces (interactions), so its momentum remains constant. In contrast, an electron interacting with a photon will inevitably alter the momentum and/or position of both particles when the electron and photon collide, thus resulting in a transfer of energy between them. Therefore the act of observing an electron changes the state it is in.

Even when the measurement does not involve a collision, such as by observing its electric field or magnetic field, it will alter its wave-function and therefore alter its momentum and/or position.

It is also necessary to distinguish clearly between the measured value of a quantity and the value resulting from the measurement process. A formula (one-dimensional for simplicity) relating involved quantities, derived from Niels Bohr's 1928 paper "The Quantum Postulate and the Recent Development of Atomic Theory, is given by the following formula:|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.
• This formula is related to the Heisenberg uncertainty principle.

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 showing how the measurement (or observation) changes the mathematical answer to the equation.

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

References

References

1. Dirac, P.A.M.. (1967). "[[The Principles of Quantum Mechanics]]". Oxford University Press.
2. Dent, Eric B.. "Managing the Complex: Philosophy, Theory and Practice".
3. Squires, Euan J.. (1994). "The Mystery of the Quantum World". Taylor & Francis Group.
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
5. Feynman, Richard. (2015). "The Feynman Lectures on Physics". Basic Books.
6. Bohr, Niels. (April 14, 1928). "THE QUANTUM POSTULATE AND THE RECENT DEVELOPMENT OF ATOMIC THEORY". Nature.
7. (1977). "Quantum Mechanics: Non-Relativistic Theory". [[Pergamon Press]].