ABSTRACT
The disturbance during a measurement on a classical system can be made arbitrarily small. One may then assume that a measurement does not change the state, e.g., the radar used by police to measure the speed of a car has a negligible effect on the speed of the car. Classical dynamics also applies to the measuring process as well. For quantum systems Postulate 26.1(OV) tells us that a measurement of a discrete observable A will yield an eigenvalue of its corresponding operator Â. Let the initial state vector be ϕ → i https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203702413/2f242b9b-104e-4a03-b478-d3f6577cf44e/content/eq6175.tif"/> at time t = 0 when the measurement begins and let the final state vector be ϕ → f https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203702413/2f242b9b-104e-4a03-b478-d3f6577cf44e/content/eq6176.tif"/> at time t = Δt when the measurement ends. A natural question to ask is
Can disturbance during a measurement on the system be made arbitrarily small so that the state can remain the same, and if not, does the initial state evolve into the final state in a unitary manner describable in terms of Postulate 29.1.2(TESP)?
