# [time 1060] abstract quant-ph/9909010

Stephen Paul King (stephenk1@home.com)
Mon, 29 Nov 1999 10:33:16 -0500

http://arXiv.org/abs/quant-ph/9909010
abstract quant-ph/9909010

# Quantum Physics, abstract quant-ph/9909010

From: Benni Reznik <reznik@post.tau.ac.il>
Date: Thu, 2 Sep 1999 13:07:10 GMT   (24kb)


## Back-Reaction of Clocks and Limitations on Observability in Closed Systems

Authors: Aharon Casher, Benni Reznik

Measurements are ordinarily described with respect to absolute "Newtonian" time. In reality however, the switching-on of the measuring device at the instance of the measurement requires a timing device. Hence the classical time $t$ must be replaced by a suitable quantum time variable $\tau$ of a physical clock. The main issue raised in this article is that while doing so, we can no longer neglect the {\em back-reaction} due to the measurement on the clock. This back-reaction yields a bound on the accuracy of the measurement. When this bound is violated the result of a measurement is generally not an eigenvalue of the observable, and furthermore, the state of the system after the measurement is generally not a pure state. We argue that as a consequence, a sub-class of observables in a closed system cannot be realized by a measurement.

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