Foundations of Physics

, Volume 11, Issue 7–8, pp 547–576 | Cite as

Measuring processes in quantum mechanics I. Continuous observation and the watchdog effect

  • K. Kraus


It is well known that successive observations of the instantaneous state of a decaying system lead to a modified decay law. In the limit of infinitely frequent observations, the modified lifetime becomes infinite (“Zeno's paradox”). We study here the behavior of decaying systems under continuous rather than successive observations. Such continuous observation is achieved by a permanent coupling of the decaying system to a counter, which is sufficiently sensitive to the presence of the decay products. For two explicitly soluble models we obtain a result very similar to Zeno's paradox: The observation again strongly modifies the decay law, and prevents the decay completely in the limit of a perfect (i.e., infinitely sensitive) counter. Like Zeno's paradox, however, this “watchdog effect” is not very likely to be detected in actual experiments, e.g., with radioactive nuclei.


Quantum Mechanic Actual Experiment Decay Product Instantaneous State Continuous Observation 
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Copyright information

© Plenum Publishing Corporation 1981

Authors and Affiliations

  • K. Kraus
    • 1
  1. 1.Center for Particle TheoryUniversity of Texas at AustinAustin

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