Entropy, irreversability, and the arrow of time

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Discussion Overview

The discussion revolves around the relationship between entropy, irreversibility, and the concept of the arrow of time, particularly in the context of quantum measurements such as electron spin. Participants explore theoretical implications and interpretations of quantum mechanics (QM) related to these concepts.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants suggest that the irreversibility of measurements, such as those involving electron spin, implies the existence of an arrow of time.
  • Others argue that the measurement process can be divided into two parts: an entanglement process described by the Schrödinger equation, which is reversible, and a selection of pointer states that is not understood and may not occur in certain interpretations of QM.
  • It is proposed that decoherence, resulting from interactions with a large number of degrees of freedom in the environment, plays a critical role in the measurement process and is associated with an increase in entropy.
  • Some participants emphasize that regardless of the interpretation of QM, the concept of decoherence is generally accepted as a key factor in understanding measurement irreversibility.

Areas of Agreement / Disagreement

Participants express differing views on the implications of measurement irreversibility for the arrow of time, with some asserting a direct connection while others highlight the complexities and uncertainties surrounding the measurement process in quantum mechanics. No consensus is reached on these points.

Contextual Notes

The discussion reflects limitations in understanding the second part of the measurement process and the implications of various interpretations of quantum mechanics, such as Many-Worlds and Bohmian interpretations, which may influence perspectives on irreversibility and entropy.

entropy1
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It is said that entropy causes an arrow of time. However, how about the irreversability of a measurement like electron spin. When measured a certain spin, the previous value gets lost. So does that also require an arrow of time?
 
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Any device measuring such things has a large enough number of mechanical degrees of freedom for thermodynamic concepts to apply to the composite electron-device system.
 
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A measurement involves (or seems to involve) two distinct processes:
  1. Through ordinary interactions between the measured system and the measuring device, the two systems become entangled. This step is presumably described completely by the Schrödinger equation (or something analogous).
  2. One of the macroscopically distinguishable "pointer states" is selected (or realized, or actualized, or chosen) according to the Born rule.
The first part process is reversible. The second is not. But we don't actually understand the second process, or even whether it happens (in Many-Worlds and Bohmian interpretations, it never happens).
 
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entropy1 said:
It is said that entropy causes an arrow of time. However, how about the irreversability of a measurement like electron spin. When measured a certain spin, the previous value gets lost. So does that also require an arrow of time?

Yes. QM assumes an arrow of time because a measurement must be irreversible.
 
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entropy1 said:
It is said that entropy causes an arrow of time. However, how about the irreversability of a measurement like electron spin. When measured a certain spin, the previous value gets lost. So does that also require an arrow of time?
Irrespective of one's favored interpretation of QM, it is now generally accepted that measurement always involves decoherence caused by interaction with a large number of degrees of freedom in the environment. It is this decoherence that involves the increase of entropy in the measurement process.
 
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Demystifier said:
Irrespective of one's favored interpretation of QM, it is now generally accepted that measurement always involves decoherence caused by interaction with a large number of degrees of freedom in the environment. It is this decoherence that involves the increase of entropy in the measurement process.

Exactly - its basically the conclusion reached in this book I recently read:
https://www.amazon.com/dp/B01L7NB4QC/?tag=pfamazon01-20

Its rather subtle though.

Thanks
Bill
 

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