# Entropy, irreversability, and the arrow of time

• B
Gold Member
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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hilbert2
Gold Member
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.

Demystifier
stevendaryl
Staff Emeritus
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 Schrodinger 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).

Lord Jestocost and entropy1
atyy
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.

bhobba and entropy1
Demystifier
Gold Member
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.

hilbert2 and bhobba
bhobba
Mentor
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