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ephen wilb

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In summary, the conversation discusses the concept of time reversibility in quantum mechanics and whether it is possible to access and control it in a setup that couples the system to a classical apparatus. It also touches on the idea of negative entropy and its relation to the arrow of time. It is mentioned that in pure quantum, there is no arrow of time and that decoherence is reversible, but this is incorrect. The conversation ends with a question about the possibility of time reversibility in the Bohmian interpretation of quantum mechanics.

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ephen wilb

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bhobba

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ephen wilb said:

Laws are symmetrical - set-ups are not as implied by entropy.

Thanks

Bill

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Khashishi

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This is related to experiments with Rabi oscillation. http://en.wikipedia.org/wiki/Rabi_cycle

If you have a carefully controlled setup, you can manipulate the quantum state of two level systems -- i.e. qubits. You can set or reset the qubit state to some superposition state. The natural tendency of a system is to decay from the upper energy state to the ground state, so you can think of these manipulations as going forward and backward in time, if you take some large literary license with the term time.

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ephen wilb

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bhobba said:Laws are symmetrical - set-ups are not as implied by entropy.

Thanks

Bill

what if the setup can be made negative entropy.. can you access the symmetrical qm law with reversible past and future?

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bhobba

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ephen wilb said:what if the setup can be made negative entropy.

I think you are confused what negative entropy means - its what's required to keep a systems entropy low. Entropy always increases - there is no escaping it - and that is the origin of the arrow of time:

http://preposterousuniverse.com/eternitytohere/faq.html

Thanks

Bill

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ephen wilb

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bhobba said:I think you are confused what negative entropy means - its what's required to keep a systems entropy low. Entropy always increases - there is no escaping it - and that is the origin of the arrow of time:

http://preposterousuniverse.com/eternitytohere/faq.html

Thanks

Bill

In pure quantum, there is really no arrow of time such that it can be made to go 10 years ago back in time? Since atoms and molecules are pure quantum and time reversible, what does it mean to make it go backwards in time? how far can it go?

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bhobba

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ephen wilb said:In pure quantum, there is really no arrow of time such that it can be made to go 10 years ago back in time?

Why do you think such applies to a classical apparatus? Why do you think classical entropy doesn't emerge in that case?

Thanks

Bill

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ephen wilb

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bhobba said:Why do you think such applies to a classical apparatus? Why do you think classical entropy doesn't emerge in that case?

Thanks

Bill

But if the universe is close system. decoherence is reversible because superposition still occur.

Which of the interpretation where decoherence is reversible.. maybe bohmian mechanics where determinism is its middle name? Then in BM, time reversibility is possible?

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bhobba

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ephen wilb said:But if the universe is close system. decoherence is reversible because superposition still occur.

That is incorrect.

Thanks

Bill

Time reversibility in quantum mechanics refers to the idea that the laws of quantum mechanics should be able to describe the behavior of a system as it evolves both forward and backward in time. This means that if we know the state of a system at a certain point in time, we should be able to predict its state at any other point in time, regardless of whether we are moving forward or backward.

In classical mechanics, the concept of entropy is often used to describe the direction of time's arrow - the idea that time moves forward and not backward. However, in quantum mechanics, time reversibility suggests that entropy does not always increase, as it does in classical systems. Instead, there are certain cases where entropy can decrease and time can flow backwards. This is a key difference between classical and quantum mechanics.

One way to access time reversibility in quantum mechanics is through quantum simulations. By controlling and manipulating the states of quantum systems, we can observe their behavior both forward and backward in time. Another approach is through time-symmetric quantum mechanics, which posits that the laws of quantum mechanics are inherently time-symmetric and can be used to describe the behavior of systems in both directions of time.

Yes, there are potential practical applications of time reversibility in quantum mechanics. For example, it could be used in quantum computing to improve the efficiency of algorithms and reduce errors. It could also have applications in quantum cryptography and secure communication, as well as in understanding and controlling quantum systems, such as in quantum sensors and metrology.

While time reversibility is a fundamental concept in quantum mechanics, it is not a universal principle. There are certain systems, such as those with strong interactions or high levels of entropy, where time reversibility may not hold. Additionally, some interpretations of quantum mechanics, such as the Copenhagen interpretation, do not fully embrace the idea of time reversibility. Ultimately, the limitations of time reversibility in quantum mechanics are an ongoing topic of research and debate in the scientific community.

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