# Decoherence and direction of causality

• I
• entropy1
In summary, the question is whether the expression ##(|A\rangle+|B\rangle)|M\rangle \rightarrow |A\rangle|M_A\rangle+|B\rangle|M_B\rangle## for decoherence implies that if the measured value is ##A##, then we will measure ##M_A##, or if we measure ##M_A##, then the measured value must have had the value ##A##. The answer may involve entanglement and an explanation in the context of the question would be appreciated.
entropy1
If I use the expression ##(|A\rangle+|B\rangle)|M\rangle \rightarrow |A\rangle|M_A\rangle+|B\rangle|M_B\rangle## for decoherence, does that mean that we can infer that, IF the measured value is ##A## that THEN we will measure ##M_A##, OR that IF we measure ##M_A## THEN the measured value must have had the value ##A##? Are both legitimate?

Last edited:
Little bump because I rephrased the question a little.

I think the answer to my question involves entanglement. If so it would be appreciated to have it explained in the context of my question. Thanks!

## 1. What is decoherence?

Decoherence is a phenomenon in quantum mechanics where a quantum system interacts with its environment, causing it to lose its quantum properties and behave more like a classical system. This results in the "collapse" of the quantum state into a definite state, which is observed as a measurement.

## 2. How does decoherence affect the direction of causality?

Decoherence plays a crucial role in determining the direction of causality in quantum systems. When a system interacts with its environment, it causes the system to lose information about its past states, making it impossible to trace back the exact cause of a particular event. This leads to the arrow of time, where causality only flows in one direction - from the past to the future.

## 3. Can decoherence be reversed?

No, decoherence is a one-way process and cannot be reversed. Once a system has interacted with its environment, it loses its quantum properties and cannot be restored. This is why we only observe the arrow of time moving forward, as the past cannot be recreated or reversed.

## 4. How does decoherence relate to the measurement problem in quantum mechanics?

The measurement problem in quantum mechanics refers to the paradox of how a quantum state can exist in multiple states simultaneously until it is observed or measured. Decoherence provides a possible solution to this problem by explaining how the interaction with the environment causes the collapse of the quantum state into a definite state, which is observed as a measurement.

## 5. What are the practical applications of understanding decoherence and the direction of causality?

Understanding decoherence and the direction of causality is crucial in various fields, including quantum computing, quantum information processing, and quantum cryptography. It also has implications in understanding the fundamental nature of time and the universe, as well as in developing new technologies and improving our understanding of the quantum world.

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