Friday07 February 2025
korr.in.ua

Classical physics principles apply in the quantum realm as well: researchers challenge a 90-year-old theory.

Quantum physics is not exempt from the second law of thermodynamics. It too encompasses chaos and disorder, albeit of a different nature.
Правила классической физики действуют и в квантовом мире: ученые поставили под сомнение 90-летнюю теорию.

According to the second law of thermodynamics, the entropy of an isolated system tends to increase over time. Everything around us adheres to this law. For instance, the melting of ice, the cooling of hot coffee, and aging are all examples of increasing entropy over time. Until now, scientists believed that quantum physics was an exception to this law. This belief stemmed from a series of articles published by mathematician John von Neumann about 90 years ago, in which he demonstrated that if we have a complete understanding of the quantum state of a system, its entropy remains constant over time. However, new research published in the journal PRX Quantum challenges this notion. Scientists now argue that the entropy of a closed quantum system also increases over time until it reaches its peak level, as reported by Interesting Engineering.

According to physicists, if we define the concept of entropy in a way that aligns with the fundamental ideas of quantum physics, there are no contradictions between quantum physics and thermodynamics.

The authors of the study highlighted an important detail in von Neumann's explanation. He claimed that the entropy for a quantum system does not change when we have complete information about the system. However, quantum theory states that it is impossible to have complete understanding of a quantum system, as we can only measure certain properties with uncertainty. This means that von Neumann's entropy is not the correct approach to consider randomness and chaos in quantum systems.

Instead of calculating von Neumann's entropy for the complete quantum state of the entire system, the authors of the study suggest calculating the entropy for a specific observable system.

This can be achieved using Shannon entropy, a concept introduced by mathematician Claude Shannon in 1948. Shannon entropy measures the uncertainty of the outcome of a specific measurement. It indicates how much new information we gain when observing a quantum system.

If there is only one possible outcome of a measurement that occurs with 100% certainty, then Shannon entropy equals zero. If there are many possible values with equally high probabilities, then Shannon entropy is large.

When we rethink the entropy of a quantum system through the lens of Claude Shannon, we start with a quantum system in a state of low Shannon entropy, which means the system's behavior is relatively predictable.

For example, imagine you have an electron, and you decide to measure its spin, which can be oriented up or down. If you already know that the spin is 100% oriented up, then Shannon entropy equals zero, and we learn nothing new from the measurement.

In a case where the spin is 50% oriented up and 50% oriented down, then Shannon entropy is high, as we could equally likely get any outcome, and the measurement provides us with new information. Over time, entropy increases, as you can never be sure of the outcome, physicists say.

Ultimately, entropy reaches a point where it levels off, indicating that the unpredictability of the system stabilizes. This reflects what we observe in classical thermodynamics, where entropy increases until it reaches equilibrium and then remains constant.

According to physicists, this case of entropy also holds true for quantum systems involving multiple particles and generating several measurement outcomes.

Thus, scientists assert that the second law of thermodynamics is also valid in a quantum system that is completely isolated from its environment. It simply requires the appropriate definition of entropy.