Debunking the Quantum Entropy Myth – How Disorder Inevitably Prevails

Researchers at TU Wien have resolved a long-standing paradox in quantum entropy, proving that even in isolated quantum systems, disorder naturally increases, aligning quantum mechanics with thermodynamics.

The second law of thermodynamics states that entropy—disorder—always increases in a closed system. This explains why ice melts and why shattered vases don’t reassemble. However, quantum physics appears to contradict this, as traditional calculations suggest entropy remains constant.

By redefining entropy in a way consistent with quantum mechanics, researchers found that disorder does, in fact, increase. Initially ordered systems become more chaotic over time, just as in classical physics.

Entropy and Time’s Direction

Entropy is often linked to disorder, but it actually measures whether a system is in a rare, specific state (low entropy) or one of many similar states (high entropy). “This defines the arrow of time,” explains Max Lock (TU Wien). “The past has lower entropy, while the future has higher entropy.” Yet, John von Neumann’s work suggested that quantum entropy remains unchanged, making time appear reversible.

“But this ignores a key detail,” says Tom Rivlin (TU Wien). “In quantum physics, we can never fully know a system. We can measure properties like position or speed, but only as probabilities, never with certainty.” This unpredictability must be factored into entropy calculations.

Unlike von Neumann entropy, Shannon entropy accounts for measurement uncertainty. “It reflects how much information we gain from an observation,” says Florian Meier (TU Wien). “If an outcome is certain, Shannon entropy is zero. If multiple outcomes are equally likely, it is high.”

Quantum Disorder Always Increases

Researchers proved mathematically and through simulations that Shannon entropy in a closed quantum system rises until it stabilizes at a maximum—just as classical thermodynamics predicts. Over time, measurements become less predictable, reinforcing the second law of thermodynamics.

For single-particle systems, this effect is negligible. But in complex quantum systems—such as those used in quantum computing—reconciling quantum mechanics with thermodynamics is crucial. “This research lays the groundwork for future quantum technologies,” Huber concludes.

Read Original Article: Scitechdaily

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