How does entanglement entropy relate to the arrow of time in quantum systems?

[Grace.S]

I’ve been trying to understand the idea that the arrow of time might emerge from increasing entanglement entropy in quantum systems. Conceptually, it feels like a big jump from classical thermodynamic entropy to a quantum information viewpoint. I struggle to see how a fundamentally time-symmetric evolution can give rise to a clear direction of time just because subsystems become more entangled with their environment.

[Emily_M]

I keep coming back to the idea that the direction of time only appears when we restrict ourselves to a small part of a much larger system. When you ignore the environment, the dynamics start to look irreversible, even though the full system evolves reversibly.

[Zoey65]

I ran into this while modeling a simple qubit coupled to a cold bath. The global state evolved unitarily, but the reduced density matrix of the qubit showed decoherence that looked very much like a one-way process, with information leaking into degrees of freedom I was no longer tracking.

[Noah.J]

That made me wonder whether the arrow of time is less about time itself and more about how we define macroscopic descriptions. Once we coarse-grain and trace out the environment, entropy growth feels inevitable, but that may be a statement about our description, not the underlying physics.

[ChloeUT]

I also looked at decoherence times versus population relaxation in a small open system. Decoherence happened much faster, but the numbers were noisy and didn’t settle into a clean story about irreversibility, which left me unconvinced that entanglement alone explains everything.

[EleanorG]

Sometimes I think entanglement entropy is acting as a bridge concept rather than a root cause. It captures how correlations spread into inaccessible degrees of freedom, which mirrors thermodynamic entropy, but I’m not sure it fully explains why time feels asymmetric.

[OliverC]

I keep asking myself whether the real asymmetry comes from initial conditions. A low-entanglement starting state already biases the evolution, and entanglement growth may just reflect that choice rather than generate the arrow of time on its own.

[Joseph.W]

To me, entanglement entropy feels like a powerful bookkeeping tool for understanding irreversibility in practice. It explains why subsystems lose information and why dynamics look one-directional, but I’m still unsure whether that’s the same as explaining the arrow of time itself.