Not all physical quantities slow down the same way before reaching equilibrium.

When isolated particles interact, they usually head toward a stable state called thermal equilibrium. Sometimes they get trapped in a long-lived intermediate pause called prethermalizationprethermalizationA phenomenon where a system remains in an intermediate state for a long time before reaching equilibrium.. Physicists often assumed that having separated energy levels in a system automatically creates this pause. The researchers investigated this assumption. They examined systems near a fully permutation-symmetric limit, using a long-range interacting spin model. They found that separated energy scales do not guarantee a prethermal pause. Instead, the pause is selective. Under the exact same system rules, some measurable properties pause while others relax straight to final equilibrium. Whether this pause occurs depends on the specific measurement and the starting state. Finally, they proved that a quantity called the Loschmidt echoLoschmidt echoA measure of how similar the system's state is after a time reversal. sets a firm lower limit on how long this pause lasts.

Why it matters

This changes how scientists design experiments to observe long-lived states in quantum systems. If they measure the wrong quantity, they might miss the prethermal plateau entirely, affecting research in condensed matter physics.

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