Thermal Equilibrium
The state when a steak reaches the same temperature as its cooking environment — the basis of sous vide precision.
Thermal equilibrium is the state where two objects in thermal contact have reached the same temperature and no net heat flow occurs between them. In cooking, a steak reaches thermal equilibrium when its internal temperature matches the temperature of the cooking environment.
In a 131°F sous vide water bath, a steak approaches thermal equilibrium when every point from surface to center reaches 131°F. In theory, it takes infinite time to reach perfect equilibrium (the temperature difference decreases asymptotically). In practice, a 1.5-inch steak reaches within 0.5°F of the bath temperature in about 90 minutes — close enough for all practical purposes.
Why equilibrium matters: It's the fundamental principle that makes sous vide work. The steak cannot exceed the bath temperature, so overcooking is impossible (assuming correct bath temperature). The water bath acts as a temperature clamp — an enormous thermal reservoir that the small steak cannot meaningfully change.
In an oven: You wouldn't want equilibrium in a 250°F oven — that would mean a 250°F steak (well past well-done). The reverse sear oven phase is a controlled approach that pulls the steak out well before equilibrium is reached, typically at 115–125°F.
Practical tip: The time to reach equilibrium depends on thickness (squared relationship), not weight. A 2-inch steak takes roughly 4x as long as a 1-inch steak to equilibrate — because heat must travel twice the distance, and the time increases with the square of the distance.