Scientists Say: Equilibrium

Equilibrium (noun, “Eek-wil-LIB-ree-um”)

Equilibrium refers to a state that remains steady or balanced over time. Such an unchanging state doesn’t have to be at a total standstill. Instead, imagine equilibrium as a tug-of-war between equally matched opponents. The state remains steady because the opposing forces balance each other out.

Imagine, for instance, a bathtub with the faucet on and the drain open. If water flows into the tub at the same rate as it flows out, the water level in the tub stays the same. That steady water level is said to be in equilibrium.

This idea of a steady state appears in many fields of science. In chemistry, equilibrium refers to balance in a chemical reaction. In reactions, starting chemicals called reactants turn into products. But products also react to form reactants again. Chemical equilibrium exists when the amounts of reactant and product remain the same, because the reaction rates match.

Biologists describe steady states in the body as in equilibrium. For example, our bodies maintain a constant temperature. When you get hot, you sweat. That cools you down. When you get cold, you shiver. That warms you back up. Biologists often use the word “homeostasis” (Ho-mee-oh-STAY-sis) to describe this type of equilibrium.

The idea of steady states is also important to ecologists. Consider predator-prey relationships. When there are more prey in an area, predators have more to eat. As a result, the number of predators spikes. Then, there are more predators to gobble up the prey, causing prey numbers to decrease. That offers less food for predators to eat, causing their numbers to drop — allowing prey numbers to rise again, and so on. These numbers change continually yet balance over time. This is a state of “fluctuating equilibrium.”

In physics, it may be hard to see the opposing forces acting on an object in equilibrium. Imagine a book resting on a desk. There is no back-and-forth movement like what we see in the predator-prey example. However, even in this “static equilibrium,” forces still balance. Gravity pushes down on the book. That force matches the “normal force” of the desk holding it up. 

Equilibrium can even refer to steady states in climate science. Earth’s global warming shows that our climate is not at a steady state. The gases entering and leaving the atmosphere are unbalanced. This results in a steady rise in global temperatures.

In a sentence

Our body’s nervous system maintains equilibrium in the body, allowing us to alternate between responding to and recovering from stressful situations.

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