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Open Systems

An important law of physics (the second law of thermodynamics) says that all systems gradually lose order. Their entropy (randomness or disorder) must increase over time. To fight this tendency, a system must actively maintain its organization. Otherwise it will decay.

One way to resist decay is to have a hard, unchanging structure, like a rock. This type of system consumes no energy, but it gradually loses its fight against disintegration.

Any system that does not consume energy will become more disordered with time, like a rock worn down to sand by pounding waves. Systems like this that do not consume energy to maintain themselves are called closed systems.

Living systems, by contrast, are open systems (a term coined by Bertalanffy in 1932). They maintain their structure by consuming energy or energy-yielding substances: food.

Energy, ultimately from the sun, is rechanneled to maintain and rebuild the system. Only open systems can grow more complex over time.

Growing, living systems appear to defy the second law of thermodynamics, because they becomes more orderly over time. But the second law of thermodynamics is not violated, because it refers only to closed systems.

Taken as a whole, the solar system is running down, increasing its entropy. However, our local biosphere on earth might continue for billions more years before the sun runs out of fuel, so systems on earth can sustain them­selves and even get more complex.

The principle of open systems: Systems can maintain order and increase complexity only by consuming energy.

All open systems interact with the environment. They have inputs and outputs. The inputs can be substances like food, energy like heat, or information like news. Living things are defined by their ability to use energy inputs to grow, resist disease, and reproduce.

Non-biological systems (such as com­puter systems) can also use inputs of energy and information to increase their complexity. If so, they are open systems.

A program that learns, such as one that selects advertising based on your internet activity, can grow more complex. It uses inputs to build and refine its knowledge base.

Bertalanffy's insight about open systems was revolutionary in the 1930s. Now it is taken for granted.

Every living system has flows of energy and information, allowing it to live and reproduce. Flows of information and energy through living systems were the focus of James G. Miller's version of General Systems, expressed in his book Living Systems (1978).

Static vs. Dynamic equilibria

Structure occurs in a system when there are stable patterns of interaction be­tween components. Something stable is firmly fixed or not likely to move or change. That is the natural condition of a closed system at rest. It will stay in its lowest energy configuration.

How can stability occur in a system that is dynamic or full of change? It only happens if there are mechanisms in the system for consuming energy to maintain organization.

A stable pattern in a system is called equilibrium. The word equilibrium refers to conditions in a system that allow it to be balanced, resulting in no net change.

There can be two ways to achieve no net change. The system can be at rest, which is called a static equilibrium, or the system can achieve a balance between equal and opposite forces, which is called a dynamic equilibrium.

A static equilibrium is a state of energy depletion (therefore maximum entropy). An example of a static equilibrium is a marble at the bottom of a round bowl.

The marble stays there, as long as no forces act upon it, because to move in any direction would go against gravity, and that would require energy. If the bowl itself is moved (an energy input) then activity occurs in the system until the marble again reaches a state of rest.

The other type of equilibrium is a steady state or dynamic equilibrium. This is a stable pattern maintained in a system using energy inputs.

A suitably organized system can compensate for decay or disorder. For example, our cells have mechanisms for detecting and repairing errors in DNA sequences.

Dynamic equilibria (steady states) can be maintained in three ways, variations on a theme:

  1. Input equals output. If you pour water into a leaky pail at exactly the rate it is leaking out, the level of water in the pail will stay the same.
  2. Creation rate equals destruction rate. Human skin is constantly sloughing off old cells and building new ones at the same rate. As long as nothing goes wrong, a normal appearance is maintained.
  3. Deviations are counteracted. A furnace maintains level heat by turning on heating elements whenever the temperature gets too low.

The principle of dynamic equilibrium: Stability (equilibrium) is maintained in an open system by consuming energy to counteract changes.


Miller, J. G. (1978) Living Systems. New York: McGraw-Hill.

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