From the perspective of the very short recorded history of humankind, the world we live in seems pretty stable. Our experience is of a physical existence having constancy and predictability. Why this rather than a universe ruled by wild, chaotic, unpredictable events?
There are some fundamental reasons we exist in such relative tranquility. But, it is important to realize the time frame of our existence is not representative of conditions that have existed during the entire history of either the universe or Earth.
Reflect on a few important time frames. Historical records date back approximately 5,000 years. Modern humans have existed for about 100,000 years and our ancestral hominid ancestors arose some 6 million years ago.
For that matter, all of the phyla (general structural organizations of organisms) that exist arose just beyond 500 million years ago. The Earth has existed for a little more than 5 billion years.
Over the course of these longer intervals of time, things have been much more tumultuous than what humankind has experienced. We know there have been times when the Earth was very much colder and very much hotter than it is now.
Places that are fertile now once were desolate and vice versa. Catastrophic events, such as huge meteors striking the Earth, have caused massive extinctions of life.
Even if things are not as stable over longer time frames than we currently experience, it is remarkable the world is no more turbulent than it is. There are some fundamental reasons for this.
The most obvious is that instability is, by definition, transient. For any number of reasons transient events arise from the convergence of rare circumstances that quickly play out. These are processes that exhaust their resources and have no means of restoring themselves.
A process transforms raw materials into something with a new structure. That is, it takes some inputs and makes a product or output. The inputs may be tapped from a reservoir or the outputs of other processes.
If the resource supplying the input is exhausted, the process comes to an end. If the output is not utilized or transported away it can accumulate to the point it acts like a pollutant interfering with further production. It gums up the works.
Such a process tends to be a "flash in the pan." It uses up its needed raw materials or becomes progressively less effective as it fouls its own environment with unused output.
The physical processes that give rise to our relatively stable environment are typically cyclical in nature. On close examination these cyclical processes are part of larger networks of processes. Moreover, the cycles are governed by feedback loops that characteristically regulate the use of resources.
Processes with real staying power are those imbedded in large networks. Not only do these processes persist, they are resilient. They have the ability to restore themselves to a normal state of operation when outside influences are pushing them away from that norm.
This type of behavior is brought about by what is generically called a feedback loop. This means some of the products of a process (some fraction of one or more outputs) are fed back to the input as information.
When the item fed back is less than the system requires, the weak signal causes the system to accelerate production. This enhanced mode is sustained until the output is adequate, at which time the fed-back signal is sufficient to moderate production.
In more complex systems the item that is fed back may come from some distant process that is interlocked with this node in a larger network. Networks typically have emergent capabilities beyond those of the individual units of which they are composed.
Obviously, big systems can exhaust raw materials just as simpler transient processes can. But big systems that survive typically have subunits that are resourceful at seeking out new resources or recycling resources that are by-products from elsewhere in the network.
Biological systems are replete with such networks. At one end of the spectrum, they govern the most basic metabolic biochemical processes. Autocatalytic feedback loops regulate production of enzymes or catalyst. Such loops enable living things to garner needed materials and energy.
At the other end of this spectrum, networks orchestrate the interplay between organisms and resources within an ecosystem. This gives rise to the stability found in such systems and their rich diversity of life.
Incredibly, development of a fertilized egg into a mature organism is accomplished under the direction of a network built around the DNA molecule. Signals generated from feedback loops abound. They ensure genes are turned on and off with perfect timing.
The stability seen in living organisms would not be possible without regularity in the broader physical universe. Physics reveals the simple fact that not all outcomes are possible. That is, things occur in predictable ways. Events are governed by physical laws.
A thrown ball follows a trajectory that can be predicted from the initial velocity and mass of the ball and the strength of the gravitational field it is subjected to. It won't take a certain path on one trial only to do something different on another trial with the same initial conditions.
Trajectories and other dynamic physical processes are governed by the "principle of least action." That is, though any number of paths could be imagined for a dynamic system, the only path that will be taken is the one that minimizes something known in physics as "action."
This principle is intimately connected to the conservation laws of energy, momentum and angular momentum. And, as has been elegantly demonstrated, each conservation law is the consequence of symmetry of some kind.
Symmetries are as simple as the fact that space and time are uniform and there is no natural bias for one direction in space over another.
It seems as if our existence in a stable universe ought to be the consequence of deeply profound principles. That this is actually true and, at some level, is the consequence of symmetries and universal laws is truly awesome. What is more, it means the universe is predictable and knowable.
Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at firstname.lastname@example.org.