Some objects appear smooth and others do not. As an example, we might characterize a silk scarf as feeling smooth, but a sheet of sandpaper as feeling rough. We also might characterize ocean waves as having smooth contours (at least until they break), while the coastline upon which they land can often be rocky and jagged (i.e., rough).
We all have an intrinsic idea of the difference between smoothness (i.e., something conveying continuity) and roughness (i.e., something manifesting discontinuities) based on our personal experiences with different objects and events in the environment (see Figure 8). Interestingly, however, despite this common familiarity with the idea of roughness, most of us choose to believe that the universe behaves in a typically smooth fashion. What’s more, if we find examples where it behaves in a discontinuous manner, we tend to think of these as being transient and anomalous deviations from smoothness, rather than as being endemic and recurrent.
Two implications arise from this underlying supposition of universal smoothness. By making this assumption, we have chosen to characterize our universe in a way that allows it to:
• React to perturbations in predictable ways, and
• Have reactions (outputs) to incremental changes (inputs) that are both commensurate and scalable (i.e., perturbations of progressively greater magnitude result in smoothly increasing effect sizes of larger magnitude).
In defense of this assumption, our everyday experience typically reinforces our impression that the universe is smooth and predictable. As a result, we generally expect that small changes in inputs will result in small changes in outputs when dealing with virtually all the systems we interact with – whether they are mechanical, social, economic, personal, or otherwise. However, it is often the (unspoken) case that the local systems we commonly interact with are not free to evolve spontaneously in many of the domains in which we operate (i.e., behave in an unconstrained way). Often, they are specifically designed (by us) to incorporate restraints that make them behave in a smooth and stable manner, even though they would not necessarily do so if they were left to evolve and develop freely by themselves.
An example helps to illustrate this point. Consider an indoor ice-skating rink where the local air temperature is well above freezing: In this case, the ice represents a forced, localized pseudo-stable state that occurs only because of the continuing expenditure of (external) energy resources to provide ongoing cooling of the water. But, there is nothing intrinsically stable about the ice in the rink. Because of the high ambient temperature around it, if the cooling mechanism were shut off, the ice would melt and become a pool of water. Thus, without the continued expenditure of external energy to cool the rink, the ice would undergo a transition from a solid to a liquid via an abrupt change in state known as a phase transition (a change that is inherently discontinuous). Thus, the ice in the rink depends for its sustenance and (pseudo-) stability (i.e., remaining frozen) on the continuing expenditure of external energy resources to force the system toward a stable-appearing operating point (i.e., ice). However, the ice would not stay that way by itself; if left alone, it would melt. To extend the analogy, if the ambient temperature were even higher and the water were to heat up further to its boiling point, it would make another (abrupt) phase transition to a gaseous state of water vapor, which would again represent a discontinuity from the pre-existing state of liquid water.
Interestingly, despite the lack of continuity associated with the behavior of many real systems, we generally believe that even when such kinds of abrupt transitions (i.e., discontinuities) occur, the changes associated with the evolution of systems are still predictable. If this were the case, then the issue of the intrinsic smoothness or roughness of the universe would make no difference with respect to our ability to predict the future. But fortunately or not, the occurrence of discontinuities has a major impact on the predictability of events.
Based on widely accepted observations, discontinuities occur often in our universe and, when they do, the results are not predictable in any precise sort of way. These can range from shattered glass, to erupting volcanos, to earthquakes, to buildings destroyed by tornados, to disjointed actions by unstable governments, to collapsing financial markets, to exploding supernovas, to people who die from one moment to the next. All of these events represent abrupt discontinuities that are irreversible. (Note: Even though some [but not all] of these occurrences may be fixable [i.e., repaired], the idea of repair is different from reversibility; reversibility involves the step-by-step retracing of events in reverse order, while reparability invokes different pathways to arrive back at a desired pre-existing state, which requires the expenditure of external energy – see Chapter 20 entitled “Thermodynamics: Laws Prohibiting the Spontaneous Reversibility of Physical Events” for further discussion).
There are many known cases where small changes in system inputs can precipitate much larger and even cataclysmic outputs. An obvious example is when an object breaks. Typically, this is not because of the application of an unexpected and overwhelming force (such as when a glass inadvertently tips off a table and smashes on the floor), but because of a subtle excess load placed on it that pushes it beyond its normal operating limits (e.g., the proverbial case of the straw that broke the camel’s back). An instance of the latter is when a previously reliable roof fails under the stress of an incremental but critical excess load that creates intolerable strain (e.g., the weight of accumulated snow – unfortunately there are many well-documented examples of this!). Another example is when system resonances amplify externally applied forces until they exceed the capacity of materials and structures to withstand them (a result of this is shown in the iconic photo from 1940 of the collapse of the Tacoma Narrows Suspension Bridge, Figure 9).
Smoothness (continuity) and roughness (discontinuity) have different implications with respect to predictability. Continuity is associated with the idea of stability and the derivative notion that any imposed change will result in an incremental reaction. In contrast, discontinuity is associated with system instability and endemic potential for rapid change, divergences, and non-predictability, as well as the risk of ultimate chaos (see Chapter 24 entitled “Chaos Theory: Implications for Macroscopic Predictability” for further discussion). These differences raise a key question: Which of these two alternatives represents the natural predisposition of the universe? If it is the former, then we can count on events unfolding in generally predictable ways. But if it is the latter, then the pockets of stability that we identify are really only pseudo-stable over certain limited domains of space and time. Beyond those defined limits, instability (and possibly chaos) may very well reign.
Another important implication arises: If different portions of the universe are not completely isolatable from each other in space and in time (i.e., if the universe is intrinsically connected in all facets, such that no portion of it is truly isolated from anything else), then the requirement of smoothness (i.e., continuity) for the successful prediction of future events would be elevated to the status of a universal imperative (see Chapter 22 entitled “Entanglement vs. Separability: The Locality Issue” for further discussion). If so, this would mean that the universe needs to be predictable