Sunday, July 8, 2012

Engineering's Grand Unified Theory

Physics has been in the news lately for what seems quite likely to be the historic discovery of the Higgs' Boson.  As the study of naturally occurring systems, Physics has exposed Nature's FOUR fundamental ways of acting: the Electromagnetic/Weak Nuclear field, the unimaginatively-named Strong Nuclear field, the Gravitational field, and the Higgs field.

A "field" is no aether-like pervasive substance, but more like a conceptual framework which can give rise to a physical force or manifestation when tickled in just the right way.  In everyday life, almost every way we interact with matter is mediated one way or the other by the Electromagnetic field.  Other than "failing to fall into space," obviously.  Even standing on the Earth and not plunging through it into the hot lava roiling beneath our feet is accomplished by the electrostatic forces between atoms resisting Gravity's pull of our bodies towards the earth's center.

Engineering is analogous to Physics in several ways.  It is also the scientific study of systems, but of man-made ones rather than strictly natural ones.  Some man-made systems are subject to the same laws of Nature that Physics has exposed to view, and these are the kinds that interest most engineers.  Other man-made systems like The Republican Party, The Stock Exchange and Glee follow no discernible pattern, and there is really no point in even trying to understand them.

Like Physics, Engineering has four fundamental phenomena:  Rocks, Sticks, String and Glue.  Anything can be made or analyzed from the standpoint of these four basic engineering elements.  However, the alert reader may be aware that this paradigm does not describe any fluid-like behaviors such as exhibited by liquids, gases, and plastics.

While Physics is still seeking a way to unify all four basic phenomena into one grand unified theory, or "Theory of Everything," Engineering has already succeeded at doing this.

The problem of designing a machine may appear on the surface to be one of getting parts to fit together and to move in a way that accomplishes something useful.  Designing a bridge may look like an exercise in making everything screwed together tightly enough to not collapse in a slight breeze.  Building a canal network, a rocket nozzle or a water pump may seem like coaxing as much fluid flow through a given point under as little pressure as possible.  What can possibly be the single idea that unifies all these goals?

It is this:  Engineering is all about the flow of stress.  Stress is merely a reduced measure of force, force per unit area.  In a bridge, forces flow from one part of the structure into others, and if you understand the flow of stress through Rocks, Sticks, String and Glue, you can build something that will support itself under all foreseeable loads.

In a machine, stress flows from one component to the next, and can carry energy with it.  The purpose of machines is to convert energy from one form to another, and so understanding how stress flows through its components reveals whether or not the machine will do its job.  And if not, why not.

In fluid systems, the flow of stress causes the flow of matter.  Fluid dynamics is not, as even fourth-year engineering students might still believe, all about stuff moving around in pipes.  In post-graduate courses one finally discovers that the real game is the diffusion of stress through the fluid and its boundaries, which merely manifests as stuff moving. In fluids, stress really causes the dispersal of momentum.

In an electrical circuit, stress in the form of voltage can build statically like stress in a bridge, or can flow in various ways like water in pipes.  The key to understanding it is to study the evolution of voltage at all points in the circuit.

There seems to be a great dividing line between solids and fluids, but even then, unification is possible.  The electronics analogy hints at this unification, since it is a system exhibiting both solid-like and fluid-like properties.

If you place a heavy brick on a table, the weight of the brick stresses the table's surface, and the stress flows through the table's top, into the legs, and down to the floor.  The stress pattern will then stay there virtually forever, if your table is made of perfect, idealized solids.

But if you place that brick on the surface of a swimming pool, something quite different happens.  As the brick makes its way to the bottom of the pool, stress flows out from it dispersing the brick's momentum  throughout the water in the pool. Once the brick is resting on the bottom, the water gradually returns to a state of complete rest, and the stress and momentum are gone.  Fluids require continual motion to sustain certain kinds of stress, while solids do not.

The key to really understanding engineering is the realization that everything is a little bit solid and a little bit fluid at the same time.  Most things are so much one or the other that you can forget about the other part.  That is, unless you want to really understand it.  In a "fluid," shearing stresses dissipate quickly because the fluid does not really stick to itself very well.  In solids, shear stress never dissipates at all, because the pieces of a solid are stuck to each other unless you pull hard enough.

And yet in both cases, stress - whether shear or compressive - always seeks the most even distribution possible. In doing so, stress causes forces to redistribute (as in a structure), energy to flow and transform (as in a machine), or momentum to disperse (as in a fluid system.)

And that is all there really is to know about engineering!

Perhaps we engineers will one day discover our version of the elusive "Higgs" particle and finally be able to understand humans as well.  Particularly humans of the female girl variety.


  1. A lot of B's add up to A. C is an example of non conservation of angular momentum.

  2. Shear stress in a fluid means nearby parts of a fluid moving at not the same speed or in different directions entirely. B is an example of a high concentration of shear stress where the two circulating areas meet, which goes completely against what stress is all about: leveling out as much as possible. You're right about C though: impossible except for a few short-lived eddy currents which quickly diffuse. Therefore, the only possible answer is... A. Try it in the sink!