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A story about bridges, progress, and hidden complexity

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There is a story, attributed to Richard Buckminster Fuller, which uses the development of bridges as an example of technological progress.  The story begins with two groups of humans separated by a large ravine.  They can shout across to each other, and they soon come to understand each other’s language.  And it turns out that there are many things they could trade with one another… if only there was some way of getting across that ravine.

Eventually, one of the groups figures out that if they fell one of the trees on the edge of the ravine, it will be long enough to provide a crossing to the other side.  And so, they cut down the tree.  This allows the two tribes to develop a lasting trading relationship which helps both groups prosper.  Unknown to the humans though, the now lifeless tree begins the long process of decomposition.  Only years later do some of the people walking along the tree notice that it has begun to creak and groan.  And if too many try crossing at once, the once sturdy tree seems to sag with the weight.

Sure enough, eventually the tree splits, taking the luckless humans who were crossing it to a watery end in the river at the bottom of the ravine.  Trade between the two groups comes to an end, and both are worse off for it.  So they begin thinking about how else they might cross the ravine.

Soon enough, someone wonders whether they might pile rocks up across the ravine, building up a wall which they would then be able to walk across.  This takes more time than felling the tree.  But eventually, the two groups are trading with one another and prospering again.

Unfortunately, damming the ravine in this way causes the water in the river to rise, and leak through the many spaces between the rocks.  Over time this erodes the dam.  And then, one stormy winter, the power of the water behind the dam becomes too great and the structure collapses.  Fortunately, however, enough of the structure is left intact to allow the groups to quickly build up the dam once more.  But collapse and rebuilding become a cyclical feature of their lives, with the dam collapsing every time there is a heavy flood.

After several cycles of flooding and rebuilding, fate intervenes.  Instead of bringing down the entire dam, the flood waters remove only the rocks from the bottom.  And while the people fear the rest of the dam will collapse, they notice that the shape of the hole – roughly a semi-circle – has not impaired the integrity of the structure.  Indeed, it appears that this arch of rocks is all the more stable despite requiring significantly fewer rocks.

Soon enough, the people decide to build a more stable stone bridge, using this arched shape to strengthen the structure.  This proves to be a boon for the two groups as they no longer have to limit their trade to goods which can be carried by people alone.  With such a strong stone bridge in place, they can harness up animals and transport waggon loads of goods at a time across the bridge.

This trade goes on for centuries.  But over time, trade increases and the load placed upon the bridge begins to weaken its structure.  Something even more sturdy is required if the volume of trade across the ravine is to continue growing.  Fortunately, by this time the people have discovered modern iron working.  Moreover, they have learned that rather than a solid arch structure, an iron bridge need only be built as a lattice of iron girders.  And so, work begins on a new iron bridge to replace the old stone bridge.

Problems emerge some years later, when the people begin to run the new railway trains across the iron bridge.  While the iron girders prove to be very strong under a downward load, they turn out to be far too brittle in the face of horizontal forces, which eventually cause the girders to crack and splinter.  By this time though, the process of industrial-scale steelmaking has been developed.  And the steel, which is no heavier than the iron, proves to be a superior material for building bridges.

Soon enough, a new steel arch-shaped bridge has been constructed to replace the iron bridge.  Trade continues to grow, and the people continue to prosper.  So much so, that the single railway crossing is no longer enough to facilitate trade demand.  And so, the people decide to construct a road bridge parallel to the railway bridge.  This time though, they use a combination of concrete and steel to form the arch across which the bridge deck will be built. 

Some years later, trade and prosperity have grown to the point at which even the road and rail bridges are not enough.  And so, they decide to build another, even bigger bridge.  This time though, their engineers have figured out that an arch shape can be just as strong when it is inverted.  Furthermore, the arch does not have to be made of solid steel but can be constructed using cables hung between two bridge towers, with the bridge deck suspended beneath; the whole structure being anchored into the rock on either side of the ravine.

And so it goes on… At each stage, the bridges get stronger even as the material required to construct them gets lighter.  And this process of optimisation can be seen across the technological landscape, propelling us to the technologically advanced economy of the modern world.


It is a comforting story, and one which I guess around 90 percent of us – at least in the developed regions of the world – have come to take for granted.  It is, indeed, the story that we reach for in times of crisis… the idea that clever people somewhere else are going to come up with some new solution to our current woes which will make the world better, brighter and more prosperous.  But there is a side to the story which has been omitted, and it is this unspoken side which I am more concerned with in this post.

Rather than looking at the bridge technology as it has developed over time, let us consider instead the bridge-making technologies.  Cutting down that first tree was a bit of a feat for those primitive humans at the start of the story.  But archaeological evidence proves that stone age humans were perfectly capable of fashioning flint axes which could fell even the largest trees.  Making rock dams was an altogether more complex endeavour.  The rocks had to be quarried – which suggests that some metal working would have had to have been developed.  At the very least, the people would have needed to have developed sleds to get the rocks from the quarry to the ravine.  And it is more likely they would have needed horses or oxen and carts, which implies they had a sufficiently developed agriculture to have spare domesticated animals and a sufficient food surplus to allow non-food specialisms like smiths and wheelwrights.

Constructing stone bridges, and stone structures in general, only happened in advanced pre-industrial economies such as those of the Romans or the Normans.  In less prosperous economies, such as in Saxon England, there was a reversion to wooden structures (which is why so few Saxon structures have survived).  The division of labour in stone-building economies is far more diverse, and the surplus food and materials required far greater.  And while bricks, made from clay and straw, came to replace wood in building construction from the sixteenth century, it was never strong enough for bridge construction.

It took an agricultural and an industrial revolution before people could begin to construct bridges out of iron.  And it required a mature industrial economy to produce the surpluses required to allow steel bridge construction.  In the same way, it is only in the oil age and with the advent of international supply chains that modern suspension bridges can be constructed.

Another way of expressing this is that each time the bridge became stronger even as the construction materials required shrunk, so the complexity of the wider economy – more discrete moving parts, performing different but essential tasks – had to increase.  But to understand this fully, we must go back to the beginning of the story one last time to talk about energy.


The people who chopped down that first tree were able to complete the task using human labour power alone.  All that was required was sufficient surplus food to allow someone to nap the flints, make the axes and to chop the tree.  Making the stone dam was an entirely different energy proposition.  To power the level of complexity needed to allow the construction of a stone dam not only needed sufficient food (calories) to power the human workers, but additional surplus food was required for the work animals.  And where metal tools were used, then at the very least, wood burning was required to fuel smelting kilns and smithies.  It may even be that they would need to know how to produce charcoal in order to make tools of sufficient quality – something that would definitely have been required in the manufacture of stone bridges.

It took the development of the Atlantic economy, and agricultural revolution to massively increase the surplus produce from farming, and the beginnings of industrial iron working before Abraham Darby III could start work on the world’s first iron bridge across the River Severn in Shropshire.  This, in turn, required that the economy had begun to make the shift from charcoal and waterpower to coal and steam power – a process which had to mature before steel bridges could become common place.

The complexity which could be achieved using coal and steam power pales in comparison to the levels of complexity that could be powered by oil.  The amount of economic activity achieved in the two decades 1953 to 1973 – corresponding to the period during which Western Europe and parts of Asia finished the transition from coal-powered to oil-powered economies – was equivalent to the economic activity that had occurred between 1800 and 1953. 

The development of alternative electricity generation technologies, from nuclear to geothermal and from wind turbines to solar panels was made possible by the massive surplus energy provided to us by oil, allowing the complexity which enables such modern wonders as smartphones, digital currencies and satellite navigation.


A more holistic view of technological improvement might start with the view that rudimentary technologies require a relatively low degree of complexity – there are few if any specialists and roles are interchangeable – which in turn requires very little surplus energy:

In order to continue making technology more efficient, however, requires ever greater complexity – more specialists doing more irreplaceable roles.  This, in turn required an ever-greater energy surplus to maintain:

Throughout history, this has been achieved in two ways.  First, productivity improvements – learning to do more with less, or making the energy available do as much work as possible – enable complexity growth within thermodynamic limits.  Second, the transition from less to more energy-dense fuels – from wood to charcoal, from charcoal to coal and from coal to oil – has periodically provided us with an additional burst of surplus energy which is associated with a technological revolution:

In light of the growing energy crisis currently engulfing us, this raises some interesting and increasingly urgent questions.  The first of these is whether there are any more productivity gains to be gleaned from technologies which, despite the hype, are mostly already mature.  If, as I would argue, the remaining productivity gains are too expensive and difficult to be worthwhile, then is there an alternative fuel source to oil which is more energy-dense and at least as versatile? 

Modern non-renewable renewable energy-harvesting technologies are not a solution because they can never concentrate enough energy to be denser than oil – even assuming a suitable battery to store the energy could be invented.  Hydroelectric won’t work at scale because all of the viable locations have already been dammed.  Geothermal in volcanically active places such as Hawaii and Iceland shows some promise, but again, cannot currently be scaled up.  Molten salt and molten metal nuclear reactors also offer a significantly greater output compared to conventional nuclear, but these have yet to make their way off the laboratory bench.  The answer, then, is that while there might be a replacement for oil at some unspecified point in the future, none is available now or for the near future.

The question this poses is, can we maintain the current level of complexity without the energy input which allowed us to create it in the first place?  The answer to this is an emphatic no!  Every other civilisation in history which burned through the energy available to it, collapsed in relatively short order.  There is no reason to believe ours will be any different.

This leaves us with a final question.  Is the technological progress in the story of the bridge – expanded to take account of complexity and energy – a reversable process or does it only work in one direction?  The answer here is that we simply don’t know.  Nobody has ever tried to do it before.  Every civilisation that went before us brought about its own extinction precisely because it attempted to maintain its complexity even as the energy available to it declined.  Were we to embark upon a process of managed de-growth, we would be the first to attempt it… and there are no guarantees of success.

As you made it to the end…

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