Brother, can you spare a calorie?

I saw a documentary a little while ago, Super Size Me, by Morgan Spurlock, in which, at one point, several people were asked in a vox-pop what a calorie actually is. None of them could answer the question. The most common answer was ‘something to do with fat?’ I am not pretending that this should be taken to mean that no-one understands what a calorie is. One of the experts interviewed on the film, a nutritionist I think it was, could provide the textbook answer of ‘one calorie is the amount of energy required to raise one litre of water one degree centigrade’.

I wondered to myself if you had asked these same people, or in fact anybody, what energy is, whether they could give a reasonable answer. Can I give a good answer? Is the question even fair? Energy is one of those concepts that we take more or less for granted, if we even think about it at all. How would you describe energy to a child, or a visitor from mars?

Would your definition concur with those of the Oxford paperback dictionary?

1. The capacity for vigorous action; 2. Force or vigour of expression; 3. The ability of matter or radiation to do work because of its motion, its mass, or its electric charge; 4. Fuel and other resources used for the operation of machinery etc.

I suspect that a lot more people would give a definition akin to the first of those listed above, as in ‘he has a lot of energy’ (meaning he has a lot of capacity for action, vigorous or otherwise). But obviously that is not what we are supposed to understand by the nutritionist telling us that an item of food has 100 calories – after all a donut has no capacity for action.

Rather the calories some of us worry about are best described by the third definition above – The ability of matter to do work. By eating the donut we harness the chemical energy stored in it and convert it into work – motion and heat primarily.

That leads us to the one truly important thing about energy: energy is conserved. This means that in a closed system (and the only truly closed system is the entire universe) the amount of energy is constant. Energy is being converted from one form to another all the time, but the total amount of energy is constant. It’s also worthwhile to note that energy is to all intents and purposes the same thing as work and is also commensurate with heat. It is even basically interchangeable with that other great constant of the universe, mass, thanks to Einstein’s famous equation E = MC2.

If you are still feeling in the dark as to what energy actually is, then you are most certainly not alone. From the preceding paragraph you might get the feeling that everything in the world is energy. After all, if mass, heat and work are just energy, what’s left? Nothing really. Energy is literally, to use a clichéd phrase, ‘the fabric of the universe’. Modern physicists have many different and esoteric ways of describing the warp and woof of this ‘fabric’ but none can get around the fact that energy is not able to be described in terms of anything more basic. Energy is just as it is.

One of the curious paradoxes about this situation is that even though we seem to be forbidden from understanding what energy actually is, we have become very skilled at manipulating it.

All life must use energy to sustain itself. Life may well have got its energy in the very beginning of prehistory from subterranean volcanoes. The sun, however, has been the primary source for the vast majority of the energy used by life on the planet.

The sun’s immense fusion reaction (turning mass into energy according to E = MC2) has provided enough energy for a huge variety of life to evolve to exploit it. The most basic type of life is an autotrophe – an organism that harnesses all of its energy from the sun; plants using photosynthesis, for example. A heterotrophe harnesses the energy gathered by autotrophes by consuming them (think of a cow nibbling grass) and some heterotrohes consume other heterotrophs. Life in this way can be thought of as a pyramid with a transfer of energy up the levels.

And who should sit at the top of the pyramid but human beings. Our ingenuity and a great deal of good fortune has meant that we use more energy than any other species on the planet. But if you stop to think about this you might wonder, how can we use more than the plants that absorb all that energy from the sun? After all, we don’t consume all of the plants on the planet. There are two words that will explain that situation: fossil fuels.

By the use of fossil fuels we are tapping into solar energy that was harvested by tiny organisms millions of years and then stored by the restless movement of the earth’s crust. You could regard it as inevitable that a species like us would evolve to exploit this incredible resource. It was a niche just waiting to be discovered, and once it was found and put to use it allowed our dreams to become reality. Our dominion over the earth has spread as our population has grown in step with our energy use. Fossil fuel energy and its products now so subsume our culture that life without them would be, to the average 21st century human, more or less inconceivable.

So, those people that were asked on that New York street what a calorie was may never have given much thought to the true nature and import of the word energy. The calories that a hamburger contains are an absolute given for them. When they are hungry they eat, and perhaps they will assiduously count the number of calories that they consume. But it troubles them little to consider the intricate web of energetic dependences that brought their hamburger into being. It would be the last thought on most people’s minds that their meal owes its existence in part to the energy gathering ability of prehistoric algae.