Do we feed on energy?
I will take a detour that might seem to have little to do with hiking, but I think it’s perfectly spot on. I think about this every time I’m preparing all the meals that I’ll need for an upcoming backpacking trip. I always try to have a reasonably good estimate of the daily caloric intake, hence I asked myself1: do I really need the energy contained in these bars or peanut butter tortillas? We need to be more meticulous if we want to draw a fairly precise conclusion.
If we’d have to answer the question “what is the essential characteristic of a living being?”, we could answer with one word: metabolism. The origin is from Greek, and it means “to change.” Or better: to exchange. Exchange of what? The two obvious candidates are matter and energy2.
It’s easy to understand why exchange of matter can’t be what characterizes our metabolism. Every atom of hydrogen, oxygen, or carbon in an energy bar or a pouch of chicken pesto pasta is absolutely identical to any hydrogen, oxygen, or carbon atom that we expel in our usual ways (sweat, breathing, excretions). How could the exchange of perfectly identical entities distinguish us from inanimate matter?
If it’s not one, it’s the other, but it’s not that simple. Energy is a strange and complicated concept. We don’t have a good definition of energy3 other than many precise ways to calculate it. And we can accurately calculate even the smallest amount of energy that is released or absorbed in many processes of our industrialized age. We know something else, one of the most fundamental laws that exist, an inviolable decree of Nature: energy is conserved; we can’t create or destroy it.
Thinking that our metabolism is the exchange of energy is almost correct. If we can’t destroy any amount of energy, and every single unit of energy – for example, one kilocalorie from our peanut bar – is equivalent to any other, where is the advantage? And if it’s only energy that we need, why don’t we lay down and warm up in the sun? Isn’t heat a form of energy? What are we really exchanging? The answer lies in a subtle detail, but we can’t move forward without mentioning the second inviolable law of the Universe we live in. We can summarize it like this: energy is conserved, but not all energy is the same. Some forms of energy are more useful than others. And the second law establishes that, without exception, in our metabolism or in any other transformation, we will produce a certain amount of useless energy.
So we need to be more precise: our metabolism allows us to extract useful energy from a tasty peanut butter bar, enables our muscle fibers to contract countless times and electrical impulses to travel through our synapses, and then exchange a certain amount of useless energy with the environment. There is even a link between our body temperature and the particular efficiency in getting rid of this unusable energy.
I know that talking about useful energy might not help many to understand. “Wouldn't it have been enough to call it energy and drop the adjective?” Sure, if we want to tell only part of the story. But to replace the term “useful” with something more accurate we’d have to dig a bit deeper and start talking about entropy.
I want to conclude with the words4 of one of the most important physicists of the 20th century, Erwin Schrödinger, who had no problem bringing up entropy to explain what we really need for living:
What then is that precious something contained in our food which keeps us from death? That is easily answered. Every process, event, happening – call it what you will; in a word, everything that is going on in Nature means an increase of the entropy of the part of the world where it is going on. Thus a living organism continually increases its entropy – or, as you may say, produces positive entropy – and thus tends to approach the dangerous state of maximum entropy, which is death. It can only keep aloof from it, i.e. alive, by continually drawing from its environment negative entropy [...]. What an organism feeds upon is negative entropy. Or, to put it less paradoxically, the essential thing in metabolism is that the organism succeeds in freeing itself from all the entropy it cannot help producing while alive.
Here’s a scientist’s answer to the initial question: man does not live by energy alone, but by negative entropy.
I’m not the first to ask this question. Many physicists of the late 19th and 20th centuries have pondered the implications of a similar question.↩
I imagine many have Einstein’s name ringing in their heads because of his most famous formula, but in this case, you can forget about it: it’s of no use here.↩
We could cite James Clerk Maxwell, who in his Theory of Heat (1872) wrote that energy is “the capacity to do work, [that is] the act of producing a change in the configuration of a system by overcoming a force opposing that change.” Is it useful? Not much, in my opinion.↩
The quote comes from the book “What is life?”, chapter 6, page 72 of Cambridge University Press edition (1967).↩