This strong nuclear force (imaginatively called ‘the strong nuclear force’) ensures that we never see stand-alone quarks. What’s more, enough of it leaks out of a proton or neutron to hold those compound particles together in the nucleus. Because of the very short range of the strong force, there is a limit to the size of the nucleus – this is why there are only a few tens of different chemical elements, because as you continue to add more protons and neutrons, a nucleus becomes increasingly unstable.§
And that’s all we need. There are other particles and other forces needed to complete the big picture of how the universe works as a whole, but to get the basics of atoms (and hence the fundamental basics of you), that extremely small number of components does the job.
THE SPACE IN BETWEEN
In one sense, it’s worth noting, though, that there is a seriously important component of what you are missing from that analysis. That is nothing. This is where language lets us down. The missing component, which is by far the biggest constituent of you, is nothing. Emptiness. The void. This isn’t a nihilistic plunge into the darkness of the soul, but a realistic assessment of your composition.
Let’s take a zoom in on those basic components existing within the simplest type of atom in your body, a hydrogen atom. If we could somehow visualise what goes on in the quantum complexity of the sub-microscopic scale, we would expect to see somewhere in the middle a single proton, made up of two up quarks, a down quark and the gluons holding it together. Around the outside in a fuzzy ball of probability would be a single electron. And in between would be empty space – absolutely loads of nothing. The hydrogen atom is around 99.9999999999996 per cent empty space. An old simile for the relative size of the nucleus and the hydrogen atom is a fly in London’s Albert Hall. Another way to look at it is that if the atom were the size of the Earth, the nucleus would be about 200 metres across – the rest of it is empty space.
CHEMICAL COMPONENTS OF YOU
What we’ve seen so far is a physicist’s view. For many, a more familiar way to look at those building blocks of you is as chemical elements. This pushes the number of basic components up quite a lot from four basic particles, but bearing in mind there are about 7 × 1027 atoms in a typical 70 kg (150 lb) human, it’s still quite a simplification to realise there are only 92 types of atom available naturally to play with on Earth, and of those, we can account for 99.95 per cent of your body weight with just eleven.
You’ve probably heard that the majority of your body is water. It might seem unlikely when you look at yourself in the mirror or rap your knuckles together. The body can feel quite solid. But bear in mind that most of you is made up of cells that are filled with water. There’s enough structure to make it unlikely that you will run down the drain, but there is certainly plenty of water there. The most common figure is that around 60 per cent of your body is water – even your oh-so-solid bones are about 30 per cent aqueous.
Knowing as we do that water is made up of hydrogen and oxygen – H2O – it might seem that this implies that the elements topping the charts for body weight should be hydrogen and oxygen, but there’s also a huge amount of that most versatile of atoms, carbon, present. All life as we know it incorporates water, and is based on carbon structures. It’s just about possible to consider life using other fluids to provide the environment that water does. Some have suggested, for example, there could be life which uses methane (CH4) instead of water on the chilly surface of Saturn’s moon Titan (averaging around –180°C/–290°F), where methane is liquid. But it is very difficult to see how any other atom could take over the role of carbon.
The reason is simple – carbon is by far the most versatile element when it comes to making a wide range of structures, from hexagonal benzene rings up to the huge carbon chains that make up polymers, such as those in plastics, and massive molecules such as DNA. It has been suggested in the past that silicon, which occupies an equivalent position to carbon in the periodic table, could be a replacement, giving us the possibility of silicon-based life. The idea was, at one point, beloved of science fiction writers.¶ But the reality is that silicon just isn’t up to the job.
While it’s true that chemists managed to make a short-lived, mangled equivalent of a benzene ring from silicon in 2009, to quote one of the team from Imperial College London responsible: ‘What is stable and normal for carbon is unstable for silicon, and by the same token what is unstable for carbon is stable for silicon. It’s an upside-down world.’ The essential molecules for life with silicon substituted are simply not able to form.
Because each carbon atom has 12 times the weight of a hydrogen atom, in practice it is carbon that comes second to oxygen by weight, with about 65 per cent oxygen, 18 per cent carbon and 10.2 per cent hydrogen in your body. With those three elements alone, we’ve made up over 93 per cent of your body weight. Throw in a small amount of nitrogen (3.1 per cent), a pinch of calcium for those bones (1.6 per cent), 1.2 per cent phosphorous, around 0.25 per cent each of potassium and sulfur, with smaller percentages of sodium, chlorine and magnesium, and you’ve hit that 99.95 per cent mark. You are likely to find traces of another 40 to 50 elements, but most contribute little to your existence.
HOW MUCH IS YOUR BODY WORTH?
One way to assess just what goes into making you up at this chemical level is to look at the value of the constituents of your body on the open market. It should be stressed that this has nothing to do with your value as an individual – or, for that matter, with the potential value of your organs and other body contents on a medical basis (some estimates put this as high as £35 million or $45 million). Here, we are considering the value of the individual elements in your body, breaking you down to your component atoms. This isn’t an easy calculation, as it can be difficult to get reasonable assessments of the market price for an element, but it has been estimated to be around £125 ($160).
Such estimates vary hugely. To see how easy it is for this to be the case, consider those highly important components of your body, oxygen and hydrogen. The estimate above used a cost per kilogram of £0.17 ($0.20) for both oxygen and hydrogen. But water (just hydrogen and oxygen) costs less than this – my latest water bill gives a charge of £0.13 per kilogram, and that’s not the cheapest way it could be obtained. In total, by this estimate, the hydrogen and oxygen in your body together are worth around £9.40 ($11.40), but this is far outpriced by the 160g of potassium in your makeup, which is given a value of £86 ($104), dominating your body’s chemical worth.
Again, if we try to buy an equivalent amount of potassium, we get a whole range of pricings. Going for lab-quality potassium metal, for example, I’d have to pay around £414 ($500) for 160g. On the other hand, a banana contains around 0.4g of potassium – so 400 bananas would give us the same amount of potassium. I can get my 160g of potassium by buying those from a supermarket at a cost of £56 ($68). If I bought the bananas wholesale, I could halve that price.
It’s clear that we’re never going to get an exact value here. Other estimates put the value of the chemical makeup of the body as anything from £0.83 ($1) to £1,650 ($2,000). This seems a bizarrely extreme range. In the figure producing the high values, for example, hydrogen dominates, because it has been priced at £83 ($100) per kilo, apparently based on the price of hydrogen fuel for cars. The low value uses relatively old data and almost certainly involves a calculation error. Even so, we get a feel for the cost of what is inside you at this simple level.