Atoms, and the four forces

Everything you can touch is made of atoms: your hand, the air, the sea, the Moon. They are far too small to see. A typical atom is about a ten-millionth of a millimetre across, and a single hair is about a million atoms wide. But they are real, and this is a picture of some. It shows the surface of a piece of gold, and each small bump is one atom. No light could show this. The picture was made by a microscope that feels its way across a surface with a needle one atom wide at the tip.

An atom is not solid. At its centre is a tiny, heavy lump called the nucleus, made of particles called protons and neutrons. Round it, far out, are much lighter particles called electrons. The nucleus holds more than 99.9 per cent of the atom’s mass, yet it is tens of thousands of times narrower than the atom. If an atom were as wide as a football stadium, the nucleus would be a pea on the centre spot. The rest is almost empty.

There are only three kinds of piece, but they make everything. What decides the kind of atom is the number of protons in the nucleus. One proton is hydrogen. Two is helium. Six is carbon, eight is oxygen, 26 is iron, 79 is gold. There are 118 kinds known, called the elements. Hydrogen and helium were made as the universe began. Nearly all the rest were built from them by stars, in their lives and in their deaths.

Four forces act on these pieces, and between them they do everything that is ever done. Gravity pulls all matter together. The electric and magnetic force holds electrons to the nucleus and atoms to each other. It is the reason you do not fall through the floor. The strong force glues the nucleus together. The weak force can turn one kind of particle into another. This chart shows how strong they are. Gravity is weaker than the others by an almost unbelievable amount.

A star is a tug of war between them. Gravity squeezes the middle of the Sun until it is nearly 16 million degrees. Protons push each other away, because they carry the same electric charge. But there, a few are driven so close together that the strong force grabs them and they stick. The weak force turns some protons into neutrons. Step by step, hydrogen becomes helium, and a little of the mass turns into the light you see. Every second, the Sun gets about four million tonnes lighter.

So why does feeble gravity rule the sky? Because it never cancels. Electric charge comes in two kinds, plus and minus, and in any big lump of matter they balance out, so the huge electric force goes quiet. The strong and weak forces reach no further than the width of a nucleus. Gravity only ever pulls, and it reaches for ever. Add up enough atoms and it wins. It rounds the planets, lights the stars, and winds galaxies like this one into spirals.