Two positive charges shoved together push back harder the closer they get. A nucleus is a knot of positive charges packed almost on top of one another. It should detonate. It doesn't.

The repulsion problem

Every atomic nucleus heavier than hydrogen is a disaster that never happens. Protons carry positive charge, and like charges repel — fiercely, and more fiercely the nearer they get. Pack two protons into the minuscule volume of a nucleus and the electric force straining to fling them apart is enormous. By that logic, no nucleus should survive its first instant.

Something is overpowering that repulsion. Whatever it is, it must be stronger than the electric force, and it must switch on only at extremely short range — otherwise we would feel it reaching out across rooms, the way we feel magnetism and electricity. Physicists gave this something a suitably blunt name: the strong force.

What is really being held together

For a while the strong force looked like a simple glue smeared between protons and neutrons. The deeper truth, worked out through the 1960s and 70s, is far stranger.

Protons and neutrons are not fundamental. Each is a bundle of smaller particles — quarks — lashed together by the strong force, which is carried between them by particles aptly named gluons. The force acts on a property called colour charge: a kind of charge with nothing to do with the familiar electric one, and nothing to do with visible colour either. And it behaves like nothing else in physics. The harder you try to pull two quarks apart, the stronger the force hauling them back, as if they were joined by an unbreakable elastic band. Pull hard enough and the energy you pour in does not free a quark — it congeals into brand-new quarks, so you end up with more particles, never a lone one in your hand. Nobody has ever isolated a single quark. They are confined, permanently.

The force that holds a whole nucleus together turns out to be just the faint spillover of this ferocious binding — a leftover reaching a little past the edge of each proton and neutron to grip its neighbours. The main event is happening inside each particle.

The weight you didn't know you carried

Here is the payoff, and it is personal. The three quarks inside a proton account for barely 1% of its mass. The other 99% is the energy of the gluon field binding them — the sheer violence of confinement, weighed as mass through the exchange rate of E = mc².

So the Higgs field hands the quarks their small intrinsic mass, but almost everything you weigh — nearly all of your body, this whole planet, the stars — is not "stuff" in any solid sense. It is bound energy, the strong force made heavy. Step on a scale and you are mostly weighing the effort of holding quarks together.

The strongest force in nature is one you will never feel and can never escape from. It does its work entirely inside particles smaller than an atom — and it is the reason there is such a thing as weight at all.

The theory that describes all this, quantum chromodynamics, is fantastically successful and fantastically hard. We can watch quarks behave exactly as predicted in the wreckage of particle collisions, and yet no one has proved, from first principles, why they must stay confined at all. It remains a formal open problem with a million-dollar prize attached. The most powerful force we know of is also the one whose basic rulebook we still cannot fully read.