Illustrated Physics
physics, drawn
Concept

Strong interaction — where it appears

The force that binds quarks into protons, neutrons and other hadrons, carried by gluons and sourced by colour charge. It weakens at short distances and grows at long ones, so quarks are never found alone.

Named by 3 essays across 3 fields — each of them below, with the objects they name alongside it.

The curve that makes both fusion and fission release energy. Binding energy per nucleon against mass number: how much energy would have to be supplied, per particle, to take a nucleus apart into free protons and neutrons. The curve is the semi-empirical mass formula, evaluated at whichever proton number binds most tightly for each mass number rather than at a guessed one; the points are measured values. It rises steeply at the light end, peaks at mass number 58, and falls slowly thereafter. Everything about nuclear energy follows from that shape and from nothing else. Two light nuclei joined move up the curve and release the difference; one heavy nucleus split moves up it too, from the other side. Both directions are downhill in energy because the peak is in the middle, and the peak is in the middle because two effects fight — the surface term, which penalises small nuclei for having most of their nucleons on the outside, and the Coulomb term, which penalises large ones because every proton repels every other. The energy released is the height climbed times the number of nucleons carried, and it is a million times a chemical bond for the same reason the vertical axis is in millions of electronvolts rather than in single ones.

The mass that is missing

A helium nucleus weighs less than the two protons and two neutrons it is made of. The shortfall is not an error in the weighing; it is the binding energy, converted at the going rate. One curve of that shortfall against size explains why both fusion and fission release energy.

relativity · Mass-energy Three identical quarks in one state, and the label that rescues them. The Δ⁺⁺ particle, mass 1,232 MeV and charge +2, drawn as the quark model describes it: three up quarks, all in the lowest orbital state, all with their spins pointing the same way, giving spin 3/2. Left: with nothing else to tell them apart, the state is unchanged by swapping any two quarks — symmetric in where they are, in their spins and in their flavour — which the exclusion principle forbids for identical fermions. Right: give each quark a label that takes three values, and put the three in the one combination of the three values that changes sign under every swap. The whole state is then antisymmetric, as it must be, and the particle is allowed. The label is called colour, and three is the smallest number of values that can do this for three quarks.

The label three identical quarks forced

In 1964 the quark model had one embarrassment it could not explain away. A particle called the Δ⁺⁺, well measured and entirely ordinary, had to be three identical up quarks in the same state with their spins pointing the same way — which the exclusion principle forbids outright. The way out was to give quarks a hidden property that takes three values, so that the three could be different after all. Nobody could see the property, and for years it looked like a bookkeeping trick. Then three independent measurements counted it, and every one came out at three. The exclusion principle, which keeps electrons in shells, had predicted the colour of the strong force.

quantum · Exclusion The strong coupling, run from the proton to the Planck mass. The inverse of the strong interaction's coupling, 1/αₛ, against energy on a logarithmic axis, run at one loop from its measured value at the Z mass (1/8.47, dot), with its slope changing at the charm, bottom and top masses as each quark joins in. On this plot the running is a set of straight lines: the coupling weakens by a fixed amount for each factor of ten in energy, about 2.57 per decade above the top mass. At the Planck mass, 1.22 × 10¹⁹ GeV, 1/αₛ is 52.5 — a coupling of 0.0191. Run down instead, it reaches zero — the coupling runs off to infinity — at 143 MeV, the three-flavour scale of the strong interaction at this order. Nineteen decades of energy are crossed by a line that falls from 52 to 0.

The nineteen decades a slow coupling buys

A proton is ten million million million times lighter than the Planck mass, and that ratio, squared, is why gravity between two protons is 10³⁶ times weaker than the electric force between them. It looks like a number demanding an explanation, and it has one. Almost none of the proton's mass comes from the masses of its quarks; it comes from the strong force, whose strength changes with energy by a logarithm. A coupling of about 1/50 at the Planck mass, weakening by a fixed step for every factor of ten in energy, takes nineteen factors of ten to become strong — and the proton's mass is where it does.

astrophysics · Planck scale

Named alongside it

The objects these essays reach for when they reach for this one.

Activation barrierAntisymmetryBaryonsBinding energyColour chargeConservation of energyCoulomb repulsionDimensional transmutationExclusion principleGravityHierarchy problemIdentical particles

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