From One Superforce to Four
Physics uses two everyday words with precision. A force is any push or pull — anything that can start, stop, speed up, slow down, or steer motion; it is measured in newtons (N), and one newton is roughly the gentle tug of a small apple resting in your hand. Energy is the capacity to cause change — to move, heat, light, or rearrange things; it can switch between forms but can never be created or destroyed, and it is measured in joules (J) — lifting that apple a metre takes about one joule. The two are linked: a force acting on something transfers energy to it, and every force stores energy in an invisible field filling the space around it. Remarkably, every push and pull in today's Universe — a falling raindrop, the fusion burning in the Sun — comes down to just four fundamental forces: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. This lesson is the story of how one original force became those four.
The story begins at the Big Bang (t = 0), a state of effectively infinite density and temperature. During the Planck Epoch — the first 10⁻⁴³ seconds (that is 1 divided by a 1 with 43 zeros after it; for scale, a 1 with just nine zeros is a billion) — the entire Universe was a chaotic quantum foam (a violent, ever-shifting churn of energy at the smallest scales) barely 10⁻³⁵ metres across, hotter than 10³² Kelvin. At that energy there were not four forces but one: a single, indistinguishable interaction physicists call the Superforce. The theory that would describe it is nicknamed a Theory of Everything (TOE) — a theory we do not yet possess, since no laboratory can recreate Planck-epoch energies.
What happened next is best pictured as freezing. In the extreme heat the forces were symmetric and interchangeable, as featureless as liquid water; as the cosmos expanded and cooled it crossed critical thresholds where that symmetry was spontaneously broken (the forces stopped being alike, all on their own), and — like water releasing latent heat as it crystallises into ice — the unified force released energy and "froze out" into separate forces, in three stages. Gravity decoupled first, at about 10⁻⁴³ seconds, leaving the other three united as the GUT force (named for the Grand Unified Theories that describe it — still unconfirmed by any experiment). Next, around 10⁻³⁵ seconds and 10²⁸ Kelvin, the strong nuclear force broke away — a transition widely linked to the trigger for cosmic inflation, the brief burst of exponential (runaway, ever-faster) expansion. Finally, at 10⁻¹² seconds (one picosecond, a trillionth of a second) and about 10¹⁵ Kelvin, the remaining electroweak force split into electromagnetism and the weak nuclear force. One picosecond in, all four forces existed as we know them today. The family tree below draws exactly this story — click any branch, or any era of the trunk itself (press 1 to 7, or walk the story with the arrow keys), to open its details.
Read from left to right: the dark trunk is whichever forces still act as one — the Superforce, then the GUT force once gravity departs, then the electroweak pair once the strong force leaves — dissolving into four separate branches as the cosmos cools from 10³² K to 10¹⁵ K. The three trunk eras also tell a story about evidence: the Theory of Everything is still a hope, the Grand Unified Theories are unconfirmed, but electroweak unification has been proven in the laboratory — its predicted W and Z carriers were found at CERN in 1983. Each branch ends in a strength bar and a carrier particle; comparing them is part two of this lesson.
The Four Forces We Live Under
The four forces that emerged are not equals. Ranked by relative strength — taking the strongest as 1 — they span from 1 down to 10⁻³⁹: nearly forty orders of magnitude (each "order of magnitude" means another factor of ten). This is exactly what the log-scale strength bars on the right of the figure compare, and each branch there also names the force's carrier particle — a boson (a force-carrying particle) exchanged between the particles that feel the force — and its range.
At the top sits the Strong Nuclear Force, with relative strength 1, carried by massless gluons. It binds quarks (the smallest known building blocks of matter — the subject of §1.2) into protons and neutrons, and a residual (leftover) part of it then binds those into atomic nuclei (the dense cores of atoms), overcoming the electric repulsion that would otherwise blow them apart. Its reach is only nuclear, about 10⁻¹⁵ metres. Predicted in 1935 by Hideki Yukawa and confirmed in 1947, its fingerprints are every stable nucleus and the hydrogen-to-helium fusion that powers the Sun and every star.
Roughly a hundred times weaker, at 10⁻², is Electromagnetism, carried by the photon (a particle of light) over infinite range. It acts on everything electrically charged: it binds electrons to nuclei and so underlies the whole of chemistry — every chemical bond, light itself, magnetism, and the electrical signals firing along your nerves. It was unified into a single theory of electricity and magnetism in 1864 by James Clerk Maxwell, building on Hans Christian Ørsted's 1820 discovery that an electric current deflects a compass needle.
Weaker still, at about 10⁻⁶, is the Weak Nuclear Force, carried by the heavy W⁺, W⁻, and Z bosons over a minuscule 10⁻¹⁸ metres (a thousandth the width of a proton). Uniquely, it transforms particles — turning one kind into another, as in beta decay (a common type of radioactive decay), first described by Enrico Fermi in 1934. It drives radioactivity and the crucial first step of the fusion that powers stars — converting a proton into a neutron — which is where the story of §1.3 begins.
Feeblest by far, at just 10⁻³⁹ (compared between two protons), is Gravity — so weak that a small magnet lifts a paperclip against the pull of the entire Earth. Its conjectured carrier, the graviton, has never been detected. Massive objects attract one another with a strength that falls off as 1/r² (twice as far apart means a quarter of the pull) — a law set down by Isaac Newton in 1687 (the Principia) and recast in 1915 by Albert Einstein as the curvature of spacetime (the combined fabric of space and time). Yet gravity has infinite range, only attracts, and never cancels out, so across cosmic distances it dominates — shaping planetary orbits, ocean tides, black holes, and the structure of every galaxy.
Strip away any one of the four and we vanish. Without gravity, the Sun and Solar System would never have formed and the Earth would hold no atmosphere; without electromagnetism, electrons would not bind to nuclei and atoms, molecules, and solid matter would disintegrate; without the strong force, no nucleus heavier than hydrogen could exist; without the weak force, the Sun's fusion could not even begin. And at the deepest level a force is more than a push or a pull: it is how the Universe manages its energy. The carrier bosons create fields that permeate space and store potential energy, and particles are forever nudged toward states of lower energy — like a ball rolling downhill — which is what every push and pull ultimately is.
One force became four. Each one holds a different scale of the Universe together — and the four of them, together, make atoms, stars, and the bodies we walk around in.