Science › Fields & particles
The four forces of nature
1973· Idea
ProvenTested many times; part of working physics.
All known interactions come down to four forces: gravity, electromagnetism, the strong nuclear force and the weak nuclear force.
See it on the Science map →How it works
Gravity pulls masses together and rules planets and galaxies. Electromagnetism holds atoms together and drives all chemistry and electronics. The strong force binds quarks into protons and neutrons, and those into nuclei. The weak force turns one kind of particle into another, driving some radioactivity and the Sun's fusion. Their strengths differ enormously: between two protons, gravity is about 10^36 times weaker than electric repulsion.
Why it matters
Every phenomenon, from friction to supernovae, reduces to these four. Uniting them is a central goal of physics.
How we know
Three are described by the Standard Model and tested at colliders; gravity by general relativity, tested from Mercury's orbit to gravitational waves (2015). No experiment has found a fifth force.
Still a mystery
Why the strengths differ so much, why gravity is so weak, and whether all four are aspects of one force are unanswered.
Connections (6)
▸General relativity: gravity is curved spacetime describes gravity, the odd one out of The four forces of natureProven
The relationship
In general relativity, gravity is not a force pulling through space but the bending of space and time by mass. The other three forces are described as quantum fields trading particles. Gravity does not fit that pattern: no one has a working quantum version, and its hypothetical carrier particle, the graviton, has never been seen.
How the link was found
Einstein published general relativity in 1915. The mismatch with quantum theory was recognized by the 1930s and sharpened in the 1970s, when calculations showed quantum gravity gives uncontrollable infinities.
Why it's proven
General relativity is confirmed by Mercury's orbit, light bending (1919), GPS clocks, and gravitational waves (2015). No quantum effect of gravity has ever been measured.
Open questions and mysteries
Is gravity a force at all, or something more basic about space and time? Gravitons may be undetectable even in principle.
▸Newton's law of gravity was the first of The four forces of natureProven
The relationship
Newton's 1687 law was the first time any force was captured by a single universal formula: every mass attracts every other, weakening with the square of distance. The same rule ran apples and planets. That set the template for finding the other forces: look for a simple law that applies everywhere.
How the link was found
Isaac Newton, in the Principia (1687). Charles-Augustin de Coulomb found a similar square-of-distance law for electric charges in 1785.
Why it's proven
Newton's law predicted planetary orbits and, in 1846, Neptune's location. Henry Cavendish measured gravity's strength in the lab in 1798.
Open questions and mysteries
Newton could not say what gravity is or how it acts across empty space. Einstein answered partly in 1915; gravity's quantum nature remains unknown.
▸The four forces of nature has three described by The Standard ModelProven
The relationship
The Standard Model describes electromagnetism, the weak force and the strong force in one consistent quantum framework. Electromagnetism and the weak force are truly merged into one electroweak force; the strong force sits alongside under the same rules. Gravity is left out entirely.
How the link was found
Assembled 1961-1975 by Glashow, Weinberg, Salam, Gell-Mann, 't Hooft, Gross, Wilczek, Politzer and many others.
Why it's proven
Every lab-scale experiment involving these three forces matches the model's predictions.
Open questions and mysteries
Why three forces with such different strengths? Whether they fully unify at very high energy, as some theories suggest, is untested.
▸The four forces of nature uniting all four is the goal of A theory of everythingOpen question
The relationship
A theory of everything would show the four forces are faces of one underlying law. There is a precedent: electricity and magnetism were united in the 1860s, and electromagnetism with the weak force in the 1960s. Joining in the strong force, and finally gravity, would complete the pattern. Gravity is the hardest step.
How the link was found
Einstein pursued a unified theory from the 1920s until his death in 1955. Grand unified theories began with Howard Georgi and Sheldon Glashow in 1974.
Is it proven?
Not achieved. Grand unified theories predict protons slowly decay; Japan's Super-Kamiokande detector has found none, showing protons last over 10^34 years.
Open questions and mysteries
Do the forces really merge at high energy? Is there one theory, or many possible ones?
▸Extra dimensions might explain why gravity is weakest of The four forces of natureSpeculative
The relationship
Gravity is roughly 10^36 times weaker than electromagnetism between two protons. One idea: the other forces are confined to our three-dimensional space, but gravity spreads into extra dimensions too, so most of its strength leaks away. Then gravity would only look weak to us, while being comparable to the other forces at short distances.
How the link was found
Arkani-Hamed, Dimopoulos and Dvali proposed large extra dimensions in 1998; Lisa Randall and Raman Sundrum offered a warped version in 1999.
Is it proven?
Not supported so far. Torsion-balance tests find gravity normal down to about 50 micrometers, and the Large Hadron Collider found no missing energy or tiny black holes predicted by these models.
Open questions and mysteries
Why gravity is so weak, called the hierarchy problem, remains one of physics' main unsolved puzzles.
▸The four forces of nature may have had three unified in Grand unification epochSpeculative
The relationship
Force strengths change with energy: the strong force weakens at high energy while electromagnetism strengthens. Extending measured trends upward, the strong, weak and electromagnetic strengths approach each other near 10¹⁶ GeV. Grand unified theories say they merge there into one force, carried by extra particles that could turn quarks into positrons (anti-electrons), letting protons decay. Gravity is thought to have separated earlier, at the Planck scale.
How the link was found
Howard Georgi and Sheldon Glashow proposed the first grand unified theory in 1974; the same year Georgi, Helen Quinn and Steven Weinberg showed the strengths could converge at high energy.
Is it proven?
Not proven. The three strengths nearly meet but miss slightly unless supersymmetry exists, shown with precise CERN data in 1991. Supersymmetry has not appeared at the LHC, and proton decay, the key prediction, is unseen.
Open questions and mysteries
Are the forces really one at high energy, and does gravity join them at the Planck scale? A proton decay seen at Hyper-Kamiokande or DUNE would be the strongest clue.