This unit zooms in past the atom to the particles and forces that everything else is built from. By the end, you should be able to compare the four fundamental forces, build protons and neutrons out of quarks, and explain what physicists mean when they say forces might unify.
Read the overview first, then work through each topic note and answer its checkpoint question before checking the common mistakes at the bottom.
Earlier units treated forces like friction, tension, and normal force as separate things. In particle physics, all of those turn out to come from just four fundamental forces acting between a small set of elementary particles. This unit sorts out which particles exist, which forces they feel, and how those forces might connect.
Honors Physics Unit 23 covers particle physics in three topics. First, you compare the four fundamental forces (gravity, electromagnetism, the strong nuclear force, and the weak nuclear force) by strength, range, and carrier particle. Next, you learn the six quark flavors, their fractional charges of and , and how quarks combine into baryons and mesons, along with antimatter and annihilation. Finally, you look at unification: the confirmed electroweak theory, Grand Unified Theories, and the indirect experiments used to test them.
Photons carry electromagnetism, gluons carry the strong force, and W and Z bosons carry the weak force. The graviton is predicted for gravity but has never been detected. A heavy carrier like the W boson makes a force short-ranged, while the massless photon gives electromagnetism infinite range.
A proton is and a neutron is , and adding the fractional quark charges gives and . Quarks are confined in color-neutral particles, which can also contain antiquarks, so an isolated quark has not been observed. Electrons belong to a different family, the leptons, and have no known internal parts.
Electroweak theory unifies electromagnetism and the weak force. CERN's 1983 discovery of the predicted W and Z bosons supported the theory. Grand Unified Theories try to add the strong force near GeV. Gravity is the hardest to include and remains an open problem.
A falling ball and a glowing light bulb involve matter and fundamental interactions. In quantum theory, boson exchange describes electromagnetic, weak, and strong interactions. Some theories propose that the forces unify at high energy, but a confirmed quantum theory of gravity is still missing.
Every interaction in nature comes from four forces: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force.
Quarks are elementary particles with no known parts inside them.
Unification is the idea that forces which look separate today are really one force seen at low energy.
Every interaction in nature comes from four forces: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. Gravity and electromagnetism have infinite range. The residual strong force between protons and neutrons acts over roughly m, while the weak force acts over roughly m. The strong interaction between quarks is described differently because quarks are confined.
In quantum theory, the electromagnetic, strong, and weak interactions can be described through exchange of carrier bosons. A quantum carrier for gravity remains hypothetical. Massless photons carry electromagnetism, gluons carry the strong force, and the heavy , , and bosons (about 80 and 91 GeV/) carry the weak force. Their large mass is why the weak force has such a short reach.
A common rough comparison sets the strong interaction's strength to 1 and lists electromagnetism near , the weak interaction near , and gravity near . These are scale-dependent estimates, not universal ratios for every pair of particles. Gravity still controls planets and galaxies because mass is always positive, so it never cancels, while positive and negative charges usually balance out.
Worked example. Compare the electric and gravitational forces between two protons. Both forces scale as , so distance cancels in the ratio: . The top is and the bottom is , so the ratio is about . The electric repulsion is roughly a trillion trillion trillion times stronger.
Check yourself: Why does electromagnetism reach across the universe while the weak force barely crosses a proton? (Answer: The photon has no mass, but the W and Z bosons are very heavy, and a heavier carrier can only be exchanged over a much shorter distance.)
| Force | Carrier | Approximate strength (strong = 1; depends on scale) | Range | Acts on |
|---|---|---|---|---|
| Strong nuclear | Gluons | 1 | About $10^{-15}$ m between nucleons | Quarks and hadrons |
| Electromagnetic | Photon | About $10^{-2}$ | Infinite | Charged particles |
| Weak nuclear | $W^+$, $W^-$, $Z^0$ | About $10^{-6}$ | About $10^{-18}$ m | Quarks and leptons |
| Gravity | Graviton (not detected) | About $10^{-39}$ | Infinite | All mass and energy |
Quarks are elementary particles with no known parts inside them. They come in six flavors: up, down, charm, strange, top, and bottom. Up-type quarks (up, charm, top) have charge , and down-type quarks (down, strange, bottom) have charge . Protons and neutrons have up and down valence quarks. Heavier quarks can appear in transient quark-antiquark pairs within hadrons, but they do not form the valence content of ordinary protons or neutrons.
Quarks can combine into hadrons, although top quarks usually decay before doing so. Ordinary baryons contain three quarks, like the proton () and neutron (); antibaryons contain three antiquarks. Ordinary mesons contain a quark and an antiquark, like the pion. Exotic hadrons can contain more quarks and antiquarks. Quarks also carry color charge (red, green, or blue), which is the source of the strong force. Every hadron is color-neutral. With enough energy, pulling quarks apart can create new quark-antiquark pairs instead of isolating a quark. This is called color confinement, and it's why no one has ever seen a free quark.
Every particle has an antiparticle with the same mass and opposite charge. The antiproton is with charge . A particle and its antiparticle can annihilate into other particles. The products carry the initial particles' total energy, including their rest energy and any kinetic energy.
Worked example. Find the charge of a meson, made of . The up quark is . An anti-down flips the down quark's to . Adding gives , so the has charge .
Check yourself: What is the charge of a neutron's antiparticle, made of ? (Answer: , so the antineutron is neutral even though its quark content differs from the neutron's.)
| Feature | Hadrons | Leptons |
|---|---|---|
| Made of | Quarks | Nothing smaller (elementary) |
| Examples | Proton, neutron, pion | Electron, muon, tau, neutrinos |
| Feel the strong force? | Yes | No |
| Feel the weak force? | Yes | Yes |
Unification is the idea that forces which look separate today are really one force seen at low energy. This has already happened once. At high energies, electromagnetism and the weak force act as a single electroweak force. Glashow, Weinberg, and Salam won the 1979 Nobel Prize for the theory, and CERN found the predicted W and Z bosons in 1983.
A Grand Unified Theory (GUT) would also include the strong force, often at an estimated energy around GeV. A theory that also includes gravity is sometimes called a Theory of Everything. Candidate theories exist, but none has been confirmed.
Some theories propose that forces were more unified in the hot early universe. A proposed gravity separation near s and a GUT transition near s are speculative. The electroweak transition is expected around s, when cooling changed how the electromagnetic and weak interactions appeared.
No accelerator can reach GUT energies, so tests are indirect. Experiments like Super-Kamiokande look for proton decay, which many GUTs predict. No decay has been seen; for some decay channels, the lower limit on the proton's partial lifetime is about years or longer. Other searches look for magnetic monopoles and check whether the force strengths, which change with energy, meet at a single value.
Check yourself: The LHC collides particles at about GeV. About how many times short of GUT energy is that? (Answer: , close to a trillion times too low, which is why tests have to be indirect.)
| Term | Definition |
|---|---|
| Fundamental force | One of the four basic interactions in nature (gravity, electromagnetism, strong nuclear, weak nuclear) that all other forces come from. |
| Fermion | A particle with half-integer spin. Quarks and electrons are examples with spin $\frac{1}{2}$. |
| Boson | A particle with integer spin. Photons, gluons, and W bosons carry interactions; the Higgs boson is also a boson but is not a force carrier. |
| Quark | An elementary particle with fractional charge that combines with other quarks to form hadrons. |
| Hadron | Any particle made of quarks, including baryons and mesons. |
| Lepton | An elementary matter particle that does not feel the strong force, such as an electron or neutrino. |
| Color charge | The property of quarks and gluons that acts as the source of the strong force, labeled red, green, or blue. |
| Antiparticle | A particle with the same mass as its partner and opposite electric charge and certain other quantum numbers. The positron is the electron's antiparticle; some particles, such as the photon, are their own antiparticles. |
| Standard Model | The tested theory that organizes all known elementary particles and describes every fundamental force except gravity. |
| Grand Unified Theory | A proposed theory that merges the strong, weak, and electromagnetic forces into a single force at extremely high energy. |
Gravity is by far the weakest force, about times the electric force between two protons. It controls large-scale motion only because mass never cancels out, while positive and negative charges usually balance. Rank the forces by coupling strength, not by how big the objects they move are.
A neutron has zero total charge, but it contains one up quark () and two down quarks ( each). Always add up the quark charges instead of guessing from the total. The same goes for the antineutron, which is also neutral but is built from antiquarks.
Protons are hadrons made of quarks, so they feel the strong force. Electrons are leptons with no known internal structure, and they do not feel the strong force. Quark content helps identify hadrons, but charge and other properties also matter when deciding which interactions a particle can experience.
Only electroweak unification has experimental confirmation. Grand Unified Theories are still unproven, and proton decay searches have already ruled out some simple versions. Candidate theories that include gravity exist, but none has been confirmed.