3 Forces






Strong Nuclear

Acts On


electric charge + -

flavor U,D,S,C,T,B

color charge R,G,B,M,C,Y

Force Carrier


virtual photon

Z0, W+, W-


There are 3 known forces



and strong nuclear

let's look at each of them in turn:


Gravity interacts with the property of an object known as mass,

There is only one kind of mass, and the gravity force between all kinds of mass is attractive. (In the next year we should finally be able to confirm what physicists suspect that an anti-atom falls under gravity, it doesn't rise.)

Every particle has mass.

Even force carrying particles like photons have mass.
That seems weird since we say that photons have no "rest mass." And indeed they have no rest mass, but, they are never at rest, so they always have mass. Photons fall under gravity.

Aside: Photons fall under gravity near the surface of the earth exactly twice as much as Newtonian gravity predicts. Einstein's general theory of relativity predicted this factor of two.

The gravitational force is carried by a particle known as a graviton. It has no rest mass. It has not been directly observed. It is a stable particle and does not decay i.e. it's half life is infinite.

A point mass or small spherical mass has a gravitational field that falls off as the inverse square of distance.


The Electroweak force is usually divided into two parts.

Electromagnetism and weak nuclear.

Even electromagnetism is usually divided into two parts:

Electricity and magnetism.

Electric forces

Electric forces act on electric charges.

There are two types of electric charge named positive and negative.

Some things, such as photons and neutrinos, have no electric charge and are not acted upon by electric forces.

(Positive and negative charges were named by Benjamin Franklin, he named the charge acquired by glass when rubbed with silk, plus and the charge acquired by amber when rubbed by rabbit fur, minus. When combined in equal quantities, positive and negative electric charges combine to make no net charge. Which is one reason that their name makes sense.)

The electric force is carried by a particle known as a virtual photon.

The virtual photon has no electric charge and is not acted upon by the electric force.

That is why photons do not exert forces on photons.

Photons are stable particles and do not decay.

The electric force from a small sphere of charge on a second small sphere of charge falls off as the inverse square of distance.


Magnetic forces are just electric forces viewed from a moving frame of reference.

So, magnetism is the relativistic transformation of electricity.

In the correct frame of reference magnetic forces can be made to disappear.

Weak Nuclear

We'll save weak nuclear forces until after we do strong nuclear forces.

Strong Nuclear

Strong nuclear forces act on the "color" charge of a particle.

There are three fundamental color charges. Named Red, Green and Blue.

Antiparticles can have three different color charges, charges with anti-colors: magenta, cyan, and yellow.

They have nothing to do with color seen by the human eye. But particles known as hadrons and mesons are always made of a collection of particles known as quarks which have color charges which add up to white. So using color charge allows people who understand color to remember the rules for adding color charge.

A proton in the nucleus of an atom will be made of three quarks: a red, a green and a blue quark, these colors add up to white.

The two quarks that make up a pi meson can be a red quark and an anti-red (cyan) quark which add up to white.

Electrons have no color charge, they do not feel the strong nuclear force. Electrons passing through the nucleus of an atom feel only the electric forces of the protons in the nucleus, not the nuclear forces. Neutrinos have no color charge they do not interact with the strong nuclear force, since we have seen that they have no charge, they do no interact with electric charges of nuclei or electron. So neutrinos have great penetrating power. they can pass through light years of lead without interacting.

The strong nuclear force is carried by particles known as gluons. Gluons have color charge themselves and so gluons interact with gluons. Physicists are looking for particles entirely made of gluons known as glueballs.

Gluons have mass and so they have a finite range of attraction, beyond 10-15 m their force falls off exponentially, much faster than inverse square. (Not all forces are inverse square.)

The strong nuclear force between nearest neighbor nuclei is on the order of 60 times stronger than the electric force between two nearest neighbor protons. When you get about 90 nucleons together in Uranium, the electrical repulsion between protons exceeds the nuclear attraction, and the nucleus spontaneously fissions.

Weak Nuclear

The weak nuclear force acts on a property of charge known as flavor.

There are six flavors with the strange names of:

up, down
strange, charm
top, bottom

The proton is made from two up quarks and a down quark, uud.

The neutron is made of one up quark and two down quarks, ddu.

The electron neutrino and the electron interact with the up and down flavors.

When a neutron spontaneously decays into a proton, an electron and an electron neutrino are emitted.

The muon and the muon neutrino are paired with with the strange and charm flavors.

The tau and the tau neutrino are paired with the bottom and top flavor.

The force carriers of the weak nuclear force are known as the W+, W- and Z0 bosons. They have mass and so have a limited range. Two have charges and so interact with electrical forces.

The main effect of the weak force interactions is not to exert forces, although they do, it is to change the flavor of a particle. Thus the weak force moderates the decay of the neutron changing one of its down quarks into an up quark.


Electrons have antiparticles called anti-electrons or positrons.

Protons have antiprotons.

Scientists recently combined positrons and antiprotons to make anti-hydrogen atoms, 1995. However these anti-hydrogens were traveling near the speed of light. In 2000 scientists at CERN will try to make anti-hydrogen and bring it to rest.

When an electron and an anti-electron combine both particles vanish and their energy is released as photons.

Protons and antiprotons annihilate each other to create energy from which other particles can appear.

Anti-electrons do not interact with protons, only electrons.

Return to Physics

Scientific Explorations with Paul Doherty

© 1999

28 June 99