These particles are combinations of quarks. They have mass and reside in the nucleus. The two most common examples of hadrons are protons and neutrons, and each is a combination of three quarks:
- Proton = 2 up quarks + 1 down quark [+1 charge on proton = (+2/3) + (+2/3) + (-1/3)]
- Neutron = 2 down quarks + 1 up quark [0 charge on neutron = (-1/3) + (-1/3) + (+2/3)]
These particles are thought to be exchanged when forces occur. A force is defined as a push or pull. But that does not tell us what it really is or how it is mediated. Richard Feynman suggested that forces occur when two particles exchange a boson, or gauge particle. Think of two people on roller skates: If one person throws a ball and the other one catches it, they will be pushed in opposite directions. In this analogy, the skaters are the fundamental particles, the ball is the force carrier and the repulsion is the force. In the case of particles, we see the force, the effect, but not the exchange.
There are four known bosons:
- Gluon - mediates the strong force, but only operates over distances of 10-13 cm
- W and Z - mediate the weak force (1/10,000 strong force), but only operate over distances of 10-15 cm
- Photon - mediates electromagnetic force (1/137 strong force) and operates over an infinite distance
A fifth gauge particle (graviton) has been proposed, but has not yet been found. The graviton is thought to mediate gravity, which is 10-39 strong force and operates over an infinite distance.
Historically, James Clerk Maxwell unified electricity and magnetism in the19th century. As physicists have constructed more powerful accelerators with higher temperatures and energies, they have seen that certain forces come together, or unify. Particle-accelerator experiments have shown that the electromagnetic force and the weak force can be combined into the electroweak force. Many physicists believe that all of the forces stem from one force that existed long ago. Theories attempting to unite the forces are called unified theories or grand unified theories (GUT). It is hoped that GUTs will tell us what the universe may have been like in its beginnings. Because accelerator experiments have simulated what are thought to be the conditions that existed just fractions of a second after the Big Bang, they may provide evidence to support or refute various GUTs.
According to the Big Bang theory:
- Prior to the Big Bang, the universe was extremely hot and small and matter existed only as free quarks.
- Once the explosion happened: Rapid inflation occurred and the universe cooled. Quarks combined into hadrons. The forces separated. Matter (atoms) formed. Matter condensed into galaxies, stars, etc.
By making bigger and bigger particle accelerators, physicists can simulate the conditions that existed within 10-43 seconds of the Big Bang!