Even while the numbers of particles became more bizarre and complex, the ways those objects interacted turned out to be surprisingly straightforward. In the 20th century, scientists discovered that objects in the universe experienced only four fundamental types of interactions:
Electromagnetism Strong nuclear force Weak nuclear force Gravity
Physicists have discovered profound connections between these forces — except for gravity, which seems to stand apart from the others for reasons that physicists still aren’t completely certain about. Trying to incorporate gravity with all the other forces — to discover how the fundamental forces are related to each other — is a key insight that many physicists hope a theory of quantum gravity will offer.
Electromagnetism: Super-speedy energy Waves
Discovered in the 19th century, the electromagnetic force (or electromag-netism) is a unification of the electrostatic force and the magnetic force. In the mid-20th century, this force was explained in a framework of quantum
mechanics called quantum electrodynamics, or QED. In this framework, the electromagnetic force is transferred by particles of light, called photons.
The relationship between electricity and magnetism is covered in topic 5, but the basic relationship comes down to electrical charge and its motion. The electrostatic force causes charges to exert forces on each other in a relationship that’s similar to (but more powerful than) gravity — an inverse square law. This time, though, the intensity is based not on the mass of the objects, but the charge.
The electron is a particle that contains a negative electrical charge, while the proton in the atomic nucleus has a positive electrical charge. Traditionally, electricity is seen as the flow of electrons (negative charge) through a wire. This flow of electrons is called an electric current.
A wire with an electrical current flowing through it creates a magnetic field. Alternately, when a magnet is moved near a wire, it causes a current to flow. (This is the basis of most electric power generators.)
This is the way in which electricity and magnetism are related. In the 1800s, physicist James Clerk Maxwell unified the two concepts into one theory, called electromagnetism, which depicted this force as waves of energy moving through space.
One key component of Maxwell’s unification was a discovery that the electromagnetic force moved at the speed of light. In other words, the electromagnetic waves that Maxwell predicted from his theory were a form of light waves.
Quantum electrodynamics retains this relationship between electromagne-tism and light, because in QED the information about the force is transferred between two charged particles (or magnetic particles) by another particle — a photon, or particle of light. (Physicists say that the electromagnetic force is mediated by a photon.)
Nuclear forces: What the strong force joins, the Weak force tears apart
In addition to gravity and electromagnetism, 20th-century physics discovered two nuclear forces called the strong nuclear force and weak nuclear force. These forces are also mediated by particles. The strong force is mediated by a type of particle called a gluon. The weak force is mediated by three particles: Z, W+, and W- bosons. (You can read more about these particles intopic 8.)
The strong nuclear force holds quarks together to form protons and neutrons, but it also holds the protons and neutrons together inside the atom’s nucleus.
The weak nuclear force, on the other hand, is responsible for radioactive decay, such as when the neutron decays into a proton. The processes governed by the weak nuclear force are responsible for the burning of stars and the formation of heavy elements inside of stars.
