Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Force Carriers

All of the known forces in our universe can be classified as one of four types: electromagnetic, strong, weak, or gravitational. These forces affect everything in the universe. The electromagnetic force binds electrons to the atoms that compose our bodies, the objects around us, the Earth, the planets, and the Moon. The strong nuclear force holds together the nuclei inside the atoms that compose matter. Reactions due to the weak nuclear force fuel the Sun, providing light and heat. Gravity holds people and objects to the ground.

Each force has a particular property associated with it, such as electric charge for the electromagnetic force. Elementary particles that do not have electric charge, such as neutrinos, are electrically neutral and are not affected by the electromagnetic force.

Mechanical forces, such as the force used to push a child on a swing, result from the electrical repulsion between electrons and are thus electromagnetic. Even though a parent pushing a child on a swing feels his or her hands touching the child, the atoms in the parent’s hands never come into contact with the atoms of the child. The electrons in the parent’s atoms repel those in the child while remaining a slight distance away from them. In a similar manner, the Sun attracts Earth through gravity, without Earth ever contacting the Sun. Physicists call these forces nonlocal, because the forces appear to affect objects that are not in the same location, but at a distance from one another.

Theories about elementary particles, however, require forces to be local—that is, the objects affecting each other must come into contact. Scientists achieved this locality by introducing the idea of elementary particles that carry the force from one object to another. Experiments have confirmed the existence of many of these particles. In the case of electromagnetism, a particle called a photon travels between the two repelling electrons. One electron releases the photon and recoils, while the other electron absorbs it and is pushed away.

Each of the four forces has one or more unique force carriers, such as the photon, associated with it. These force carrier particles are bosons, since they do not obey the exclusion principle—any number of force carriers can have the exact same characteristics. They are also believed to be fundamental, so they cannot be split into smaller particles. Other than the fact that they are all fundamental bosons, the force carriers have very few common features. They are as unique as the forces they carry.