Quantum field theory is a set of basic ideas that physicists use to explain the laws of nature. The two most important of those ideas are: (1) interactions between objects are transmitted by fields, and (2) those interactions obey the principles of quantum mechanics.
A field is an influence that an object creates in the region around it. For example, a magnet creates a magnetic field in the space around it. This field, in turn, can make another magnet move. Scientists say that magnets interact by means of the magnetic force and that a magnetic field transmits the force.
Consider a magnetic field created by a bar magnet. Suppose a compass lies far from the magnet. If the compass is far enough away, the field due to the bar magnet will be too weak to affect the compass needle—a small magnet itself. Now, imagine that the compass comes closer and closer to the bar magnet. When the compass reaches a certain point, the field will make the needle rotate by a measurable amount.
An electric field transmits the electric force between electrically charged objects. Electric fields and magnetic fields usually occur together, so they are usually referred to jointly as electromagnetic fields. Other kinds of fields transmit three major forces: (1) the strong force, which holds particles called quarks together in protons, neutrons, and other objects; (2) the weak force, which is responsible for certain kinds of radioactivity; and (3) the gravitational force, by which all types of objects are attracted to one another.
The fundamental principle of quantum mechanics is that energy transmitted by fields comes in packets or bundles. Physicists refer to the packets as field particles. Each kind of field particle has a name. For example, the field particles of electromagnetic energy are known as photons. Visible light is a form of electromagnetic energy, and so its field particles are photons.
