The physical behaviour of everything in the universe is determined by four fundamental forces: gravity, the electromagnetic force, and the strong and weak nuclear forces. The electromagnetic and weak nuclear forces are part of a single electroweak force at very high temperature.
Physicists are searching for a "theory of everything" which will describe all four forces with a single set of equations. In this, part one of a two-part article, I describe gravity and the electromagnetic force.
The universe is composed of atoms and atoms are made of a relatively small number of subatomic particles. Atoms and subatomic particles interact with each other through the four basic forces to produce the universe as we see it.
Gravity is the weakest of the four forces, but it has an infinite range of influence. The force of gravity is associated with an object's mass and its effects are also determined by the distance between two bodies, as described by Newton's law of gravitation.
The force of gravity explains why objects fall to earth, it keeps the planets in orbit around the sun, it causes matter to condense together to form stars, and it holds whole galaxies together.
The modern concept of gravity pictures it as a field pervading space and time, mapping out the strength and direction of the force. The field is bent or warped in the vicinity of matter, the extent of the warping being proportional to the size of the matter's mass.
All atoms have a central nucleus surrounded by an orbiting cloud of electrons. The electromagnetic force holds the atom together internally and also binds atoms together externally. The strength of this force determines physical properties such as melting and boiling points, hardness and elasticity, and it allows atoms to combine together in various ways, thereby determining chemistry and biochemistry.
The force of gravity between two objects is always attractive, but electrical and magnetic forces can attract or repel. Electrical charges come in two varieties: positive and negative. Opposite charges attract each other, like charges repel.
Magnetic "charges" also come in two varieties: north and south poles - opposite poles attract each other, like poles repel.
The French physicist Charles Coulomb (1736-1806) worked out in 1785 that the force between charged objects obeys a law similar to Newton's law of gravitation. The size of the force between two charged objects is proportional to the product of the two charges, but reduces over distance. A similar law describes the force between magnetic poles.
Many readers will be familiar with the concept of a force field from secondary school physics, when they followed Michael Faraday's instructions for visualising lines of magnetic force around a bar magnet.
Take a flat bar magnet (rectangular piece of flat iron with north and south poles) and lay a flat piece of cardboard over it. Now tap out fine iron filings over the cardboard. The filings line up along the lines of force. The lines radiate out from each pole and follow a curved trajectory to the opposite pole.
Electrical and magnetic forces are intimately interconnected. Moving electric charges produce magnetic fields, and moving magnets can induce electric currents. The laws which describe the force between motionless charges are only part of the whole picture, which was put together by the Scottish physicist James Clark Maxwell (1831-1879) who described the entire electromagnetic force field in four equations.
Maxwell's theory also turned out to be a theory of light. Light is composed of electric and magnetic fields vibrating in unison but at right angles to each other in a wave motion. The wavelength of the light determines whether it is Xray, ultraviolet light, visible light, infrared, microwave or radio-wave.
The force that binds atoms together is electrical. This is most easily understood for ionic compounds whose atoms are ionised. A common example is sodium chloride (table salt), consisting of charged atoms (ions) of sodium and chlorine. Atoms are electrically neutral, having equal numbers of positive charges (protons) and negative charges (electrons).
In ionic compounds one type of atom borrows electrons from the other type, becoming negatively charged and leaving the other atom positively charged. These oppositely-charged ions attract each other and line up in a regular three-dimensional array of alternating positive and negative ions. The attraction is strong, and such compounds are solid at room temperature and have a high melting temperature.
Maxwell's laws explain many of the properties of matter, but they do not explain many more, e.g., the detailed structure of atoms and how they emit or absorb light. These matters are explained by quantum mechanics, developed in the 1920s.
Max Planck had shown in 1900 that when matter emits or absorbs light it does so by emitting or absorbing chunks of energy called photons. Quantum mechanics treats light as composed of photons whose energy is inversely proportional to the wavelength of the light.
The theory of quantum electrodynamics (QED) explains how electromagnetic force is transmitted. It is considered that the carrier of the force is a photon - not an ordinary one as in light, but a ghostly virtual photon. An electric charge emits these virtual photons. If a similar charge is in the vicinity it absorbs the virtual photon and is repelled.
It may be that gravity also has carrier particles similar to photons - gravitons - but a quantum field theory similar to QED has yet to be developed.
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC