The natural world consists basically of particles (quarks, electrons, etc.) acted on by forces. There are four fundamental forces - gravity, electromagnetic, the strong nuclear force, and the weak nuclear force. I described these forces in two recent articles.
Physicists have long dreamed of a theory of everything (TOE), i.e., a theory that provides a unified description of all the forces in nature. In addition to providing a succinct summary of fundamental physics, a TOE might also explain why the laws of physics take the form they have.
Gravity is the force of attraction that operates between any lump of matter and all other matter in the universe. It is the force that causes stars to condense from clouds of gas and dust, and rocky planets to form by the aggregation of many small clumps of solid material. The electromagnetic force binds atoms together to form molecules and causes molecules to aggregate together to form the solid world we see. The strong and weak nuclear forces act only within the nucleus of the atom.
The strong force binds the subatomic particles (protons and neutrons) together in the nucleus. The weak nuclear force is responsible for a number of nuclear processes including certain radioactive processes.
The history of physics encourages us to think it will be possible to formulate a TOE. Thus, in 1689 Isaac Newton formulated the law of universal gravitation that explains in one sweep the fall of an apple to earth and the orbits of all the planets in our solar system around the sun.
In 1873 the Scottish physicist James Clerk Maxwell formulated a theory of electromagnetism that gave a unified explanation for electrical, magnetic and optical phenomena. And, in the late 1960s Steven Weinberg and Abdus Salam independently showed that the weak nuclear force and the electromagnetic force are simply different manifestations of an underlying electroweak force. Also, ongoing work hints at a future unification of the electroweak force and the strong nuclear force.
So far, gravity has stubbornly resisted attempts to join with the other forces into a single explanatory tent. However, the theory of superstrings offers the prospect that it can include gravity with the other forces in a single description, a TOE.
The standard model that physicists use to analyse the behaviour of fundamental particles (e.g. quarks and electrons) treats these particles as points. Mathematically, points have no size and this causes problems when analysing forces. Consider the force of electrical attraction between a positive and a negative charge. It obeys the inverse square law, i.e., halve the distance between the charges and you quadruple the force. You can see that as two dimensionless points approach each other very closely the attractive forces between the particles increases to infinity.
In the real world these infinite forces do not arise despite the fact they appear in the mathematical equations. Physicists deal with unreasonable infinities by carrying out a process known as "renormalisation" in which experimentally measured values are substituted in the equations for the theoretical infinities.
Re-normalisation works well for all the forces except gravity. This is a consequence of the uncertainty principle which states that the more knowledge you have of the position of an atomic particle the less well defined is its velocity (or its energy of motion). When two particles are very close together this largely pinpoints their position, so their velocities must therefore fluctuate widely.
Because mass and energy are related (E=MC, this means the mass of the particles fluctuates widely and in particular can sporadically get very large. This on average increases the gravitational attractive force, setting up a vicious circle and preventing the effective application of the re-normalisation process. The only solution is to transform the fundamental point sources of mass into extended objects which smears out the effect of gravitational attraction. Strings do just that.
According to the superstring theory, everything in the universe, i.e. all particles and forces, and perhaps space-time itself, consists of stupendously small vibrating and spinning strings under fantastic tension. The strings can be closed loops like rubber bands or open pieces like bits of thread. The sizes of the threads, if they exist, are fantastically small - a trillion trillion times smaller (10/24) than an atom. The strings vibrate in an analogous fashion to violin strings and emit the "sounds" of their fundamental pitches and overtones.
The "subatomic orchestra" plays a symphony whose notes, passages and movements are the various kinds of fundamental particles. The strings can split and recombine and so produce the many interactions observed by physicists who follow the behaviour of elementary particles.
ALBERT Einstein spent most of his life in a quest for a "unified field theory" that would give a single explanation for all the forces in nature. Einstein failed in that quest. Undoubtedly the most appealing feature of string theory is its unique ability to include gravity in the description of the forces of nature.
But this facility exacts a steep price in theoretical difficulty. For example the theory requires that the universe has at least 10 dimensions (but, for reasons that are not yet understood, only three space dimensions and one time dimension are apparent), and the interactions of the strings are described by the conceptually difficult mathematics of doughnuts and twisted surfaces. The theory is very difficult to calculate with and, as far as I am aware, has yet to yield testable predictions.
This is not the easiest stuff to understand. Of course we are talking about fundamental things, and I like to comfort myself in such matters with the thought expressed by St Thomas Aquinas about the difficulty of comprehending the nature of God: "The dimmest conception of the highest thing is worth more than the surest grasp of a lesser thing."
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC