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Four Forces, The

  The ultimate aim of 20th-century physics is to show that the four forces are all different aspects of the same force.

They are gravity, electromagnetism, the weak force and the strong force. They are believed to be the only forces in Nature, although some suggestions that a fifth exists have been made. Sensitive experiments to detect this fifth force have not discovered it, although its existence has not been entirely ruled out.

The forces differ widely in every respect. The weak force and the strong force have such short ranges that they only interact when the separations involved are smaller than the width of an atom, but gravity and the electromagnetic force take effect at intergalactic distances.

The strength of each force is another factor by which they are distinguished. Gravity is the weakest of the four, needing a mass the size of the Earth for us to feel its effects. The weak force is the next strongest, followed by the electromagnetic and then the strong force.

We are familiar with the concept of forces diminishing with distance, but there is an exception to this rule. The strong force, acting between quarks, grows greater with distance, rather as if the objects were connected by elastic.

Under the right circumstances all forces except gravity can repel. Gravity only attracts objects together, never pushing them apart.

All forces are ‘carried’ by field particles. The force affects objects by exchange of these particles. A common conceptual difficulty is encountered when one attempts to understand how attractive forces can operate in this way.

Physical scientists believe that at the beginning of the universe, when very high temperatures existed, there was only one force and one field particle. As the universe cooled, the forces became differentiated, and ‘branched away’ from each other. Gravity separated very early, which is why it is very different to the other forces. The next to go was the strong force, followed by the weak force. This leaves us with the electromagnetic force which we see today.

Scientists hope to show that all the forces are aspects of one fundamental force by attempting to re-create the conditions at the beginning of the universe in high energy particle accelerators. Today, due to these experiments, we can say that the weak force and the electromagnetic force are indeed aspects of a single force, known as the electroweak interaction. The theory which combines the weak and the electromagnetic forces into the electroweak interaction is called the Standard Model, and was discovered in the 1960s.

An early example of the unification of forces was provided by James Maxwell (1831 - 1879). He showed that electricity and magnetism are intimately related, and combined the separate ideas about the two into electromagnetic theory, without which no electrical apparatus would be understood today. We now consider electricity and magnetism as different aspects of the electromagnetic force. JJ

See also unified field theory.



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