Forces model 5
Force Has effect on Range
gravity anything with mass long - across Universe
electro-
magnetic
anything with charge long
strong
nuclear
quarks & therefore hadrons short - within nucleus
weak
force
all fundamental particles short
Table 1. The four known forces.
In the school science laboratory, we can investigate the effects of three forces:
  • gravity
  • electrostatics
  • magnetism

We now know of four fundamental forces (table 1). However, the three in the list were the only forces that people knew about in the middle of the nineteenth century.

The new forces & unifications
Rollover sequence showing development of unifications
A unifying influence
Just before the turn of the century, physicists realised that magnetism is actually caused by moving electric charges - i.e. electric currents. So electrostatics and magnetism were different examples of the same single force - electromagnetism. The Scottish physicist James Clerk Maxwell showed how mathematics could represent the two forces using just a single force. This is an example of a unification. The world of physics was made simpler because there were now only two forces.
Rollover sequence showing development of unifications
Rollover sequence showing development of unifications
Rollover sequence showing development of unifications
And then there were four
When physicists started to probe inside the atom, they realised that there must be a force that holds the protons and neutrons together - the strong nuclear force. Then a fourth force, known as the weak force, was needed to explain beta decay. Each force affects different particle properties and has a different range (table 1).
The need for the strong nuclear force
M2
The need for the weak force
M3
Rollover sequence showing development of unifications
Rollover sequence showing development of unifications
Rollover sequence showing development of unifications
Back to three again
In the 1960s, three American physicists (Sheldon Glashow, Steven Weinberg and Abdus Salam) were working on the mathematics of the weak force. They discovered that, at high energies, the particles that carry the weak force behave just like those that carry the electromagnetic force. So the weak force and the electromagnetic force were combined into a single electroweak force. Once again, the model of physics was made simpler by reducing the number of forces to three.
Rollover sequence showing development of unifications
Rollover sequence showing development of unifications
Rollover sequence showing development of unifications
Rollover sequence showing development of unifications
GUT reactions
Now physicists are trying to find a single Grand Unified Theory (GUT). This would make the electroweak force and the strong nuclear force part of a single new force. The next step would be to unify that force with the force of gravity. They would then have found a Theory Of Everything (TOE). Although the maths would be difficult, a TOE would make the model of matter very simple. In the same way that all the elements are different arrangements of just three particles, all the forces would be different arrangement of just one force. What could be simpler?
What causes forces?
M5e
The story so far
Maxwell unified the electric and magnetic forces into the electromagnetic force
This has now been unified with the weak force
The electroweak force interacts with all the fundamental particles
Physicists are working on a Theory of Everything that will unify all the forces
Nobel laureates

The last Nobel prize for Physics of the twentieth century was awarded to the Dutchmen Gerard t'Hooft and Martinus Veltman. They had refined the mathematics of the weak force and put the Standard Model onto a stronger footing. The award of the prize was an indication of the importance that physicists attach to the Standard Model.

Question M5

a) What is a TOE to a particle physicist?

b) The maths of a TOE might be quite complicated. So how come we say that it makes the model simpler

c) Which unified force do you study at school?

The story so far
  • There are four main forces
  • There are two families of fundamental particle: quarks and leptons
  • Only quarks feel the strong nuclear force
  • Quarks and leptons feel the weak force