Atoms - the inside story. Forces of nature.

6. The Standard Model

page 23

Animation showing repulsive force from exchange of a ball

Picture 6.1 The ice skaters can push each other apart by throwing a ball between them. The juggler on the left is pushed back when he throws it. The juggler on the right is pushed back when he catches it.

	Force of nature

		We currently think of four main forces of nature: electromagnetism weak force strong nuclear force gravity (not a part of The Standard Model) The first two have been unified to make the electroweak force and gravity is not strictly a part of The Standard Model.

	Where do forces come from?

		Physicists think that all forces are caused by the exchange of particles. Imagine two jugglers playing on a frozen lake. When they start throwing their batons at each other, they will be pushed apart. The batons carry momentum from one juggler to the other. This momentum pushes each juggler away from the other across the slippery ice.

Animation showing repulsive force from exchange of a virtual photon

Picture 6.2 Two electrons push each other apart by exchanging virtual photons.

	The electromagnetic force

		The electromagnetic force is carried by particles called virtual photons. They are passed between the charged particles like the batons between the jugglers. Two negatively charged electrons will exchange these virtual photons and push each other apart. Notice that they are sending virtual photons out in every direction. The intensity of the virtual photons gets smaller with distance, because they are spreading out. This is why the force gets weaker with distance. It is even possible to explain why the force decreases with the square of distance. The electromagnetic force is described by the theory of quantum electrodynamics (QED), which was developed by Richard Feynman. He developed a graphical method of showing the forces – now known as Feynman diagrams (see page 24). All the forces of nature are carried by virtual particles. Virtual particles are particles that cannot be observed in their own right. They come into existence by borrowing energy from the particles on which they act.

	Strong nuclear force

		We have seen that the strong nuclear force was proposed to explain how a nucleus binds. The electrostatic repulsion between the positively charged protons is enormous within the tiny nucleus of an atom. In order to overcome this repulsion, there has to be a stronger, attractive force. This is the strong nuclear force. We know it holds protons and neutrons together in the nucleus and it actually acts between all hadrons. More fundamentally, the stong nuclear force is a force between quarks - hence its acting on all hadrons, which are made up of quarks. It has no effect on leptons (including the electron). To explain what is happening in the nucleus, we have to think about the properties of quarks. Quarks have an electric charge and a colour charge (The quarks don't actually have a colour, it is just to distinguish this property from electric charge). The strong nuclear force is a force between these colour-charged particles and is extremely strong – hence the name. The theory to describe it is quantum chromodynamics (see page 24). The particle that carries the force is called the gluon. (The physicists who labelled this particle decided against a poetic Greek name!) The quarks exchange gluons to produce the forces between them.


neutron	to	proton

Picture 6.3 Beta decay of a neutron.

	Weak force

		The weak force is the interaction that explains beta decay – the decay of a neutron into a proton, an electron and an anti-neutrino. Beta decay is fundamentally different from alpha decay. An alpha particle is made of two protons and two neutrons. These particles were already in the nucleus. However, in beta decay, a fast moving electron is fired out of the nucleus. The nucleus did not originally contain any electrons so there must have been a change within the nucleus to produce an electron. On one level, we can think of the change as being a neutron turning into a proton plus an electron (picture 6.3). However, we now know that neutrons and protons, like all hadrons, are made up of quarks (see page 17). Beta decay happens when one of the down quarks in a neutron changes into an up quark, making it a proton. The flavour of one of the down quarks changed. This change of flavour represents one fundamental particle changing into a different fundamental particle. This is different from any other change that we know. We can explain alpha decay and chemical reactions by the movement of particles rather than a fundamental change in a particle.

	A new force

		When a down quark changes into an up quark and an electron, there must be a force that acts on all of the particles involved. It cannot be the strong nuclear force because this has no effect on electrons. Neither, as physicists know, can it be the electromagnetic force. In order to explain it, we need to identify a new force called the weak force. The weak force is very short range and, as the name implies, it is not at all strong. Its effects are felt by all fundamental particles – quarks and leptons – and it joins gravity, the strong force and the electromagnetic force in our the list of known forces.

. . . joins an old force

In the 1960s Sheldon Lee Glashow, Abdus Salam and Steven Weinberg were independantly working on some mathematical problems with the weak force. They solved the problems by combining the weak force with the electromagnetic force and produced the theory of the electroweak force. This is an example of unifying a force (see page 22).

There are three exchange particles that carry the weak force, they are the Z⁰ (zed-zero ), W⁺ (double-you-plus and W^- (double-you-minus). By now physicists could not think of any more inventive names and stuck with the letters.

Table 12 shows the forces of nature.

Force	Particles it affects	Exchange particle
Electromagnetic	anything with charge	virtual photon
Weak	all fundamental particles	W⁺, W^-, Z⁰
Strong nuclear force (SNF)	quarks	gluon
(Gravity)	anything with mass	(graviton)

Table 12 The four forces of nature. Gravity is not described by The Standard Model. The darker square is yet to be discovered.

	Gravity

		Gravity is not a part of The Standard Model, which essentially deals with the tiny fundamental particles. The current theory of gravity is the General Theory of Relativity. Gravity is another inverse square force, therefore physicists think that there is an exchange particle that causes it. This particle would be radiated by anything with mass and its intensity would decrease with the square of distance. Physicists have already named this particle the graviton but have not found one yet.