John D. Reid's Page on The Standard Model

John D. Reid

Research Interests

Standard Model

Fig 1. Two electrons interacting by exchanging a photon.

A goal of physics is to describe and understand natural phenomena in the most fundamental terms. The current theories that are used are collectively called the standard model. These theories do a good job of describing all natural phenomena in terms of 4 fundamental forces between fundamental particles. Fundamental interactions means that any interaction observed can be reduced to one of the fundamental forces.

For example, suppose a biological interaction involved an interaction between two cells. (Some biological interactions may, of course, involve other interactions, such as interactions betwen cells and light.) The interaction between the two cells could consist a chemical interaction. A chemical interaction is ultimately the interaction between the electric charges associated with atoms and molecules. Thus the fundamental force of interaction is the electric force. In this example, no further reduction is possible.

The process of finding out what the most fundamental interactions are is a continuous one. For example, Isaac Newton observed nearby objects falling straight down to the ground (for example, an apple falling from a tree). He also observed the moon appearing in the sky and taking about 28 days to move completey around the Earth. It is by no means obvious, but he was able to develop a theory, called the theory of gravity, that described both observations as being due to a fundamental interaction, that is, the interaction between two objects with mass that causes them to attract. Thus his theory simplified two apparently different phenomena into one fundamental interaction, called gravity.

At this stage in physics, it seems that we are able to describe all natural phenomena as being the result of four basic interactions, or forces. They are listed in table 1.

Table 1. Four Fundamental Forces

Force Strength Compared to Gravity* Examples

Gravity 1 Apple falling. Planets orbiting.

Weak Force 10³⁷** Some forms of radioactive decay.
Some thermonuclear reactions taking place in the Sun.

Electricity & Magnetism 10⁴¹ Electricity in house.
Lightning.
Molecular bonds.

Stong Force 10⁴³ This force binds things called quarks together. Quarks are what neutrons and protons are made of. This force also has the affect of binding the neutrons and protons together inside an atomic nucleus. (This is no small feat given that the protons inside the nucleus repel each other due to the electric force.) Some of this binding energy from the strong force is released in nuclear reactors.

Table 1. Four Fundamental Forces
Force	Strength Compared to Gravity*	Examples
Gravity	1	Apple falling. Planets orbiting.
Weak Force	10³⁷**	Some forms of radioactive decay. Some thermonuclear reactions taking place in the Sun.
Electricity & Magnetism	10⁴¹	Electricity in house. Lightning. Molecular bonds.
Stong Force	10⁴³	This force binds things called quarks together. Quarks are what neutrons and protons are made of. This force also has the affect of binding the neutrons and protons together inside an atomic nucleus. (This is no small feat given that the protons inside the nucleus repel each other due to the electric force.) Some of this binding energy from the strong force is released in nuclear reactors.

* Comparison of the strength of each force between two protons separated distance of 10^-15meters
** This is scientific notation. For example, 10⁶ is equal to a 1 with 6 zeros after it, or, 1,000,000.
Another example, 10^-2 is equal to 1/10², or 1/100.

Table 1 lists the way in which things interacts. Table 2 lists the most fundamental objects known. That is, these objects do not consist of any smaller objects, as far as we know. Furthermore, all objects we observe (from stars to people to cells, atoms, etc) are made up of the constituents listed listed in table 2.

Table 2.
Fundamental Constituents of Matter

PARTICLES
Move cursor over particle to see more information.

quarks leptons

u c t e μ τ

d s b ν_e ν_μ ν_τ

For each particle above there also exists an anti-particle.

EXCHANGE PARTICLES

particle interaction

graviton gravity

W⁺,W^-,Z⁰ weak

γ electromagnetic

g strong

Table 2. Fundamental Constituents of Matter
PARTICLES Move cursor over particle to see more information.
quarks	leptons
u	c	t	e	μ	τ
d	s	b	ν_e	ν_μ	ν_τ
For each particle above there also exists an anti-particle.
EXCHANGE PARTICLES
particle	interaction
graviton	gravity
W⁺,W^-,Z⁰	weak
γ	electromagnetic
g	strong

These particles are described, in the standard model, as interacting by "exchanging" particles. Consider, for example, two electrons interacting. One way to think of how they interact is that each electron experiences the electric field of the other and thus experiences a force due to the other electron. In the standard model the interaction is described as the two particles "exchanging" photons (γ). In the process of the exchange, their momenta will be altered, that is, their paths are deflected. An analogy would be two people standing on roller skates and throwing a ball back and forth. Whenever one person throws the ball they will recoil. When a person catches the ball, it will make them move back as well. The result is that the two people move farther and farther away from each other while they exchange a ball, just as two electrons would move farther and farther away while exchanging photons.

Figure 1 shows a representation of this interaction. This type of figure is called a Feynman diagram. These diagrams are used to describe all kinds of fundamental particle interactions and the diagrams have a one-to-one correspondence with mathematical equations used to calculate the probability for such reactions to occur. (Feynman got the Nobel Prize in Physics, in part, due to his development of this technique.)

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