The Standard Model and Ordinary Matter
So, we used to think that ordinary, atomic matter dominated the mass inventory
of the universe. Even then, we knew that other forms of matter were
possible. A zoo of other particles have been created in the lab and
have been produced by cosmic rays. You may have heard of some of them:
mesons, positrons, muons...
Ordinary matter is almost the only stable known form of matter
So, how do those other particles relate to ordinary atomic matter, and
why don't they contribute to the mass of the universe?
First, the easy answers:
Of all the possible types of particles from the zoo (the zoo is made
of members and combinations of members of the "Standard Model"), the
only stable particles (stable meaning they don't decay very rapidly)
- Ordinary atomic matter components:
- protons: They have never been observed to decay.
- neutrons: They are stable -- only if they are bound in a nucleus.
A free neutron will decay.
- electrons: Electrons have not yet been observed to decay.
- neutrinos: There are many neutrinos in the cosmos. But, like the electrons,
they are very light and probably don't contribute much to the mass inventory
of the universe. They are important, however, in stellar nuclear reactions.
So, to summarize: The combinations of particles which form ordinary atomic
matter happen to be the only particles (except for the ghostly neutrinos)
which are long-lived enough to be important in considerations of the \
cosmological mass scales.
But, what of the others and what are their relationships to ordinary atomic
The Standard Model
Let's see if we can cover the entire Standard model as simply and succintly
as possible. According to the Standard Model, there are two types of things
(besides empty space, which may not be really 'empty'):
Let's first talk about the force carriers. An example of a force carrier
is the photon, which is a packet of light. It actually carries the
electromagnetic force. There are others which correspond to the other forces:
- fundamental particles
- force carriers
- gluons: carry the strong force which holds protons and neutrons together
and keeps them bound to each other in the nucleus
- 'W' and 'Z' bosons which are involved in the 'weak force' which is
associated with forcing some particles to decay and change form.