The idea of finding a cosnerved quantity is extrememly
useful- it can greatly simplify our calculations as well as provide some
insight into how the unvierse works.
As you might guess there are other conservation laws
beyond energy and momentum.
All of these conservation laws are consequences of the
Standard Model of particle interactions. As you can see the conservation
of energy, momentum and angular momentum are very fundamental to the study
of physics. As far as we know, these seven quantities are the only
ones that are always conserved.
Energy- Recall that along with the traditional
forms of energy (kinetic, gravitational, light, electrical, chemical, etc.)
there is also the energy associated with mass.
Momentum- This has been shown to be conserved
even on the subatomic scale.
Angular momentum- We'll study this next week
as part of this unit. Not only does it include macroscopic objects
that spin, but also the particle spin or intrinsic angular momentum found
in subatomic particles.
Electric charge- if the system is positively
charged in the beginning, it must be positively charged in the end (we
do not get to create any net charge- we can create a pair of particles,
one negatively charged and the other equally positively charged)
Color charge- This behaves much like electrical
charge, but instead quarks can come in three, not two varieites- red, green
or blue (don't attach too much significance to the names, they're just
Quark number- how ever many (quarks - anti-quarks)
we have in the beginning, we much have the same at the end.
Lepton number- Electrons are the most common
lepton, but there are also muons and taus which are very similar to electrons,
but are more massive. Also, there are small, nearly massless particles
know as neutrinos within this family of particles. This counts not
only electrons in the interaction, but also positrons (anti-electrons)
and electron neutrinos (and anti-electron neutrinos).
Like most "laws" in science, we can never really know
if they are true, only if they are proven to be incorrect. Perhaps
there is a very rare process that we have not yet observed that may not
respect these conservation laws. If such processes are found, we will have
to downgrade at least one of these laws to an approximate conservation
For example, it was only a few years ago that we
learned that the various neutrinos have mass. Because of that we
now know that the electron, muon or tau numbers don't have to be conserved
individually, rather the total lepton number is conserved.
Similarly, some quantities are conserved in all but
a few unusually situations. Some of these are strangeness, topness
and bottomness. (The names come from the various flavors of quarks.)
There is a page within our web site that describes
more about the four fundamental forces
and some of the particles that are related to these forces.
Places on the internet ot learn more about
these new conservation laws:
Science of matter, space and time at FermiLab. This is a great
place to start your tour of "particle physics". It offers a qualitative
overview of what the world is made of and what the standard model
is. They even have a short video
that gives an overview of the standard model.
Adventure- This is a great site that explains the physics behind quarks,
electrons and other fundamental particles. (A bit more technical
than the previous site.)
Stanford Linear Accelearator Center (SLAC) Virtual
Visitor Center- This site not only has information on the fundamental
physics, it also describes the methods that physicists use to study the
physics of small particles.