Re: People say that matter is neither created or destroyed. Explain.

What you're asking about is a classical law of physics, called the 
"conservation of matter".  It has a law partner, called the "conservation 
of energy".

I'll explain the classical laws first, then touch on Einstein's refinement 
to these laws, and then discuss the implications of nuclear physics.

The classic view on conservation of matter is this: all matter is composed 
of atoms of the various elements, and the total number of atoms in the 
universe remains constant.  That is, you can rearrange the existing atoms 
in any way you like, but you cannot cause additional atoms to materialize 
nor can you cause existing atoms to disappear.  

This has the practical implication that for any kind of physical process 
you can conceive the total mass (weight) of the ingredients will equal the 
total mass of the products.  Now, you may have to be very careful in 
accounting for where the ingredients are coming from and the products are 
going to, but if you are careful, the mass you start with equals the mass 
you end with.  

For example, if you burn a piece of wood, you need to measure not only the 
mass of the woor but how much oxygen from the air was used; in terms of 
product, you have ash, as well as smoke and various gasses (carbon dioxide, 
carbon monoxide, and various hyrdocarbons).   Measuring this carefully may 
be a bit tricky, but if you're careful, you will see a balance between 
ingredients and products.

Another approach is to burn this wood inside a closed container with some 
oxygen supply inside the container; the mass of the closed container and 
contents is the same before the wood burns as after.  

The point is that all of the oxygen atoms and various atoms of the wood 
have been simply rearranged, but they all still exist somewhere.  This is 
the classic law of conservation of matter.

The classic law of conservation of energy is similar, but a bit harder to 
visualize.  It simply says that the amount of energy (whatever *that* is!) 
in the universe remains constant; it may take on different aspect, such as 
kinetic energy (the energy of motion), potential energy (energy stored by 
gravitational attraction or the electrical attraction between atoms), 
electromagnetic energy (e.g., electricity, radio waves, light, etc.), or 
heat.

The bottom line says that no matter what you do, the energy you obtain by 
any conceivable process equals the energy you put in plus whatever stored 
energy you extract.  You can neither increase nor decrease the total sum of 
energy in the universe; all you can do is change energy from one form to 
another.  For example, falling water over a dam converts the gravitational 
potential energy of the water to kinetic energy, which can be imparted to a 
turbine turning a generator providing electric power.  Some of the water's 
potential energy becomes electricity, the other becomes heat lost by 
friction in the mechancial equipment and inefficiencies in the electrical 
equipment, and even to the viscosity and friction of the water itself!

These are powerful and extremely practical ideas for scientific 
understanding the physical world.  However, they are not quite correct. 

Einstein showed that a consequence of his special theory of relativity is 
that matter and energy are coupled, and that one can indeed create or 
destroy matter (or energy) PROVIDED an appropriate exchange is made between 
matter and energy.  That is, energy can be converted to matter, subtracting 
from the universe's store of energy and adding to the total number of 
atoms.  Conversely, atoms can be destroyed, subtracting from the total 
count but thereby adding to the universe's store of energy.

In other words, what Einstein discovered is that the universe's total 
matter plus energy is constant -- you can convert between matter to energy 
and energy to matter, much as one can convert U.S. dollars to Deutschmarks 
and back again, provided the proper conversion rates are used.  Thus, 
Einstein's contribution was to combine the two independent laws of 
conservation into a single law of mass-energy conservation.

The conversion factor between energy and matter is Einstein's famous

E = mc**2

This tells us how many joules of energy are produced for every kilogram of 
matter converted to energy, or how many kilograms of matter are produced 
for every joule of energy converted (c is the speed of light, which is 
300,000,000 meters per second)

This means that the mass of an object increases as its potential energy 
increases.  Conversely, as we extract potential energy, the mass of the 
products is less than the mass of the ingredients (that is, when we burn 
the wood, the mass of the ashes and gasses is slightly less than the mass 
of the wood and oxygen we started with).  However, the increase or decrease 
is so small it almsost always negligible for all practical purposes, except 
one.  For example, my watch weighs a bit more after I wind it.  But the 
increase in weight is about one millionth of one trillionth of a gram.  
That's pretty close to zero, for most practical intents and purposes.

Except where nuclear physics comes in.

Inside the atom are forces holding the nucleus together, called, without 
too much imagination, the strong nuclear force and the weak nuclear force. 
This force represents a potential energy within the atom. When a 
radioactive atom decays, it spontaneously breaks up into two or more 
smaller atoms which have less mass in total than the parent atom; the 
missing mass has been converted to energy (kinetic energy of speeding 
particles plus the electromagnetic energy of photons).  The missing mass 
equals (via E= mc**2) the potential energy of the nuclear forces that has 
been liberated; the liberated nuclear potential energy equals the kinetic 
and electromagnetic energy.  

We can stimulte this decay process in a nuclear fission reactor by 
bombarding certain unstable atoms (types or uranium or plutonium, for 
example) with neutrons, causing them break up and convert some of their 
nuclear potential energy to heat and light.  Or energy can be liberated 
through a fusion process, where certain simple atoms such as hyrdrogen are 
combined to form helium.  In either case if you add up the mass you start 
with, it will equal the total of mass plus energy you wind up with.  For 
example, the Sun and all other stars use nuclear fusion to generate their 
heat and light.  These basic fission and fusion processes are also the 
basis of nuclear weapons.
  
To give you a another example of how immense this conversion factor between 
energy and matter is, consider this: a huge nuclear power plant might 
produce 1 GW (gigawatt) of electricity, enough to power a medium size city. 
 Over the course of a year, producing this much energy "burns" (converts 
from matter to energy) about 350 grams (3/4 of a pound) of nuclear fuel.

By comparison, a coal-fired power plant would use hundreds, if not 
thousands of railroad carloads of coal.  

But if we were to carefully measure all the coal ash, and all the gasses 
produced during the burning, and compare it to the hundreds of thousands of 
tons of coal and oxygen fed to the boilers, we would also find ourselves 
350 grams short at the end!
 
Steve Czarnecki