Everything we can see around us, including our own bodies, is made
of atoms. They're the things that combine together to make
molecules, which in turn make up everything from tables to
turkeys.
The ancient Greeks invented the term "atom" to mean
something that is as small as possible, and can't be broken down
even further. But, as modern physicists have shown, there is
something even smaller than the atom. In fact, there are lots of
things. These are called subatomic particles. (In fact, subatomic
means "smaller than the atom.")
Each atom has an inner structure made of many smaller
particles, and some of those particles have an inner structure of
even smaller particles. The differences among the inner structures
of atoms cause the differences between elements like hydrogen,
gold, neon and lead.
Let's take a look inside a
typical atom.
The picture at top left may look familiar -- it's the way atoms
are often depicted. The blue lines represent particles called
electrons, which orbit the yellow center, called the nucleus (the
plural of nucleus is nuclei). The electrons aren't important to
RHIC, so let's give attention to the yellow nucleus.
In the center of the picture, you can see a magnified nucleus.
And, you can see that a nucleus has many things inside it! In
general, the particles inside the nucleus are called nucleons. But
each kind of nucleon has its own name. The red circles represent
protons, and the blue circles are neutrons.
There are even smaller particles inside the protons and
neutrons; the green circles are quarks, while the yellow squiggles
represent particles called gluons. Just like Elmer's glue holds
paper together, gluons hold quarks together. You can also see
arrows inside the quarks -- these show the type of quark. Protons
always have two "up" quarks and one "down"
quark, while neutrons have two "down" quarks and an
"up" quark.
This atom has nine protons, nine neutrons, and nine electrons.
But atoms can have many different combinations of particles. A
hydrogen atom, for example, has just one proton and one electron.
A typical gold atom has 79 protons, 79 electrons, and 118
neutrons. That's a heavy atom!
Now, at RHIC, physicists use only the nuclei of atoms -- they
remove the electrons. Whenever an atom has fewer electrons than
protons, it's called an ion. RHIC utilizes ions of gold.
How small are atoms and subatomic particles? If you tried to
measure them in inches or centimeters with a ruler, you'd have a
lot of zeros to deal with! For example, a typical atom is
0.000000001 meters across -- that's one billionth of a meter!
So, instead of getting mixed up with all those zeros, let's use
comparisons to see how incredibly small these things are.
Let's start by imagining
an enlarged atom, magnifying it millions of times until it fills
the distance from the Earth to the moon. That's a massive atom --
10,000,000,000 inches across!
Now, how
wide would the nucleus be on this scale? About 10,000 inches, the
length of a golf course. So, how big would a proton be? You
guessed it -- about as big as a football field (1,000 inches). In
measuring the size of a proton in an earth-sized atom, we've gone
from the distance between the Earth and moon, down to one football
field. And at this
scale, a quark would be about the size of a mere golf ball
(approximately one inch wide).
It's pretty incredible,
isn't it? If a quark is that small when an atom is enlarged millions
of times, imagine how small it is in reality. For the record, a
quark actually measures 0.000000000000000001 meters.
So now you have an idea
how small the collisions at RHIC are. And what a difficult task it
is to cause them to successfully collide -- and examine the products
of those collisions, which are just as small.
Everyone knows that ice is
frozen water, and that steam is water vapor. To
put it another way: ice, water and steam are three different forms
of the same thing. We call those three forms solid, liquid and gas.
And we know that one form can turn into another form, if the
conditions are right.
For example,
an ice cube will melt if we leave it on the counter at room
temperature. Or, a pot of water will boil and give off steam if we
put it on a hot stove. Or, steam from a hot shower will condense
back into water droplets when it hits a cold bathroom wall.
But did you
know that there's a scientific name for what happens when ice turns
to water, or water turns to steam? It's "phase transition"
-- the process through which one form of matter turns to another
form of matter. It happens when conditions like temperature and
pressure change just enough to cause a change in the way the atoms
interact.
Here's an
illustration of the phase transitions for water. Of course, just as
you can go from ice to water to steam by adding more and more heat,
you can also go in reverse, by taking away heat.
Water isn't the only
thing that goes through phase transitions -- everything can, given
the right conditions. In fact, RHIC is designed to create another
kind of phase transition -- one that's much more rare than melting
ice or boiling water.
The phase transition that
physicists want to create at RHIC is something like melting. But
instead of ice, the melting will happen to atoms. RHIC will create
extremely high temperatures and pressures by colliding atomic nuclei
together at high speeds.
When they hit, the nuclei
may create just the right conditions for quark-gluon plasma to form.
This plasma will consist of "melted" protons and neutrons,
the particles that make up the center of atoms. If the protons and
neutrons melt, they'll release the quarks and gluons inside
themselves. The quarks and gluons will be able to flow freely for
just an instant -- almost like flowing water.
This phase transition
from normal, everyday matter to quark-gluon plasma is just the
opposite of what scientists believe occurred immediately after the
Big Bang. Just like with ice that melts and then freezes again, this
phase transition can occur in both directions.
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