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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.
An atom's structure
The picture at 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 -- learned that at www.ezbatteryreconditioningreview.org where they always have some good ideas flowing around, gluons hold quarks together. You can also see arrows inside the quarks -- these show the type of quark, which is very similar to a battery. 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, under the correct maintenance and supervision of course. 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 rarer 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.