In the “Making Arrangements” activity, you found that some combinations of colored squares resulted in net charges of plus one while others had a net charge of zero.

In other words, you modeled the formation of protons and neutrons from quarks. Protons and neutrons are called nucleons because they form the nuclei of atoms.

As late as the nineteen sixties, protons and neutrons were thought to be fundamental particles like electrons. It was thought that, like electrons, protons and neutrons could not be divided into smaller particles.

In the nineteen sixties, physicists working at the Stanford Linear Accelerator Center in California found that electrons traveling almost the speed of light sometimes changed their direction abruptly and lost much of their energy when they collided with matter.

It was as though the electrons were encountering small, but very hard pieces of matter on their voyage through nucleons.

The verbal description of the figure on Contact Card One starts here. Have your card available so you can follow the description.

This figure shows the path of an energetic electron as it collides with a proton. It is a representation of what the Stanford scientists found.

In the center of the figure you will feel a circle outlined with a dashed line. Inside you will feel three smaller raised circles in it. This represents a cross section of a spherical proton with three smaller particles inside it.

Most of the proton, however, is empty space.

Now place your finger on the small open circle at the left side of the figure close to the left margin line. There is a horizontal line connected to it that moves to the right. Track the line until you feel it cross the boundary of a proton, come close to one of the smaller particles and change direction to exit the proton at almost a ninety-degree angle to the direction in which it entered.

The verbal description of Contact Card One ends here.

What could have made the negatively-charged electron change direction?

It was certainly attracted to the positively-charge proton, but it was traveling too fast to just attach itself to the proton, and something inside the proton made it change direction. It appears that something may have repelled it.

If so, whatever repelled it would have been negatively charged. Neither protons nor neutrons have net negative charges, but think back to what you discovered about the quarks that make up proton and neutrons?

Combinations of positively-charged “up” quarks and negatively-charged “down” quarks produced nucleons with a charge of either plus one or zero.

Could an electron have been repelled by a negatively-charged “down” quark, sending it off in another direction? This is the kind of question the Stanford scientists were asking.

You may have wondered why you used combinations of three quarks to form protons and neutrons. The answer to that question is that quarks are very different from any other type of particle you may have studied.

Quarks have properties called “flavors," but not the kind you can taste. There are six flavors, or varieties, of quarks. Just as we concentrated on one pizza ingredient, we will focus on only two varieties of quarks, “up” and “down” quarks.

Quarks also have assigned characteristic called “color," but their “colors” refer to the kind of strong forces that hold them together, not to their ability to reflect light. It takes one red, one blue, and one green quark to form a stable particle.

Quarks of the same color repel each other because they have similar forces, so only one quark of a given color is found in a stable particle.

In addition, no two quarks can form a particle because their “color” forces would not be balanced, in the same way that you cannot balance a three-legged stool on two legs.

Quark’s electrical charges are different from most charges we are familiar with. They have positive and negative charges, but the charges are fractions rather than whole numbers.

An “up” quark has a charge of positive two-thirds elementary charge and a “down” quark has a charge of negative one-third elementary charge.

The charges of protons and neutrons are the sums of their quark charges. You learned how to calculate these sums in the “Making Arrangements” activity.

Quarks are held to one another by gluons. Gluons are sometimes called the “glue that holds the world together” because gluons carry strong nuclear force.

This force is also called a “chromoelectric” force because gluons also have red, blue, or green “color.”

Gluons of all three colors must also be present to make a stable particle. This strong nuclear force acts only through very small distances, like ten to the negative twelfth centimeters, and it does not diminish when the distance between the quarks increases.

In fact, there is some evidence that it tends to increase with small increases in distance and then remain constant.

So quarks have another different characteristic. When quarks are closely packed, they behave as if they were free—almost as if they are content that others of their kind are nearby.

But any attempt to separate them is counteracted immediately by their pulling each other more and more powerfully together.

Scientists think that this strong nuclear force is the reason that quarks do not exist as free, directly observable particles in the universe today.

This strong force acts not only between quarks, but also between two or more protons, between two or more neutrons, or between neutrons and protons.

In other words, the strong nuclear force not only binds quarks into protons and neutrons, but it also holds protons and neutrons tightly together to form the nuclei of atoms.

This force does not apply to electrons that are outside the atom’s nucleus.

At the point you might be asking, “Why are we focusing on these tiny quarks in the study of the origin of the universe?”

Quarks and electrons may have been among the first stable particles formed in the early universe.

The quarks and electrons that were present in the infant universe could possibly be the same fundamental particles that make up the atoms of all matter that we can observe today.

That includes all matter in and on earth, in our solar system, in the Milky Way galaxy, and in all the galaxies of the universe.

By the way, that includes you! A one hundred sixty-five pound person is made of seven times ten to the twenty-eighth power “up” quarks, six and one half times ten to the twenty-eighth power “down” quarks, and two and a half times ten the twenty-eighth power electrons.

You are made of fundamental particles—quarks and electrons.