As she listened to her father, Saeko sat with her chin propped in her hand. He prodded her elbow.
“Take how you’re leaning your elbow against the table. Why do you suppose your elbow doesn’t travel right through it?”
“What do you mean? That’s just how things are,” Saeko blurted, and then immediately regretted her response. She had walked right into her father’s trap. “Wait, no. Um, let me see …” Saeko wracked her brain, hemming and hawing. “Because it’s made up of matter,” she finally concluded, knocking on the tabletop with her fist for emphasis.
“Because it’s made up of matter? But Saeko, the fact that objects exist is actually much more difficult to explain. The real mystery is, why does the universe have any structure at all? And yet you take physical objects completely for granted.”
As he zeroed in on the point he wanted to make, Saeko’s father’s eyes sparkled. She had always enjoyed watching the changes in her father’s eyes.
“The fact that objects exist is a mystery?” she asked back.
Saeko still didn’t see what her father was getting at. He sensed her confusion and broke the idea down into simpler terms.
“The elements that make up this table and the elements that make up your body are different. Right now, there are 111 elements that we know of. How do we classify them? Basically, we distinguish them based on the number of protons and neutrons that make up their core, and the number of electrons that orbit that core. The element with the least mass is hydrogen, with one proton and one electron. The heaviest element is uranium, with 92 protons, 146 neutrons, and 92 electrons. I’ve already told you that the number of protons and electrons is always the same. Each proton is made up of two up quarks and one down quark, and each neutron has two down quarks and one up quark. You said that your elbow doesn’t go through the table because it’s made of matter, right? When you said that, what sort of configuration of electrons did you imagine?”
Until her father had posed the question, Saeko had never imagined how anything’s electrons were configured. After all, she’d never before thought to question why a part of her body didn’t pass through solid matter.
“You were probably envisioning something like this, weren’t you? The electrons orbiting the nucleus of the atom, forming a sort of sphere. A ball, if you will. And these balls are all packed together to make up a three-dimensional object. So let’s say the table is made up of black balls, and your elbow is made up of white balls. Both types of balls are packed together so tightly, there’s no way one could pass through the other.”
Saeko nodded decisively. It wasn’t exactly what she had imagined, but it wasn’t very far off. It was a pretty good description of how she conceived of matter.
“But the reality of the situation is totally different. If we had a microscope that could enlarge an atom to the size of a baseball, you’d be surprised by what it looks like. There wouldn’t be much to see. Basically, matter is made up of a whole lot of nothing.”
“Nothing?”
“Empty space.”
Saeko’s father opened the rice-seasoning container on the table and extracted a single sesame seed, holding it up on his index finger for Saeko to see.
“Say the nucleus of our atom was the size of this sesame seed — about a millimeter in diameter. The electrons would orbit it at a distance of fifty meters, and they would be so tiny as to be invisible to the naked eye.”
From where Saeko’s father was sitting, the living room wall was easily ten meters away. But the electrons’ orbit would be much further away.
“That’s all there is?” Saeko asked.
Her father nodded, grinning. “That’s it. Nothing more.”
If the sesame seed on her father’s finger was the nucleus, and the electrons were orbiting it at a distance of fifty meters, the atom really was mostly empty space.
If the outer shell of the atom were large enough to hold our whole apartment, there would be a nucleus in the middle the size of a sesame seed and nothing else!
As she came to terms with that concrete image, Saeko was seized by a wave of fear. Here she sat at the dining table, but if the atoms that made up the chair she was sitting on were mostly empty space, what was keeping her from falling through the chair and the floor, and straight through the earth’s crust?
Finally understanding her father’s question, a doubt resonated deep in Saeko’s mind. “Why, then? If matter is mostly nothing, why don’t we fall? Why don’t we pass through things?”
“Who knows? Maybe if everything shifted just a millimeter, we’d be in an entirely different universe,” her father teased. “But don’t worry. At the moment, the chances that you’re going to fall through the table and into an endless void are … null. But why? If matter is mostly empty space, why can’t objects pass through each other? Now, I want you to think about it and come up with the answer on your own. Why doesn’t one type of matter slip right through another?”
Until that day, whenever Saeko had watched scenes where people walked through walls on TV or in movies, she’d assumed they were ghosts. But now that she understood more about the structure of matter, she found herself wondering about things from a different angle. It seemed to make more sense for people to be able to pass through matter, and the real mystery was the fact that they didn’t.
Saeko had spent the entire rest of the day pondering the question her father had posed. None of the books she picked up offered an answer; they didn’t even go so far as to ask the question. She would have to come up with the answer for herself.
First, she reviewed what she knew about the structure of atoms in her mind. Compared to protons and neutrons, electrons were so tiny as to be almost irrelevant, and the mass of the atom was computed solely based on the protons and neutrons. The electrons whizzed wildly and unpredictably around the nucleus. If their orbits were neatly contained the way an eggshell contained an egg, they would be easier to picture. But that wasn’t how it worked. And yet, no electron ever encroached on another electron’s territory.
The first idea that popped into Saeko’s mind was that of a force field. In science fiction movies set in outer space, sometimes the heroes had an invisible force field around their space ships that blocked the attack beams of enemy ships. Perhaps there was a sort of force field that prevented electrons from entering each other’s shells. An invisible force field just like in sci-fi movies.
When Saeko ran the idea by her father at dinner that night, his response was encouraging. “A force field, hmm? You’re on the right track. But what do you think creates that force field? I’ll give you a hint: think about the four interactive forces of the natural world.”
Now she was getting somewhere. Saeko looked up the forces that governed sub-atomic particles. With a more specific area of focus, it was easy to find what she was looking for. Almost any physics book contained information about the four interactive forces of the natural world: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. Saeko wasn’t entirely clear on the differences between the four, but she understood that electromagnetism, the strong nuclear force, and the weak nuclear force functioned on an atomic and quantum level, while gravity applied to large expanses of space, like the solar system and the universe. In other words, the former three forces were the ones that were relevant to the question at hand.
As she continued to read, she came across a passage that stated, “The strong nuclear force binds together protons and neutrons and causes a powerful electric force between the nucleus and the electrons.” That’s it! Saeko was jubilant. “While the atom itself carries no electric charge, it contains strong electric fields and charges within. These electric fields and charges are what give structure to matter.”