How Do We Know Protons, Electrons, and Quarks really Exist?

Bill Robertson

Question:

How do we know protons, electrons, and quarks really exist?

Answer:

Scientific explanations often make use of things we cannot see or feel, such as protons, electrons, and quarks. Do these things really exist? If so, how do we know they exist?

Imagine you’re enclosed in a completely dark room with no light at all and can’t see a thing. You are chained to a chair somewhere in the room, and you have a pile of rocks at your disposal. While trying to decide whether or not you are part of a bad horror film, you decide to figure out the size of the room you’re in and its shape.

Good thing you have those rocks. You can throw the rocks in all directions. Judging from how long it takes for a rock to hit a wall in a particular direction, you can get a rough idea of how far away the walls are. If you have enough rocks and patience, you can probably get a good idea of what sort of room you’re in. If there’s a door (no doubt handy if you actually are in a horror film), you can probably tell where the door is by the different sound made when the rocks hit it.

That’s “Consistent With”. . .

Suppose it is not possible to ever turn on the lights in the room. Can you be 100 percent certain the walls are where you think they are? Was that really a door that you hit? For that matter, can you be completely certain those were walls you hit? Is it possible that the sounds were made by something else?

No, you can’t be 100 percent certain your surroundings are what you think they are, because you can never turn on the lights to see for sure. One thing you can say, though, is that all of your observations (i.e., the sounds you heard) are consistent with there being walls at varying distances away from you. You could even draw a map of your room. You could use the time of travel for the rocks thrown in each direction to estimate how far away the walls are in each direction. Abrupt changes in wall distance might be difficult to determine, so your map might be a bit fuzzy near those changes, but you still could get a general idea.

Let’s move on to atoms—those tiny little things that you’ve heard about since you were in the second grade. You’ve seen drawings of atoms. How did people decide what those drawings should look like? Part of the answer has to do with experiments similar to throwing rocks at the walls of a dark room.

In a famous experiment performed in the early twentieth century, Ernest Rutherford fired alpha particles (other tiny things you can’t see!) at gold foil and looked at the pattern the particles made after hitting the foil. This pattern indicated that the atoms of gold had a closely packed, positively charged nucleus. The reasoning behind this picture of an atom had to do with the fact that alpha particles are positively charged and would be repelled by a positively charged nucleus. Although most alpha particles went straight through the gold foil, some “bounced off” at sharp angles, even to the point of heading back in the direction from which they came. Only a closely packed, positively charged, nucleus of a gold atom could accomplish this.

This experiment, plus many others like it, plus observations of chemical reactions, nuclear reactions, and the light that substances emit, all combine to give us a particular picture of what atoms look like. Just as with that dark room in which you can’t turn on the lights, though, we cannot isolate a single atom and look directly at it to see if our drawings are correct.

The important thing is that everything we observe is consistent with our current view of atoms. The same statement goes for all sorts of other scientific models, such as quarks, electric and magnetic fields, and black holes. They’re useful models because they explain our observations and help us predict new observations.

Who Needs Reality?

What about the issue of whether or not these models are real? Are electrons real? Are atoms even real? Fortunately for us, it doesn’t matter whether or not these things are real. If you ask this question of a room full of scientists, you will find some who say these models are so well supported by evidence that they are, in fact, real. You will also find some who will say that these models are just constructs that have been invented by people. All of those scientists, however, make use of the models. Your philosophy regarding the reality of models doesn’t affect your ability to use the models to guide your scientific investigations. If you believe these models represent reality, great. If you don’t believe these models represent reality, also great. You can use the models effectively no matter what you believe.

Finally, how does this issue apply to your classroom teaching? Well, for starters it makes for a great classroom discussion. It does more than that, though. It helps put the knowledge of science into perspective. A great physics educator by the name of Arnold Arons has argued that two of the most important questions we can ask in the teaching of science are, “How do we know?” and “Why do we believe?” If you weave those questions into your science lessons, you will help students see science as much more than a collection of facts and formulas.

Now there’s something worth doing!

Bill Robertson (wrobert9@ix.netcom.com) is the author of the NSTA Press book series, Stop Faking It! Finally Understanding Science So You Can Teach It. He is a former college physics instructor who has conceptualized and written science curricula and science kits for BSCS, the Wild Goose Company, the Chicago Museum of Science and Industry, and the Exploratorium. In addition to his writing, Bill reviews science materials and conducts inservice workshops for elementary and middle school teachers across the country. Bill has an M.S. in physics and a Ph.D. in science education.

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