9-12 > Physical Science
 Grade level: 9-12 Subject: Physical Science Duration: Two class periods
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Understanding: Uncertainty

Students will understand the following:
 1 The Heisenberg uncertainty principle, formulated by the German scientist Werner Heisenberg, states that in the world of subatomic particles, the very act of observing alters the reality being observed, and therefore, in that world of subatomic particles, one can never measure all properties exactly. 2 The “uncertainty” in the uncertainty principle cannot be done away with by better observation techniques; rather, it is part of the nature of reality itself. 3 The uncertainty principle does not apply to the world of ordinary objects, since in that world, the effect of observation on the reality observed is so small as to be negligible.

For this lesson, you will need:
 • Research materials on Heisenberg and the uncertainty principle • Computer with Internet access • Materials students may need to create models

 1 Share the following background information with your students: In 1927, the young German physicist Werner Heisenberg was working at the Danish physicist Niels Bohr’s research institute in Copenhagen, Denmark. The two scientists worked together on theoretical investigations into quantum theory and the nature of physics. When Bohr was away on vacation, Heisenberg had an insight into the limits of physical knowledge:The act of observing alters the reality being observed. 2 Go on to explain that Heisenberg was thinking about reality at the subatomic level. His reasoning went like this: To measure the properties (position and momentum) of a particle such as an electron, one must use light, or radiation, as a measuring device. But the energy in the radiation affects the electron being observed. If you adjust the light beam to accurately measure position, the energy of the light beam will change the momentum of the electron; if you adjust the light beam to measure the momentum of the electron, the energy of the light beam will move the electron, throwing off its position. 3 To simplify further, explain that to determine the position of an electron, we shine light on it. The light strikes the electron and causes it to move a very tiny amount. This means that the “image” of the electron (if we could see it) is slightly blurred, like a photograph in which a person moved while the picture was being taken. We can defineprobablelocations for the electron but cannot locate it exactly. 4 Clarify one more concept before moving on: Uncertainty is not due to any fault on the part of the observer; rather, it is part of the nature of reality. 5 Let students know that the Heisenberg uncertainty principle applies only to the subatomic world, not to the ordinary world of macroscopic objects. It is only in the quantum mechanical world that in order to make a measurement, one must disturb the system. In other words, in order for us to know something is there, we must bump into it; in order to locate an electron, the electron must encounter a photon. In the ordinary world, this does not hold true. 6 Once students have grasped the foregoing simplified explanation of Heisenberg’s uncertainty principle, divide your class into groups. Challenge group members to design a model that would help others understand the uncertainty principle. Here are three examples of the sorts of models students might come up with: One person acts as a particle, such as an electron, circling in its orbit around the nucleus of an atom. Another person acts as a photon, bumping into the “electron,” thus moving it as the two encounter each other. A Ping-Pong ball represents an electron, and a hair dryer represents a beam of light. When the “beam” (i.e., the stream of air) encounters the “electron,” it causes it to move. A Ping-Pong ball in an open shoe box lined with cotton represents an electron. The only way a blindfolded person can locate the ball in the box is by dropping paper clips into the box and listening for the difference in sound when a paper clip hits the ball or hits the cotton-lined bottom of the box. However, when the paper clip hits the ball, it causes the ball to move. 7 Students in each group should accompany their model with a written explanation of what each component in the model represents and how the model helps to explain Heisenberg’s principle.

 Give students the simplest possible explanation of Heisenberg’s uncertainty principle. Rather than asking them to come up with their own ideas for ways to model the principle, illustrate the principle for them by one of the example models given.

 1 Philosophers of science (physicists) now consider several realities when explaining the nature of the universe. Give examples of how other major philosophies (such as political, religious, economic) also recognize alternate realities. 2 What is meant by the phrase, “collapse of the wave function,” when physicists are trying to decide whether or not to think of an electron as a wave or as a particle? 3 “Schroedinger’s Cat” is a scientific parable explaining how physicists create reality to explain outcomes. Discuss the probabilistic nature of a wave, the certainty of a particle, and Heisenberg’s Uncertainty Principle in the context of this parable. 4 Near the end of “Understanding: Uncertainty,” the narrator asks you, the viewer, who may be taking a hard look at this program, if you feel annoyed, stupid, or have a sense of being on the edge of a new understanding. Explain how you feel after viewing this program.

 Students’ models and explanations should reflect their own grasp of the Heisenberg uncertainty principle.

 Take a Cyberspace Field Trip into the Nucleus of an Atom “The Particle Adventure” is an award-winning multimedia interactive Web site that will take your students on a tour of the inner workings of the atom. Your students will explore the particles that exist within the atom and the nucleus, and the forces that hold them there. Frequent quizzes check your students’ understanding of what they have toured. Particle Adventure may be found atadventure. Schroedinger’s Cat “Schroedinger’s Cat” is a scientific parable explaining how physicists create reality to explain outcomes. Have students do research to find and understand the parable. Then invite them to discuss the probabilistic nature of a wave, the certainty of a particle, and Heisenberg’s uncertainty principle in the context of the parable.

 A Journey into the Mysteries of Relativity and Quantum Mechanics Alphonse J. Sistino. Orland Park, Illinois: Beaudoin, 1997This book captures the essences of ancient, classical, and modern physics, including quantum mechanics and its development, in an exploratory narrative. Order, Chaos, Order: The Transition from Classical to Quantum Phillip Stehle. New York: Oxford University Press, 1994Discover the history of quantum theory in this fascinating book.

 A Perspective on Interconnectedness from Quantum Mechanical Physicist and Philosopher, David Bohm (1917-1992) “Understanding: Uncertainty” could be your physics students’ first exposure to a deep and serious philosophical discussion on the modern view of reality among physicists. Print out this brief essay by quantum physicist David Bohm. The Particle Adventure This award-winning mutlimedia interactive site will in a very interesting way help allow your students to explore the particles that exist within the atom and the nucleus, and the forces that hold them there. Students are quizzed frequently. The Page of Uncertainty To paraphrase the author, “This page is dedicated to that enigmatic scientific law, the Heisenberg Uncertainty Principle.” Fermi Lab: Discovering the Nature of Nature Nobel laureate Leon Lederman was the former director of The Fermi Lab featured at this site. On your tour of one of the largest particle accelerators be sure to check out “Particle Physics” and “Education.” Introduction to Cosmology Certainly it is with uncertainty that we completely understand the fundamental structure of matter, space and time, and yet we allow our imagination to pull together that which we do know into interconnected cosmological theories.

 Click on any of the vocabulary words below to hear them pronounced and used in a sentence. Definition:A science that deals with matter and energy and their interactions. Context:Physics, like science itself, is a disciplined way of coming to know and then explaining the nature of the physical world. A world made of matter in motion. Definition:A body of three laws, proposed by Sir Isaac Newton in the 17th century, that describes the interaction of matter and motion in cause and effect type relations. Context:The founders of America looked for a way of governing that was based on natural principles with a government that would run objectively and mechanically as did Newton’s mechanical universe. Definition:A theory of matter that is based on the concept of the possession of wave properties by elementary particles, that affords a mathematical interpretation of the structure and interactions of matter on the basis of these properties, and that incorporates within it quantum theory and the uncertainty principle. Context:As physicists peered into the atom they were forced to accept new realities of matter and energy, thus a new view, a quantum mechanical view, emerges in the 20th century. Definition:Existing or occurring as fact, actual rather than imaginary, ideal, or fictitious. Context:What is reality to a physicist is that which is measurable. Definition:A theory that allows us to give the reality of a wave when it is successful in explaining observed and measurable phenomena, and to choose to give it the reality of a particle when that choice yields a logical explanation for observed phenomena. Context:When photons of light pass through a diffraction grating, they interfere constructively and destructively as if they were waves. When photons bounce off a mirror, we can think of them as behaving as particles. Definition:A resulting wave form caused by the interaction of two or more waves, sometimes in a destructive manner (crest to trough) and sometimes in a constructive manner (crest to crest). Context:The bright and dark pattern of light seen as a light wave passes through a diffraction grating is called an interference pattern.