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Parallel Worlds, Parallel Lives

Classroom Activity

Activity Summary
Students investigate the double-slit experiment and research historical conflicts involving scientific theories.

Learning Objectives
Students will be able to:

  • describe the setup and results of the double-slit experiment.

  • gain an understanding of how scientific theories become established in the scientific community, and of the challenges new theories often face when they are introduced.

Suggested Time
Three class periods if all parts are completed


Multimedia Resources

Additional Materials

  • reference materials, such as textbooks, library books, and/or online materials

Quantum theory is a branch of theoretical physics that developed as a means of understanding the fundamental properties of matter. As it took shape in the early part of the 20th century, quantum mechanics was viewed primarily as a way of making sense of the host of observations—at the level of single electrons, atoms, or molecules—that could not be explained in terms of classical physics. One question in particular centered on whether light is a wave or a particle.

The double-slit experiment is a classic physics experiment first performed in the early 1800s by the English scientist Thomas Young in an attempt to resolve whether light is a particle or a wave. Using sunlight diffracted through a small slit, Young projected the light rays emanating from the slit onto another screen containing two slits placed side by side. Light passing through the pair of slits was then allowed to fall onto another screen. Young observed that when the slits were large, spaced far apart and close to the next screen, then two separate patches of light formed on the screen. However, when he reduced the size of the slits and brought them closer together, the light passing through the slits and onto the screen produced distinct bands of color separated by dark regions in a serial order, known as interference patterns. These patterns could only be produced if light were acting like a wave.

Then in 1905, Albert Einstein showed that light is a collection of discrete particles, which that he called "photons." When the double-slit experiment is repeated using single photons, an interference pattern is also seen, despite the notion that a single particle shot toward the screen should not be able to interfere with itself. The fact that light sometimes behaves as a wave and sometimes behaves as a particle is known as the wave-particle duality. Subatomic particles like electrons exhibit a similar wave-particle duality. The attempt to discover why this occurs has generated numerous theories, from Niels Bohr's Copenhagen interpretation to Hugh Everett's Many-Worlds interpretation.


Part A: The Double-Slit Experiment

Before the Lesson

  1. Do all the steps of the interactive activity prior to using it with students (some instructions may vary slightly depending on your computer, operating system, and plug-in version).

  2. Bookmark the PhET Quantum Wave Interference Web site. You can lead a demonstration or have students work with the application in teams.

The Lesson

  1. Tell students they will be learning about a famous experiment in physics known as the double-slit experiment. Explain that this experiment was originally designed by Thomas Young in the early 1800s to investigate the nature of light. Young was trying to determine whether light is composed of particles or waves, which at that time were thought to travel through some sort of ether.

  2. Discuss the following concepts with students if they are not familiar with them: wavelength, frequency, destructive interference, and constructive interference.

  3. Either conduct the following as a demonstration or organize students into teams to explore Young's experiment and how Hugh Everett used it as evidence for the possibility that parallel universes exist.

  4. Have students go to the PhET Quantum Wave Interference Web site and click the "Run Now" button to launch the program from their Web browser.

  5. Once the interactive is loaded, have students set the following parameters:

    1. Click on the "High Intensity" tab at the top of the applications if it does not show up as the default
    2. Set "Screen" parameters: no fade, 0.5 brightness
    3. Set "Display" parameters: select Hits
    4. Set particle type (from pull down menu above "Gun Controls"): Photons
    5. Set "Gun Controls" parameters:" position slider in middle
    6. Click the "Double Slits" button and set the following:
      • "Slit Width" position slider to far left
      • "Slit Separation" position slider in middle
      • "Vertical Position" position slider three-quarters to the right

  6. Tell the class that once the photon gun is turned on, points of light will appear on the back screen (detector) indicating where the photons hit the screen. Ask students what they would expect to show up if light were to behave as a particle. (The screen would show two concentrated regions where the particles build up in intensity as they hit the detector behind each slit.) What would students expect if light were to behave as a wave? (Three or more regions would appear on the detector as the light waves constructively and destructively interfere with one another.) What do students think they will see? (Students will see three regions, supporting the hypothesis that light is a wave.)

  7. Tell students they will now fire the gun. Have them:

    1. Turn the photon gun on by clicking the red button.
    2. Click the "Start" button in the onscreen clock and then the "Play" button at the bottom of the screen; fire the gun for about 5 seconds (count manually or use a watch or clock with a secondhand) and then click "Pause" to stop the firing.

  8. Did the pattern on the screen match students' predictions? Tell your class that, like the experiment they conducted online, Young's experiment showed interference patterns, which led him to conclude that light travels in waves (in ether). It was not until Albert Einstein used the photoelectric effect nearly a century later to show that light travels in discrete units (called quanta) that scientists came to realize that light behaves like both a wave and a particle, depending upon the conditions of the experiment.

  9. Now have students click on the "Single Particles" tab at the top of the application. They should then set the same parameters as they did in the high-intensity beam experiment, but click "Auto Repeat" in the Gun Controls box and "Rapid" at the bottom of the screen. Students will now be shooting single photons through the two slits. What kind of pattern do students predict will occur on the detector?

  10. Have students click the "Play" button and allow the gun to shoot single photons for about 3 minutes. Did the pattern on the screen match students' predictions? (Students likely will have predicted that the detector will show the single photons appearing in two places behind the slits, thus supporting the quantum—or particle—nature of light. However, the detector actually shows hits all across the screen, suggesting the beginnings of a wave interference pattern like that seen with the high-intensity beam. Hugh Everett proposed that the particle has traveled through both slits simultaneously and come back together again by the time it hits the detector. The same behavior occurs with single electrons and atoms. Everett postulated that if atoms can behave this way and humans are made up of atoms, then humans as well could exist in two—or more—places at once.)

  11. As a class, discuss the computer simulation of the double-slit experiment. What were the simulation's strengths and weaknesses? (The simulation does a good job modeling the actual experimental setup and results; however, one disadvantage of the model is that some students may think that when the single photon is shot, it splits and goes through the two slits, which it does not. Rather, the simulation shows what scientists hypothesize is happening—that the wave corresponding to the photon goes through both slits and interferes with itself.)

Part B: Famous Conflicts in Science

  1. Tell students that throughout the history of science, there have been "maverick" individuals have come along who challenge currently held scientific theories. Sometimes new theories were accepted, sometimes they were rejected. Ask students if they can think of any scientific theories that were replaced with new theories over time.

  2. Explain that Hugh Everett's ideas about quantum mechanics challenged the accepted ideas developed by Niels Bohr. Tell students they are going to be investigating other famous challenges in science history. Group the class into teams, and assign each team one of the following conflicts:

    • Earth-centered vs. sun-centered solar system
    • Big Bang theory vs. steady state theory of how the universe began
    • plate tectonics vs. fixed continental plates
    • impact theory vs. climate change as a reason for the extinction of dinosaurs
    • organisms inherit acquired characteristics vs. natural selection as a mechanism for evolution

  3. Have teams research their conflict and write a summary that includes the following:

    • who the key players were
    • what the main disagreement was
    • which theory was considered the standard at the time and why
    • arguments for and against each of the two theories
    • which theory, if either, was ultimately accepted, and why
    • a time line reflecting when each of the two theories was proposed and a general timeframe focusing on when the currently held theory seemed to be accepted

  4. When students have finished, have them present their findings to the class. Use the conflicts to initiate a broader discussion on the nature of scientific theory. Help students recognize that most theories frequently undergo examination and modification, and are sometimes even replaced by new theories. Discuss some of the reasons why this occurs. What does it take to replace a widely accepted theory?


Use the following rubric to assess students' work.




Needs Improvement

Part A: The Double-Slit Experiment

Students can use the software independently and accurately. They are able to describe their results and draw conclusions.

Students many need assistance using the software. They demonstrate a general understanding, but may need help drawing conclusions about their results.

Students have difficulties using the software. They have difficulties describing their results and/or drawing conclusions.

Part B: Famous Conflicts in Science

Students produce a detailed paper. They demonstrate an understanding of the scientific conflict, the key players, and the arguments for and against the different theories. They also demonstrate an understanding that scientific theories are frequently being modified and sometimes even replaced.

Students produce a paper but have difficulty explaining the scientific conflict, the key players, and/or the arguments for and against the different theories.

Students spend little time on their research, and their paper lacks detail. They have trouble explaining the scientific conflict, the key players, and/or the arguments for and against the different theories.


The "Parallel Worlds, Parallel Lives" activity aligns with the following National Science Education Standards.

Grades 5-8
Physical Science

  • Properties and changes of properties of matter

History and Nature of Science

  • Science as a human endeavor
  • Nature of science
  • History of science

Grades 9-12
Physical Science

  • Structure of atoms
  • Interactions of energy and matter

History and Nature of Science

  • Science as a human endeavor
  • Nature of scientific knowledge
  • Historical perspectives

Classroom Activity Author

Margy Kuntz has written and edited educational materials for more than 24 years. She has authored numerous educational supplements, basal text materials, and trade books on science, math, and computers.

Teacher's Guide
Parallel Worlds, Parallel Lives

InteractivePhET Quantum Wave Interference Java Interactive

Koch Foundation