Class time: Approximately 30 to 60 minutes
Purpose: This activity uses water waves to model how gravitational waves are created and behave, and also how two light waves can interfere with each other.
Notes to the teacher: Waves on the surface of water in a clear container are most easily visible if the container is on an old-fashioned overhead transparency projector that is focused on a screen, so the ripples show up on the screen. If you only have one projector or none, students can still see the waves by setting the clear container on a dark surface and shining a flashlight at an angle at the water’s surface. If you have an ELMO projector, use it to illuminate the water’s surface and project the waves. The related activity “Dropping in with Gravitational Waves” by NASA’s Jet Propulsion Laboratory can be used for additional background information.
Use the Wave Generation Activity Guide for the Student to help guide your students though the wave exploration.
- Overhead transparency projector(s), ELMO projector or flashlights (in lieu of projectors and screens if those are not available)
- If using a projector: projection screens or blank walls, whiteboards or white poster boards
- on which to project
- Shallow clear glass baking dishes or clear plastic containers with flat bottoms (as large as possible)
- Water from a classroom sink, pitchers or beakers
- Marbles (or other small objects) to drop into the water
- Corks, Ping-Pong balls or other small objects to float in water
- Stiff rulers
- Small binder clips or large paper clips
- Tuning forks (if available)
- Optional: students’ cellphones to make video recordings of the ripples
- Help the students fill the clear containers about halfway full with water and visualize the water’s surface by projecting it on a screen or by shining a flashlight across the surface.
- Explain that waves produced on the water’s surface can be a model for exploring the production of ripples in spacetime. Explain to your students that water does not bend with gravity, as spacetime does, rather water is displaced by objects set on its surface. This model is best used for conceptualizing wave generation and interference.
- Explain that objects disrupting the water’s surface can generate waves, just as masses disrupting spacetime can generate gravitational waves.
- Help the students understand that objects that are just sitting there create alterations in the surface (spacetime) but no ripples (gravitational waves in spacetime).
- Guide the students to understand that objects must move around in the right ways to create ripples or gravitational waves. In this simple model, any motion will generate ripples. Under general relativity, acceleration or deceleration of a massive object is required to produce gravitational waves.
- Let the students try many different ways of creating ripples, as outlined in the Wave Generation Activity Guide for the Student. Have students observe the effects of each.
- Help the students understand how water waves affect the relative positions of two corks or Ping-Pong balls floating on the water. (In this model, the water’s surface moves and the objects will bob up and down as a wave passes.) How does this compare and contrast with how objects will move as gravitational waves pass by in spacetime. (In general relativity, the empty space between two objects can slightly expand and contract, even if the two objects stayed right where they were the entire time. See the earth oscillating in the Stephen Colbert and Brian Greene video.)
- Guide the students to understand that the water ripples are analogous to light waves, and that two water waves can interfere with each other just as two light waves can interfere with each other. Two water waves can produce constructive and destructive interference patterns, just as two light waves can produce interference patterns in a laser interferometer.
- When finished, have the students carefully pour the water down a sink or into containers and dry everything off.
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