Class time: Approximately 30 to 60 minutes

Purpose: In this activity, students will design, build, test and optimize a simple optical seismometer, as an analogy for a gravitational-wave detector.

Notes to the teacher: If you have plenty of time, this is a great opportunity for the students to have the freedom to create their own designs using a variety of materials and techniques with minimal guidance. If you have less time, you can guide the students toward specific design choices. For additional background information, explore this Physics Central page on Gravitational Waves from the American Physical Society.

Use the Wave Detection Activity Guide for the Student to help guide your students through the wave detection exploration.

Materials:

• Stationary stands from which to hang a pendulum. Ring stands with a protruding arm, burette clamp or ring are ideal. Music stands will also work. If necessary, pendulums could hang from camera tripods, stools, chairs or so on.
• String to hang a pendulum. If you would like to give your students more options and more time, you could also provide wires of varying stiffness, ranging from electromagnet wire to coat hanger wire.
• Scissors and/or wire cutters to cut the string or wire.
• Flat mirrors. Small classroom optical mirrors or makeup mirrors are ideal. Alternatives are anything that is very flat and very shiny. Shiny butter knives work well, as do shiny pie servers. If necessary, students could wrap aluminum foil shiny-side-out around a small piece of flat cardboard.
• Weights to weigh down the mirror as a pendulum. Metal washers, fishing weights or pennies work well.
• Small plastic bags (Ziploc snack bags, for example) to contain the weights.
• Tape. Blue painter’s tape is by far the easiest to get off mirrors and silverware, but Scotch or masking tape could be used if necessary. Duct tape will leave a lot of sticky residue.
• Flashlights with a tightly focused beam. Many newer LED flashlights can be focused down to a very tight beam. Red laser pointers are also a possibility, but ensure that the students never accidentally or intentionally shine the pointer or its reflection in their or someone else’s eyes. Green laser pointers are much too bright and should not be used.
• Books, small boxes or other props to position the flashlights.
• Light-colored screens, wall, poster boards or sheets on which the light can shine.
• Rulers
• Optional: students’ cellphones to make video recordings of the output light signal
• Optional: clear glass or plastic sheets to shield the pendulum from air currents

Directions:

1. Students should set up the mirror as a pendulum, with the mirror approximately vertical and hanging by a string or wire from the stand (see diagram).
    
2. Students should set the flashlight on its side on books or other props, so that its tightly focused beam bounces off the mirror (at some angle) and hits the screen.
    
3. After everything settles down, the pendulum should be essentially motionless, and the light beam should essentially stay in one spot on the screen.
    
4. Help the students realize that small vibrations in the classroom can create small motions of the mirror, which can produce much larger and more easily detected changes in the position of the light beam on the screen.
    
5. Allow the students to optimize their designs to make them more or less sensitive to vibrations, for example by adjusting the length or stiffness of the string/wire, attaching weights to the hanging mirror, adjusting the angle and path length of the light beam and so on. Use the Engineering Design Process protocol by Advancement Courses for additional design support.
    
6. Discuss how to make the designs as sensitive as possible to vibrations of interest (such as talking close to the mirror or a light tap on the table) and how to make them as resistant as possible to other stimuli such as air currents.
    
7. Discuss the similarities and differences between the students’ designs and a laser interferometer for detecting gravitational waves. [In both systems, a very small wave can cause a much larger and more easily detected change in a light beam. In the students’ system, there is one beam of light containing many different wavelengths, and it indicates wave vibrations by changing its position on the screen. In the LIGO gravitational wave detectors, there are two beams of laser light at the exact same wavelength, and they indicate gravitational waves when they interfere with each other in certain ways.]