Title Bar - Cosmology

Activity: Finite Wave Speed

An easy and effective everyday way to experience the finite speed of light is in an analogy to sound. Sound is also a type of wave with finite speed. The speed that a sound wave travels is, however, much slower than the speed at which light waves travel. For this reason it is much more easily measured by humans. Take students to a large field with pencils, paper, stopwatches and walkie-talkies if available. The other important equipment includes something which makes a loud noise, some examples include air horns, starter pistol, or even a car horn. First have students measure out the distance between the noise-making item and their position on the field; they should try to be as far away as possible but 300 meters at least. While you stand at the far end of the field with the noise making device have the students stand at the far end of the field with stopwatches and recording devices. If you have walkie-talkies, give one to a student and keep one yourself. Turn on the walkie-talkies. If you don't have walkiee-talkies work out a system before hand with your students so that there is a visual signal which indicates that the sound wave has been started. Simultaneously, give the signal and make the sound. Students should start stop watches when the first signal is given and stop them when they hear the sound. If you have walkie-talkies the time between the sound over the walkie talkie and the sound through the air should be measured. The more times this is measured the less likely that individual differences in measurement will throw off the calculation. It also may take students a few practice measurements to get the hang of the stopwatch. From their data have the students calculate the speed of sound. The approximate speed of sound in air under "normal" conditions is about 340 m/s, but this is very dependant upon the temperature and humidity of the air. Discuss the variability of the speed with the students, but as with their scaling projects it is not so important that their answers be "right" so much as consistent. There is no measurement of the air on that field on that day at that temperature more accurate than their own. Therefore the answers they get, if they are confident in their measurements, are right. In addition, have them suggest ways that their measurements could be improved and in what ways the accuracy of their experiments are limited, for example the limitation of human reaction time.

The main source of information that astronomers can collect from the universe is electromagnetic radiation. Loosely speaking, this information is all light. All the information available to astronomers travels from distant galaxies and other objects at the speed of light, 3 x 108 meters/second, to Earth, where it is received. Because the objects studied by the astronomers study are so far away, the light these objects emit takes minutes, days or even years to reach the Earth. For example, the Sun is about eight light minutes away. That means that the light emitted by the Sun travels at 3 x 108 meters per second for eight minutes before it hits the Earth. When that light is collected on the Earth by astronomers with a telescope or in a photograph, they receive the light that the Sun emitted eight minutes ago. They are actually seeing the Sun as it was eight minutes in the past! If they wait eight more minutes, they will see the Sun as it was when they took the first photograph. With objects that are much farther away, the light takes longer to reach the Earth and therefore we can see further into the past.

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