Speed of Sound: Why Galileo Couldn't Measure the Speed of Light?

The Idea

The speed of sound, like any other sound, can be found by measuring the time it takes to go a certain distance. This simple and straightforward measurement can give a reasonable ballpark estimate, but not highly accurate results. We will, however, be limited by the large distances we need to work with and the small times we need to accurately measure. We measure the speed of sound using a more accurate method in the following projects.

What You Need

• long tape measure (or some other way to estimate a long distance, such as counting cinder blocks of a known length on a building or clocking the distance of several blocks using the odometer of a car)
• means of generating a loud sound (such as an air horn, garbage-can lid, or baseball bat, or someone with a loud voice)
• stopwatch
• partner (which may not be needed if you can set up an echo)

Method

1. Measure or estimate a course of known or estimated distance without visual obstruction. A football field or possibly multiple lengths can work. You can also use a building or natural geographic feature, such as a cliff to reflect an echo. This effectively doubles the distance the sound travels.
2. Generate the sound and note the difference in time between when the sound was generated and when it is heard at a distant location. (This can be accomplished by observing when the garbage-can lid was struck or observing at a distance when the air horn is sounded.)
3. To get the speed of sound, divide the distance by the time. See Figure 72-1.

Expected Results

The speed of sound is about 343 meters per second or about 1096 feet per second at 20°C. It is unlikely this technique will give an accurate value for the speed of sound, but it should provide a ballpark estimate.

Why It Works

Velocity is distance divided by time. Because the speed of light is so much greater than the speed of sound, the time it takes light to travel the distance can be considered essentially zero and is insignificant compared to the speed of sound.

Note that Galileo tried to measure the speed of light using a similar method. Light moves so quickly, however, it requires extremely large distances to measure the time it takes to travel using a stopwatch. Rather than saying that Galileo failed in his attempt, we like to say he succeeded in proving that light was much faster than he could measure.

Other Things to Try

The distance to a lightning strike can be determined by counting the number of seconds between seeing the lighting and hearing the thunder. Using a known value for the speed of sound multiplied by time can provide an estimate of the distance to the lightning strike. Similarly, the speed of sound can be determined if the distance to the lightning strike is known (or can be measured, such as by driving to where the lightning was observed to hit) and divided by the time between seeing the lightning and hearing the thunder.

The Point

Speed is distance divided by time. However, the accuracy of any experiment is limited by the resolution of the least certain measurement. Even if distance can be measured accurately, the time measurements are limited by the reaction time of the observer.

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