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How can we measure the speed of light?
Question Date: 2015-02-24
Answer 1:

The speed of light is considered to be 299,792,458 m/s. Not 299,792,457 m/s. Not 299,792,459 m/s. That’s because the official size of a meter is the distance that light travels in 1/299,792,458 seconds. I hope this is an unsatisfactory answer. Even though it’s true, it would be like me telling you that pizza is the best food because the best food is pizza.

A better explanation would involve breaking down the experiments that have been used to determine the speed of light. Experiments performed in the 1970s and 1980s are what provide the current value for the speed of light. These experiments were made possible by the invention of lasers.

All light is a wave, which means it can be described by a wavelength or a frequency. For example, red light has a wavelength of 650 nm while blue light has a wavelength around 450 nm. If you could measure the wavelength (a unit of distance) and the frequency (a unit of 1/time), you could multiple them together and get distance/time, which is the velocity. You would ideally want only one wavelength and one frequency of light that can’t change or else there would be errors in the measurement. Lasers made this possible!

Scientists designed experiments with lasers to accurately measure the frequency and the wavelength and obtain the speed of light.

The speed of light was a puzzling challenge for a long time. E = mc2 uses c, which is the speed of light, and so scientists have wanted to make this value as accurate as possible. Even before E=mc2, scientists were still curious about how fast light traveled. Some people believed that it traveled infinitely fast, which isn’t so unreasonable. You couldn’t actually observe how fast it traveled. Galileo cleverly thought that the speed of light actually had a value and that he could figure it out. If you wanted to measure your mile speed, you would measure a mile, have a friend wait at the end with a stopwatch, and take the distance (1 mile) divided by the time (maybe 9 minutes or 0.15 hour) to get about 6.7 mph.

Galileo thought he could do the same thing. He and a friend took lanterns and climbed two hills about a mile apart. Galileo and his friend had their lanterns lit but closed by shutters. Galileo opened his shutter and when his friend saw the light, he would open his own shutter. Galileo would count how long it took to see the light from his friend’s lantern and estimate the speed of light. Although this was a clever experiment, light would take about 0.000005 seconds to travel a mile, making it too hard to detect how fast the light actually was.

Other experimentalists tried different tricks to measure the speed of light. A Danish astronomer measured the speed of light 50 years after Galileo by watching Jupiter’s moon Io and predicting when it would arrive. When his predictions were off, he realized that Jupiter and Earth were moving away from each other as they traveled in orbit, changing the distance light traveled from Io. From this, he was able to calculate the speed of light but still not very accurately. It wasn’t until technological improvements in mirrors, lenses, light, and finally lasers allowed for an accurate measurement of the speed of light. And while scientists of olden days would climbs hills and watch moons through telescopes for months, you could actually measure the speed of light in a couple of minutes.

There are putties that change color with heat that you can put in a microwave (with permission) with a cup of water (so you don’t ruin the putty or microwave) and measure the distance between different colored spots (the wavelength). A microwave will list what frequency waves it uses, and you can take that number multiplied by the wavelength to get a pretty good estimate of the speed of light. An example of how this experiment works is on this website: click here to read

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