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Section 1: Introduction

Saint Hans bonfire—Midsummer celebration in Skagen, Denmark.

Figure 1: Saint Hans bonfire—Midsummer celebration in Skagen, Denmark.

Source: Painting by P.S. Krøyer: Sct. Hans-blus på Skagen, 1906; owned by Skagen Museum. More info

Light has fascinated humans for thousands of years. In ancient times, summer solstice was a celebrated event. Even to this day, the midsummer solstice is considered one of the most important events of the year, particularly in the northern-most countries where the contrast between the amount of light in summer and winter is huge. Visible daylight is intense due to the proximity of the Earth to the Sun. The Sun is essentially a huge nuclear reactor, heated by the energy released by nuclear fusion. When something is hot, it radiates. The surface of the Sun is at a temperature of roughly 6000 K (roughly 10,000°F). Our eyes have adapted to be highly sensitive to the visible wavelengths emitted by the Sun that can penetrate the atmosphere and reach us here on Earth. The energy carried in sunlight keeps the Earth at a comfortable temperature and provides the energy to power essentially everything we do through photosynthesis in plants, algae, and bacteria—both the processing happening right now as well as what took place millions of years ago to produce the energy stored in fossil fuels: oil, gas, and coal.

The interaction of light with different substances has long been a subject of great interest, both for basic science and for technological applications. The manipulation of light with engineered materials forms the basis for much of the technology around us, ranging from eyeglasses to fiber-optic cables. Many of these applications rely on the fact that light travels at different speeds in different materials.

The speed of light therefore plays a special role that spans many aspects of physics and engineering. Understanding and exploiting the interaction between light and matter, which govern the speed of light in different media, are topics at the frontier of modern physics and applied science. The tools and techniques employed to explore the subtle and often surprising interplay between light and matter include lasers, low-temperature techniques (cryogenics), low-noise electronics, optics, and nanofabrication.

Flight controllers Charles Duke (Capcom), Jim Lovell (backup CDR), and Fred Haise (backup LMP) during lunar module descent.

Figure 2: Flight controllers Charles Duke (Capcom), Jim Lovell (backup CDR), and Fred Haise (backup LMP) during lunar module descent.

Source: © NASA. More info

Light travels fast, but not infinitely fast. It takes about two and a half seconds for a pulse of light to make the roundtrip to the Moon and back. When NASA's ground control station was engaged in discussions with the Apollo astronauts at the Moon, the radio waves carrying the exchange traveled at light speed. The 2.5 second lag due to the radio waves moving at the finite speed of light produced noticeable delays in their conversations. Astronomers measure cosmic distances in terms of the time it takes light to traverse the cosmos. A light-year is a measure of distance, not time: it's the length that light travels in a year. The closest star to the Sun is about four light-years away. For more down-to-Earth separations, a roundtrip at light speed from London to Los Angeles would take about 0.06 seconds.

So, if the upper limit to the speed of light is c, an astonishing 300 million meters per second (186,000 miles per second), how much can we slow it down? Light traveling in transparent materials moves more slowly than it does in a vacuum, but not by much. In air, light travels about 99.97% as fast as it does in a vacuum. The index of refraction, n, of a material is the ratio between light's speed in empty space relative to its speed in the material. In a typical glass (n = 1.5), the light is traveling about 67% as fast as it does in a vacuum. But can we do better? Can we slow light down to human speeds? Yes!

This unit explores the fascinating quest for finding ways to slow down and even stop light in the laboratory. Perhaps this extreme limit of manipulating light in materials could give rise to novel and profound technological applications.