Light

• 1. Introduction

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  Light, in vacuum, moves at exactly 299,792,458 metres per second. That number is not a measurement anymore. It is one of the fundamental constants of nature, and the metre itself is now defined in terms of it. Every form of electromagnetic radiation, from radio waves to gamma rays, travels at precisely this same speed through empty space.

  Yet the slice of that radiation a human eye can actually perceive is narrow. Visible light spans wavelengths of roughly 400 to 700 nanometres, frequencies of 750 to 420 terahertz. On one side sits infrared, on the other ultraviolet, both invisible to us. Why does the eye stop where it stops? Why did some of the sharpest minds in history spend centuries arguing whether light was a particle or a wave? And how did experiments eventually bring a beam of light to what looked like a complete standstill? The answers begin with a single question: what is light made of?

• 2. The Particle Wave War

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  René Descartes, who lived from 1596 to 1650, rejected the older ideas of forms and species and declared light a mechanical property of the luminous body. In 1637 he published a theory of refraction that assumed, incorrectly, that light travelled faster in a denser medium than in a thinner one. He reached this by analogy with sound waves. He was wrong about the speed, but right that light behaved like a wave.

  Pierre Gassendi, who lived from 1592 to 1655, proposed a particle theory of light, published after his death in the 1660s. Isaac Newton studied that work young and preferred it. In his Hypothesis of Light of 1675 he argued light was made of corpuscles emitted in all directions from a source. One of his arguments against waves was simple. Waves bend around obstacles, while light travels only in straight lines.

  Robert Hooke, who lived from 1635 to 1703, took the other side. In his 1665 work Micrographia he compared the spreading of light to waves in water. Christiaan Huygens worked out a mathematical wave theory in 1678 and published it in his Treatise on Light in 1690. He proposed that light was emitted as a series of waves through a medium called the luminiferous aether.

  Thomas Young broke the deadlock around 1800. He showed through diffraction experiments that light behaved as waves, and first stated his general law of interference in January 1802. The decisive blow came from speed. Newton's corpuscles implied light sped up in a denser medium; Huygens implied the opposite. In 1850, Léon Foucault made a measurement accurate enough to settle it, and his result supported the wave theory.

• 3. Maxwell's Unification

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  Michael Faraday discovered in 1845 that the plane of polarization of linearly polarized light rotates when light travels along a magnetic field through a transparent dielectric, an effect now called Faraday rotation. This was the first evidence that light was related to electromagnetism. By 1847 he proposed that light was a high-frequency electromagnetic vibration that could travel even without a medium like the ether.

  James Clerk Maxwell took up Faraday's work and found that self-propagating electromagnetic waves would move through space at a constant speed. That speed happened to equal the previously measured speed of light. He concluded light was a form of electromagnetic radiation, stating the result first in 1862 in On Physical Lines of Force. In 1873 he published A Treatise on Electricity and Magnetism, containing the full mathematical description still known as Maxwell's equations.

  Heinrich Hertz confirmed the theory by generating and detecting radio waves in the laboratory. He showed these waves reflected, refracted, diffracted and interfered exactly like visible light. That confirmation led directly to modern radio, radar, television and wireless communications.

• 4. Quanta And Photons

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  Max Planck, in 1900, was trying to explain black-body radiation when he suggested something strange. Light was a wave, but these waves could gain or lose energy only in finite amounts tied to their frequency. He called these lumps quanta, from a Latin word for how much. In 1905 Albert Einstein used light quanta to explain the photoelectric effect and argued these quanta had a real existence.

  Arthur Holly Compton showed in 1923 that the wavelength shift seen when low-intensity X-rays scatter from electrons could be explained by a particle theory of X-rays, but not a wave theory. In 1926 Gilbert N. Lewis gave these light quanta their name. He called them photons. Quantum mechanics eventually pictured light as in some sense both particle and wave, and in another sense neither.

  Satyendra Nath Bose showed in 1924 to 1925 that light followed different statistics from classical particles. With Einstein, he generalized this to a whole set of integer-spin particles called bosons, after Bose, that follow Bose-Einstein statistics. The photon is a massless boson of spin 1. Paul Dirac quantized the electromagnetic field in 1927, and by the end of the 1940s a full theory of quantum electrodynamics was built on the work of Julian Schwinger, Richard Feynman, Freeman Dyson and Shinichiro Tomonaga.

• 5. Chasing The Speed

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  Galileo attempted to measure the speed of light in the seventeenth century. The first real experiment came from Ole Rømer, a Danish physicist, in 1676. Using a telescope, he watched Jupiter and its moon Io. Noting discrepancies in Io's apparent orbital period, he calculated that light takes about 22 minutes to cross the diameter of Earth's orbit, though that orbit's size was unknown at the time.

  Hippolyte Fizeau performed a more accurate measurement in Europe in 1849. He aimed a beam at a mirror several kilometres away and placed a rotating cog wheel in its path. At a certain rate of rotation, the beam passed through one gap going out and the next gap coming back. Knowing the distance, the number of teeth and the rotation rate, he calculated the speed.

  Léon Foucault used rotating mirrors in 1862 to obtain a value of 298,000,000. Albert A. Michelson worked on the speed of light from 1877 until his death in 1931. In 1926 he refined Foucault's methods, timing light on a round trip from Mount Wilson to Mount San Antonio in California, yielding 299,796,000. In transparent matter, light slows. In water its speed is about three quarters of that in vacuum.

• 6. Stopping A Beam

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  Two independent teams of physicists were said to bring light to a complete standstill by passing it through a Bose-Einstein condensate of the element rubidium. One team worked at Harvard University and the Rowland Institute for Science in Cambridge, Massachusetts. The other worked at the Harvard-Smithsonian Center for Astrophysics, also in Cambridge.

  The popular description of stopped light is misleading. The light was stored in the excited states of atoms, then re-emitted at an arbitrary later time, triggered by a second laser pulse. During the time it had stopped, it had ceased to be light at all. Reaching that condensate required its own breakthroughs in handling light and matter.

  Short and ultrashort laser pulses, created by Q switching and modelocking, opened the study of ultrafast processes. The mechanical forces of light on matter led to levitating and positioning clouds of atoms, or even small biological samples, in an optical trap or optical tweezers. That technique, with Doppler cooling and Sisyphus cooling, was the crucial technology needed to achieve Bose-Einstein condensation.

• 7. The Pressure Of Photons

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  Light exerts physical pressure on the objects in its path. The effect can be deduced from Maxwell's equations, but it is easier to explain through particles. Photons strike an object and transfer their momentum. The pressure equals the power of the beam divided by c, the speed of light. Because c is so large, the force is negligible for everyday objects.

  A one-milliwatt laser pointer exerts a force of about 3.3 piconewtons on what it illuminates. You could lift a U.S. penny with laser pointers, but it would take roughly 30 billion of them. At nanometre scales, the story changes. Light pressure becomes significant in nanoelectromechanical systems, where it is studied to drive mechanisms and flip nanometre-scale switches in integrated circuits.

  At larger scales, light pressure can spin asteroids faster, pushing on their irregular shapes like wind on the vanes of a windmill. Solar sails that would accelerate spaceships are under investigation. Einstein in 1909 predicted radiation friction, a force opposing the movement of matter. He wrote that a moving plate reflects more radiation on its front surface than its back, leaving a force that counteracts its motion and increases with its velocity.

• 8. How Light Is Made

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  A body at a given temperature emits a characteristic spectrum of black-body radiation. Sunlight is a simple thermal source, radiation from the Sun's chromosphere at around 6000 K. Solar radiation peaks in the visible region when plotted in wavelength units, and roughly 44 percent of what reaches the ground is visible. An incandescent light bulb emits only around 10 percent of its energy as visible light, the rest as infrared.

  The peak of the black-body spectrum sits in the deep infrared, around 10 micrometres, for cool objects like human beings. As temperature rises, the peak shifts toward shorter wavelengths, producing first a red glow, then white, then a blue-white as it moves into the ultraviolet. The pure-blue colour in a gas flame or welder's torch is not thermal. It comes from molecular emission, notably CH radicals emitting a band around 425 nm.

  Atoms emit and absorb light at characteristic energies, producing emission lines. Sodium in a gas flame emits its characteristic yellow. Deceleration of a free charged particle yields cyclotron, synchrotron and bremsstrahlung radiation. Particles moving through a medium faster than light moves in that medium produce Cherenkov radiation. Fireflies make light by bioluminescence, and boats can disturb plankton into a glowing wake. Sunlight powers photosynthesis, which provides virtually all the energy used by living things, while the vampire squid uses its own light to hide from prey.