Knocking on Heaven's Door by Lisa Randall Read Free Book Online Page A

Geometrical optics, though clearly an approximation, is nonetheless a good description for a wave with wavelength small enough for interference effects to be irrelevant, and for the light to be treated as a linear ray. In other words, geometrical optics is an effective theory, valid in a limited regime.
    That doesn’t mean we keep every idea that has ever been developed. Sometimes ideas are just proved wrong. Euclid’s initial description of light, resurrected in the Islamic world in the ninth century by Al-Kindi, which claimed that light was emitted by our eyes, was one such example. Although others, such as the Persian mathematician Ibn Sahl, correctly described phenomena such as refraction based on this false premise, Euclid and Al-Kindi’s theory—which predates science and modern scientific methods—was simply incorrect. It wasn’t absorbed into future theories. It was simply abandoned.
    Newton didn’t anticipate a different aspect of the theory of light. He had developed a “corpuscular” theory that was inconsistent with the wave theory of light developed by his rival Robert Hooke in 1664 and Christian Huygens in 1690. The debate between them lasted a long time. In the nineteenth century, Thomas Young and Augustin-Jean Fresnel measured light interference, providing a clear verification that light had the properties of a wave.
    Later developments in quantum theory demonstrated that Newton was correct in some sense too. Quantum mechanics now tells us that light is indeed composed of individual particles called photons that are responsible for communicating the electromagnetic force. But the modern theory of photons is based on light quanta, the individual particles of which light is made, that have a remarkable property. Even an individual particle of light, a photon, acts like a wave. That wave gives the probability of a single photon being found in any region of space. (See Figure 5.)
    Geometrical Optics
    Light travels in straight lines .

    Wave Optics
    Light travels in waves .

    Photons
    Light is transmitted by photons, particles that can act like waves.

    [ FIGURE 5 ] Geometrical optics and waves were precursors to our modern understanding of light, and still apply under appropriate conditions.
    Newton’s corpuscular theory reproduces results from optics. Nonetheless, Newton’s corpuscles, which don’t have any wavelike nature, are not the same as photons. So far as we now know, the theory of photons is the most basic and correct description of light, which consists of particles that can also accommodate a wave description. Quantum mechanics gives our currently most fundamental description of what light is and how it behaves. It is fundamentally correct and survives.
    Quantum mechanics is now much more of a frontier research area than optics. If people continue to think about new science with optics, they are primarily thinking about new effects possible only with quantum mechanics. Modern science, though no longer advancing the science of classical optics, does therefore include a field of quantum optics, which studies the quantum mechanical properties of light. Lasers rely on quantum mechanics, as do light detectors such as photomultipliers, and photovoltaic cells that convert sunlight into electricity.
    Modern particle physics also encompasses the theory of quantum electrodynamics (QED), which Richard Feynman and others developed and which includes not only quantum mechanics but also special relativity. With QED, we study individual particles including photons—particles of light—as well as electrons and other particles that carry electric charge. We can understand the rates at which such particles interact and at which they can be created and destroyed. QED is one of the theories that is heavily used in particle physics. It also has made the most accurately verified predictions in all of science. QED is a far cry from geometrical optics, yet both are true in their appropriate domain of