Stephen Hawking by John Gribbin Read Free Book Online Page B

theory, transforming our understanding of the very small. Attention focused chiefly on the development of the quantum theory over the next few decades, with relativity and cosmology (the study of the Universe at large) becoming an exotic branch of science investigated by only a few specialist mathematicians. The union of large and small still lay far in the future, even at the end of the 1920s.

    As the nineteenth century gave way to the twentieth, physicists were forced to revise their notions about the nature oflight. This initially modest readjustment of their worldview grew, like an avalanche triggered by a snowball rolling down a hill, to become a revolution that engulfed the whole of physics—the quantum revolution.
    The first step was the realization that electromagnetic energy cannot always be treated simply as a wave passing through space. In some circumstances, a beam of light, for example, will behave more like a stream of tiny particles (now called photons). One of the people instrumental in establishing this “wave-particle duality” of light was Einstein, who in 1905 showed how the way in which electrons are knocked out of the atoms in a metal surface by electromagnetic radiation (the photoelectric effect) can be explained neatly in terms of photons, not in terms of a pure wave of electromagnetic energy. (It was for this work, not his two theories of relativity, that Einstein received his Nobel Prize.)
    This wave-particle duality changes our whole view of the nature of light. We are used to thinking of momentum as a property to do with the mass of a particle and its speed (or, more correctly, its velocity). If two objects are moving at the same speed, the heavier one carries more momentum and will be harder to stop. A photon does not have mass, and at first sight you might think this means it has no momentum either. But, remember, Einstein discovered that mass and energy are equivalent to one another, and light certainly does carry energy—indeed, a beam of light is a beam of pure energy. So photons do have momentum, related to their energy, even though they have no mass and cannot change their speed. A change in the momentum of a photon means that it has changed the amount of energy it carries, not its velocity; anda change in the energy of a photon means a change in its wavelength.
    When Einstein put all of this together, it implied that the momentum of a photon multiplied by the wavelength of the associated wave always gives the same number, now known as Planck’s constant in honor of Max Planck, another of the quantum pioneers. Planck’s constant (usually denoted by the letter h ) soon turned out to be one of the most fundamental numbers in physics, ranking alongside the speed of light, c . It cropped up, for example, in the equations developed in the early decades of the twentieth century to describe how electrons are held in orbit around atoms. But although the strange duality of light niggled, the cat was only really set among the pigeons in the 1920s when a French scientist, Louis de Broglie, suggested using the wave-particle equation in reverse. Instead of taking a wavelength (for light) and using this to calculate the momentum of an associated particle (the photon), why not take the momentum of a particle (such as an electron) and use it to calculate the length of an associated wave?
    Fired by this suggestion, experimenters soon carried out tests that showed that, under the right circumstances, electrons do indeed behave like waves. In the quantum world (the world of the very small, on the scale of atoms and below), particles and waves are simply twin facets of all entities. Waves can behave like particles; particles can behave like waves. A term was even coined to describe these quantum entities—“wavicles.” The dual description of particles as waves and waves as particles turned out to be the key to unlocking the secrets of the quantum world, leading to the