How to Destroy the Universe by Paul Parsons Page A

both the slits. In other words, a wave peak passes through one slit at the same time as a wave peak passes through the other, while a trough passes through one at the same time as a trough passes through the other. Physicists call light that has this property “coherent.” If, on the other hand, the light was incoherent, its waves would be hopelessly out of synch and there would be no chance of witnessing any interference fringes. Most light sources in nature are incoherent. When Thomas Young first performed the double-slit experiment in the early 19th century, he was able to generate reasonably coherent light by shining an incoherent source through a pinhole. Because the pinhole is to all intents and purposes a dot of zero size, it filters out the incoherent variations from point to point along the light’s wavefront. But this wasn’t the only demand placed on the light used in Young’s experiment. It was also crucial that the light should be made up of waves of just one wavelength (see How to make the loudest sound on Earth ).
    Shown here is the double-slit experiment. When coherent light is shone through two slits and onto a screen, a pattern of bright and dark fringes is formed as peaks and troughs in the waves traveling from each slit meet and interfere with one another.
Monochrome vision
    If light of all different wavelengths is passing through the slits, each wavelength will make a different interference pattern on the screen, with different spacings between the fringes, and the clean pattern of bright and dark bands is lost. Instead, the light must all be of a single wavelength. Because the wavelength of a light beam is what determines its color (for example, red light has a wavelength of 650 billionths of a meter while the wavelength of blue light is shorter, at 450 billionths of a meter), light of a single wavelength is called monochromatic (from the Greek “mono,” meaning single, and “chroma,” meaning color). Making monochromatic light is easier said than done. Ordinary light sources—such as filament lightbulbs—give off light at a large range of wavelengths. The range is determined by the temperature of the source, in this case the hot filament inside the bulb. Germanphysicist Max Planck used quantum theory (also known as quantum mechanics) to work out the spectrum of radiation from a hot body. The spectrum is just a graph with the range of wavelengths of the electromagnetic radiation given off along the bottom and the intensity of the radiation at each wavelength plotted vertically. For a filament bulb most of this radiation is at infrared and visible light wavelengths, but there’s a large spread either side.
    Young overcame this problem by using light from a mercury vapor lamp. This is a source of monochromatic light that operates on the principles of quantum theory—although Young didn’t realize it at the time, because the theory was yet to be formulated. It works using a glass bulb filled with mercury vapor, through which a large electric current is passed. Energy from the current is absorbed by the mercury atoms, which makes electrons in the outer shell of each atom jump up to a higher energy level. Energy levels are one of the key features of quantum theory, and their existence was one of the predictions of the Schrödinger equation which lies at the theory’s heart (see p.116). At very small scales, the energy of an electron in an atom is only allowed to take one of a discrete range of values. An atom raised to a higher energy level soon drops back down again, re-releasing the energy as a packet of light—called a photon. The energy of the photon is determined by its wavelength, so electrons all dropping down from thesame energy level will have the same wavelength—in other words, they are monochromatic.
Lasers
    Nowadays, scientists perform demonstrations of the double-slit experiment using lasers. The word laser is an acronym,