How to Destroy the Universe by Paul Parsons Page B

standing for “light amplification by the stimulated emission of radiation.” A laser works in a similar way to a mercury lamp. A material known as the lasing medium—ruby is a common choice—is first pumped with energy from a source such as an ordinary flash tube. This raises electrons in the lasing medium to a particular energy level. As they start to drop back down, monochromatic light is released with a characteristic wavelength given by the gap between the energy levels involved. When a photon released by an atom in the lasing medium passes near to another atom with an electron in an energized state, it can trigger the electron to drop down and release another photon. Not only does this new photon have exactly the same energy and wavelength but the peaks and troughs of its waves are naturally synchronized, making it coherent too. This process is called “stimulated emission,” the theory of which was developed by Albert Einstein in 1917.
    The laser was invented by US scientist Charles Townes in the late 1950s. It consists of a cylinder of the lasingmedium with mirrors at each end to bounce photons back and forth inside it, so their numbers become amplified by stimulated emission. One of the mirrors is only half-silvered, allowing a proportion of the light to escape as a tightly collimated beam. Lasers proved to be one of the greatest inventions of the 20th century, underpinning devices such as CD players, fiber optics and high-capacity data storage. Today, you can buy cheap lasers as pointers and even key fobs. You can even recreate Young’s experiment by plucking a hair from your head and shining a laser at it. The hair acts as the obscuring gap between the two slits, and on the wall behind it you will see a pattern of bright and dark fringes.
Quantum double slits
    When Young produced interference fringes in 1801, it seemed like fairly conclusive evidence that light is a wave. But here’s where it starts to get really interesting. In the late 19th century and early 20th century, experimental observations began to roll in suggesting that light can still exhibit some degree of particle-like properties. Max Planck’s accurate explanation of the radiation from hot bodies relied on this—as did Einstein’s explanation of the photoelectric effect (see How to harness starlight ). So what was going on? Surely light had to be either one thing or the other?
    To get the bottom of the mystery, experimentalists resolved to repeat Young’s experiment—but with a twist. This time, rather than shining a whole beam of light through the apparatus, they would send just one particle—that is, one photon—at a time. Common sense would dictate that this single particle of light can only pass through one slit or the other, and so it should be impossible to generate any kind of interference from light passing through both slits. What actually happens is quite remarkable. The scientists fired a photon through the apparatus and recorded the position of the resulting dot on the second screen. Then they repeated the process over and over. As time passed, and more dots accumulated on the screen, a pattern began to take shape—the original interference pattern. But how can this be? At any one time there was only one photon in the system. There’s nothing else there for it to interfere with. The only way a single photon can give rise to interference is if the photon somehow interferes with itself. Put simply, it must pass through both slits—it is in two places at once.
Schrödinger’s equation
    This single experiment laid bare the inherent weirdness of the quantum world. Down at this level there is no such thing as a pure particle that is in one place at one time, or a pure wave that is spread out in space—just a strange mixture of the two. Today, physicistsinterpret the wave aspect as a wave of probability. Peaks of the wave correspond to where you’re most likely