Beyond Contact: A Guide to SETI and Communicating with Alien Civilizations - Hardcover

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Section of Chapter 7 Lightwave (Laser) Communication Optical SETI (OSETI) communication can be best compared to the light signals used to send coded messages between ships at sea. The equipment used to generate the coded flashes of light is more sophisticated, and the flashes of light are much more brief (billionths versus fractions of a second). However, the basic concept is not all that different from the communication technique employed by mariners for generations. This chapter discusses the techniques used to generate light signals that can be detected across interstellar distances, as well as the systems used to detect these signals on the receiving end. Just as we can use radio waves to transmit information, we can do the same thing with visible and infrared light. While the basic principle is the same (we're using photons to convey information), the equipment we use to generate and detect these signals is different than what we use to transmit and detect radio waves. Interstellar semaphores OSETI currently looks for two types of laser signals: a pulsed beacon, or a steady, continuous signal. The approach is fairly straightforward. The transmitting civilization aims a tightly focused laser beam at a distant star. Because lasers can be turned on and off within an extremely short period of time (billionths of a second or less), they can be focused into a very tight beam, which can outshine an entire star, if only for an instant. A pulsed beacon would flash, in strobe-light fashion, at the target star. A continuous (always on) beacon works a bit differently. This type of laser is tuned to shine at a very precise wavelength (color). In both cases, the light from the laser beam focuses on a very small region of the sky, so even at great distances, it's apparent strength is detectable to an observer within the focus of the beam. Either type of signal can be detected over interstellar distances and used to transmit large amounts of information. The physics of starlight The light emitted by stars (also known as starlight), carries an incredible amount of information. We can learn a great deal about a distant object by studying its spectrum (the color of its light). By shining the star's light through a prism, we can split its light into a rainbow of individual colors. Then, by analyzing the different colors of light emitted by a star, we can learn: ·

The chemical composition of the star ·

The temperature of the star's surface (which allows us to infer its size and weight) ·

The approximate age of the star (which can be inferred from a star's temperature and chemical composition) ·

Whether the star is orbited by large planets or a dim companion star (brown dwarf ) We can also detect an intelligent civilization that is attempting to communicate with us via a laser beacon. Photographing chemistry Since each chemical element absorbs light at a specific wavelength, we can determine the chemical composition of the star's outer atmosphere by examining the color content of a star's light. In a sense, a star transmits its own chemical "bar code," enabling astronomers to measure the chemical composition of a star. One of the things we're interested in learning is distant stars' metal content. By analyzing a star's spectrum, we can determine how much carbon, nitrogen, oxygen, iron, and other heavy elements it has. If the star is rich in heavy elements, the star may have a greater chance of developing rocky, Earth-like planets and carbon-based life. Taking a star's temperature Since the color and intensity of light closely correlates with temperature, we can measure a star's surface temperature by analyzing the color and intensity of its light. The light emitted by a star follows the rules that govern blackbody radiation, which varies according to temperature. As an object's temperature rises, it emits more light overall, and peak intensity occurs at shorter (bluer) wavelengths. When an object reaches a temperature of several hundred degrees Fahrenheit, it emits nearly all of this energy as infrared (invisible) light. As its temperature increases above this threshold, the object emits some of its energy as red light, which is why molten steel glows red. As the temperature increases to several thousand degrees, its color will shift from red to yellow to white, and eventually to blue. If the object gets hot enough (millions of degrees), it will emit most of its light as ultraviolet or X-ray radiation. To measure a star's temperature, we must look at its spectrum to find the wavelength (color) where light intensity is highest (brightest). Weighing a star Since a star's surface temperature and brightness are closely related to the rate at which the star burns its fuel, and the burn rate is, in turn, directly related to a star's mass, once we know a star's brightness, temperature and chemical composition, we can estimate its mass (similar to its weight). Massive stars burn their nuclear fuel at a much faster rate than do smaller stars. As a result, they emit much more light than their less massive counterparts. We're primarily interested in stars whose mass is similar to that of our sun. These stars belong to the main-sequence category of stars, and have a life span of several billion years. A star's mass is a critical factor in determining its ability to host life, primarily because its life span directly correlates with its mass. Stars that have more than 10 times the mass of our sun will burn much more brightly (which is not necessarily a problem since their habitable zones will simply be further out). They also have a much shorter life span--a billion years (or less) compared to about 10 billion years for our sun. This shorter life span is a problem because life takes time to evolve from single-celled bacteria to animals. Conversely, stars that are much less massive than our sun, although they have extremely long life spans, have tiny or non-existent habitable zones.