Where to look in the electromagnetic spectrum?
Based on our present knowledge of physics, if we are to detect a signal from space it will be as electromagnetic waves, somewhere in the spectrum between gamma rays at one end and wavelengths longer than AM radio transmissions at the other. This is because only gravitational and electromagnetic waves can travel through a vacuum. Sound waves, for example, need a material (air, water, etc) to vibrate. The light we see from the stars has traveled trillions of kilometers through the vacuum of space. As we also know, geometry makes the intensity of an electromagnetic wave fall off with the square of the distance traveled. Hence if the distance is doubled, the area reached is quadrupled and the signal strength is quartered (Figure 7.1). If a transmitter has a certain power, then it is advisable to make the beam as narrow as possible in order to maximise the signal at the receiver. It turns out that the higher the frequency of transmission, the easier it is to narrow the beam. This is why a laser appears as a sharp narrow beam of light, whereas a radio signal fans out over a wide angle.
KeywordsRadio Frequency Electromagnetic Wave Electromagnetic Spectrum Antenna Gain Optical Wavelength
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