For Extraterrestrial Intelligent Life

As, from our above discussions, it appears to be quite reasonable to expect the existence of a sizeable number of intelligent civilizations in our galaxy, is there a way to detect them? Let us discuss the search efforts for extraterrestrial intelligence (SETI). The only practical way by which we can obtain knowledge about our surrounding universe is by electromagnetic radiation. Except for material inscribed on matter (Rose and Wright 2004), other messengers from the universe do not deliver the huge amount of information that is transmitted in the electromagnetic spectrum. Such alternate messengers are cosmic rays, charged particles, primarily protons and electrons that are produced in supernova explosions or in outbursts on the Sun, gravitational waves, which are generated by the collapse of massive stars or star mergers, and neutrinos, generated by nuclear processes in the core of stars. Today, the entire electromagnetic spectrum, ranging from gamma rays and X-rays over the ultraviolet-, visible-, and infrared light range, up to microwaves and radio waves, can be observed either from the Earth's surface or from space.

There are parts of this spectrum that are particularly suitable for interstellar communication. The higher the frequency of the electromagnetic waves, the more easily they are absorbed by the interstellar gas and dust. In visible light most of the galaxy is heavily obscured, whereas in infrared light star formation regions or the galactic center can be observed much better. The galaxy is essentially transparent in the microwave region. Here the range of frequencies from 1 to 100 GHz (30 cm to 3 mm wavelength) shown in Fig. 10.4 presents a vital window, where the amount of background radiation is at a minimum and where it might be particularly easy to carry out interstellar communication (Oliver 1977).

For frequencies below 1 GHz, as seen in Fig. 10.4 (left side of the figure), the galactic synchrotron radiation gives an increasingly larger background contribution the smaller the galactic latitude angle |b| is against the galactic plane. This synchrotron radiation is produced by high-energy cosmic ray electrons, which are forced to orbit around magnetic fields which lie in the galactic plane. There is also a high-frequency limit because of the spontaneous emission of photons in galactic clouds of various temperatures (right side of the figure). Figure 10.4 shows the microwave window outside the Earth's atmosphere. When observed from the Earth's surface, the spectrum from 20 to 1000 GHz is contaminated by H2O and O2 emission from the terrestrial atmosphere. This leaves a microwave window from 1 to 20 GHz from the

Fig. 10.4. The microwave window in terms of an emission temperature, for outside the Earth's atmosphere. The more intense the background radiation of the galaxy, the greater is the emission temperature. On the ground the window is narrowed to the left of the dotted line because of emission by H2O and O2 molecular bands in the atmosphere (modified after Oliver 1977)

Fig. 10.4. The microwave window in terms of an emission temperature, for outside the Earth's atmosphere. The more intense the background radiation of the galaxy, the greater is the emission temperature. On the ground the window is narrowed to the left of the dotted line because of emission by H2O and O2 molecular bands in the atmosphere (modified after Oliver 1977)

ground with a very low background, called the "water hole", although atmospheric water is no longer a limitation when observing from space. But even in the center of the "water hole" in Fig. 10.4 there is still noise from the 3 K cosmic background radiation, which is the remnant of the Big Bang fireball (Chap. 1). It is fortunate that in the "water hole" the very important 1.4024 GHz (21 cm) neutral hydrogen radio line is located, which because of the low galactic background noise has allowed us to map our galaxy and determine its spiral structure.

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