Possible Contact in the not too Distant Future

The current SETI programs can detect 3 kW (a small radio station) at a distance of 100 Ly if its transmission is beamed toward Earth. Therefore, to detect a similar signal from an extraterrestrial civilization at a distance of 1700 Ly would be beyond our present technical capabilities. In addition to the signal strength, there is another important point that must be considered when trying to detect radio transmissions from extraterrestrial civilizations: the age distribution of the emitting societies. From the above estimate, that about 2 million intelligent societies have formed in the past 5 billion years, and assuming that star formation was a continuously ongoing process over all this time, one finds an average interval of 2500 years between the birth of intelligent societies. This birth interval means that the societies that are closest to our present state are either 2500 years more advanced or lag 2500 years behind. In the latter case, they would not yet be able to communicate.

If the next more advanced society, our closest temporal neighbor, also started to communicate with a profusion of radio waves like us, would there be a way detecting it? The difficulty here is the great size of our galaxy. Because the creation of the planet that carries the next society was a chance event, the likelihood that it formed nearby is small. Statistically, one must assume that this next society is located roughly a galactic radius, 50 000 Ly, away from us, and because of the finite speed of light, their first radio signals will not reach us until 47 500 years in the future. This shows that with our present receivers we will not be able to detect the initial radio transmissions from our closest temporal neighbors, but only signals from older more advanced societies that live closer.

Fig. 10.7. Distances of extraterrestrial societies (dots) emitting radio signals. The dashed circle marks the galactic disk, the cross its center and the dot marked Sun is the position of the Earth. Solid circles indicate distances from which travel times of radio waves to the Earth are multiples of 10 000 years

If we assume that as soon as an extraterrestrial society becomes communicating, it starts with an outburst of radio emission then, in principle, we should be able to detect societies that are up to 76 000 years older (see Fig. 10.7). These signals would come to us from the opposite edge of the galaxy and would have spent all this time traveling the 76 000 Ly toward Earth. Figure 10.7 shows our galaxy (dashed) from above the galactic plane

Fig. 10.7. Distances of extraterrestrial societies (dots) emitting radio signals. The dashed circle marks the galactic disk, the cross its center and the dot marked Sun is the position of the Earth. Solid circles indicate distances from which travel times of radio waves to the Earth are multiples of 10 000 years

Distance from Earth (1000 Ly)

Distance from Earth (1000 Ly)

and the Earth (dot marked Sun) at a distance of 26 000 Ly from the galactic center (cross). Also shown are circles (solid) that mark positions from which one has the same travel times of radio waves (in multiples of 10 000 years) to Earth. Possible radio-emitting societies somewhere in the galaxy are shown as dots. If intelligent societies are born every 2500 years, with the above mentioned maximum travel time, initial radio bursts should be observable from at most 30 (= 76 000/2500) older societies. Yet this number is probably too high, because the density of stars is greater in the inner parts of the galaxy (see Fig. 10.7) where the star formation rate was higher. A guess is that there may be about 25 societies from which the initial radio outburst could be observable.

But for how long will intelligent civilizations emit radio waves, and would these advanced societies eventually stop this conspicuous emission voluntarily? If we assume that radio emissions continue at least over a thousand years before they are stopped, then the probability of detection until another society commences radio emission is 0.4 (= 1000/2500), which means, that we should presently be able to detect the initial radio emissions of about 10(= 25 x 0.4) extraterrestrial societies from our entire Milky Way, which were roughly in a technological state like ours when the signals were sent off. For shorter radio emission times, we would detect even fewer societies. Yet with such small numbers one always has the chance that because of an unequal distribution in birth times and distances one might have no detection at all. On the other hand, if the extraterrestrials do not hesitate using radio waves for their entire lifetimes, we should detect at least 4000 radio sources from extraterrestrial societies.

As mentioned above, there is also the problem of detecting such relatively weak radio sources at the present time over large galactic distances. Here, luckily, our rapidly advancing radio technology will greatly improve this situation in the near future. One day, with still more advanced technology, we might even detect initial radio transmissions from other galaxies. This indicates that extraterrestrial intelligent civilizations will not be hidden from us forever and that SETI searches will eventually be successful. However, because these radio communications would originate from civilizations that lived many thousands of years ago in our Milky Way, and 2-40 million years ago in nearby galaxies, there will clearly be no chance of a two-way conversation.

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