Free Astronomy Magazine November-December 2020

47 NOVEMBER-DECEMBER 2020 advanced civilization may have already built it. Having enough power available, ET would be able to send light pulses into space that would add to the brightness of the parent star, causing small variations in magni- tude in the entire visible spec- trum (white light) or in certain colors. By suitably modulating the impulses, a sequence “easily” dis- tinguishable from the variations attributable to natural phenom- ena would be generated. What can we expect to see in that star’s light curve? Probably short and simple numerical sequences (it is reasonable that ET knows how to count), represented by impulses of varying duration, intensity and amplitude. If the sole purpose of the message is to inform us that we are not alone, the beginning of some basic numerical se- quences may be enough, where each subsequent number is repre- sented with light pulses of a du- ration or intensity proportionally higher than the previous one. We can perhaps expect the most basic 1, 2, 3, 4, 5, 6, 7... or a se- quence of the prime numbers 2, 3, 5, 7, 11, 13, 17... or the Fi- bonacci sequence 1, 1, 2, 3, 5, 8, 13... all probably known by a technological alien civilization. The preference of one sequence over the others may depend on the amount of energy required to produce and keep the signal transmitted for a long time. Intuitively, higher numbers would re- quire more energy, and for this reason the sequences might appear short but re- peated. Long sequences, as well as being excessively expensive, might also be less legible. Curiously, the duration of the inter- vals between successive pulses could also be revealing, representing, for example, it- self a numerical sequence. Waiting for more targeted SETI projects, let’s turn on the radio − you never know what might happen! dio domain is truly amazing. Perhaps it would be preferable to launch more diver- sified surveys, massively including visible frequencies in these studies − a choice that could restore meaning to the dozens of op- tical telescopes that lie unused in less ad- vanced professional observatories. An alien civilization could send a periodic message in visible light through powerful lasers powered by large photovoltaic sys- tems much more efficient than ours. From a conceptual point of view, we earth- lings already have an idea of how to make a transmitter of that type. A slightly more !