Interstellar Communication If interstellar travel is too difficult, let's use Photons instead! Not surprisingly, most scientists have explored the possibility of communicating by radio, but using light may also be possible (read about OSETI. Much power needed It is not difficult to calculate the power that would have to be transmitted by a civilisation on a star 100 light years away so that we could detect it with a radio telescope such as that at Jodrell Bank. If the signal was sent simultaneously in all directions at once, the power required would have to be a million times more powerful than the most powerful transmitter that we have here on Earth - not very practicable! So the remote civilisation would need to use a large antenna in order to direct the power towards a star where they might suspect another civilisation resides. Thus, unless they have a vast number of such transmitting systems, each pointing towards a different star, we would have to hope that we are directing our antenna at them at the right moment, that is, whilst they are pointing at us! But at least, in principle, we could expect to communicate over distances of 3000 light years, using only the technology we have at present here on Earth. Future technology will greatly extend this range, perhaps across the whole galaxy, and a new generation of receiving antennas that we are just beginning to build, will be able to receive weak signals from several directions simultaneously - so we could, in future, keep watch on many stars at once. What will the messages look like? One point to note is that the calculations which give the possible detection range assume a very simple and slowly transmitted signal - such as morse code with only one dot or dash every second or so. We could not expect to detect television type signals even from the nearest star systems so we do not have to worry that other civilisations might be judging our intelligence on the basis of typical TV programmes radiated from our Earth! Once we have received a signal, we will face another problem. How might we be able to understand that message?