Interstellar Communications System

The article is excessively pessimistic.

Using diode lasers at 2% efficiency is abusrd when there exist xenon difluoride lasers that are 40% efficient.

Second, the amount of power required by an appropriately sensitive receiver is MUCH smaller than specified.

Third, the assumed expansion of the beam is calculated incorrectly. In addition, by careful beam shaping, using a Bessel function wave front, the beam expansion can be dramatically reduced.

In fact, for a reasonable bit rate, a laser with output of a mere 100 megawatts would suffice. Assuming 40% efficiency, the input power would be only 250 megawatts. Such a laser would easily outshine the nearby star in the narrow frequency band produced by the modulated laser. Assuming power generation at 40% (easily achieved using steam turbines) then total thermal input is only 625 megawatts, many orders of magnitude less than described in the web site. This is comparable to the larger (but very compact) ship reactors now in use.

I will leave it as an exercise for the reader to find the errors in the calculations in the web site referred to.

In other words, it should be relatively practical to transmit information via xenon-difluoride laser light over instellar distances at extremely high bit rates with a transmitting mirror of modest (10 meter) diameter. The bit rates achieved will depend on the distance and the diameters of the transmitting and receiving mirrors, and, of course, the transmitting power. The bigger the mirrors, the longer the range, or the lower the power requirements. The receiver is assumed to be able to detect one out of every three photons, which has been achieved in practice. Communication at hundreds of light-years should be possible.

However, absolutely NOTHING can be done about the "speed of light" limitation. At a distance of four light-years, round trip time for a message and reply is eight years. This sort of hampers dialog.

For further information, see http://coseti.org, the official web site of the Optical SETI movement. It contains references to purlished and refereed papers that demonstrate the practical feasibility of interstellar communication.

However, it still remains very unlikely that we will detect a signal from an ETI source in the foreseeable future. It is a very large Universe in which we live. Other civilizations are likely to be tens of thousand of light-years away, in unknown directions.