Interstellar Communications...

Figure 2, page 3 of the paper (arXiv link below)

TECHNOLOGY REVIEW: Here’s an interesting question. If we ever want to communicate with civilisations around other stars, what will be the best way to send a message, given that we will know nothing about how they intend to receive it?

That’s the question considered today by David Messerschmitt at the University of California, Berkeley. It turns out that the laws of physics, the nature of interstellar space and a little common sense place surprisingly strict bounds on how communication can take place. So if extraterrestrials think in a way that is anything like us, communication of one kind or another is distinctly possible.

Messerschmitt begins by listing the way in which any form of communication is likely to be limited. To begin with, he says that the power of any signal falls with the square of the distance traveled. Assuming that energy is likely to be a limiting factor for a civilisation, an important property of any interstellar transmitter will be to minimise the energy per bit in any signal–while still allowing the reliable extraction of the information it contains, of course.

One potential shortcoming is that the science and technologies that Messerschmitt invokes were all developed on Earth within the last century. That’s a blink of an eye in cosmological terms.

Back in the 1970s, the American astronomer Carl Sagan pointed out that any alien civilisation is likely to be at a very different stage in its evolution. Should it be less mature than us, however, this civilisation will not have developed radio technology in the first place.

That means that our potential contacts are likely to be much more advanced, probably centuries or millennia ahead of us. Sagan asked whether it is possible that these civilisations will have stumbled across a better form of interstellar communication technology, one that seems like magic to us.

"Any sufficiently advanced technology is indistinguishable from magic." Sir Arthur C. Clarke

Physics arXiv: How to Design an Interstellar Communications System