Reporting on a new exhibit at the Science Museum in London, The Guardian has a fun article about the question of extraterrestrial life, its probabilities, its possible forms, and its role in books, film and television.
Amid the historical data, the popular speculations and the observations on Captain Kirk and his penchant for big-haired babes from beyond, are plenty of reasonable comments and a little physics. For example
We don’t know for sure because aliens continue not to visit. One in 100 Americans may believe they have been abducted (cue for a short lecture on sleep paralysis, which manifests itself in pressure on the chest and the experience of being taken away) but if the laws of physics extend beyond Alpha Centauri, then they have not. The distances to the nearest stars are awesome, and the energy costs literally astronomical.
The article ends with a summary of recent work done by people like Clive Trotman, a biologist at the University of Otago in New Zealand.
You can’t just broadcast a message saying, “Is anybody out there?” The signal dissipates as the square of the distance. By the time you get to Pluto, it’s already vanishingly faint.
So you send an ultra-powerful signal as a focused laser beam. How much energy would that take? How long could you afford to transmit? How many directions must you point the transmitter to cover the whole sky? (The answer to that one is 100,000 trillion). And what chance a citizen of an alien civilisation is tuned in when your one-second message whistles by at the speed of light? The arithmetic, says Trotman, predicts one-way communication with both antennae pointing at each other will happen for one second every 10 billion billion years. Assuming, that is, both civilisations are using the same wavelength.
Ah, the inverse square law; cornerstone of physics and astronomy classes everywhere, and a perfect beginning to the first week of my Fall semester.