There are few questions in science as simple to state and profound in their implications as “are we alone?”—are humans the only species with a technological civilisation in the galaxy, or in the universe? This has been a matter of speculation by philosophers, theologians, authors of fiction, and innumerable people gazing at the stars since antiquity, but it was only in the years after World War II, which had seen the development of high-power microwave transmitters and low-noise receivers for radar, that it dawned upon a few visionaries that this had now become a question which could be scientifically investigated.
The propagation of radio waves through the atmosphere and the interstellar medium is governed by basic laws of physics, and the advent of radio astronomy demonstrated that many objects in the sky, some very distant, could be detected in the microwave spectrum. But if we were able to detect these natural sources, suppose we connected a powerful transmitter to our radio telescope and sent a signal to a nearby star? It was easy to calculate that, given the technology of the time (around 1960), existing microwave transmitters and radio telescopes could transmit messages across interstellar distances.... [Read More]