Wednesday, November 30, 2005


Dying to talk to someone

I was imagining having an Uncle Joe across the galaxy: being able to talk to him over the radio, but unable ever to see him. Then I realized that scenario does not make sense. The incongruity between my intuition and my understanding of physics was delightful.

Radio waves are light, and hence subject to the speed of light, so a single conversation at interstellar ranges could take lifetimes. Moreover, since visible light and infrared light are both light, my inability to see Uncle Joe is practically identical to my inability to exchange radio messages with him.

One difference is that radio signals are serial, whereas visual signals are series-parallel. If we needed to transmit radio messages in parallel, though, we could do it by using more than one frequency (“channel”).

A stranger difference between radio and visual is the potential for broadcasting at different volumes. To broadcast a visual image of Joe more “loudly”, Joe’s engineers would have to shine more light on him, and to distinguish him from background radiation I would require more light than his body could reflect without combusting. Still, intergalactic radio and intergalactic semaphore are essentially the same process.

My Earth-formed concept of distance does not predict well what happens at interstellar ranges. That tickles me. The Earth-formed concepts of number, or Euclidean geometry, those seem to hold at any range in the universe. (Non-Euclidean geometries have been sought by astronomers, but none found.) However, other intuitions just don’t scale.

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