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I'm trying to understand how even places that are no longer sustaining life can continue to give off some sort of signal. Is that right? Can we really listen to parts of the universe that are not currently sustaining life, but may perhaps still transmit something detectable, and of intelligent origin? Chong, San Francisco
The Doctor Responds: Analogy: the speed of sound. If lightning strikes a mile away, the thunder doesn't reach your ears for about five seconds. That's because sound travels at a speed of about 1000 feet per second. So, by the time you receive the audio message (thunder), the transmitting entity (lightning) is already dead! Well, light (and radio waves, which are a form of light) travel much faster than sound, but they are not instantaneous. The speed of light (and SETI signals) is about a million times faster than the speed of sound (or roughly a billion feet per second). So, a signal from the nearest star would take four years to reach us. Echoes off the Moon (such as from The SETI League's moonbounce beacon) take a couple of seconds. And signals reaching us from the far side of the Milky Way have been travelling tens of thousands of years. Certainly, if we receive such signals from planets orbiting distant stars, we can assume that the individual sender (if not its whole civilization) may be long dead. So, SETI is really not about communications, but rather about interstellar archaeology. What we expect to unearth will be artifacts, that will tell us something about a (possibly) long-gone civilization.
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