Halpern, Paul, Wesson, Paul. "1 To See the World in a Grain of Sand: What We Can Observe from Earth." Brave New Universe: Illuminating the Darkest Secrets of the Cosmos. Washington, DC: The National Academies Press, 2006.
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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos
with those signals. Suppose intelligent life is rare enough that none of these nearby stars have planets inhabited by civilizations capable of radio communication. Then presently no other world in space could possibly know about Felix and his human creators.
If we now imagine intelligent life so uncommon that the nearest communicative civilizations lie in remote galaxies, we can see how vast distances could preclude contact. A culture millions or billions of light-years away would have had to be broadcasting for eons in order for us to know about them. If we use the history of life on Earth as the model, many planets would have been too primitive to support advanced beings that long ago. Therefore, no communicative civilization would be ancient enough for its signals to have already reached us.
Moreover, on galactic scales the expansion of the universe would greatly exacerbate the time delay. Because the alien races would be situated in galaxies fleeing from ours, their radio broadcasts would need to cross ever-widening gulfs. Hindered by the currents of outward galactic movement, any messages sent out would wash up on our shores far, far later than they otherwise would in a static universe.
If the closest civilizations are even farther away, we would never learn of their existence. As cosmology tells us, beyond an invisible barrier called the particle horizon—defined as the greatest distance any incoming particle could have traversed in the universe’s current age—alien signals wouldn’t stand a chance of reaching us. They’d face the situation of Alice in the looking-glass world; though they’d travel as fast as they could, they wouldn’t be able to outrace the expansion of the universe.
Indeed, it’s entirely possible that a cornucopia of worlds could reside beyond the curtain of invisibility. Some of these planets might even be Earth’s near twin. Others could house technologies exceeding our wildest speculations. Yet unless any of these societies find a way of circumventing the speed of light, we would remain as separate from them as prisoners confined forever to solitary cells.