All that night long, the boy slept and the man waked, gazing forward steadily into the dark. There were no stars. – Ursula K. Le Guin, The Farthest Shore.
In San Francisco on Labor Day Weekend, the folks at the California Academy of Sciences screened a special live show called “Tour of the Universe”. Here, a narrator took the audience from Earth into Outer Space, beyond our solar system, beyond the limits of our galaxy and galactic cluster, to the edge of the observable universe. As eager observers of many a starry night, P and I watched the digital projections with interest. When we filed out of Morrison planetarium, P, whose curiosity is not easily contained, wondered aloud what the narrator had meant by the concept of the Earth’s Radiosphere and why any intelligent life outside the Radiosphere has no chance of knowing about us.
Between the two of us, the explanation that we developed at the time was as follows: the Radiosphere is an imaginary sphere whose center is the same as the center of the earth, and whose radius equals the distance traveled by the first radio signals emitted by human beings in the 1930s. If you remember the opening sequence of the movie Contact, the camera is taking to the far reaches of outer space, where we here radio signals documenting several historical events of the last century, becoming older as the camera zooms further out from the Earth. In order for another intelligent civilization in the Universe to know about our existence, they must necessarily have to intercept the transmissions that human beings have inadvertently sent out into space – all manner of radio signals, soaring arias, documentaries, bad reality TV programs, tragedies and comedies, nuclear explosions, the 11 minutes of athletic activity embedded inside three hours of advertising static that constitutes a game of American football. This necessarily means that the listening civilization has to be within the imaginary Radiosphere. Or, in other words, if there is intelligent life on an exoplanet that lies outside the Earth’s Radiosphere, then it would be impossible for them to know of our existence.
P’s question was that, since the Radiosphere will grow outwards at the rate of 1 light year per year in all directions, would an intelligent civilization currently outside the Radiosphere eventually hear about us? And not thinking carefully enough, I said, “Yes, eventually”. However, as is often the case, her questions are harder and deeper than they initially seem; I was wrong. Since we live in an expanding universe, the correct answer is “Not necessarily”. This has bizarre implications, if we allow our minds to wander a little. First, let us see why the Radiosphere cannot grow fast enough for its surface to reach any given exoplanet: To be specific, let this exoplanet belong to a star in a different galaxy from ours, one that is far far away as in Star Wars lore. Given that we live in an expanding universe – as opposed to a stationary or a contracting universe – every galaxy is moving away from every galaxy owing to the expansion of space time. Furthermore, if the physicists are right, then the farther two bodies are, the faster they are receding from each other. Thus, our chosen exoplanet is moving away from us faster and faster. Much to my surprise, physics does not put the usual constraints on the speed of this expansion, i.e., the expansion of spacetime can take place faster than the speed of light! I don’t know why exactly this is true, and I must find this out from an advanced physics textbook or from a physicist. The Radiosphere is, of course, growing at the speed of light. Now, if our exoplanet was far enough to being with, and was moving away fast enough, then it is possible that the expanding Radiosphere may never grow fast enough to swallow it.
Unless I am mistaken, the implications of this thought experiment – academic though they may be – are both somber and fascinating: Assuming that some intelligent species does not kill itself off in a nuclear holocaust, survives the interplanetary billiards of meteor and comet collisions, weathers volcanic eruptions and similar catastrophes, evolves, migrates to other distant worlds to escape the inevitable expansion and death of their parent star, they will eventually be alone in a practically limitless sea of empty space. Owing to the expansion of space time, every other star and every other planet has receded far away out of reach, so far away that even light cannot catch up, and therefore any communication with any outside world is impossible. The odds of any earthbound species surviving many billion or more years into the future are exceedingly slim, but if it does, what a strange life awaits it? For, other than the parent star system which gives it life during the day, and a possible dim moon or two, the night sky will be utterly dark – No visible stars, no constellations, no galaxies, no nebulae. Without these keepers of cultural lore, these signposts for voyagers, these pointers in a coordinate system, what would a species’ culture be like? Their stories? Their myths? Their gods? Their science? Their place in the universe? Where will they put their heroes and villains, if not in imagined patterns in the skies?
[I wonder though, won’t a universe in that incredible state of dissipation, where everything is so far away from everything else, be utterly, mind-numbingly cold; too cold to support any kind of life as we know it? But let us set that interruption out of our minds for a few more moments of irrational speculation :-)]. Now, what if there was no evolutionary continuity between our time and the time of this hypothetical species that we have dreamed up? What if they evolve long after every currently thriving species and currently thriving archival technology has long been extinguished? How could they ever know that there once was a time when billions of celestial pinpoints made awesome patterns in the sky? Could their Einsteins and Newtons ever know that they were once part of a galactic conglomeration of stars, planets and interstellar gas and dust? Could their Sagans and their Asimovs figure that, many eons before them, star-stuff had come to life and contemplated the cosmos? In the absence of celestial signposts, will they again return to thinking, as the pre-Copernicans did, that their planetary home is the center of the universe?!
More information on the Radiosphere from the Hayden Planetarium.