We all live under the same sky, but we don't all have the same horizon.
Michael Hart has an interesting way of considering the paradox that he has done so much to promote. To fully appreciate his argument, we have to understand the notion of a particle horizon.175
A particle horizon is easiest to explain in a static Universe. (The Universe is, of course, not static. It began in the Big Bang, has been expanding ever since, and recent findings suggest that it will expand for eternity. Taking into account the expansion of the Universe makes a discussion of particle horizons rather subtle. Fortunately, nothing is lost if we discuss the idea in terms of a static Universe.) Imagine, then, a Universe that is infinite in extent and throughout which galaxies are uniformly distributed. Furthermore, this model Universe came into existence about 15 billion years in the past; perhaps the galaxies already existed, and some supreme intelligence "threw the switch" and turned on all the stars at precisely the same instant. What would such a Universe look like to an observer on an Earth-like planet, some 15 billion years after this creation event? Would the night sky be blindingly bright, the result of light reaching the planet from the infinite number of galaxies? Surprisingly (at least to those unfamiliar with Olbers' paradox), this infinite static Universe would look similar to our own Universe. The point to remember is that nothing can travel faster than light. So no influence — no light, no gravitational waves, nothing — could have reached the observer from regions more distant than 15 billion light years. This distance — the distance to the particle horizon — is the effective size of the observable Universe. Nothing from beyond the horizon has had time to reach the observer.
Hart makes the following argument. First, suppose our Universe is infinite. Since the Universe began some 15 billion years ago, however, the size of the observable Universe is given by the distance to the particle horizon. Second, suppose biogenesis — the development of life from non-living material — is an exceedingly rare occurrence. (We shall discuss the problem of biogenesis in more detail later, but at this point it is sufficient to say that Hart believes the probability of generating the characteristic molecules of life through the random shuffling of simpler molecules is exceptionally small. Most biologists think biogenesis must be common, because life arose so quickly on Earth; nevertheless, our knowledge of these things is so sketchy that Hart's contention cannot be ruled out.) It follows that in an infinite Universe there will necessarily be an infinite number of planets with life, but within any given particle horizon there might only be one planet with life. According to this argument, there is a sense in which there is nothing particularly special about Earth: in an infinite Universe there will be an infinite number of other Earths teeming with life. But within our particle horizon — within our observable Universe — only Earth spontaneously gave rise to life.
As Hart points out, his idea can be falsified quite easily. For example, extraterrestrials could visit Earth; or SETI might succeed and detect signals; or astrobiologists might prove that life arose spontaneously on Mars and independently of Earth. Any of these developments would disprove the notion of biogenesis being a rare, once-in-a-Universe event. In the absence of these developments, though, Hart argues that the Fermi paradox leads to a chilling conclusion: we are the only civilization within our particle horizon. Although the Universe contains an infinite number of advanced civilizations, for all practical purposes we are alone.
The conclusion that we are alone in the Universe — the third class of solution to the Fermi paradox — is the subject of the next chapter.
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