We have yet to discover any signs of aliens, a troubling observation that has led to much speculation. One possible solution to the Great Silence is that nobody's out there. It's a conclusion that sounds impossible to believe, but there may be something to it. Here's why we may be alone in the universe. Ever since physicist Enrico Fermi posed the question — where is everybody? — people have been wondering why we haven't seen any signs of extraterrestrial civilizations. As Fermi pointed out, the math just doesn't add up. Our galaxy, at 13 billion years old, has been around long enough for aliens to explore and colonize it many times over by now (recent work shows it should take less than a billion years, perhaps even as little as a few tens of millions of years). Clearly, we should have seen somebody by now. This surprising observation led astronomer Michael Hart to conclude that spacefaring life in the Milky Way should be either galaxy-spanning or non-existent. But the exclusive presence of "non-existent spacefaring" aliens could be attributable to any number of things, including a reluctance to explore space, or owing to technological intractability. But it could also imply that aliens simply don't exist. Indeed, despite all the recent discoveries of potentially habitable exoplanets, along with the general feeling that our universe is primed for life, there are many reasons to suspect we're truly unique in the large scheme of things. The Right Place at the Right Time As astronomer Paul Davies has said, "If a planet is to be inhabited rather than merely habitable, two basic requirements must be met: the planet must first be suitable and then life must emerge on it at some stage." Indeed, life is dependent on the presence of five critical elements, or metals in the parlance of astronomers: sulfur, phosphorus, oxygen, nitrogen, carbon (or SPONC for short). These heavier elements were cooked in nuclear reactions inside stars and became part of the interstellar medium only when stars reached the end of their energy-producing life. So, as time went by, the concentration of metals in the universe gradually increased. But here's the thing — these heavier elements only recently became sufficiently concentrated in the interstellar medium to allow life to form. Planets around older stars, therefore, are likely to be low in SPONC. Only around relatively young stars, like ours, can life emerge. So humanity would thus be among the first civilizations — perhaps the first — to arise. But as Stephen Webb points out in his book, Where is Everybody?, the suggestion that chemical enrichment explains our solitude is, by itself, way too overstated — it's insufficient to completely explain the Great Silence. For example, we don't know the degree of metallicity required of a star for it to possess viable planets, and we know that the metallicity of stars vary considerable between different classes of stellar populations. Simply put, we don't know enough about this variable to make a definitive conclusion about it. Much more via Gizmodo.
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