Not Only Is Earth One Nice Planet Among Many, but Our Entire Universe Is Lost in a Crowd
Your patience is earnestly requested. We're a bit delayed finding those habitable planets, their myriad life forms, and that dense throng of alien civilizations. Fine-tuning remains a problem. So, more boldly go. What if not just Earth, but our whole universe, is seen as one mere Copernican blip?
Should the fine-tuning turn out to be real, what are we to make of it? There are two widely-discussed possibilities: either God fine-tuned the universe for us to be here, or there are (as string theory implies) a large number of universes, each with different laws of physics, and we happen to find ourselves in a universe where the laws happen to be just right for us to live. After all, how could we not?A large number of hitherto unnoticed universes? No sooner asked than granted: Nima Arkani-Hamed and others have proposed over 10^500 universes because fewer of them would not obviate fine-tuning. Why believe in them? As a New Scientist writer has explained
But the main reason for believing in an ensemble of universes is that it could explain why the laws governing our Universe appear to be so finely turned for our existence ... This fine-tuning has two possible explanations. Either the Universe was designed specifically for us by a creator or there is a multitude of universes -- a multiverse.Cosmologists deserve credit for making the choice so clear. In that spirit, Discover Magazine offers the multiverse as "Science's Alternative to an Intelligent Creator" (2008). And New Scientist's editors promise, in addition, a "multiverse-fuelled knowledge revolution". As David Berlinski puts it, "The Big Fix has by this maneuver been supplanted by the Sure Thing."
Three multiverse concepts worth knowing (because they turn up time and again in pop science media) are inflation, string theory, and M-theory:
inflation: Cosmologist Alan Guth champions a particle, the "inflaton," which caused the universe to expand faster than the speed of light, thus (accidentally) smoothing the Big Bang into its present delicate balance between too loud and too soft. In Guth's eternally inflating universe:
... anything that can happen will happen; in fact, it will happen an infinite number of times. Thus, the question of what is possible becomes trivial -- anything is possible, unless it violates some absolute conservation law.Guth's signature statement is "The recent developments in cosmology strongly suggest that the universe may be the ultimate free lunch."
string theory: In a theory that became popular in the late 1970s, to reconcile gravity with quantum mechanics in a ten-dimensional spacetime, the building blocks for all the universes are strings of vibrating energy. The extra six spatial dimensions curl up inside our four conventional ones (3D plus time). The resulting string landscape is a huge increase in possible solutions to equations. Key string theorist Leonard Susskind is blunt about one attraction of the theory: "Without any explanation of nature's fine-tunings we will be hard pressed to answer the ID critics."
Nobelist Steven Weinberg agrees:
Just as Darwin and Wallace explained how the wonderful adaptations of living forms could arise without supernatural intervention, so the string landscape may explain how the constants of nature that we observe can take values suitable for life without being fine-tuned by a benevolent creator.String theory has been known at times to sound like the Darwin of the cosmos, underwriting a God-free universe.
M-theory is a composite of various versions of string theory, with an eleventh dimension added in the mid-1990s, in the hope of finding the "grand design" of the multiverse, as set out by Stephen Hawking in his 2010 book with physicist Leonard Mlodinow, The Grand Design (2010).
Cosmologist Max Tegmark offers the most elaborate multiverse scheme to date, first introduced in Scientific American in 2003. Starting with the assumption that fine tuning is best explained by the existence of infinite undetected universes, he goes on to point out that therefore there is an infinite number of people just like you:
The simplest and most popular cosmological model today predicts that you have a twin in a galaxy about 10^1028 meters from here. This distance is so large that it is beyond astronomical, but that does not make your doppelg�nger any less real...That's only a Level I multiverse. We must take it on faith, because we will never see these faraway selves. Level II multiverses, which can have different space, time, and laws of physics, explain fine-tuning: " ... it's not surprising that we find ourselves in one of the rare universes that are inhabitable, just like it's not surprising that we find ourselves living on Earth rather than Mercury or Neptune."
Level III multiverses are created every time you or anyone (or anything) takes one path instead of another in an infinite dimensional space (Many Worlds theory). In the Level IV multiverse, all alternative realities exist at the same time, and mathematical concepts are real.
If you are tempted by doubt, Tegmark offers a simple explanation: "Evolution provided us with intuition for the everyday physics that had survival value for our distant ancestors, so whenever we venture beyond the everyday world, we should expect it to seem bizarre."
So Darwin can explain both the multiverse (Weinberg) and why we doubt it (Tegmark). Darwinism is more important in upholding multiverse cosmologies than many realize. But that makes sense. Just as humans are merely one accidentally evolved animal among many, our universe is simply one accidentally evolved universe among many.
The Copernican Principle is similarly flexible: When conjuring habitable planets, it assumes ours is one among countless winners. Yet when conjuring a multiverse, it assumes that our universe is a lonely winner among countless flops. The choice seems to depend on which assumption is required as a defense against design. That feature, as we shall see, can once again transform speculation into orthodoxy.