The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [87]
In this way of thinking, to ask why the constants have their particular values is to ask the wrong kind of question. There is no law dictating their values; their values can and do vary across the multiverse. Our intrinsic selection bias ensures that we find ourselves in that part of the multiverse in which the constants have the values with which we’re familiar simply because we’re unable to exist in the parts of the multiverse where the values are different.
Note that the reasoning would fall flat if our universe were unique because you could still ask the “lucky coincidence” or “deeper explanation” questions. Much as a potent explanation for why the shop has your shoe size requires that the shelves be stocked with many different sizes, and much as a potent explanation for why there’s a planet situated at a bio-friendly distance from its host star requires planets orbiting their stars at many different distances, so a potent explanation of nature’s constants requires a vast assortment of universes endowed with many different values for those constants. Only in this setting—a multiverse, and a robust one at that—does anthropic reasoning have the capacity to make the mysterious mundane.*
Clearly, then, the degree to which you are swayed by the anthropic approach depends on the degree to which you are convinced of its three essential assumptions: (1) our universe is part of a multiverse; (2) from universe to universe in the multiverse, the constants take on a broad range of possible values; and (3) for most variations of the constants away from the values we measure, life as we know it would fail to take hold.
In the 1970s, when Carter put forward these ideas, the notion of parallel universes was anathema to many physicists. Certainly, there’s still ample reason to be skeptical. But we’ve seen in the previous chapters that although the case for any particular version of the multiverse is surely tentative, there’s reason for giving this new view of reality serious consideration, Assumption 1. Many scientists now are. Regarding Assumption 2, we’ve also seen that, for example, in the Inflationary and Brane Multiverses, we would indeed expect physical features, such as the constants of nature, to vary from universe to universe. Later in this chapter we’ll look at this point more closely.
But what about Assumption 3, concerning life and the constants?
Life, Galaxies, and Nature’s Numbers
For many of nature’s constants, even modest variations would render life as we know it impossible. Make the gravitational constant stronger, and stars burn up too quickly for life on nearby planets to evolve. Make it weaker and galaxies don’t hold together. Make the electromagnetic force stronger, and hydrogen atoms repel each other too strongly to fuse and supply power to stars.13 But what about the cosmological constant? Does life’s existence depend on its value? This is the issue Steven Weinberg took up in his 1987 paper.
Because the formation of life is a complex process about which our understanding is in its earliest stages, Weinberg recognized that it was hopeless to determine how one or another value of the cosmological constant directly impacts the myriad steps that breathe life into matter. But rather than give up, Weinberg introduced a clever proxy for the formation of life: the formation of galaxies. Without galaxies, he reasoned, the formation of stars and planets would be thoroughly compromised, with a devastating impact on the chance that life might emerge. This approach was not only eminently reasonable but also useful: it shifted the focus to determining the impact that cosmological constants of various sizes would have on galaxy formation, and that was a problem Weinberg could solve.
The essential physics is elementary. While precise details of galaxy formation are an active area of research, the broad-brush process involves a kind of astrophysical snowball