Theoretical physicist Tasneem Zehra Husain has an excellent article on the multiverse in this month’s Nautilus. In this age of the expert whom we must trust to give us the truth, Husain’s transparent and clear explanation of some of the underlying philosophical concerns regarding the multiverse is refreshing. I only wish that her writing was more aware of the historical plenitude traditions. Many of the philosophical concerns regarding the multiverse interact heavily with, or even are mandated by, plenitude thinking. Husain makes this quite clear, and locating this thinking in the historical matrix of plenitude traditions would further enrich and elucidate her explanation of the multiverse hypothesis.

Plenitude thinking holds that everything that can exist will exist. As Aruther Lovejoy observed, it had an obvious influence on a range of thinkers since antiquity, including Bruno’s infinity of worlds (read extra-terrestrials) and Leibniz’s view that the species are “closely united,” and “men are linked with the animals.”

Though I don’t suspect plenitude thinking had a direct influence on the initial development of the multiverse hypothesis, it doesn’t take a physicist to see a fairly obvious connection. If everything that can exist will exist, then why should there be only one universe?

But a more interesting interaction comes in how physicists evaluate and justify the multiverse hypothesis which, after all, isn’t very satisfying. With the multiverse, difficult scientific questions are answered not with clever, enlightening, solutions but with a sledgehammer. Things are the way they are because things are every possible way they could be. We are merely living in one particular universe, with one set of circumstances, so that is what we observe. But every possible set of circumstances exists out there in the multiverse. There is no profound explanation for our incredible world. No matter how complicated, no matter how unlikely, no matter how uncanny, our world is just another ho-hum universe. All outcomes exist, and all are equally likely. Nothing special here, move along.

As Princeton cosmologist Paul Steinhardt puts it, the multiverse is the “Theory of Anything,” because it allows everything but explains nothing. Given this rather unsatisfying aspect of the multiverse, how can it be defended?

Enter plenitude thinking. An important theme in plenitude thinking is that there should be no arbitrary designs in nature. If everything that can exist will exist, then no particular designs will exist where others are also possible.

This has become a powerful element in evolutionary philosophies of science. As Leibniz explained, the entire, continuous, range of designs should be manifest in nature, rather than a particular, arbitrary design. That would be capricious.

This rule holds unless there is sufficient reason for it not to (Leibniz’s PSR). If only one design can arise in the first place, due to some reason or technicality, then all is good — the design is no longer viewed as arbitrary. The problem is, we can find no such reason or technicality for our universe. It seems any old universe could just as easily arise.

Plenitude thinking mandates that the designs we find in nature should fill the space of feasible designs. We should not find particular designs where others are possible. But this seems to be precisely what we find in our universe. It is a particular design where others are possible. Theoreticians have been unable to find any reason for why this design should have occurred.

If we say the universe was designed, then it is a design that is arbitrary, and that violates the Principle of Plenitude. The solution to this conundrum is the multiverse.

This is how physicists can learn to love the multiverse. Yes, it is a sledgehammer approach, but it satisfies plenitude thinking. Our universe is no longer arbitrary. Instead, the full range of universes exists out here. Husain beautifully explains this, and here is the money passage:

For decades, scientists have looked for a physical reason why the [universe’s] fundamental constants should take on the values they do, but none has thus far been found. … But to invoke design isn’t very popular either, because it entails an agency that supersedes natural law. That agency must exercise choice and judgment, which — in the absence of a rigid, perfectly balanced, and tightly constrained structure, like that of general relativity — is necessarily arbitrary. There is something distinctly unsatisfying about the idea of there being several logically possible universes, of which only one is realized. If that were the case, as cosmologist Dennis Sciama said, you would have to think “there’s [someone] who looks at this list and says ‘well we’re not going to have that one, and we won’t have that one. We’ll have that one, only that one.’ “

Personally speaking, that scenario, with all its connotations of what could have been, makes me sad. Floating in my mind is a faint collage of images: forlorn children in an orphanage in some forgotten movie when one from the group is adopted; the faces of people who feverishly chased a dream, but didn’t make it; thoughts of first-trimester miscarriages. All these things that almost came to life, but didn’t, rankle. Unless there’s a theoretical constraint ruling out all possibilities but one, the choice seems harsh and unfair.

Clearly such an arbitrary design of the universe is unacceptable. (By the way, Husain also adds the problem of evil as an associated problem: If the universe was designed, then “how are we to explain needless suffering?”)

The multiverse solves all this. True, the multiverse is an unsatisfactory, sledgehammer approach. But it saves plenitude, and that is the more important consideration.

Husain’s article is a thoughtful, measured explanation of how physicists today are reckoning with the multiverse hypothesis. But make no mistake, religion does the heavy lifting. The centuries-old plenitude thinking is a major theme, running all through the discourse. That, along with a sprinkling of the problem of evil, make for decisive arguments.

The multiverse is another good example of how religion drives science in ways that are far more complex than is typically understood.

Cross-posted at Darwin’s God.

Image credit: ESA/Hubble & NASA.