Is the universe one or many?

For decades physicists have dreamed of a theory of everything that would precisely give all known physical particles and their interactions. Physicists have also hoped that this theory would be so simple and compelling as to be the self-evident “natural” way the universe must be constructed.

Physicists have definitely made progress on the first objective. While there is not a single theory that yet does the trick, a combination of quantum mechanics (in particular quantum chromodynamics) for the very small and general relativity for the very large appear to predict the vast majority of known physical processes with spectacular precision. For example, the magnetic moment of the electron computed via the theory agrees to 10 digit accuracy with the best experimental measurements.

String theory offers a hope of unification even of these two remaining theories, as it seamlessly includes both quantum mechanics and gravitation. However, in spite of decades of effort, string theorists have not been able to make testable predictions from it, a situation that has led some physicists to reject string theory. Research continues, though, and many remain optimistic.

In any event, the second objective, that of finding a unique theory, now appears more elusive and questionable than ever.

To begin with, the masses of some particles, such as the Higgs boson recently discovered at the Large Hadron Collider (LHC) in Europe, are much different than a “natural” reading of the symmetry laws thought to underpin nature. And the expected particles predicted by “supersymmetry” have not appeared in the LHC data, dashing hopes that the supersymmetry theory would explain things.

An even more fundamental question is why the universe did not either collapse back on itself within microseconds after the big bang, or explode with such ferocity as to forever doom the gravitational formation of stars, planets and us. Why is our universe on such a knife edge of long-lived expansion?

The cosmological constant paradox is even more daunting. By a straightforward application of existing theories, the vacuum energy density of the universe (which is related to the cosmological constant) should be a staggering 1090 grams per cc, yet it is measured to be only about 10-30 grams per cc, a discrepancy factor of 10120. This has been called the worst prediction in the history of physics. Some physicists have hypothesized that the contribution to the vacuum energy density has both positive terms (from bosons) and negative terms (from fermions), so that when all are added together, they magically cancel to 120-digit accuracy. But other researchers roll their eyes at this sort of explanation.

Such inexplicable “cosmic coincidences” have led a growing number of physicists to embrace the multiverse, namely the notion that our universe is just a single speck in an enormous ensemble of universes, the vast majority of which are utterly inimical to any form of life. For example, some string theorists have calculated that there are 10500 possible configurations of space-time. Thus, so some physicists speculate, the reason our universe is so absurdly fine-tuned and “unnatural” is because in such a huge ensemble, inevitably one universe (ours) beats the enormous odds and is livable. Otherwise we wouldn’t be here to talk about it.

Physicist Nathan Seiberg, for instance, declared

Ten or 20 years ago, I was a firm believer in naturalness. … Now I’m not so sure. My hope is there’s still something we haven’t thought about, some other mechanism that would explain all these things. But I don’t see what it could be.

But other physicists remain extremely wary of this type of “anthropic” reasoning, as well as the extravagant hypotheses of countless other universes that we cannot observe. Lee Smolin, for instances, writes

We physicists need to confront the crisis facing us. A scientific theory [the string theory/multiverse] that makes no predictions and therefore is not subject to experiment can never fail, but such a theory can never succeed either, as long as science stands for knowledge gained from rational argument borne out by evidence.

Along this line, British physicist David Deutsch argues that what makes a good scientific theory is that it is “hard to vary.” By this metric the string theory / multiverse approach does rather poorly — virtually any set of physical laws and any universe could be constructed by the presently understood approach.

Some religious scholars have claimed that these developments “prove” that a transcendent being “designed” the universe. Theologian Neil Manson, for instance, described the multiverse as “the last resort for the desperate atheist.” But other, more sanguine, observers point out the dangers of jumping to such conclusions.

For one thing, the “God of the big bang” typically leaves most religious-minded persons rather cold. Did Johann Sebastian Bach have the “God of the big bang” in mind when he composed over 1000 works of mostly sacred music, among the greatest in the classical repertoire? Was the “God of the big bang” the being that inspired Albert Schweitzer, Mohandas Ghandi and Mother Teresa to surrender their careers and fortunes, and devote themselves to the poor and downtrodden? Probably not.

In any event, the lessons from the creationism-intelligent design controversy are clear: claims that one can “prove” God (in any religious tradition) via arguments based on apparent design or seemingly inexplicable phenomena in the natural world are likely to disappoint in the long run. And invoking a Creator or Designer every time unexplained phenomena arise is a “thinking stopper,” burying grand questions in the inaccessible mind of an inscrutable being. So considerable caution is in order.

Many of these issues are nicely summarized in a recent Simons Foundation article by Natalie Wolchover. Additional information is given by the present author at Anthropic principle, Cosmic coincidences, Cosmological Constant and Multiverse.

Comments are closed.