Written by: Dimitri Leggas
Edited by: Hsin-Pei Toh
Today’s popular scientific discourse is filled with discussions on “the multiverse,” with headlines like “The Case for Parallel Universes” and “Looking for Life in the Multiverse” filling journals such as Scientific American. Many of these articles focus on the excitement of numerous universes rather than their theoretical underpinnings. The idea that infinitely many of “you” exist is impressive, if daunting, and the idea of a multiverse has become increasingly juxtaposed with divine creation. But to accept the multiverse may come at the cost of a wider scientific understanding of our universe as well.
It is well known that slight adjustments to any fundamental physical parameters would drastically change our universe, more particularly our solar system, making life as it occurs on Earth impossible. Take for instance the force of gravity. Were gravity only a few percentage points stronger, the sun would have burned out in under one billion years, eliminating any possibility for the evolution of complex life. On the other hand, if gravity were a few percentage points weaker, the center of the sun would not reach the temperatures necessary to catalyze the nuclear reactions that produce sunlight that most life relies upon. A host of examples like these suggests that our universe is somehow a special place: one that is able to accommodate life hospitably. In 1973, Brandon Carter, a British physicist, offered an explanation in the form of the anthropic principle, which states that the parameters of our universe are fine-tuned for life as we experience it, precisely because we are here experiencing it.
Still, the anthropic principle does not examine why fundamental quantities in physics take the values that they do. That is, why is the gravitational constant 6.67408 × 10-11 m3 kg-1 s-2 and not another number that would make life as we understand it impossible? Adherents of Intelligent Design and Creationism seize the scientific recognition of fine-tuning in order to proselytize religious ideology. They contend that these coincidences that make life possible point to our creation by a superior being. Scientists are not eager to accept these claims. After all, if countless universes existed, each with different parameters, then it would come as no surprise that one with our “fine-tuned” settings will exist as well, according to the laws of probability. According to Tim Maudlin, a philosopher from NYU interested in cosmology, most physicists don’t use the fine-tuning argument, but instead rely on mechanisms within physical frameworks to explain the multiverse. Two such theories are inflation and string theory.
He proposed a rapid expansion of the universe, due to an energy source like dark energy, in the first trillionth of a trillionth of a trillionth of a second. This theory explains the uniformity of our universe on a large scale. A revised form of inflation called eternal inflation argues that the dark energy source for inflation is not uniform, but “marbled.” One corollary of eternal inflation is that many big bangs occur, so many universes come into being. The notion of a multiplicity of universes finds acceptance in string theory as well. String theory aims to bridge gaps in theory between general relativity and quantum mechanics. Proponents of string theory argue that in addition to the three conventional spatial and single temporal dimensions, an additional six spatial dimensions exist. Mathematics predicts an extraordinarily large (10500) number of possible ways to fold these dimensions, which would be small and tucked into the more familiar ones. Each of these options corresponds to a different universe.
Some scientists are quick to accept the multiverse as an explanation for the anthropic principle. In fact, it is common to accept un-testable predictions of well-tested theories. For example, scientists generally accept ideas about what occurs inside a black hole since general relativity has been repeatedly tested and verified. Because string theory makes predictions at small scales, it remains untestable and not falsifiable. However, many scientists are satisfied with the accuracy of predictions made by inflationary theory and take very seriously the prediction of a multiverse. Often, the mathematically simplest theories, however ontologically absurd they may seem, suggest a multiverse, whereas a great deal of mathematical maneuvering is necessary to yield a single universe.
While a multiverse may seem mathematically appealing and impressive for its sheer vastness, it would change current philosophical approaches to theoretical physics. Researchers would no longer be able to seek a single unifying theory that could elegantly describe everything, because our universe would behave completely differently from others. In a widely debated lecture, Stephen Hawking stated that philosophy is dead because it has failed to keep up with science, in particular physics. Tim Maudlin made the opposite claim, pointing out that physicists do not fully understand the relationship of fundamental constants to one another and how they might shift in order to accommodate changes in others. If it turns out that multiple universes do not exist, he argues that this should not point to intelligent design as the only remaining option. Instead, we may only need to look more deeply into the infrastructure our universe is built upon.