As a child, one of the topics I frequently pondered and discussed with my parents was the scale of the universe. I became accustomed to the line of reasoning with which we’ve all become familiar: if the universe is finite, what’s outside that limit? Like many, I’ve never felt completely comfortable with that logic. Instead, I assumed that our limited understanding of the physical world must be the reason we’re forced to assume the notion of infinity.
Fast forward 30 years. One of the beautiful things about having children is reliving one’s own childhood. My son has been similarly-intrigued by this question for the past few years and I’ve done my best to address it without burdening him with too many preconceptions that would limit his imagination about what’s possible. I’m certainly not an astrophysicist, nor do I play one on TV. However, I felt that our combined understanding of science should provide us enough pretext to have a rational discussion and to reach some pragmatic conclusions. This post describes the rationale we’ve used to come to the conclusion that the universe must be infinitely large.
An empirical view of the world lends us to believe that objects, both living and otherwise, exist on a “scale continuum”: We start with enormous galaxies that have millions of star systems that in turn have one or more stars, planets, moons, and many other transient objects. Once we get down the sub-planetary scale, the continuum continues from relatively-large blue whales to still-large elephants to “our-sized” humans. The scale continues downward, yet again, to cats, mice, bees, gnats, and then down to multi-cell microorganisms, single-cell microorganisms, and viruses (which, by the way, are debated as to whether or not they should be considered living). Eventually, we get down to what was once perceived to be an object that is no longer divisible, the famed “atom.”
For hundreds of years and well into the late 1800s, scientists believed that “atoms” were aptly-named objects that could not be further subdivided. In the early 1900s, we discovered electrons and later understood the relationship they had to neutrons and protons. Since then, advancement has continued at a tepid pace. Over the past few decades, it seems the science of quantum physics introduces new, increasingly-smaller objects, some of which defy our contemporary understanding of physics.
So, we now get to the pragmatic bit. In my humbly-simple analysis, I believe that we will continue to discover increasingly smaller entities with no end in sight. That is, we’re only limited by our ability to break down whatever element we currently believe to be the most microscopic. Once technological advances allow it, it is my contention that any infinitesimally-small object can be broken down further.
This premise is the basis for our layperson’s rationale: on a galactic scale, a planet is microscopic; on a planetary scale, a human is microscopic. It is then our contention that it would be reasonable to conclude there is a scale that makes even our entire “known universe of galaxies” seem microscopic. And so on forever.
So, said another way, why do we believe the universe is infinitely large? Because we believe that it is infinitely small. While it is completely possible that the universe has an upper size bound but no lower size bound, nature’s elegance and apparent “recursive nature” would lead me to posit this hypothesis. For example, note the parallels between how we envision our solar system and how we envision an atom consisting of a sun-like nucleus orbited by planet-like electrons. Is it reasonable, then, to hypothesize that the universe is concentric, and “recursively repeats” infinitely in a similar fashion?
An interesting sub-thesis of this is that the study of the incredibly-small is likely just as interesting as the incredibly-large. We can likely learn a lot about the composition of our universe by studying the increasingly small sub-atomic objects that exist right here on Earth. It is plausible to consider the possibility of individual atoms themselves constituting a universe with incredibly microscopic beings that live within; to them, we would seem unfathomably large. These beings could well be wondering if their universe is infinite. What a world in which we live.
Further reading: Did You Know: We Know The Age of the Universe.