Question: Are the physical properties of the universe tuned for fart smell?
Short answer: Yes.
Long Answer: Physicists have discovered that our universe can be characterized by a number of fundamental physical constants. Examples include gravity, electrical charge, and perhaps two dozen others. If these constants took on different values, the universe would be similarly altered, and it is of some philosophical interest that even small changes would render life as we know it impossible. This observation has led to the concept of the fine-tuned universe.
Our own contribution to this field has been the discovery of the flatological constant, which relates the smelliness of farts to the distance they travel in the absence of any external perturbation. The governing equation is:
x = kM
where x is distance from the source in inches, M is the amount of fart material, and k = 2.42 is the flatological constant. This value arises from the fundamental fact that fart particles, once released, are bombarded by the surrounding air molecules. As a result, farts undergo Brownian motion, a random trajectory through space that is shown here (simulation courtesy of Wikipedia):
Remarkably, this aspect of the microscopic universe was discerned as early as 60 BCE, by the philosopher Lucretius, who described it in his classic poem De rerum natura (On the Nature of Things).
More recently, the precise dynamics were characterized by Einstein and Smoluchowski, in the form of the famous diffusion equation.
Given the intense interest in fine-tuning arguments, it is worth considering what the world would be like if the flatological constant took on different values. Qualitatively, one can say that very small values of k would render fart smell largely undetectable, while very large values would place people at risk of hydrogen sulfide toxicity. These possibilities are depicted here, for different values of k:
For small values of k (left), farts would be totally safe but very difficult to detect. Because people generally enjoy the smell of their own farts, this situation would be far from ideal. In contrast, if the value of k were much higher (right), most farts would be lethal once inhaled, so that human life would be effectively impossible. Our current universe, with k = 2.42 (middle), provides a comfortable middle ground, in which farts can be smelled with very minimal risk.
We can therefore say that the physical properties of the universe are tuned in such a way as to allow the safe enjoyment of fart smell. Because this is possible over a large range of values for the flatological constant, our findings are unlikely to provide much support for fine-tuning arguments. But they are consistent with more modest variants of the same principle, in which our universe necessarily has properties compatible with our ability to observe them.
Our results also suggest that there could be other universes in which fart smell is nearly absent or nearly intolerable. This multiverse theory was also advanced by Lucretius:
“Now in no way must it be thought to be like the truth—
…that this was the only world and heavens created,
…again and again it is necessary to admit
that there are other groupings of matter such as this,
which the aether holds in its greedy grip.”