Finely Tuned Skepticism
On Boltzmann Brains, the Past Hypothesis, the Anthropic Principle, etc.
Some Preamble About Entropy and The Past Hypothesis
Most everyone reading this knows something about entropy, which is usually characterized as a given system’s level of disorder (the more disorderly it is, the greater its entropy), but that’s somewhat misleading: it’s really just about how improbably a system is configured. Energetic particles are unlikely to simultaneously converge within a small area by aimlessly roving about; there are vastly more arrangements whereby those particles haplessly intermix in unremarkable ways. This same logic explains why heat diffuses and equilibrates across systems (hence the Second Law of Thermodynamics that entropy increases over time): when you combine hot and cold water, it eventually produces a pretty evenly lukewarm solution because most of the potential configurations of the water molecules are like that.
The ultimate outcome is something fairly homogeneous rather than disorderly, so distinguishing entropy from complexity is also crucial. It’s much simpler to relay the characteristics of a system with maximum entropy (e.g., with constant temperature) or minimal entropy than, for example, one with a complicated amalgam of temperatures distributed unpredictably. So, forming anything super complex (like our world) requires some intermediate level of entropy between these extrema, and since entropy increases over time, fostering complex worlds necessitates beginning with special low-entropy conditions. From these principles alone, you might infer that the universe used to be jam-packed, independent of the astronomical clues about its expansion, cosmological constants, dark energy, etc.
Either way, according to the Past Hypothesis, our universe existed in a radically low-entropy state in the past (viz., the Big Bang), but why/how was the universe ever configured in such an improbable state? One partial explanation for why we find ourselves in a region of reality with a low-entropy past could be the Anthropic Principle, which posits that observers can only observe observable conditions, basically. Or, inversely, as it’s sometimes put, we naturally don’t find ourselves in uninhabitable situations because we couldn’t exist there: we wouldn’t exist in such conditions because we couldn’t exist in such conditions; hence, we don’t.
Thus, the suitability of our environment is unsurprising, even if that environment is atypical, because you wouldn’t find creatures like us elsewhere. As I’ve written before, “Earth (or our solar system) is a popular/useful analogy: planets must be somewhat special to be a suitable environment for complex life, but it’s unsurprising that we find ourselves populating one of these endurable ones, even if they’re more rare.” Thus, if complexity requires some intermediate degree of entropy, and entropy only ever increases, then we’d logically expect complex, telescope-wielding lifeforms like us to observe a universe that had lower entropy in the past.
Theism v. The Multiverse
This anthropic reasoning (that we’d naturally observe circumstances that facilitate our existence) is frequently deployed to counteract fine-tuning arguments for theism, which claim that our universe is so well-calibrated to be habitable and engender human life that it’s downright suspicious and that our propitious cosmological circumstances are compelling evidence of divine involvement—here’s Ross-Douthat-whisperer Bentham’s Bulldog’s version:
If there is a God, then the universe being finely tuned makes sense. God would want to create a universe capable of sustaining life. He’d be decently likely to make a universe like this one, with predictable, stable physical laws that we can scientifically explore. If there is no God, then the constants, laws, and initial conditions could be anything, so it’s absurdly unlikely that they’d fall in the ridiculously narrow range needed to sustain life.
The Anthropic Principle reduces the fishiness our cosmologically queer predicament, but it doesn’t alone explain the origin of these uncommon circumstances (only the otherwise curious contents of our observations, given that we make them). What’s the mechanism for fabricating the strange conditions needed for complex life? Anthropic reasoning explains why we would find ourselves in an improbable setting given that we exist, but if a hospitable setting is so improbable and precarious, then maybe we just shouldn’t exist at all—it doesn’t completely unravel why/how the universe was originally in such a low-entropy state.
One potential explanation—other than theism—is that our universe is part of a vast multiverse that comprises an unending diversity of cosmological situations. Pairing up that variation in circumstances with the selection effects of the Anthropic Principle provides a more complete account of how we could wind up with access to the requisite conditions for a habitable universe, despite requiring such odd/specific tweaking of the dials.
So, just like with evolution, we’re confronted with a convenient fit between ourselves and the world, and we’re presented with competing possible explanations to divide our credence between: was this fitness effectuated by purposeful divine tailoring or by the brute force of randomness and the Anthropic Principle? I’m especially circumspect about fine-tuning arguments because of their resemblance to controversies over Darwinism, and I’m loath to be convinced by theism this way, given how humanity was plainly duped in a parallel manner by the trickiness of evolution:
The similarities here with evolutionary theory are obvious: these arguments about stumbling upon something unlikely and inferring a designer look awfully familiar, as does the alternative theory that there’s something like natural selection for universes. . . . [T]he fact that this entire song and dance has played out before (about how natural selection got started, etc.) without dislodging the Theory of Evolution instills skepticism within me about what are essentially a bunch of shopworn anti-Darwinian arguments being redeployed here wearing fake mustaches.
Other arguments about about why fine-tuning arguments are less convincing than their supporters believe are available, too. For example, to the extent they succeed, they’re much better evidence for the Simulation Hypothesis than they are for the existence of God. Again, this is probably old hat for some readers, but I wanted to delineate this background material a little before some upcoming posts about them.
Boltzmann Brains and Skeptical Responses to Fine-Tuning
Another potential solution to fine-tuning conundra, however, is that our strange observations are unreal/misleading and therefore spurious. If we’re being fooled (à la The Matrix) by superhuman entities—or maybe we’re being duped by an Evil Demon or we’re Brains in Vats, etc.—who can impose impressions of odd cosmological details or conjure arbitrary perceptions within our minds, then maybe puzzling over the strangeness or likelihood of our observations is a misadventure.
Like with the this theism-versus-multiverse debate, there’s also a probabilistic option, purged of top-down guidance. As mentioned above, the Past Hypothesis claims that the universe used to possess less entropy; conversely, in the deep future, our universe will eventually become smoothed out and uninteresting as entropy maximizes; but, per modern physics, fluctuations of particles in that otherwise uninteresting high-entropy blend should occasionally generate weird and improbable stuff, such as brain-like objects with the false impression that they’re human. These objects—Boltzmann Brains—would sometimes briefly have subjective experiences indistinguishable from our own, and if our universe continues to generate them over a sufficiently lengthy timeline, then Boltzmann Brains should vastly outnumber the population of real people, and it’s arguable that we should therefore presume that we’re, in fact, Boltzmann Brains (more about this in an upcoming post).
But if we really are Boltzmann Brains, then—contra the Past Hypothesis—we actually do live in a high-entropy setting, just as we’d naively assume according to probability, the Second Law of Thermodynamics, and so on. Thus, maybe our cosmological circumstances aren’t so exotic; maybe physics doesn’t even work how we suppose it does. If our perceived evidence about residing within a low-entropy universe with strangely hospitable attributes is unreliable, then we can abandon the Past Hypothesis and other fine-tuning-like enigma.
Confusingly, however, just as the notion of Boltzmann Brains could motivate overturning the Past Hypothesis, the Past Hypothesis can be marshaled to counteract the very reasoning used to surmise that we’re likely Boltzmann Brains,1 since even though Boltzmann Brains may outnumber regular observers over the universe’s total lifespan, they don’t patently outnumber non-Boltzmann-Brain observers in a low-entropy early-universe setting like ours. So, if we accept the evidence that motivates forwarding the Past Hypothesis, then we shouldn’t readily infer that we’re Boltzmann Brains, and resorting to that concept to escape the Past Hypothesis would be erroneous. Thus, Boltzmann Brains don’t provide a straightforward pathway for demystifying that situation (again, more on this confusing, tangled relationship in an upcoming post).
Moreover, although we can potentially neutralize fine-tuning arguments by embracing skepticism, doing so engenders a new batch of fine-tuning-ish riddles about why, for example, the demon/neuroscientist controlling the fictitious observations in your mind is routinely choosing to fool you into accumulating beliefs about living in a finely tuned and coherent world.2 Also, unlike before, this new fine-tuning mystery cannot be defanged using anthropic reasoning, because we’ve conceded that our observations are undependable.
In sum, fine-tuning arguments naively assume that our observations accurately reflect our environment; if your observations can be radically misleading—because you’re a Boltzmann Brain or trapped within other skeptical scenarios—then the world might not be so finely tuned—at least, not with the precision supposed by that style of theistic argumentation; however, our faulty evidence about the world would be highly improbable (and maybe deliberately curated) instead, even if the world’s actual characteristics were not, so maybe fine-tuning can’t be so easily defeated.
Also, if these skeptical scenarios are plausible, then the Anthropic Principle is a less reliable antidote to fine-tuning predicaments than is normally assumed, since a Boltzmann Brain can observe stuff without those observations needing to correspond to a cosmological scenario that sensibly accommodates its presence. Given the potential for such fantastical/unscientific observations without any explanatory connection between those observations and the observer’s presence, what good does the Anthropic Principle do? If Boltzmann Brains can observe pretty much anything, what does it matter if it’s potential observations are restricted to whatever’s observable?
Conclusion
Ultimately, just as we’re stuck deciphering whether our experiences are counterfeit or genuine, we also must determine whether the fitness of our world is the product of some unguided trial-and-error process (like evolution), or if it’s somehow purposefully calibrated: is our world and/or our existence manufactured according to God’s blueprints or is it a fluky outcome downstream of manifold randomness—or even some combination of these?
The interplay between these topics is subtle, but either way humanity itself seems unplanned, prima facie: even if the universe being habitable is fortuitous, its overwhelming emptiness doesn’t resemble what anyone should expect from a designer who is uniquely preoccupied with beings like us. In fact, so long as we credit God with attributes like omniscience and omnipotence, then fine-tuning logic supplies a more powerful argument against theism than for it, because no matter how obvious you think it is that our universe had an architect, it’s even more obvious that it’s imperfect.
See the linked article (“What Follows from the Possibility of Boltzmann Brains” by Matthew Kotzen) at page 31; cf., Ron Avni, The Boltzmann Brains Puzzle, 57 Noûs 958, 7-9 (2022).
Michael Huemer relies on similar points to argue that we probably aren’t Brains in Vats.