The Fine-Tuning Argument (FTA) is one of those standard arguments for the existence of God. The argument goes that humans can only arise when the parameters of the universe are tuned exactly right. And while it’s possible that we just got lucky, the argument goes that it’s far more likely that God did the tuning.
The standard way to talk about the FTA is delve into a bunch of math equations. Not that there’s anything wrong with math, but here I wanted to write an in-depth overview that doesn’t talk about the math. There will, however, be a lot of physics. The goal here is not to refute the FTA (although refutations will occur incidentally), but to explore it, and to understand how we test hypotheses about the universe.
(Links to be added later)
1. The Fine-Tuning Argument: A walkthrough
2. Prediction distributions and inflation
3. Ignorant hypotheses
4. Anthropic reasoning
The parameters of the universe
The core premise of the FTA is that the universe is fine-tuned. Which is to say, the probability of life looks like this:
The “universal parameters” on the horizontal axis can represent multiple things. There are the 26 fundamental constants of nature, such as the masses of elementary particles, and strength of the fundamental forces. There are derived constants which could in principle be calculated from fundamental constants. Finally, there are historical facts about the universe, such as the energy density at the largest scales.
The claim is that for at least some of these universal parameters, the probability of life is sharply peaked. Which is to say that if you change the parameter by a small amount, then the probability of life drops significantly.
What does it mean to change a parameter by a “small” amount? By one standard, the change in a parameter is “small” if it’s a small percentage of the parameter’s value. For example, if we change the inverse fine structure constant (~137) by 1, that’s small; if we change the cosmological constant (~10-122) by 1, that’s large. It’s not clear that this standard is always the most appropriate one, but it’s the one we use in most cases.1
What if it’s not fine-tuned?
As a former physicist, my impression of fine-tuning is, “reasonable hypothesis, but dreadfully difficult to prove”. In the actual world, we know that we have one set of parameters for the universe, and we know that it’s fairly difficult to make predictions from those parameters. In fact, most physicists dedicate their careers to making such predictions. There are far fewer physicists who work on counterfactual parameters of the universe, and those physicists can’t even rely on experiments for help. Thus, you might expect that the scholarly work proving that our universe is fine-tuned is relatively superficial. I say “relatively”, because perhaps the work is still deep enough to get lost in, deep enough that neither you nor I have enough time to review and understand the scientific literature. But we need to remember that this is an inherently difficult problem full of pitfalls, and we may never know the true answers.
One version of the FTA claims that life is very sensitive to the amount of energy released by hydrogen-to-helium fusion. If this were to change by 1 part in 7, then we’d either have all hydrogen or no hydrogen, and life would be impossible.2 But other physicists argue that if we were to change this parameter by tweaking the strong coupling constant, this would also change the binding energy of deuteron, which also affects the density of hydrogen–and before long we’re in the weeds.
I think it’s very easy to tweak one universal parameter by a little bit, and see that it negatively impacts some particular process that was important to life as we know it. But if the behavior of the universe is so sensitive to small changes, doesn’t that just demonstrate how hard it is to make these predictions? What if we tweak the parameter even more, and some new substitute process appears? What if we tweak multiple parameters at once? What if life is still possible, but it occurs on different kinds of planets near different kinds of stars, at different times in the history the universe? What if there were entirely different forms of life, unrecognizable to us? In short, maybe the graph looks more like this:
It doesn’t particularly matter how sharp the peak is, if you don’t know how many peaks there are. In fact, the sharper the peak, the more it demonstrates the difficulty of the problem, the more it demonstrates our ignorance of the landscape.
Granted, if the cosmological constant is so large that everything gets pulled to pieces, then it’s hard to imagine life existing in any form. So it’s still possible that the landscape is as flat as we had initially presumed.
Of course, when we talk about the FTA, usually nobody wants to dig into the physicists’ arguments. So we just take fine-tuning for granted and move on. And that’s what I’m going to do too in this series. But let’s take this moment between posts to dwell on the humility of our knowledge.
1. One common exception is the cosmological constant. Since the cosmological constant is on the order of 10-122, you’d think a change of 10-120 would be very large. However, some theories of physics predict that the cosmological constant should be on the order of 1, and it’s actually very surprising that the cosmological constant should be so small. Thus, any change less than 1 is considered small, and any change larger than 1 is considered large. This standard is used in fine-tuning arguments, and it’s used to argue in favor of alternative physics theories. (return)
2. This argument comes from astrophysicist Martin Rees, in his book Just Six Numbers. The energy release of hydrogen fusion is 0.7% of the mass of the initial hydrogen atoms. Rees claims that if it were as low as 0.6%, everything would be hydrogen; if it were as high as 0.8%, hydrogen would all disappear. This is an example of a “derived constant”, which can in principle be calculated from the fundamental constants. (return)