The Fine-Tuning Argument Can't Get Off the Ground

Is it impossible for something indefinable to exist in universes that are unimaginable?

Recently, theists, or believers in God, have grown very bullish on the idea that the fine-tuning of the universe provides evidence for God’s existence. According to this fine-tuning argument (FTA), if the physical constants of the universe had been even slightly different, the universe could not have given rise to life, so God must have designed the universe for life to be possible.

In Believe: Why Everyone Should Be Religious, the New York Times columnist Ross Douthat talks as though it’s virtually certain that cosmic fine-tuning proves that we live in a “cosmos made for us” by God.¹ Many Substack writers enthusiastically agree with Douthat. Bentham’s Bulldog has declared his faith in the FTA in an article confidently titled “Fine-Tuning, Made by God”!

Theists should curb their enthusiasm. Many physicists believe that there’s no good evidence that the universe is fine-tuned for life. As astrophysicist Fred C. Adams writes:

Astrophysicists have discussed fine-tuning so much that many people take it as a given that our universe is preternaturally fit for complex structures… But in fact the fine-tuning has never been rigorously demonstrated. We do not really know what laws of physics are necessary for the development of astrophysical structures, which are in turn necessary for the development of life. Recent work on stellar evolution, nuclear astrophysics, and structure formation suggest that the case for fine-tuning is less compelling than previously thought. A wide variety of possible universes could support life. Our universe is not as special as it might seem.

In fact, the FTA is based on reckless overconfidence in what it is possible for humans to know. We can’t be confident that the universe is finely tuned for life because we can’t even define what life is and can’t model what the universe would be like if its physical constants were altered.

I consulted with astrophysicist Coel Hellier while writing this article, and he has reviewed it for errors about physics. (Check out Coel’s Substack!) Evaluating the FTA requires a deep knowledge of physics, and I am not a physicist—neither are Douthat and Bentham’s Bulldog. I am interested in the FTA, as they are, because I am interested in religion. Fortunately, many physicists have written accessible overviews of the FTA and these permit non-specialist readers to gain some understanding of the debate.

What is the fine-tuning argument?

Proponents of the FTA, whom I will call “fine-tuners,” argue that if the physical constants of the universe had been even slightly different, the universe could not have given rise to life. There are multiple examples here. Quarks are the particles that make up protons and neutrons, and they come in various flavors, including up and down. If the relative masses of up and down quarks had been different, then a very different universe would have emerged that would be hostile to life as we know or can imagine it. In some cases, the only element to form would have been helium, in others only hydrogen, and in others no atoms could have formed at all.² Similarly bizarre universes would have arisen if the relative strength of physical forces or the cosmological constant had been different. Fine-tuners are persuaded that the precise settings of these parameters cannot be a matter of chance. Rather, there must be some explanation of why the universe is so precisely calibrated to the emergence of life. One popular theory is that fine-tuning has to be explained by God or some other supernatural force.

The basic problem with the fine-tuning argument

It is not controversial that life as we know it would be impossible if the physical parameters of our universe took on different values. However, the most effective criticism of the FTA is that we can’t know whether all and any life would be impossible under different conditions. The FTA assumes that physicists can model alternate universes with different physical constants, and the problem is that they can’t.

That’s the case made by physicist Sean Carroll in this interview and in The Big Picture: The Origin of Life, Meaning, and the Universe Itself³:

when it comes to most of the numbers characterizing physics and astronomy, it’s very hard to say what would happen were they to take on other values. There’s little doubt that the universe would look quite different, but we don’t know whether it would be hospitable to biology. (305)

He strengthens this argument by noting that the chemistry and biology of our universe could not be deduced from knowledge of its basic physics.

if we didn’t know anything about the universe other than the basic numbers of the Core Theory and cosmology, would we predict that life would come about? It seems highly unlikely. It’s not easy to go from the Core Theory to something as basic as the periodic table of the elements, much less all the way to organic chemistry and ultimately to life. (305)

By Core Theory, Carroll means the theory of the fundamental forces that make up and explain our universe, “the quantum field theory of the quarks, electrons, neutrinos, all the families of fermions, electromagnetism, gravity, the nuclear forces, and the Higgs” (176). This Core Theory has been the most successful in accounting for the results of physics experiments, and it counts as physicists’ best guess at how our universe works.

The point is that, even if we know the basic physical forces at work in an alternate universe, it is impossible to know what kinds of chemistry and other higher order phenomena might emerge. Given this fact, we can’t know whether life would be possible or not in alternate universes.

I asked astrophysicist Coel Hellier to evaluate Carroll’s argument. He replied:

Yes, I do like and agree with Carroll’s criticism. We simply don’t know enough to predict what universes would be like if we started changing physical constants and other things. We can say it would be different, but we can’t then think through all the consequences. There could well still be life forms, just different ones.

This “unimaginable alternate universes” argument seems obviously true. Of course, we know far more about the physics and chemistry of this universe than those of alternate universes. Even here, though, we can’t predict chemistry from physics. Much about the chemistry of this universe remains unknown and chemists must do empirical research to gain new knowledge. As an example, novel experimental techniques have enabled chemists to see how chemicals behave under conditions of high pressure, which might inform us about chemistry in the interior of planets. Chemicals behave very differently in high-pressure and normal conditions. The nature of chemical bonding changes, so that some chemicals that are normally stable become unstable and vice versa.

It seems to me that if the logic of the fine-tuners were correct, we could simply predict what chemistry would be like from physics and would not need empirical research. If physics doesn’t allow us to predict chemistry in this universe, how can we have any idea what kind of chemistry and other emergent traits would arise in alternate universes? To know what such universes would be like and whether they could support life, we would have to observe them, and that is not currently possible.

If true, Carroll’s criticism is devastating to the FTA. If scientists can’t model what alternate universes would be like, then the FTA can’t get off the ground.

I’ll use analogy to illustrate Carroll’s argument. Imagine a primitive tribe that lives in a very arid region. Sometimes temporary ponds will form, but they have no fish or other aquatic animals in them. The tribe usually gets its water from a well. The tribe understands some things about the water, like drinking, swimming, and drowning.

I submit that this tribe would be unable to conceive of any water-dwelling animal, like a fish. If they even managed to conceive of the question of whether any animal could live in water, they would find the whole idea absurd and impossible. They would be sure that there could be no form of aquatic life. After all, they would reason, animals have to breathe air, so how could any animal live in water? The tribe’s knowledge of some facts about water would not provide them with enough information to imagine what a fish would be like. To understand how a fish lives in water, you have to actually observe a fish. In the same way, the existence of life in a universe can’t be predicted from knowledge of its basic physics.

What is life?

Carroll discusses another fundamental problem with the fine-tuning argument: fine-tuners have no definition of what life is. Life is notoriously impossible to define. Astrobiologists, or scientists who study what extraterrestrial life would be like, admit that there is no satisfactory definition of the term:

A common approach is to list life’s characteristics, which include reproduction, growth, energy utilization through metabolism, response to the environment, evolutionary adaptation, and the ordered structure of cells and anatomy. Unfortunately, this way of defining life is unsatisfactory for a couple of reasons. First, the list describes what life does rather than what life is. Second, most of these aspects of life are not unique. Life has structural order such as cells, but salt crystals are also ordered. Some of my friends have no children but they’re alive, I think, as are mules that cannot reproduce. Growth and development apply to living entities, but also to spreading fires. All life metabolizes but so does my car. Life reacts to its environment, but a mercury thermometer also responds to its surroundings.⁴

The lack of definition of the term “life” would seem to make nonsense of the FTA. How can you define the conditions for life if you can’t even define what life is? Can an explanation be valid if you don’t even know what you’re trying to explain?

The problem here is again illustrated by my analogy. One reason that the tribe wouldn’t be able to conceive of a fish is that they have an overly restrictive theory of what constitutes life. Since the only animals that they could observe breathed air, they wouldn’t be able to imagine how a fish could extract oxygen from water.

The carbon question

To my knowledge, fine-tuners have never addressed Carroll’s unimaginable alternate universes argument. However, they have addressed the issue of the definition of life. In A Fortunate Universe: Life in a Finely Tuned Cosmos, physicists Geraint F. Lewis and Luke A. Barnes, defenders of the FTA, contend that it is all but certain that life is impossible without carbon. Of course, all life on earth is carbon-based, and one reason why is that carbon can form a wide variety of molecules, which can be used as building blocks for life. While Lewis and Barnes admit that it’s possible that silicon might form the basis of life, they are skeptical, and they think that no other elements could give rise to life.⁵

For this reason, Lewis and Barnes focus heavily on the cosmic conditions necessary for the production of carbon. The fine-tuners note that the production of carbon is highly problematic given the physics of stars and go on to argue that the existence of large quantities of carbon in our universe is evidence of cosmic fine-tuning. For carbon to be formed, three helium nuclei must fuse. The way that this happens is that two helium nuclei fuse to create the isotope beryllium-8, after which another nucleus must fuse with the beryllium-8. The problem is that beryllium-8 is unstable and rapidly decays, making it hard for the fusion to take place. Stars can produce large quantities of carbon only because this element has a quantum energy level that matches that of beryllium-8 plus helium. Since this is so, when beryllium and helium fuse, they create a carbon atom in one of its excited states. This energy level is known as the Hoyle Resonance. Lewis and Barnes believe that this process shows fine-tuning because it is so unlikely that the carbon atoms would resonate at the level necessary for them to form. They argue that if the physical constants of the universe were only a little different, then much less carbon would be produced by stars, making the genesis of life unlikely.

Subsequent research has not been kind to Lewis and Barnes’ argument since they made it in 2016. The astrophysicists Fred C. Adams and Evan Grohs have shown that in many alternate universes, beryllium-8 would be stable, which would, in some cases, make carbon production unproblematic and the Hoyle Resonance irrelevant.⁶ The broader point is that if you changed the values of universe’s parameters, you might make it less friendly to life in one way, but more friendly in others, which is a point that fine-tuners don’t consider.

For this reason and others, Adams believes that there is no evidence of fine-tuning:

the parameters of our universe could have varied by large factors and still allowed for working stars and potentially habitable planets. The force of gravity could have been 1,000 times stronger or 1 billion times weaker, and stars would still function as long-lived nuclear burning engines. The electromagnetic force could have been stronger or weaker by factors of 100. Nuclear reaction rates could have varied over many orders of magnitude. Alternative stellar physics could have produced the heavy elements that make up the basic raw material for planets and people. Clearly, the parameters that determine stellar structure and evolution are not overly fine-tuned. Given that our universe does not seem to be particularly fine-tuned, can we still say that our universe is the best one for life to develop? Our current understanding suggests that the answer is no.

It’s also possible that carbon is not necessary for the genesis of life. Biochemists have discovered that many chemical elements could form the basis for life, not just carbon. One biological process that is likely to be at the origin of life is autocatalysis, or a self-sustaining chemical feedback loop where the product of a chemical reaction can serve as the catalyst of the same reaction. Autocatalytic loops are essential to life as we know it. While most research on autocatalysis has been confined to carbon, biochemists Zhen Peng and colleagues have shown that a dozens of chemical elements, including chlorine, copper, and calcium, can give rise to autocatalysis. Peng et al. conclude, “the conditions under which life originated could be dramatically different from what living organisms are dealing with today, and extraterrestrial life, if it exists, could be very different from life as we know it.”

These scientific developments illustrate how rash it is for fine-tuners to assume that they can define the conditions for life. Since we know so little about what life is and how it might originate, trying to defend the FTA is a fool’s errand.

I hope that Carroll develops his brief and schematic criticisms of the FTA into a richer account of the overconfidence of the fine-tuners. Many other criticisms of the FTA are also valid. For one thing, the theory is frustratingly vague and incomplete. Many questions about the theory are natural, and it provides no satisfying answers. Why would God value life so much that he would fine-tune a universe so that it could arise? Why did God choose the particular physics of this universe when others would presumably have been possible for an all-powerful being? If the universe is fine-tuned for life, why are so much of the earth and the universe apparently uninhabitable? The FTA provides no principled, scientific grounds for answering these questions.

If physicists like Carroll, Adams, and Hellier are right, the whole debate around the fine-tuning argument is pointless because the argument can’t get off the ground. It would appear that all the fine-tuners are saying is that it is impossible that something indefinable exists in universes that are unimaginable. And that is not a thesis worth taking seriously.

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1. Douthat, Ross. 2023. Believe: Why Everyone Should Be Religious. Grand Rapids, MI.: Zondervan, p. 24.

2. Lewis, Geraint F., and Luke A. Barnes. 2016. A Fortunate Universe: Life in a Finely Tuned Cosmos. Cambridge: Cambridge University Press, pp. 46-50.

3. Carroll, Sean M. 2016. The Big Picture: On the Origins of Life, Meaning, and the Universe Itself. New York: Dutton.

4. Catling, David C. 2013. Astrobiology: A Very Short Introduction. Oxford: Oxford University Press, p. 6.

5. Lewis and Barnes, p. 114.

6. For an accessible presentation of this idea, see here. For the long version, see here, preprint here.