Journal of Creation 38(1):31–36, April 2024
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Fine-tuned light
A review of: Children of Light: The astonishing properties of sunlight that make us possible by Michael Denton
Discovery Institute Press, Seattle, WA, 2018
Children of Light is the third book in the Privileged Species series by Michael Denton, a Senior Fellow at the Discovery Institute’s Center for Science and Culture.1 He has an M.D. from Bristol University in the UK and a Ph.D. in biochemistry from King’s College in London, and has commented extensively (and critically) on evolution.2
The Privileged Species series focuses on the empirical fact of fine-tuning. There are many factors about our situation that have a ‘Goldilocks’ property—i.e., if they were even slightly different in any way from what they are, life as we know it would not be possible. In Children of Light, Denton focuses on the many facets and functions of visible light that are ‘just right’ for human life.
Put simply, the book is a tour de force on the fine-tuning of light for human-like life. I will simply give the reader a taste of what Denton covers throughout his chapters; I cannot cover all the elements he mentions, but these few details I think show why this is a topic that deserves our full attention. (All quotes, unless otherwise marked, are from Children of Light.)
The miracle of sunlight
Many of our ancestors worshipped the sun. Denton agrees with Carl Sagan, who said that “they were far from foolish”. They built many monuments to line up with the movement of the sun. This included Stonehenge, the Sphinx, some structures at Angkor Wat, and even whole cities such as Teotihuacan.
Today, we don’t worship or build monuments to the sun. Nonetheless, we’re still often struck by a sunrise or sunset. And the eerie twilight of a solar eclipse still amazes us. But, compared to so many of the ancients, we don’t give the sun much thought. Denton avers that familiarity strips us of our awe. I also suspect a little bit of scientific knowledge does too. We know the sun is ‘just’ a big ball of plasma like any other star. So, while we know that without it we couldn’t live, the sun itself rarely gets much special attention.
However, this underestimates just how special our ‘light’ circumstances are. As Denton explains:
“Less widely known, however, is the existence of an extraordinary suite of coincidences in the nature of things which render the Earth’s surface a supremely fit habitat for advanced carbon-based life forms like ourselves—coincidences that are, on any consideration, ludicrously improbable.”
He mentions that the sun has just the right properties for photosynthesis and high-acuity vision; Denton explains the significance:
“… these are elements of natural fitness exclusively for our type of life—for beings possessing the gift of sight, breathing oxygen (aerobic), and inhabiting the terrestrial surface of a planet like the Earth [emphasis original].”
The light of life
Most stars, including our sun, emit most of their radiation as light and heat. But this very fact is an amazing element of fitness. Electromagnetic radiation (EMR) of different frequencies interacts with matter in different ways. And the range of EMR frequencies is incredibly large—on the order of 1025. However, visible light makes up an incredibly small portion of that frequency spectrum, falling in a range of about 400–800 nm. This also happens to be roughly the EMR frequency range that enables photochemistry. It is energetic enough to enable most chemical reactions, as well as excite valence electrons to higher orbital levels, as is required for photosynthesis. Higher frequency EMR, such as UV, X-rays, and gamma rays, strip electrons from atoms completely and denature biological molecules. On the other hand, lower frequency EMR, such as far-infrared, microwave, and radio waves, does not have a high enough energy to excite electrons to higher orbital levels; they only vibrate or rotate atoms. Denton notes an important thing to realize:
“… it is not that life adapted to the right light but that the right light is the only light that provides the correct energy levels for photochemistry [emphasis original].”
However, heat is also a significant element of fitness. It warms things well beyond what they would otherwise be without heat radiation. But not too much! If atoms are moving too fast, chemical reactions in organic compounds become too energetic and common to sustain a stable organism. And the most efficient way to supply that heat is precisely the way the sun provides it: in the near-infrared spectrum:
“The essential heat that prevents the Earth’s hydrosphere from freezing solid and that animates matter for chemical reactions is provided by electromagnetic energy in another region of the EM spectrum—the IR region, or more specifically the near infrared. This region lies adjacent to the visual band, between it and the far infrared and microwave regions, or between about 0.8 microns and 14 microns. This is the only region of the EM spectrum which can provide safe heat to warm the Earth, preventing it from freezing, providing sufficient kinetic energy to move molecules and promote chemical reactions but not enough to cause uncontrolled chemistry.”
Together, the visual and near-infrared bands of the EM spectrum are extremely small in comparison to the range of possible EM frequencies. Denton’s conclusion is apt:
“That the Sun should emit radiation in the only infinitely small region of the EM of utility to life is a truly extraordinary coincidence!”
Letting the light in
Light in the right EM frequency range is necessary for life like us to thrive, but it’s not sufficient. We also need the right sort of atmosphere to let in the right light in the right amounts. As Denton points out:
“The life-giving light of the Sun must penetrate the atmosphere right down to the ground to work its magic, and a proportion of the Sun’s IR radiation (heat radiation) must be absorbed by and held in the atmosphere to warm the Earth above the freezing point of water and animate the atoms of life for chemistry.”
And our atmosphere is amply fit for such a task. Notice the ranges over which the atmosphere lets through the most EM radiation (figure 1). It blocks completely any wavelengths shorter than about c. 200 nm (middle of the UV range), is highly (though not completely) transparent to visible light, is ‘patchy’ in its transparency to infrared light from 0.8–15 μm wavelength, but it blocks infrared wavelengths 15–1,000 μm; it is impervious to all wavelengths longer than this besides long-wave microwave and short-wave radio waves (c. 4 cm–15 m), to which the atmosphere is completely transparent.
If we shift the absorbance spectrum in figure 1 (i.e., if we imagine the absorbance by the atmosphere had covered a slightly different region of the EM spectrum), ‘light eaters’ would be impossible. Shifting it to the left even a little would expose the surface to much more UV radiation, potentially absorb visible light, and absorb a lot more short-wave infrared radiation; the UV would destroy biological tissues, and the increased short-wave IR absorption would heat the atmosphere too much, producing a runaway greenhouse effect and making biochemistry impossible. Shifting it slightly to the right would also result in a lot more visible and near-IR light being absorbed, creating a runaway greenhouse effect that would make biochemistry untenable.
Moreover, water displays a similar element of fitness in the atmosphere. Of all wavelengths, it is most transparent to visible light (figure 2), not just as a liquid but also as ice3 and vapour (figure 3). This means that photosynthesis is possible in the air, in the water, and even under a frozen lake!
In the IR range, however, things are dramatically different from the visible range. There are strong absorption bands in the IR region, with the vast majority of the region completely absorbed by the atmosphere. Denton notes that this absorption raises the temperature at the surface about 33 °C above what it would otherwise be. Since the average global surface temperature is about 15 °C, and without the atmosphere it would be about –18 °C, Earth’s atmosphere is the difference between H2O normally being water rather than ice on the surface! However, it also insulates Earth’s surface from extreme daily temperature changes. It protects us both from the heat of the sun in the day and the cold of its absence at night, evening out the temperatures considerably. It’s important to note, though, that the parts of the spectrum that let IR radiation through are just as important as those that absorb it. This allows much of the Earth’s own radiation to escape into space, which helps modulate the temperature.
And to top it all off, the dips in the absorption spectra in the IR region are due to the particular gases in the atmosphere that are essential for the existence of aerobic life, for reasons independent of their absorbance characteristics. Denton explains:
“The fact that the combined absorbance characteristics of these five gases provide just the right absorbance characteristics necessary for advanced aerobic life on the earth’s surface, letting through the right light for photosynthesis and absorbing sufficient heat to raise the earth’s temperature to within the ambient range, is an extraordinary fact—one of the most astonishing elements of fitness for life in all nature. Why? Because the five atmospheric gases N2, O2, O3, H2O, and CO2, four of which—N2, O2, H2O, and CO2—form the bulk of the atmosphere, must exist on any planet hosting complex carbon-based biological life. That their absorbance characteristics should be of such vital benefit for life is therefore a coincidence of stunning fortuity.”
The gift of the leaf
One cannot talk about the importance of light for life without talking about photosynthesis. It is practically the sole source of oxygen production for the atmosphere (and long-agers believe photosynthesis was the origin of a significantly oxygenated atmosphere).
But Denton focuses his attention on the leaf and its importance for complex terrestrial life like us. As he mentions:
“By providing reduced carbon fuels for land-based life, the gift of the leaf had the enormous consequence of enabling aerobic life forms not only to leave the water, but to become air-breathing—taking up oxygen directly from the atmosphere.”
Indeed, air-breathing is an important precondition for complex life. Denton explains:
“Only by taking in oxygen directly from an atmosphere enriched in oxygen (as is our current atmosphere on Earth) can we obtain the necessary 250 milliliters of oxygen we need every minute even at rest.
“And there is little doubt that this requirement (being air-breathing) will also apply to all advanced, complex carbon-based aerobes throughout the universe. … It is far more difficult to obtain oxygen from water than from air, and this puts a ceiling on the metabolic rate aerobic water-breathing organisms can attain and on the consequent complexity (in the broadest sense) that aquatic organisms may achieve compared with air-breathing organisms.”
High oxygen levels in the air is a precondition for complex life. Moreover, photosynthesis is a precondition for the maintenance of high levels of oxygen in the atmosphere, and photosynthetic plants are the food necessary for terrestrial aerobes to survive and thrive. Therefore, photosynthesis and plants are necessary for complex terrestrial life like us.
Denton then proceeds to briefly explain the amazing phenomenon of photosynthesis. He offers a helpful summary:
“In essence, the process involves the use of light energy to draw electrons and protons (H+) from water (H2O), oxidizing the water to oxygen (O2) which is released into the atmosphere, and reducing carbon dioxide to sugars and various reduced carbon compounds (CH). The overall reaction can be written thus:
CO2 + H2O ⇨ CH + O2”.
He notes that it is an incredibly complex and specific process, and cannot fully describe it in a short book like this. However, it relies on many specific preconditions to be possible. Some have already been mentioned, such as the particular radiation properties of the sun and the absorbance properties of the atmosphere. But it is crucially dependent on liquid water as well—not just its optical properties, but many other properties that Denton explored in the second book of this series, The Wonder of Water.4 Water alone exists in all three phases at ambient temperatures, and has appropriate viscosity to produce and maintain soil, the matrix in which most terrestrial plants grow. The surface tension and viscosity properties of water also make plant transpiration possible, which is a much more efficient means of transporting water from the soil to the atmosphere than mere soil evaporation.
The complex web of interlocking preconditions needed for photosynthesis and the physical structure of plants, which, in turn, provide the preconditions for complex aerobic life, are so fortuitously aligned that it looks rigged.
Fitness for vision
Sight is so crucial to understanding the world around us that ‘to see’ is often synonymous with ‘to understand’. Specifically, high-acuity vision of the camera eye (made up of a lens, retina, and tubes filled with photon-detecting molecules) allows us to see to a far horizon, to focus on fine details up close, or to observe stars light-years away from us, and everything in between. While there are some other cool ways of sensing the world around us in the animal kingdom (e.g., echolocation), they would be useless for mastering fire or cataloguing the movements of the heavens, which formed the prelude for science.
But vision shares a commonality with photosynthesis, as Denton explains:
“All biological light-detecting devices depend on the fundamental fact that the energy levels of EM radiation in the visual region are just right for photochemistry.”
However, there are other properties of visible light that make it uniquely fit for high-acuity vision. Our eyes are marvellous seeing devices, capable of handling trillion-fold changes in luminescence (i.e., the difference between a fresh snowfield on a clear day and on a moonless night). However, they have limits. For instance, they are diffraction limited. When light is focused through a small opening, it interacts with the edges of the opening and creates an interference pattern on the opposite side of the opening, called an ‘Airy disc’ (figure 4). As Denton explains:
“The formation of the disc, whether in the eye or a telescope, reduces the resolving power of the optical device, because, when two point sources in the visual field are close together, their Airy discs may overlap and the two sources cannot be resolved.”
What’s crucial about this is that the Airy disc diameter provides a physical limit to the image-resolving capacity of a camera, and it is dependent on several parameters, such as the aperture diameter, the focal length (the distance between the aperture and the retina), and the wavelength of the light. Denton points out that it roughly corresponds to this formula:
Airy disc diameter (in microns) = 2.44λF/A (where λ is wavelength, F is focal length and A is aperture).
From this, we can calculate that the maximum resolving power of the human eye corresponds to an Airy disc diameter around 2.5 μm. This corresponds well to the diameter of many photoreceptors, which range from c. 1.5–6 μm.5 And few animals on Earth have better resolving power than humans—raptors (birds of prey) and the like are among the few.
But if our eyes responded to shorter wavelengths, could we have higher-acuity vision? No. First, shorter wavelengths (into the UV range and beyond) excite electrons too much for photon-detecting molecules to function properly. Second, to increase the resolving power, we’d need to keep the same number of photosensitive chemicals per cell while reducing the size of the cells. But that’s physically impossible because we can’t reduce the size of the photo-sensitive molecules. This would seriously reduce, for example, the span of luminescence over which our eyes could function. Plus, it’s simply not possible, given the size of atoms, to make all the complex biochemical processes occur in a vessel much smaller than photoreceptors actually are.
But maybe we could achieve better acuity with longer wavelengths? No. Greater wavelengths mean greater Airy diameters, and thus decreased resolving power. Plus, to function as our eyes do, eyes would have to be orders of magnitude larger than they are. That creates clear biomechanical problems according to the square-cube law, where an object’s surface area increases by n2 as its volume increases by n3. As Denton summarizes:
“In short, given the basic constraints of biology, the wavelength of light is almost exactly what it needs to be for high-acuity vision in organisms of our approximate size and biological design, inhabiting a planet of the right size and gravity to maintain an oxygen-rich atmosphere capable of sustaining advanced carbon-based life.”
The anthropocentric thesis
Science fiction loves to imagine the possibility of life of all sorts of shapes, sizes, and biochemistries being possible. However, Denton points out a key underlying assumption of such ideas:
“Such scenarios are, of course, pure science fiction, but the underlying notion that the cosmos is fit for a vast zoo of alien life-forms of wildly differing biologies and biochemistries as well as intelligent mechanical forms, is not science fiction but a world view that suggests that there is no special fitness in nature for intelligent, conscious agents like ourselves [emphasis in original].”
This sort of materialistic anti-teleology is the key assumption of our culture. However, the more we look into the amazingly improbable confluence of conditions necessary for the existence of advanced ‘light eaters’ capable of technology, the more absurd this thesis becomes. As Denton says:
“No matter how unfashionable the notion may be in many intellectual circles, the evidence is unequivocal: Ours is a cosmos in which the laws of nature appear to be specially fine-tuned for our type of life—for advanced, carbon-based ‘light eaters’ who possess the technologically enabling miracle of sight!”
What about dark life?
It may be that more than half of the biomass on Earth actually doesn’t need light to run. Denton mentions the life-forms deep underground and at the bottom of the oceans that live exclusively apart from light. They don’t even need to interact with any ‘light eaters’ to survive! However, a curious fact about these organisms is that they are almost all unicellular. Denton explains:
“We now know there can be a cosmos replete with carbon-based life; yet, without the additional elements of fine tuning for us energy-hungry aerobes, it would be devoid of complex, advanced, carbon-based organisms remotely comparable with ourselves.”
Assessment
As with the previous books in the Privileged Species series, Denton stops short of affirming a personal designer, and often relies on a long-age framework to make his case. However, the long-age emphasis in Children of Light is somewhat muted relative to Fire Maker and The Wonder of Water. Meanwhile, the key strength of those books shines through this one too: Denton has assembled an amazing array of scientific facts to support his foundational conclusion that our conditions are fine-tuned for advanced life like us. I have only briefly surveyed a few that Denton speaks of, and these alone would make Denton’s case. However, the book mentions so many more. That alone makes the book worth the read.
References and notes
- See my reviews of the previous two in Doyle, S., Fine-tuned fire, J. Creation 37(2):22–25, 2023; and Doyle, S., Fine-tuned water, J. Creation 37(3):29–32, 2023. Return to text.
- See, for example, Denton, M., Evolution: A theory in crisis, Adler & Adler, MD, 1985; Denton, M., Nature’s Destiny: How the laws of biology reveal purpose in the universe, Free Press, New York, 1998; Denton, M., Evolution: Still a theory in crisis, Discovery Institute Press, Seattle, WA, 2016. Return to text.
- Warren, S.G. and Brandt, R.E., Optical constants of ice from the ultraviolet to the microwave: a revised compilation, J. Geophys. Res. 113:D14220, 2008 | doi:10.1029/2007JD009744. Return to text.
- Denton, M., The Wonder of Water, Discovery Institute Press, Seattle, WA, 2016. Return to text.
- Kolb, H., Photoreceptors; in: Kolb, H., Fernandez, E. and Nelson, R. (Eds.), Webvision: The organization of the retina and visual system; webvision.med.utah.edu, Jul 2013. Return to text.
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