The Anthropic Principle
In the last forty years, it has become increasingly clear to scientists that the laws of nature (such as gravity) and the properties of our earth are particularly well suited to the existence of life. This has obvious philosophical implications for the origin of our extraordinary universe. Besides discussing these, this post gives an overview of the main examples of fine-tuning and some quotes from famous scientists to illustrate its impact.
In 1961, the American physicist Robert H. Dicke gave the anthropic principle its name when he noted that the laws of nature describing how matter behaves (such as Maxwell's laws for electromagnetism) must be such as to enable our existence in the universe. English astronomer John D. Barrow and American physicist Frank J. Tipler elaborated on this idea in their 1986 book The Anthropic Cosmological Principle by figuring out all the limitations that result from this. Laws of nature are expressed in mathematical formulas that have a certain form and that contain certain parameters (or constants), numbers with a specific value [NB in applying the (universal) laws of nature to a specific system, additional initial conditions are necessary, parameters that only apply to that system. However, if this system is the universe as a whole, the distinction between the two kinds of parameters disappears]. Both this form as the value of the parameters are objects of scientific research. Research on the anthropic principle mainly focuses on varying the parameters because this is the most obvious. It then follows that a remarkable fine-tuning of parameters (an alternative name of the anthropic principle) is needed to make the existence of life even possible. Randomness does not cause fine-tuning hence this outcome strongly suggests that the universe was purposefully made for life in general and for humans in particular.
universes can contain life
§ Before discussing the actual fine-tuning (also called coincidences) of the key parameters, here a few notes to clarify their context:
• the real task of (natural) science is to explain measurable phenomena in the universe (such as the falling of an apple from a tree or the trajectory of a rocket) in terms of the laws of nature, not to explain those laws themselves (in terms of ??).
• related to this is that the concept of probability is strictly spoken only applicable to something that repeats itself over and over again (like throwing dice), not on laws which are by definition what they are (but it is applicable to finding an earth among all the planets for the existence of many planets can be interpreted as repetition)
• however, a better understanding of how the relatively few and simple laws make extremely complex life forms (us!) possible has made many scientists wonder whether and how much it could have been otherwise: one imagines a generator of universes and planets within those universes, all with different laws and their parameters, and then determines whether life could exist somewhere there for a longer period of time. The exact probabilities certainly depend on which model generator one considers (which distribution of parameters or which mechanism), but strikingly enough, the final conclusion hardly depends on that: practically any change of laws or their parameters, or of the properties of the earth for that matter, would make the existence of complex life forms impossible; so we could not exist in any other place or under any other circumstances!
• the increase in knowledge about the laws of nature and about the functioning of life since roughly 50 years ago, continues to reveal more dependencies thus adding to an ever-growing list of fine-tuning (more than 200 examples are already listed); this reinforces the conclusion more and more
• some scientists argue that unifying the laws of nature would eliminate fine-tuning, as all parameters would become interdependent; they fail to see, however, that striving for fewer and simpler laws further adds to the wonder (and doesn't change having a very special earth)
• the last naturalistic attempt at explanation (the 'multiverse') proposes a real generator of universes, but (apart from its being unscientific because it attempts to describe unobservable phenomena for the purpose of explaining the laws of nature) its problem remains which laws does that satisfy (with their own fine-tuning?!) and where did generator and laws come from?
§ Then some quotes from scientists active in the field as a confirmation that the anthropic principle is not just an invention of some individual, but generally impresses:
∗ Fred Hoyle (the astronomer who named the Big Bang): ”[The discovery of the fine-tuning of the carbon nuclear energy levels] shook my atheism to its foundations.”
∗ Paul Davies (famous atheistic physicist): ”The impression of design is overwhelming.”
∗ Leonard Susskind (father of string theory): “Our own universe is an extraordinary place that seems fantastically well designed for our own existence. This speciality is not something we can attribute to lucky coincidence, that is far too improbable. The apparent coincidences cry out for an explanation.”
∗ Tony Rothman (theoretical cosmologist): “The medieval theologian who gazed at the night sky through the eyes of Aristotle and saw angels moving the spheres in harmony, has become the modern cosmologist who gazes at the same sky through the eyes of Einstein and sees the hand of God, not in angels but in the constants of nature. [. . . ] Faced with the order and beauty of the universe and the strange coincidences, it is very tempting to make the leap of faith from science to religion. I'm sure many physicists want that. I just wish they would admit it.”
∗ Arno Penzias (Nobel Laureate for Cosmic Microwave Radiation): “Astronomy is leading us to a unique event, a universe created out of nothing, with the very delicate balance needed to provide exactly the conditions that life needs, and with an underlying (one might say 'supernatural') plan.”
∗ George Ellis (leading theoretical cosmologist): “Superb fine-tuning is found in the laws which makes this [complexity] possible. Realizing how complex the outcome is, makes it very difficult not to use the word 'miraculous' without taking a stand as to the ontological status of that word.”
∗ Robert Griffiths (Heinemann Prize winner in Mathematical Physics): ”When we need an atheist for a debate, I go to the philosophy department; the physics department isn't much use.”
∗ Robert Jastrow (self-proclaimed agnostic astronomer): “For the scientist who lived by his belief in the power of reason, the story ends like a nightmare. He has climbed the mountains of ignorance; he is about to reach the highest peak; then as he hoists himself over the last rock, he is greeted by a band of theologians who have been sitting there for centuries.”
§ The anthropic principle, as mentioned, is most clearly expressed in the discrepancy between the small number of parameters that can be tuned and the large number of properties that must be achieved before life can exist. The universal parameters are:
-the critical mass-energy density of the universe (Ω, estimated fine-tuning level 10^−15),
-its fraction of normal matter (Ω_n),
-its degree of expansion (λ, estimated fine-tuning level 10^−120 and for the present value 10^−55),
-the gravitational force constant (α_g, with value 10^−38),
-the electromagnetic force constant (α, with value 10^−2),
-the weak force constant (α_w, with value 10^−6),
-the strong force constant (α_s, with value 1),
-the ratio of proton / electron masses (µ),
-of electron / proton numbers (#, estimated fine-tuning 10^−37),
-of proton / neutron masses (ν, estimated fine-tuning 0.1%)
-and the proton half-life (τ , not much known about its fine-tuning or value; it must be small, but nonzero for matter to dominate over antimatter).
So that's only 11 numbers! Note that all these numbers are dimensionless; constants with dimension (the gravitational constant G, speed of light c, Planck's constant h and the electric charge e) are not parameters, but serve to define the units of length, time, mass-energy and charge-current.
§ The key attributes to capture for the universe as a whole are:
-its size (which determines the development of its expansion), its homogeneity (which determines the degree of 'clumping' of matter), coolness (for the thermal stability) and age (for the degree of its development; all four depend on Ω and λ);
-the formation and stability of galaxies (otherwise stable planets cannot exits; both properties depend on Ω_n);
-the density of the galaxies (for the degree of star formation and the stability of their orbits; depends on α_g);
-the density of the planetary systems (for planet formation and the stability of their orbits; also dependends on α_g);
-the dimensions and longevity of stars (life needs stable, not too big stars; both depend on α_g and α, estimated fine-tuning level 10^−40);
-multiple coincidences of fusion energy levels (to get elements heavier than H; depending on α_s and α, estimated fine-tuning levels 1-2% each);
-the relative abundance of the elements (many elements are needed, each in its own proportion, to fulfill important life functions; also depend on α_g, α_w and µ);
-chemistry in general (the basis of life; dependent on α and µ, estimated fine-tuning level 4%)
-and specifically the polarity of water (for the temperature and energy of its phase transitions and the drifting of ice?!)
-or the degree of quantum tunneling of hemoglobin (for O2 transport)
-or the magnitude of the relativistic corrections in Cu (copper) & V (vanadium; both for nervous system functioning and bone development)
-or the functioning of DNA itself (nothing is known about the needed fine-tuning).
Conclusion: the exact number of properties [NB or even their combined fine-tuning] is unclear, but it is clearly quite more than the amount of parameters.
§ There are also Earth-specific properties to be fixed (that may not apply to other planets and so the concept of probability can, in principle, be applied to them):
-the co-rotating position of our solar system between the outer arms of our galaxy (for its stability),
-the type and size of our single sun (relevant for its stability and brightness, UV photosynthesis, the width of the habitable zone around it and the Earth's orbital period in it),
-gas giants far from the sun (such as Jupiter, for shielding against comets and stabilizing the Earth orbit),
-a large moon (which stabilizes the Earth's rotation, and the Earth–moon system stabilizes the inner planets?!),
-the right distance from the sun (even the right position within the habitable zone, and with the right albedo of the earth, for a good temperature),
-a very circular orbit (for thermal stability),
-the correct rotational speed [NB caused by a bizarre collision that also formed the moon???] (to distribute the solar heat),
-correct tilt plus the aphelion pointing north (which has the most continents for a VERY even heat distribution),
-a strong magnetic field (originating from the large iron/nickel core, to protect life and water against cosmic rays),
-the right size (for good gravity and cooling time),
-enough sulfur in the core (to help it solidify),
-sufficient minerals containing iron, chlorine and sulfur (important trace elements for life),
-the right amount of tectonic, volcanic & seismic activity [NB and even ice ages?] (to recycle minerals and form continents),
-sufficient water (so that the oceans are connected and the climate is nowhere too extreme), with sea salt aerosols for cloud formation,
-a thin atmosphere (which allows the 3 water phases to coexist), with the right concentrations of CO2, N2, O2, H2O and O3, the right amounts of lightning (to form nitrates) and forest fires (to recycle plants).
The combined fine-tuning needed for all properties of our earth is estimated to be more than 10^−99. Even if that might be an exaggeration, there are 'only' an estimated 10^23 planets in the universe, giving our Earth a negligible chance of existing at all! Good luck SETI ;-) [NB Search for ExtraTerrestrial Intelligence, see www.SETI.org]
Conclusion
Estimates of the combined fine-tuning of the universal parameters (including the initial conditions) are strongly model dependent. The numbers already mentioned indicate that this fine-tuning can be even greater than that specific to the Earth.
In this context, it is interesting that Sir Roger Penrose (famous mathematical physicist and atheist) in his book The Emperor's New Mind from 1989 calculated the fine-tuning (ordering) of the universe in the Big Bang. This concerns only the initial conditions for which he arrives at the unimaginably small number of 1 in 10^10^123. This is the most extreme number that has ever been calculated [NB except for some bizarre numbers from abstract mathematics such as the first number of Skewes or the number of Graham, which is so large that it needs its own notation to even be written down], surpassing even the googolplex [NB 10^googol; with the googol (search engine namesake) = 10^100 which is interestingly comparable to the estimated fine-tuning needed for the earth itself].
PS The concepts of ordering, fine-tuning and information (content) are related: information written on paper corresponds to a precise ordering of the ink and opening a combination lock requires precise knowledge of its required fine-tuning.
The laws of nature and parameters of the universe as well as the properties of the earth thus represent a large amount of information. What about human DNA? The classic maximum of information that more than 3 billion base pairs can hold is 10^{2•10^9} (750 MB) [NB so lying somewhere between the googol and the googolplex]; it just fits on a normal CD. It was long thought that most DNA was 'junk', that it was of no use and that it contained no information [NB yet less than 1% of our DNA already corresponds to more information than the googol]. Recent findings, however, show that there is still much to be learned about our DNA, moreover those strongly suggest that none of it is junk. Indeed, multiple layers of coding have been discovered: the once thought 'degenerate' codons are now called duons for regulating the pausing & the binding site when copying DNA, “intron/exon” boundary conditions make that a piece of DNA encodes for multiple proteins, as do overlapping genes and backward-oriented promoters. All in all, this would make our DNA contain even more information than the classic maximum (and thus make it look more like a DVD?)! Are you sure you want to suggest that your favorite HD movie could have evolved 'magically' from the primordial soup? {;-)
Main sources used: Luke Barnes (ETH Zurich) quantitatively discusses how small differences in the constants of nature, or in the laws of nature themselves, would have enormous consequences for the existence of life and why philosophical counterarguments fail (The fine-tuning of the universe for intelligent life, 76 pages, www.arXiv.org) & (more critically) Hugh Ross (Dr of Astronomy) lists 154 parameters necessary for the existence of life with 226, mainly scientific, references
(www.reasons.org/articles/fine-tuning-for-life-on-earth-june-2004).
My approach tries to be a middle ground between the detail and the completeness (and thus reliability) of both sources.
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