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The term fine-tuned universe refers to a collection of ideas that state the existence of life in the Universe is the result of the universe's physical constants relating to one another in remarkedly exactly the fashion that is required in order for the universe to be hospitable for life. The fine-tuned universe argument is related to the weak anthropic principle, which states that any valid theory of the universe must be consistent with our existence as human beings at this particular time and place in the universe.

While the concept of the remarkable precise interplay of physical constants being necessary for known life is widely accepted within mainstream science, the argument that they imply that the universe was purposely fine-tuned to support life is not. Though there are fine tuning arguments that are naturalistic, the assertion that the universe was designed to be fine-tuned is largely promoted by advocates of intelligent design and other forms of creationism. This fine-tuning of the universe apparent to some is cited as evidence for the existence of God or some form of intelligence capable of manipulating the basic physics that govern the universe, or even evidence that the universe is a simulated reality.

The premise of the fine-tuned universe assertion is that any small change in the approximately 26 dimensionless fundamental physical constants would make the universe radically different: if, for example, the fine-structure constant (which can be thought of as a function of the electron charge relative to the Planck charge) were slightly different, or if the strong nuclear force were only 2% stronger, diprotons would be stable and hydrogen would fuse too easily, making stars as we know them impossible and prevent the universe from developing life as we know it. Being dimensionless, these 26 fundamental constants would take on the same values independent of the system of units that any culture would use to measure physical quantity with.

As modern cosmology developed, various hypotheses have been proposed (including an oscillatory universe or a multiverse) where physical constants are postulated to resolve themselves to random values in different iterations of reality, resulting in separate parts of reality with wildly different characteristics. In such scenarios fine-tuning does not exist at all as only those "universes" with constants hospitable to life (such as what we observe) would develop life capable of pondering the question.

Critics of both the fine-tuned universe assertion and the anthropic principle argue that they are essentially a tautology; life as we know it may not exist if things were different, but a different sort of life might exist in its place. The claim of a fine-tuned universe has also been criticized as an argument by lack of imagination for assuming no other forms of life are possible (see also carbon chauvinism). In addition, it appears to be an example of backwards reasoning since it asserts that the universe is adapted to humans instead of that humans are adapted to the universe through the process of evolution through natural selection. Finally, it is an example of the logical flaw of hubris or anthropocentrism in its assertion that humans are the purpose of the universe. The argument loses much of its meaning and appeal if it is restated in the alternative terms "If the constants of the universe weren't exactly what they are, Saturn wouldn't have rings around it."

Nature of the constants

Modern science as practiced since Rene Descartes is reductionist, meaning that it attempts to discover the most fundamental objects and rules governing the observable behavior of the universe. In descriptions of the physical universe, fundamental rules take the form of laws (usually equations relating physical quantities and properties) involving physical constants, while the fundamental objects are elementary particles with constant mass, charge, and other physical properties. This reductionism is a pragmatic approach that obtains results and is not a philosophical position on ontology. The nature of these constants is a much debated topic in physics and metaphysics (see string theory).

Meaning of "universe"

Both popular and professional research articles in cosmology often use the term "universe" to to refer to observable universe. The reason for this usage is that only observable phenomena are scientifically relevant. Since unobservable phenomena have no perceptible effects, physicists argue that they "causally do not exist". Since unobservable parts of the universe cannot be measured, hypotheses about them are not testable, and thus inappropriate for a scientific theory.

In metaphysics, "universe" refers to everything that exists. This encompasses both observable and unobservable phenomena. Metaphysics seeks to describe everything that is knowable about existance.

All arguments that refer to evidence refer to the observable universe but may not apply to the unobservable parts of reality sometimes called "other universes". A larger Multiverse may exist where different parts have different parameters. Our observable universe, according to this has the parameters necessary for carbon based life. Other parts of the multiverse may be sterile or may contain different types of Self-aware systems or life.

Known physical constants and possible examples of fine tuning

(Sources for this section: , , , , , )

• The nuclear strong force holds together the particles in the nucleus of an atom. If the strong nuclear force were slightly weaker, by as little as 2%, multi-proton nuclei would not hold together and hydrogen would be the only element in the universe. If the strong force were slightly stronger, by as little as 1%, hydrogen would be rare in the universe and elements heavier than iron (elements resulting from fusion during the explosion of supernovae) would also be rare.

• The nuclear weak force affects the behavior of leptons (e.g. neutrinos, electrons, and muons) that do not participate in strong nuclear reactions. If the weak force were slightly larger, neutrons would decay more readily, and therefore would be less available, and little or no helium would be produced from the big bang. Without the necessary helium, heavy elements sufficient for the constructing of life as we know it would not be made by the nuclear furnaces inside stars. If the weak force were slightly smaller, the big bang would burn most or all of the hydrogen into helium, with a subsequent over-abundance of heavy elements made by stars, and life as we know it would not be possible.

• The intensity of the force binding electrons to protons in atoms depends on the electromagnetic coupling constant. The characteristics of the orbits of electrons about atoms determines to what degree atoms will bond together to form molecules. If the electromagnetic coupling constant were different atoms and molecules would be significantly different.

• The ratio of electron to proton mass also determines the characteristics of the orbits of electrons about nuclei. A proton is 1836 times more massive than an electron. If the electron to proton mass ratio were different, atoms and molecules would be significantly different.

• The entropy level of the universe affects the condensation of massive systems. The universe contains about one billion photons for every baryon. This makes the universe extremely entropic, i.e. a very efficient radiator and a very poor engine. If the entropy level for the universe were slightly larger, no galactic systems would form (and therefore no stars). If the entropy level were slightly smaller, the galactic systems that formed would effectively trap radiation and prevent any fragmentation of the systems into stars. In either case, the universe would be devoid of stars and solar systems.

• The force of gravity affects the interaction of particles. In order for life as we know it to form, the force of gravity must be 10 (10 to the 40th power) times weaker than the force of electromagnetism. The relationship of gravity to electromagnetism as it currently exists is this: The positively charged particles must equal in charge the numbers negatively charged particles or else electromagnetism will dominate gravity, and stars, galaxies and planets will not form. The numbers of electrons must equal the numbers of protons to better than one part of 10 (10 to the 37th power).

• The more probable explanations of Multiverse models themselves require fine-tuning.

These and other examples are often given as evidence of the universe being fine tuned. Whether they actually are proof of fine tuning is a matter debated between proponents of the fine-tuning argument and critics who feel that such reasoning is a subjective anthropomorphism of natural physical constants and even if it is improbable that they occurred by chance, that by itself, improbability is insufficient reason to conclude that they occurred by design.

Explaining fine-tuned universe

Fine-Tuning comes with caveats. The fact that a universe with different physical constants might be inhospitable to life as we know it does not necessarily mean that it is inhospitable to any form of life. Currently, there is no way of experimentally determining if a universe allows for life or not. Further, most of this universe, especially the interstellar vacuum, appears to be devoid of life; other physical constants may exist that allow a much greater density of life than in this universe.

If it is accepted that the universe is fine-tuned, there are many major explanations for the occurrence.

• Random chance: It could be that through sheer random circumstance, this universe is the one that was created, and that there is no further explanation. Some, like Stephen Jay Gould, believe that fine-tuning does not need any more explanation than that a particular roll of dice would result in a double six (i.e. an extremely lucky event). Our universe had to have physical constants, and they just happen to be the ones that permit our existence, as opposed to no living creatures, or different ones. Had there been other sapient and sentient beings in a totally different universe living in totally different bodies they would have asked the exact same apparently meaningless question.

• Multiverse: This assumes the existence of a mechanism that has created many universes with different physical constants, some of which are hospitable to intelligent life. Because we are intelligent beings, we are by definition in a hospitable one. This approach has led to considerable research into the anthropic principle and has been of particular interest to particle physicists because theories of everything do apparently generate large numbers of universes in which the physical constants vary widely. As of yet, there is no evidence for the existence of a multiverse, but some versions of the theory do make predictions which some researchers studying M-theory and gravity leaks hope to see some evidence of soon. Multiverses are not necessarily falsifiable, and thus some are relunctant to call multiverses a "scientific" idea.

• Cosmological natural selection (CNS), was created by physicist Lee Smolin as a testable alternative to string theory predictions of an enormous landscape of possible universes. CNS holds that the creation of a black hole often (always?) entails the creation of baby universes, and that through a process of selection that in some ways mimics evolutionary natural selection, universes are created that are optimized for creating black holes. By extension, these same universes are optimized for creating stable atoms, long lived stars, and lots of stable carbon atoms. This last point also happens to explain why our universe seems to be biophillic.

• The Ekpyrotic universe. Brane cosmology assumes that the visible universe lies on a three-dimensional brane which moves in higher dimensional space. Our brane may be one of innumerable others moving through these extra dimensions. The ekpyrotic scenario was proposed by Khoury, Ovrut, Steinhardt and Turok in 2001. It suggests that the visible universe was empty and contracting in the distant past. At some time, our brane collided with another, parallel "hidden" brane, which caused the contracting universe to reverse and begin expanding. Hot matter and radiation was created in the collision, which started the hot big bang from which the present-day universe originated. The brane collision, from the four-dimensional perspective of the visible brane, looks like a big crunch followed by a big bang. Over a long enough period of time it should not be surprising that some of these universes would be biophillic.

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Naturalism and the fine tuning argument

That life as we know it would not be possible if the physical constants of the universe were even slightly different from what they are and may appear to be "fine-tuned" is an uncontroversial position within the mainstream scientific community. But conclusions drawn from that observation or appeals to the improbability of life that some intelligence intentionally "fine-tuned" the universe for life are not widely accepted. There is controversy over whether such conclusions can even be considered within natural science or whether they are better considered as matters of metaphysics or religion.

Some conjectures of fine-tuning may not by necessity violate the naturalistic underpinnings of natural science, but that any specific argument has not yet been established. Those that leave the question of agent or first cause unaddressed are as equally appealing to those who posit a theistic or deistic first cause as it is to those who are to strict materialists.

Cosmologist Sir Martin Rees expresses a point of view common in the scientific community: "People used to wonder: why is the earth in this rather special orbit around this rather special star, which allows water to exist or allows life to evolve? It looks somehow fine-tuned. We now perceive nothing remarkable in this, because we know that there are millions of stars with retinues of planets around them: among that huge number there are bound to be some that have the conditions right for life. We just happen to live on one of that small subset. So there's no mystery about the fine-tuned nature of the earth's orbit; it's just that life evolved on one of millions of planets where things were right." Rees states a personal preference for a multiverse: "We seem to have three choices'... We can dismiss it as happenstance, we can acclaim it as the workings of providence, or (my preference) we can conjecture that our universe is a specially favoured domain in a still vaster multiverse.’ If this multiverse contained every possible set of laws and conditions, then the existence of our own world with its particular characteristics would be inevitable." .

Naturalistic fine-tuned universe arguments

Theoretical physicist Lee Smolin is attempting to verify a theory which he calls "cosmological natural selection" in which the universe fine-tuned itself. By his reasoning, self-organizing critical systems are capable of fine-tuning all by themselves, following only a simple set of physical laws — thus making it likely that the parameters are "fine-tuned" the way we see them, but as the result of natural processes, not an intelligence. This is similar to the self-regulating system proposed in Gaia theory, but on a cosmological scale.

Ikeda-Jefferys argument

A Bayesian probabilistic discussion by mathematician Michael Ikeda and astronomer William H. Jefferys (2006); web version available at argues that the traditional reasoning about intelligent design from the presence of fine-tuning does not properly condition on the existence of life and is also based on an incorrect reversal of conditional probabilities: it is an example of the prosecutor's fallacy, which in this form erroneously claims that if fine-tuning is rare in naturalistic universes, then a fine-tuned universe is unlikely to be naturalistic. (In this context, "naturalistic" is taken to be synonymous with "not intelligently designed".)

Ikeda and Jefferys offer a proof which, they argue, indicates one should in fact draw a conclusion opposite to the traditional reasoning: instead of implying intelligent design, the presence of fine-tuning actually argues against such design. Their proof hinges on the assumptions that

1. our universe exists and contains life (L),
2. our universe is "life friendly" (F), in that its conditions are compatible with life existing naturalistically, and
3. life can exist in a "naturalistic" (N) universe only if that universe is "life-friendly" (N&L ⇒ F: the weak anthropic principle).

The Ikeda-Jefferys fine-tuning theorem states that, under these assumptions, the probability that our universe is naturalistic, given it contains life, is less than, or equal to, the probability that our universe is naturalistic, given that it contains life and is also fine-tuned — in probabilistic notation, P(N|L) ≤ P(N|L&F). In other words, the existence of fine tuning increases (or rather, cannot decrease) the probability that our universe is naturalistic, given that we already know it contains life. Thus, Ikeda and Jefferys argue ironically, supporters of intelligent design should try to prove that our universe is not fine-tuned. The philosopher of science Elliott Sober makes a similar argument.

The Ikeda-Jefferys argument arrives at a different conclusion from that of standard Cosmological Intelligent Design due to a differing assumption held by the two arguments concerning the nature of omnipotence. Cosmological Intelligent Design arguments assume that an intelligent designer has chosen to work through "natural" laws (which he can modify) while the Ikeda-Jefferys argument does not make this assumption. Thus the conclusion most Cosmological Intelligent Design arguments draws is that one should look for a low probability of randomness producing life-friendly conditions, and the Ikeda-Jefferys conclusion is that one should look for life that is not supported by natural law. If the Ikeda-Jefferys argument holds the same assumptions as Cosmological Intelligent Design, fine-tuning provides no new information about the likelihood or unlikelihood of design; using the variables above, we would obtain P(N|L) = P(N|L&F), because we would have L -> F: life could exist only in a life-friendly universe, regardless of how that universe came to be, or whether it was subject to continued divine intervention.

A similar argument is made by Elliott Sober (2004). One should note that Richard Swinburne reaches the opposite conclusion using Bayesian probability (Swinburne 1990).

In fiction

The second part of The Gods Themselves by Isaac Asimov deals with a parallel universe with a different Strong nuclear force.

References

• John D. Barrow and Frank J. Tipler, 1986. The Anthropic Cosmological Principle. Oxford Univ. Press. ISBN 0-19-282147-4
• John D. Barrow, 2003. The Constants of Nature, Pantheon Books, ISBN 0-375-42221-8
• Nick Bostrom, 2002. Anthropic Bias: Observation Selection Effects in Science and Philosophy, Routledge, New York, ISBN 0-415-93858-9
• Paul Davies, 1982. The Accidental Universe, Cambridge University Press, ISBN 0-521-24212-6
• Michael Ikeda and William H. Jefferys, "The Anthropic Principle Does Not Support Supernaturalism," in The Improbability of God, Michael Martin and Ricki Monnier, Editors, pp. 150-166. Amherst, N.Y.: Prometheus Press. ISBN 1-59102-381-5
• Simon Conway Morris, 2003. Life's Solution: Inevitable Humans in a Lonely Universe. Cambridge Univ. Press.
• Martin Rees, 1999. Just Six Numbers, HarperCollins Publishers, ISBN 0-465-03672-4
• Elliott Sober, 2004. The Design Argument, in The Blackwell Guide to the Philosophy of Religion, W. E. Mann, Editor. Blackwell Publishing, ISBN 0-631-22129-8
• Richard Swinburne, 1990. Argument from the fine-tuning of the universe, in Physical cosmology and philosophy, J. Leslie, Editor. Collier Macmillan: New York. pp. 154-73.
• Ward, P. D., and Brownlee, D., 2000. Rare Earth: Why Complex Life is Uncommon in the Universe. Springer Verlag.

See also

• Anthropic principle
• Fine-tuning
• Intelligent design
• Origin of life
• Rare Earth hypothesis
• Ultimate fate of the universe

External links

• Does the Cosmos Show Evidence of Purpose?
• Fine-Tuning Argument links to online references, at the Secular Web
• Cosmological fine-tuning
• Design and the Anthropic Principle by Hugh Ross
• The Anthropic Principle Does Not Support Supernaturalism by Michael Ikeda and Bill Jefferys
• Overview of the Cosmological Intelligent Design argument Stephen M. Barr. July 2001. First Things, the Journal of Religion, Culture, and Public Life.
• The Design Argument by Elliott Sober (Adobe PDF format)
• Home page of Templeton Foundation project on fine-tuning
• Sharpening Ockham's razor on a Bayesian strop by William H. Jefferys and James O. Berger (Adobe PDF format)
• The cosmos is fine-tuned to permit human life at the talk.origins index to creationist claims.
• Is the Universe fine-tuned for us? (Adobe PDF format)
• Interview with Charles Townes discussing science and religion.
• Evidence For Design In The Universe

• Intelligent design
• Pseudophysics
• Creation Science