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The fine-tuned universe is the hypothesis that, because "life as we know it" could not exist if theconstants of nature – such as theelectron charge, thegravitational constant and others – had been even slightly different, the universe must be tuned specifically for life.[1][2][3][4] In practice, this hypothesis is formulated in terms ofdimensionless physical constants.[5]
In 1913,chemistLawrence Joseph Henderson wroteThe Fitness of the Environment, one of the first books to explore fine tuning in the universe. Henderson discusses the importance of water and the environment to living things, pointing out that life as it exists on Earth depends entirely on Earth's very specific environmental conditions, especially the prevalence and properties of water.[6]
In 1961, physicistRobert H. Dicke argued that certain forces in physics, such asgravity andelectromagnetism, must be perfectly fine-tuned for life to exist in the universe.[7][8]
AstronomerFred Hoyle argued for a fine-tuned universe: "From 1953 onward, Willy Fowler and I have always been intrigued by the remarkable relation of [...] and your fixing would have to be just where these levels are actually found to be. [...] A common sense interpretation of the facts suggests that a superintellect has monkeyed with physics, as well as with chemistry and biology, and that there are no blind forces worth speaking about in nature."[9] In his 1983 bookThe Intelligent Universe,[10] Hoyle wrote, "The list of anthropic properties, apparent accidents of a non-biological nature without which carbon-based and hence human life could not exist, is large and impressive."[11]
Belief in the fine-tuned universe led to the expectation that theLarge Hadron Collider would produce evidence ofphysics beyond the Standard Model, such assupersymmetry,[12] but by 2012 it had not produced evidence for supersymmetry at the energy scales it was able to probe.[13]
PhysicistPaul Davies said: "There is now broad agreement among physicists and cosmologists that the Universe is in several respects 'fine-tuned' for life. But the conclusion is not so much that the Universe is fine-tuned for life; rather it is fine-tuned for the building blocks and environments that life requires".[14] He also said that"'anthropic' reasoning fails to distinguish between minimallybiophilic universes, in which life is permitted, but only marginally possible, and optimally biophilic universes, in which life flourishes becausebiogenesis occurs frequently".[15] Among scientists who find the evidence persuasive, a variety ofnatural explanations have been proposed, such as the existence ofmultiple universes introducing asurvivorship bias under theanthropic principle.[5]
The premise of the fine-tuned universe assertion is that a small change in several of the physical constants would make the universe radically different.Stephen Hawking observed: "The laws of science, as we know them at present, contain many fundamental numbers, like the size of the electric charge of the electron and the ratio of the masses of the proton and the electron. ... The remarkable fact is that the values of these numbers seem to have been very finely adjusted to make possible the development of life".[4]
For example, if the strong nuclear force were 2% stronger than it is (i.e. if thecoupling constant representing its strength were 2% larger) while the other constants were left unchanged,diprotons would be stable; according to Davies, hydrogen wouldfuse into them instead ofdeuterium andhelium.[16] This would drastically alter the physics ofstars, and presumably preclude the existence of life similar to what we observe on Earth. The diproton's existence would short-circuit the slow fusion of hydrogen into deuterium. Hydrogen would fuse so easily that it is likely that all the universe's hydrogen would be consumed in the first few minutes after theBig Bang.[16] This "diproton argument" is disputed by other physicists, who calculate that as long as the increase in strength is less than 50%, stellar fusion could occur despite the existence of stable diprotons.[17]
The precise formulation of the idea is made difficult by the fact that it is not yet known how many independent physical constants there are. TheStandard Model of particle physics has 25 freely adjustable parameters andgeneral relativity has one more, thecosmological constant, which isknown to be nonzero but profoundly small in value. Because physicists have not developed an empirically successful theory ofquantum gravity, there is no known way to combine quantum mechanics, on which the standard model depends, and general relativity.[18]
Without knowledge of this more complete theory suspected to underlie the standard model, it is impossible to definitively count the number of truly independent physical constants. In some candidate theories, the number of independent physical constants may be as small as one. For example, the cosmological constant may be a fundamental constant but attempts have also been made to calculate it from other constants, and according to the author of one such calculation, "the small value of the cosmological constant is telling us that a remarkably precise and totally unexpected relation exists among all the parameters of theStandard Model of particle physics, the bare cosmological constant and unknown physics".[18]
Martin Rees formulates the fine-tuning of the universe in terms of the following sixdimensionless physical constants.[1][19]
Max Tegmark argued that if there is more than one time dimension, then physical systems' behavior could not be predicted reliably from knowledge of the relevantpartial differential equations. In such a universe, intelligent life capable of manipulating technology could not emerge. Moreover,protons andelectrons would be unstable and could decay into particles having greater mass than themselves. This is not a problem if the particles have a sufficiently low temperature.[24]
An older example is theHoyle state, the third-lowest energy state of thecarbon-12 nucleus, with an energy of 7.656 MeV above the ground level.[25] According to one calculation, if the state's energy level were lower than 7.3 or greater than 7.9 MeV, insufficient carbon would exist to support life. To explain the universe's abundance of carbon, the Hoyle state must be further tuned to a value between 7.596 and 7.716 MeV. A similar calculation, focusing on the underlying fundamental constants that give rise to various energy levels, concludes that thestrong force must be tuned to a precision of at least 0.5%, and the electromagnetic force to a precision of at least 4%, to prevent either carbon production or oxygen production from dropping significantly.[26]
Some explanations of fine-tuning arenaturalistic.[27] First, the fine-tuning might be an illusion: more fundamental physics may explain the apparent fine-tuning in physical parameters in the current understanding by constraining the values those parameters are likely to take. AsLawrence Krauss put it, "certain quantities have seemed inexplicable and fine-tuned, and once we understand them, they don't seem to be so fine-tuned. We have to have some historical perspective".[23]Victor J. Stenger has shown that random selection of physical parameters can still produce universes capable of harboring life.[28] Some argue it is possible that a final fundamentaltheory of everything will explain the underlying causes of the apparent fine-tuning in every parameter.[29][23]
Still, as modern cosmology developed, various hypotheses not presuming hidden order have been proposed. One is amultiverse, where fundamental physical constants are postulated to have different values outside of the known universe.[30][31]: 3–33 On this hypothesis, separate parts of reality would have wildly different characteristics. In such scenarios, the appearance of fine-tuning is explained as a consequence of the weakanthropic principle andselection bias, specificallysurvivorship bias. Only those universes with fundamental constants hospitable to life, such as on Earth, could contain life forms capable of observing the universe who can contemplate the question of fine-tuning.[32] Zhi-Wei Wang andSamuel L. Braunstein argue that the apparent fine-tuning of fundamental constants could be due to the lack of understanding of these constants.[33]
If the universe is just one of many (possibly infinitely many) universes, each with different physical phenomena and constants, it is unsurprising that there is a universe hospitable to intelligent life. Some versions of the multiverse hypothesis therefore provide a simple explanation for any fine-tuning,[5] while the analysis of Wang and Braunstein challenges the view that this universe is unique in its ability to support life.[33]
The multiverse idea has led to considerable research into the anthropic principle and has been of particular interest toparticle physicists becausetheories of everything do apparently generate large numbers of universes in which the physical constants vary widely. Although there is no evidence for the existence of a multiverse, some versions of the theory make predictions of which some researchers studyingM-theory and gravity leaks hope to see some evidence soon.[34] According toLaura Mersini-Houghton, theWMAP cold spot could provide testable empirical evidence of aparallel universe.[35] Variants of this approach includeLee Smolin's notion of cosmologicalnatural selection, theekpyrotic universe, and thebubble universe theory.[34]: 220–221
It has been suggested that invoking the multiverse to explain fine-tuning is a form of theinverse gambler's fallacy.[36][37]
Stephen Hawking andThomas Hertog proposed that the universe's initial conditions consisted of asuperposition of many possible initial conditions, only a small fraction of which contributed to the conditions seen today.[38] According to thetop-down cosmology theory, the universe's "fine-tuned" physical constants are inevitable, because the universe "selects" only those histories that led to the present conditions. In this way, top-down cosmology provides an anthropic explanation for why this universe allows matter and life without invoking the multiverse.[39]
Some forms of fine-tuning arguments about the formation of life assume that only carbon-based life forms are possible, an assumption sometimes calledcarbon chauvinism.[40] Conceptually,alternative biochemistry or other forms of life are possible.[41]
Thesimulation hypothesis holds that the universe is fine-tuned simply because the more technologically advanced simulation operator(s) programmed it that way.[42]
Graham Priest,Mark Colyvan,Jay L. Garfield, and others have argued against the presupposition that "the laws of physics or the boundary conditions of the universe could have been other than they are".[43]
Some scientists,theologians, andphilosophers, as well as certain religious groups, argue thatprovidence orcreation are responsible for fine-tuning.[44][45][46][47][48] Christian philosopherAlvin Plantinga argues that random chance, applied to a single and sole universe, only raises the question as to why this universe could be so "lucky" as to have precise conditions that support life at least at some place (the Earth) and time (within millions of years of the present).
One reaction to these apparent enormouscoincidences is to see them as substantiating the theistic claim that the universe has been created by a personal God and as offering the material for a properly restrained theistic argument – hence the fine-tuning argument. It's as if there are a large number of dials that have to be tuned to within extremely narrow limits for life to be possible in our universe. It is extremely unlikely that this should happen by chance, but much more likely that this should happen if there is such a person as God.
— Alvin Plantinga, "The Dawkins Confusion: Naturalismad absurdum"[49]
William Lane Craig, a philosopher andChristian apologist, cites this fine-tuning of the universe asevidence for the existence ofGod or some form ofintelligence capable of manipulating (or designing) the basicphysics that governs the universe.[50] Philosopher and theologianRichard Swinburne reaches the design conclusion usingBayesian probability.[51] Scientist and theologianAlister McGrath observed that the fine-tuning of carbon is even responsible for nature's ability to tune itself to any degree.
The entire biological evolutionary process depends upon the unusual chemistry of carbon, which allows it to bond to itself, as well as other elements, creating highly complex molecules that are stable over prevailing terrestrial temperatures, and are capable of conveying genetic information (especially DNA). [...] Whereas it might be argued that nature creates its own fine-tuning, this can only be done if the primordial constituents of the universe are such that an evolutionary process can be initiated. The unique chemistry of carbon is the ultimate foundation of the capacity of nature to tune itself.[52][53]
Theoretical physicist and Anglican priestJohn Polkinghorne stated: "Anthropic fine tuning is too remarkable to be dismissed as just a happy accident".[54]
Theologian and philosopherAndrew Loke argues that there are only five possible categories of hypotheses concerning fine-tuning and order: (i) chance, (ii) regularity, (iii) combinations of regularity and chance, (iv) uncaused, and (v) design, and that only design gives an exclusively logical explanation of order in the universe.[55] He argues that theKalam Cosmological Argument strengthens the teleological argument by answering the question "Who designed the Designer?".[55]
CreationistHugh Ross advances a number of fine-tuning hypotheses.[56][57] One is the existence of what Ross calls "vital poisons", which are elementalnutrients that are harmful in large quantities but essential for animal life in smaller quantities.[58]
Philosopher and theologianRobin Collins argues that theism entails the expectation that God would create a reality structured to allow for scientific discovery to easily happen. According to Collins, various physical constants such as thefine-structure constant allowing for efficient energy usage, thebaryon-to-photon ratio allowing for thecosmic microwave background to be discovered, and the mass of theHiggs boson allowing it to be detected are examples of the laws of physics being fine-tuned for scientific discovery.[59]
Evolutionary biologistRichard Dawkins dismisses the theistic argument as "deeply unsatisfying" since it leaves the existence of God unexplained, with a God capable of calculating the fine-tuning at least as improbable as the fine-tuning itself.[60] Against this claim, it has been argued that theism is a simple hypothesis, allowing theists to deny that God is at least as improbable as the fine-tuning.[61][62][63]
Douglas Adams satirized the theistic argument in his 2002 bookThe Salmon of Doubt:[64]
Imagine a puddle waking up one morning and thinking, "This is an interesting world I find myself in, an interesting hole I find myself in, fits me rather neatly, doesn't it? In fact, it fits me staggeringly well, must have been made to have me in it!"
Contrary to a common argument that a small increase in the strength of the strong force would lead to destruction of all hydrogen in the Big Bang due to binding of the diproton and the dineutron with a catastrophic impact on life as we know it, we show that provided the increase in strong force coupling constant is less than about 50% substantial amounts of hydrogen remain.
