T H E A N T H R O P I C P R I N C I P A L
"The intelligent beings in these regions should therefore not be surprised if they observe that their locality in the universe satisfies the conditions that are necessary for their existence. It is a bit like a rich person living in a wealthy neighborhood not seeing any poverty."
STEPHEN HAWKING
STEPHEN HAWKING
In physics and cosmology, the anthropic principle is the philosophical argument that observations of the physical universe must be compatible with the conscious life that observes it. Some proponents of the argument reason that it explains why the universe has the age and the fundamental physical constants necessary to accommodate conscious life. As a result, they believe that the fact that the universe's fundamental constants are within the narrow range thought to allow life is not remarkable.
The strong anthropic principle as explained by Barrow and Tipler (see variants) states that this is all the case because conscious life, in some sense, needed to exist. On the other hand, in a sufficiently large universe, some worlds might evolve conscious life regardless of adverse conditions. Douglas Adams used the metaphor of a living puddle examining its own shape, since, to those living creatures, the universe may appear to fit them perfectly (while in fact, they simply fit the universe perfectly). Critics argue in favor of a weak anthropic principle similar to the one defined by Brandon Carter (see variants), which states that the universe's ostensible fine tuning is the result of selection bias, e.g. in the long term, only survivors can report their location in time and space.
The principle was formulated as a response to a series of observations that the laws of nature and parameters of the Universe take on values that are consistent with conditions for life as we know it rather than a set of values that would not be consistent with life as observed on Earth. The anthropic principle states that this phenomenon is a necessity because living observers wouldn't be able to exist, and hence, observe the Universe, were these laws and constants not constituted in this way.
The term anthropic in "anthropic principle" has been argued to be a misnomer. While singling out our kind of carbon-based life, none of the finely tuned phenomena require human life or some kind of carbon chauvinism. Any form of intelligent life would do, specifying carbon-based life, per se, is irrelevant.
The anthropic principle has given rise to some confusion and controversy, partly because the phrase has been applied to several distinct ideas. All versions of the principle have been accused of discouraging the search for a deeper physical understanding of the universe. The anthropic principle is often criticized for lacking falsifiability and therefore critics of the anthropic principle may point out that the anthropic principle is a non-scientific concept, even though the weak anthropic principle, "conditions that are observed in the universe must allow the observer to exist",[5] is "easy" to support in mathematics and philosophy, i.e. it is a tautology or truism. However, building a substantive argument based on a tautological foundation is problematic. Stronger variants of the anthropic principle are not tautologies and thus make claims considered controversial by some and that are contingent upon empirical
Fine-tuned Universe
In 1961, Robert Dicke noted that the age of the universe, as seen by living observers, cannot be random.[8] Instead, biological factors constrain the universe to be more or less in a "golden age," neither too young nor too old.[9] If the universe were one tenth as old as its present age, there would not have been sufficient time to build up appreciable levels of metallicity (levels of elements besides hydrogen and helium) especially carbon, by nucleosynthesis. Small rocky planets did not yet exist. If the universe were 10 times older than it actually is, most stars would be too old to remain on the main sequence and would have turned into white dwarfs, aside from the dimmest red dwarfs, and stable planetary systems would have already come to an end. Thus Dicke explained away the rough coincidence between large dimensionless numbers constructed from the constants of physics and the age of the universe, a coincidence which had inspired Dirac's varying-G theory.
Dicke later reasoned that the density of matter in the universe must be almost exactly the critical density needed to prevent the Big Crunch (the "Dicke coincidences" argument). The most recent measurements may suggest that the observed density of baryonic matter, and some theoretical predictions of the amount of dark matter account for about 30% of this critical density, with the rest contributed by a cosmological constant. Steven Weinberg[10] gave an anthropic explanation for this fact: he noted that the cosmological constant has a remarkably low value, some 120 orders of magnitude smaller than the value particle physics predicts (this has been described as the "worst prediction in physics").[11] However, if the cosmological constant were more than about 10 times its observed value, the universe would suffer catastrophic inflation, which would preclude the formation of stars, and hence life.
The observed values of the dimensionless physical constants (such as the fine-structure constant) governing the four fundamental interactions are balanced as if fine-tuned to permit the formation of commonly found matter and subsequently the emergence of life. A slight increase in the strong nuclear force would bind the dineutron and the diproton, and nuclear fusion would have converted all hydrogen in the early universe to helium. Water and the long-lived stable stars essential for the emergence of life would not exist. More generally, small changes in the relative strengths of the four fundamental interactions can greatly affect the universe's age, structure, and capacity for life.
Origin
The phrase "anthropic principle" first appeared in Brandon Carter's contribution to a 1973 Kraków symposium honouring Copernicus's 500th birthday. Carter, a theoretical astrophysicist, articulated the Anthropic Principle in reaction to the Copernican Principle, which states that humans do not occupy a privileged position in the Universe. As Carter said: "Although our situation is not necessarily central, it is inevitably privileged to some extent."[12] Specifically, Carter disagreed with using the Copernican principle to justify the Perfect Cosmological Principle, which states that all large regions and times in the universe must be statistically identical. The latter principle underlay the steady-state theory, which had recently been falsified by the 1965 discovery of the cosmic microwave background radiation. This discovery was unequivocal evidence that the universe has changed radically over time (for example, via the Big Bang).
Carter defined two forms of the Anthropic Principle, a "weak" one which referred only to anthropic selection of privileged spacetime locations in the universe, and a more controversial "strong" form which addressed the values of the fundamental constants of physics.
Roger Penrose explained the weak form as follows:
"The argument can be used to explain why the conditions happen to be just right for the existence of (intelligent) life on the earth at the present time. For if they were not just right, then we should not have found ourselves to be here now, but somewhere else, at some other appropriate time. This principle was used very effectively by Brandon Carter and Robert Dicke to resolve an issue that had puzzled physicists for a good many years. The issue concerned various striking numerical relations that are observed to hold between the physical constants (the gravitational constant, the mass of the proton, the age of the universe, etc.). A puzzling aspect of this was that some of the relations hold only at the present epoch in the earth's history, so we appear, coincidentally, to be living at a very special time (give or take a few million years!). This was later explained, by Carter and Dicke, by the fact that this epoch coincided with the lifetime of what are called main-sequence stars, such as the sun. At any other epoch, so the argument ran, there would be no intelligent life around in order to measure the physical constants in question — so the coincidence had to hold, simply because there would be intelligent life around only at the particular time that the coincidence did hold!"
The Emperor's New Mind
One reason this is plausible is that there are many other places and times in which we can imagine finding ourselves. But when applying the strong principle, we only have one Universe, with one set of fundamental parameters, so what exactly is the point being made? Carter offers two possibilities: First, we can use our own existence to make "predictions" about the parameters. But second, "as a last resort", we can convert these predictions into explanations by assuming that there is more than one Universe, in fact a large and possibly infinite collection of universes, something that is now called a multiverse ("world ensemble" was Carter's term), in which the parameters (and perhaps the laws of physics) vary across universes. The strong principle then becomes an example of a selection effect, exactly analogous to the weak principle. Postulating a multiverse is certainly a radical step, but taking it could provide at least a partial answer to a question which had seemed to be out of the reach of normal science: "why do the fundamental laws of physics take the particular form we observe and not another?"
Since Carter's 1973 paper, the term "Anthropic Principle" has been extended to cover a number of ideas which differ in important ways from those he espoused. Particular confusion was caused in 1986 by the book The Anthropic Cosmological Principle by John D. Barrow and Frank Tipler,[13] published that year which distinguished between "weak" and "strong" anthropic principle in a way very different from Carter's, as discussed in the next section.
Carter was not the first to invoke some form of the anthropic principle. In fact, the evolutionary biologist Alfred Russel Wallace anticipated the anthropic principle as long ago as 1904:
"Such a vast and complex universe as that which we know exists around us, may have been absolutely required ... in order to produce a world that should be precisely adapted in every detail for the orderly development of life culminating in man."[14] In 1957, Robert Dicke wrote: "The age of the Universe 'now' is not random but conditioned by biological factors ... [changes in the values of the fundamental constants of physics] would preclude the existence of man to consider the problem."
Variants
Weak anthropic principle (WAP) (Carter): "we must be prepared to take account of the fact that our location in the universe is necessarily privileged to the extent of being compatible with our existence as observers." Note that for Carter, "location" refers to our location in time as well as space.
Strong anthropic principle (SAP) (Carter): "the Universe (and hence the fundamental parameters on which it depends) must be such as to admit the creation of observers within it at some stage. To paraphrase Descartes, cogito ergo mundus talis est."
The Latin tag ("I think, therefore the world is such [as it is]") makes it clear that "must" indicates a deduction from the fact of our existence; the statement is thus a truism.
In their 1986 book, The Anthropic Cosmological Principle, John Barrow and Frank Tipler depart from Carter and define the WAP and SAP as follows:
Weak anthropic principle (WAP) (Barrow and Tipler): "The observed values of all physical and cosmological quantities are not equally probable but they take on values restricted by the requirement that there exist sites where carbon-based life can evolve and by the requirements that the Universe be old enough for it to have already done so."
Unlike Carter they restrict the principle to carbon-based life, rather than just "observers." A more important difference is that they apply the WAP to the fundamental physical constants, such as the fine structure constant, the number of spacetime dimensions, and the cosmological constant —, topics that fall under Carter's SAP.
Strong anthropic principle (SAP) (Barrow and Tipler): "The Universe must have those properties which allow life to develop within it at some stage in its history."
This looks very similar to Carter's SAP, but unlike the case with Carter's SAP, the "must" is an imperative, as shown by the following three possible elaborations of the SAP, each proposed by Barrow and Tipler:
"There exists one possible Universe 'designed' with the goal of generating and sustaining 'observers.'"
This can be seen as simply the classic design argument restated in the garb of contemporary cosmology. It implies that the purpose of the universe is to give rise to intelligent life, with the laws of nature and their fundamental physical constants set to ensure that life as we know it will emerge and evolve.
"Observers are necessary to bring the Universe into being."
Barrow and Tipler believe that this is a valid conclusion from quantum mechanics, as John Archibald Wheeler has suggested, especially via his participatory universe and Participatory Anthropic Principle (PAP).
"An ensemble of other different universes is necessary for the existence of our Universe."
By contrast, Carter merely says that an ensemble of universes is necessary for the SAP to count as an explanation.
Modified anthropic principle (MAP) (Schmidhuber): The 'problem' of existence is only relevant to a species capable of formulating the question. Prior to homo sapiens' intellectual evolution to the point where the nature of the observed universe - and humans' place within same - spawned deep inquiry into its origins, the 'problem' simply did not exist.
The philosophers John Leslie[22] and Nick Bostrom reject the Barrow and Tipler SAP as a fundamental misreading of Carter. For Bostrom, Carter's anthropic principle just warns us to make allowance for anthropic bias, that is, the bias created by anthropic selection effects (which Bostrom calls "observation" selection effects) — the necessity for observers to exist in order to get a result. He writes:
"Many 'anthropic principles' are simply confused. Some, especially those drawing inspiration from Brandon Carter's seminal papers, are sound, but... they are too weak to do any real scientific work. In particular, I argue that existing methodology does not permit any observational consequences to be derived from contemporary cosmological theories, though these theories quite plainly can be and are being tested empirically by astronomers. What is needed to bridge this methodological gap is a more adequate formulation of how observation selection effects are to be taken into account."
—Anthropic Bias, Introduction.,
Strong self-sampling assumption (SSSA) (Bostrom): "Each observer-moment should reason as if it were randomly selected from the class of all observer-moments in its reference class."
Analysing an observer's experience into a sequence of "observer-moments" helps avoid certain paradoxes; but the main ambiguity is the selection of the appropriate "reference class": for Carter's WAP this might correspond to all real or potential observer-moments in our universe; for the SAP, to all in the multiverse. Bostrom's mathematical development shows that choosing either too broad or too narrow a reference class leads to counter-intuitive results, but he is not able to prescribe an ideal choice.
According to Jürgen Schmidhuber, the anthropic principle essentially just says that the conditional probability of finding yourself in a universe compatible with your existence is always 1. It does not allow for any additional nontrivial predictions such as "gravity won't change tomorrow." To gain more predictive power, additional assumptions on the prior distribution of alternative universes are necessary.[21][25]
Playwright and novelist Michael Frayn describes a form of the Strong Anthropic Principle in his 2006 book The Human Touch, which explores what he characterises as "the central oddity of the Universe":
"It's this simple paradox. The Universe is very old and very large. Humankind, by comparison, is only a tiny disturbance in one small corner of it - and a very recent one. Yet the universe is only very large and very old because we are here to say it is... And yet, of course, we all know perfectly well that it is what it is whether we are here or not."
anthropic reasoning
Carter chose to focus on a tautological aspect of his ideas, which has resulted in much confusion. In fact, anthropic reasoning interests scientists because of something that is only implicit in the above formal definitions, namely that we should give serious consideration to there being other universes with different values of the "fundamental parameters" — that is, the dimensionless physical constants and initial conditions for the Big Bang. Carter and others have argued that life as we know it would not be possible in most such universes. In other words, the universe we are in is fine tuned to permit life. Collins & Hawking (1973) characterized Carter's then-unpublished big idea as the postulate that "there is not one universe but a whole infinite ensemble of universes with all possible initial conditions". If this is granted, the anthropic principle provides a plausible explanation for the fine tuning of our universe: the "typical" universe is not fine-tuned, but given enough universes, a small fraction thereof will be capable of supporting intelligent life. Ours must be one of these, and so the observed fine tuning should be no cause for wonder.
But how seriously can we take the multiverse? And which specific multiverse should we assume? — this question must be answered before any quantitative anthropic predictions can be made. Although philosophers have discussed related concepts for centuries, in the early 1970s the only genuine physical theory yielding a multiverse of sorts was the many worlds interpretation of quantum mechanics. This would allow variation in initial conditions, but not in the truly fundamental constants. Since that time a number of mechanisms for producing a multiverse have been suggested: see the review by Max Tegmark. An important development in the 1980s was the combination of inflation theory with the hypothesis that some parameters are determined by symmetry breaking in the early universe, which allows parameters previously thought of as "fundamental constants" to vary over very large distances, thus eroding the distinction between Carter's weak and strong principles. At the beginning of the 21st century, the string landscape emerged as a mechanism for varying essentially all the constants, including the number of spatial dimensions.[29]
The anthropic idea that fundamental parameters are selected from a multitude of different possibilities (each actual in some universe or other) contrasts with the traditional hope of physicists for a theory of everything having no free parameters: as Einstein said, "What really interests me is whether God had any choice in the creation of the world." Quite recently, proponents of the leading candidate for a "theory of everything", string theory, proclaimed "the end of the anthropic principle" since there would be no free parameters to select.
Excerpt from “I Don’t Have Enough Faith to Be an Atheist”
by Norman L. Geisler and Frank Turek.
Scientists are now finding that the universe in which we live is like a diamond studded Rolex, except the universe is even more precisely designed than the watch. In fact, the universe is specifically tweaked to enable life on earth. A Planet with scores of improbable and inter-dependent life- supporting conditions that make it a tiny oasis in a vast and hostile universe. The extent of the universe’s fine-tuning makes the Anthropic Principle perhaps the most powerful argument for the existence of God.
These highly precise and interdependent environmental conditions (which are called “Anthropic Constants” make up what is known as the “Anthropic Principle.” “Anthropic” comes from the Greek word that means “human” or “man”. The “Anthropic Principal” is just a fancy title for the mounting evidence that has many scientists believing that the universe is extremely fine-tuned (designed) to support human life here on earth.
It’s not that there are just a few broadly defined constants that may have resulted by chance. No, there are more than 100 very narrowly defined constants that strongly point to an intelligent Designer.
Astrophysicist Hugh Ross has calculated the probability that these and other constants (122 in all) would exist today for any planet in the universe by chance (I.e., without Divine design). Assuming there are 1022planets in the universe (a very large number: 1 with 22 zeros following it), his answer is shocking; one chance in 10138, that’s one chance in one with 138 zeros after it. There are only about 1070 atoms in the entire universe.
In effect, there zero chance that any planet in the universe would have the life-supporting conditions we have, unless there is an intelligent Designer behind it all.
Here are fifteen of them.
Anthropic Constant 1: Oxygen Level
On earth, oxygen comprises 21 percent of the atmosphere. That precise figure is an Anthropic Constant that makes life on earth possible. If oxygen were 25% fires would erupt spontaneously, if it were 15%, human beings would suffocate.
Anthropic Constant 2: Atmospheric Transparency
If the atmosphere were less transparent, not enough solar radiation would reach the earth’s surface. If it were more transparent we would be bombarded with far roo much solar radiation down here. (In addition to atmospheric transparency, the atmospheric composition of precise levels of nitrogen, oxygen, carbon dioxide and ozone are in themselves Anthropic constants).
Anthropic Constant 3: Moon-Earth Gravitational Interaction
If the interaction were greater than it currently is, tidal effects on the oceans, atmosphere, and rotational period would be too severe. If it were less, orbital changes would cause climatic instabilities. In either event, life on earth would be impossible.
Anthropic Constant 4: Carbon Dioxide level
If the CO2 level were higher than it is now, a runaway greenhouse effect would develop (we’d all burn up). If the level were lower than it is now, plants would not be able to maintain efficient photosynthesis (we’d all suffocate).
Anthropic Constant 5: Gravity
If the gravitational force were altered by 0.00000000000000000000000000000000000001 percent, our sun would not exist, and, therefore neither would we. Talk about precision.
Anthropic Constant 6: Centrifugal Force
If the centrifugal force of planetary movements did not precisely balance the gravitational forces, nothing could be held in orbit around the sun.
Anthropic Constant 7: Rate Of Expansion
If the universe had expanded at a rate one millionth more slowly than it did, expansion would have stopped and the universe would have collapsed on itself before any stars had formed. If it had expanded faster, then no galaxies would have formed.
Anthropic Constant 8: Speed Of Light
Any of the laws of physics can be described as a function of the velocity of light (now defined to be 299,792,458 meters per second). Even a slight variation in the speed of light would alter the other constants and preclude the possibility of life on earth.
Anthropic Constant 9: Water Vapor Levels.
If water vapor levels in the atmosphere were greater than they are now, a runaway greenhouse effect would cause temperatures to rise too high for human life. If they were less, an insufficient greenhouse effect would make the earth to cold to support human life.
Anthropic Constant 10: Jupiter.
If Jupiter were not in it’s current orbit, the earth would be bombarded with space material. Jupiter’s gravitational field acts as a cosmic vacuum cleaner, attracting asteroids and comets that might otherwise strike earth.
Anthropic Constant 11: The Earth’s Crust.
If the thickness of the earth’s crust were greater, too much oxygen would be transferred to the crust to support life. If it were thinner, volcanic and tectonic activity would make life impossible.
Anthropic Constant 12: The Earth’s Rotation.
If the rotation of the earth took longer than 24 hours, temperature differences would be too great between night and day. If the rotation period were shorter, atmospheric wind velocities would be to great.
Anthropic Constant 13: Axis Tilt.
The 23-degree axis tilt of the earth is just right. If the tilt were altered slightly, surface temperatures would be too extreme on earth.
Anthropic Constant 14: Atmospheric Discharge.
If the atmospheric discharge (lightning) rate were greater, there would be too much fire destruction; if it were less there would be little nitrogen fixings in the soil.
Anthropic Constant 15: Seismic Activity.
If there were more seismic activity, much more life would be lost; if there were less, nutrients on the ocean floors and in river runoff would not be cycled back to the continents through tectonic uplift. (yes, even earthquakes are necessary to sustain life as we know it).
The strong anthropic principle as explained by Barrow and Tipler (see variants) states that this is all the case because conscious life, in some sense, needed to exist. On the other hand, in a sufficiently large universe, some worlds might evolve conscious life regardless of adverse conditions. Douglas Adams used the metaphor of a living puddle examining its own shape, since, to those living creatures, the universe may appear to fit them perfectly (while in fact, they simply fit the universe perfectly). Critics argue in favor of a weak anthropic principle similar to the one defined by Brandon Carter (see variants), which states that the universe's ostensible fine tuning is the result of selection bias, e.g. in the long term, only survivors can report their location in time and space.
The principle was formulated as a response to a series of observations that the laws of nature and parameters of the Universe take on values that are consistent with conditions for life as we know it rather than a set of values that would not be consistent with life as observed on Earth. The anthropic principle states that this phenomenon is a necessity because living observers wouldn't be able to exist, and hence, observe the Universe, were these laws and constants not constituted in this way.
The term anthropic in "anthropic principle" has been argued to be a misnomer. While singling out our kind of carbon-based life, none of the finely tuned phenomena require human life or some kind of carbon chauvinism. Any form of intelligent life would do, specifying carbon-based life, per se, is irrelevant.
The anthropic principle has given rise to some confusion and controversy, partly because the phrase has been applied to several distinct ideas. All versions of the principle have been accused of discouraging the search for a deeper physical understanding of the universe. The anthropic principle is often criticized for lacking falsifiability and therefore critics of the anthropic principle may point out that the anthropic principle is a non-scientific concept, even though the weak anthropic principle, "conditions that are observed in the universe must allow the observer to exist",[5] is "easy" to support in mathematics and philosophy, i.e. it is a tautology or truism. However, building a substantive argument based on a tautological foundation is problematic. Stronger variants of the anthropic principle are not tautologies and thus make claims considered controversial by some and that are contingent upon empirical
Fine-tuned Universe
In 1961, Robert Dicke noted that the age of the universe, as seen by living observers, cannot be random.[8] Instead, biological factors constrain the universe to be more or less in a "golden age," neither too young nor too old.[9] If the universe were one tenth as old as its present age, there would not have been sufficient time to build up appreciable levels of metallicity (levels of elements besides hydrogen and helium) especially carbon, by nucleosynthesis. Small rocky planets did not yet exist. If the universe were 10 times older than it actually is, most stars would be too old to remain on the main sequence and would have turned into white dwarfs, aside from the dimmest red dwarfs, and stable planetary systems would have already come to an end. Thus Dicke explained away the rough coincidence between large dimensionless numbers constructed from the constants of physics and the age of the universe, a coincidence which had inspired Dirac's varying-G theory.
Dicke later reasoned that the density of matter in the universe must be almost exactly the critical density needed to prevent the Big Crunch (the "Dicke coincidences" argument). The most recent measurements may suggest that the observed density of baryonic matter, and some theoretical predictions of the amount of dark matter account for about 30% of this critical density, with the rest contributed by a cosmological constant. Steven Weinberg[10] gave an anthropic explanation for this fact: he noted that the cosmological constant has a remarkably low value, some 120 orders of magnitude smaller than the value particle physics predicts (this has been described as the "worst prediction in physics").[11] However, if the cosmological constant were more than about 10 times its observed value, the universe would suffer catastrophic inflation, which would preclude the formation of stars, and hence life.
The observed values of the dimensionless physical constants (such as the fine-structure constant) governing the four fundamental interactions are balanced as if fine-tuned to permit the formation of commonly found matter and subsequently the emergence of life. A slight increase in the strong nuclear force would bind the dineutron and the diproton, and nuclear fusion would have converted all hydrogen in the early universe to helium. Water and the long-lived stable stars essential for the emergence of life would not exist. More generally, small changes in the relative strengths of the four fundamental interactions can greatly affect the universe's age, structure, and capacity for life.
Origin
The phrase "anthropic principle" first appeared in Brandon Carter's contribution to a 1973 Kraków symposium honouring Copernicus's 500th birthday. Carter, a theoretical astrophysicist, articulated the Anthropic Principle in reaction to the Copernican Principle, which states that humans do not occupy a privileged position in the Universe. As Carter said: "Although our situation is not necessarily central, it is inevitably privileged to some extent."[12] Specifically, Carter disagreed with using the Copernican principle to justify the Perfect Cosmological Principle, which states that all large regions and times in the universe must be statistically identical. The latter principle underlay the steady-state theory, which had recently been falsified by the 1965 discovery of the cosmic microwave background radiation. This discovery was unequivocal evidence that the universe has changed radically over time (for example, via the Big Bang).
Carter defined two forms of the Anthropic Principle, a "weak" one which referred only to anthropic selection of privileged spacetime locations in the universe, and a more controversial "strong" form which addressed the values of the fundamental constants of physics.
Roger Penrose explained the weak form as follows:
"The argument can be used to explain why the conditions happen to be just right for the existence of (intelligent) life on the earth at the present time. For if they were not just right, then we should not have found ourselves to be here now, but somewhere else, at some other appropriate time. This principle was used very effectively by Brandon Carter and Robert Dicke to resolve an issue that had puzzled physicists for a good many years. The issue concerned various striking numerical relations that are observed to hold between the physical constants (the gravitational constant, the mass of the proton, the age of the universe, etc.). A puzzling aspect of this was that some of the relations hold only at the present epoch in the earth's history, so we appear, coincidentally, to be living at a very special time (give or take a few million years!). This was later explained, by Carter and Dicke, by the fact that this epoch coincided with the lifetime of what are called main-sequence stars, such as the sun. At any other epoch, so the argument ran, there would be no intelligent life around in order to measure the physical constants in question — so the coincidence had to hold, simply because there would be intelligent life around only at the particular time that the coincidence did hold!"
The Emperor's New Mind
One reason this is plausible is that there are many other places and times in which we can imagine finding ourselves. But when applying the strong principle, we only have one Universe, with one set of fundamental parameters, so what exactly is the point being made? Carter offers two possibilities: First, we can use our own existence to make "predictions" about the parameters. But second, "as a last resort", we can convert these predictions into explanations by assuming that there is more than one Universe, in fact a large and possibly infinite collection of universes, something that is now called a multiverse ("world ensemble" was Carter's term), in which the parameters (and perhaps the laws of physics) vary across universes. The strong principle then becomes an example of a selection effect, exactly analogous to the weak principle. Postulating a multiverse is certainly a radical step, but taking it could provide at least a partial answer to a question which had seemed to be out of the reach of normal science: "why do the fundamental laws of physics take the particular form we observe and not another?"
Since Carter's 1973 paper, the term "Anthropic Principle" has been extended to cover a number of ideas which differ in important ways from those he espoused. Particular confusion was caused in 1986 by the book The Anthropic Cosmological Principle by John D. Barrow and Frank Tipler,[13] published that year which distinguished between "weak" and "strong" anthropic principle in a way very different from Carter's, as discussed in the next section.
Carter was not the first to invoke some form of the anthropic principle. In fact, the evolutionary biologist Alfred Russel Wallace anticipated the anthropic principle as long ago as 1904:
"Such a vast and complex universe as that which we know exists around us, may have been absolutely required ... in order to produce a world that should be precisely adapted in every detail for the orderly development of life culminating in man."[14] In 1957, Robert Dicke wrote: "The age of the Universe 'now' is not random but conditioned by biological factors ... [changes in the values of the fundamental constants of physics] would preclude the existence of man to consider the problem."
Variants
Weak anthropic principle (WAP) (Carter): "we must be prepared to take account of the fact that our location in the universe is necessarily privileged to the extent of being compatible with our existence as observers." Note that for Carter, "location" refers to our location in time as well as space.
Strong anthropic principle (SAP) (Carter): "the Universe (and hence the fundamental parameters on which it depends) must be such as to admit the creation of observers within it at some stage. To paraphrase Descartes, cogito ergo mundus talis est."
The Latin tag ("I think, therefore the world is such [as it is]") makes it clear that "must" indicates a deduction from the fact of our existence; the statement is thus a truism.
In their 1986 book, The Anthropic Cosmological Principle, John Barrow and Frank Tipler depart from Carter and define the WAP and SAP as follows:
Weak anthropic principle (WAP) (Barrow and Tipler): "The observed values of all physical and cosmological quantities are not equally probable but they take on values restricted by the requirement that there exist sites where carbon-based life can evolve and by the requirements that the Universe be old enough for it to have already done so."
Unlike Carter they restrict the principle to carbon-based life, rather than just "observers." A more important difference is that they apply the WAP to the fundamental physical constants, such as the fine structure constant, the number of spacetime dimensions, and the cosmological constant —, topics that fall under Carter's SAP.
Strong anthropic principle (SAP) (Barrow and Tipler): "The Universe must have those properties which allow life to develop within it at some stage in its history."
This looks very similar to Carter's SAP, but unlike the case with Carter's SAP, the "must" is an imperative, as shown by the following three possible elaborations of the SAP, each proposed by Barrow and Tipler:
"There exists one possible Universe 'designed' with the goal of generating and sustaining 'observers.'"
This can be seen as simply the classic design argument restated in the garb of contemporary cosmology. It implies that the purpose of the universe is to give rise to intelligent life, with the laws of nature and their fundamental physical constants set to ensure that life as we know it will emerge and evolve.
"Observers are necessary to bring the Universe into being."
Barrow and Tipler believe that this is a valid conclusion from quantum mechanics, as John Archibald Wheeler has suggested, especially via his participatory universe and Participatory Anthropic Principle (PAP).
"An ensemble of other different universes is necessary for the existence of our Universe."
By contrast, Carter merely says that an ensemble of universes is necessary for the SAP to count as an explanation.
Modified anthropic principle (MAP) (Schmidhuber): The 'problem' of existence is only relevant to a species capable of formulating the question. Prior to homo sapiens' intellectual evolution to the point where the nature of the observed universe - and humans' place within same - spawned deep inquiry into its origins, the 'problem' simply did not exist.
The philosophers John Leslie[22] and Nick Bostrom reject the Barrow and Tipler SAP as a fundamental misreading of Carter. For Bostrom, Carter's anthropic principle just warns us to make allowance for anthropic bias, that is, the bias created by anthropic selection effects (which Bostrom calls "observation" selection effects) — the necessity for observers to exist in order to get a result. He writes:
"Many 'anthropic principles' are simply confused. Some, especially those drawing inspiration from Brandon Carter's seminal papers, are sound, but... they are too weak to do any real scientific work. In particular, I argue that existing methodology does not permit any observational consequences to be derived from contemporary cosmological theories, though these theories quite plainly can be and are being tested empirically by astronomers. What is needed to bridge this methodological gap is a more adequate formulation of how observation selection effects are to be taken into account."
—Anthropic Bias, Introduction.,
Strong self-sampling assumption (SSSA) (Bostrom): "Each observer-moment should reason as if it were randomly selected from the class of all observer-moments in its reference class."
Analysing an observer's experience into a sequence of "observer-moments" helps avoid certain paradoxes; but the main ambiguity is the selection of the appropriate "reference class": for Carter's WAP this might correspond to all real or potential observer-moments in our universe; for the SAP, to all in the multiverse. Bostrom's mathematical development shows that choosing either too broad or too narrow a reference class leads to counter-intuitive results, but he is not able to prescribe an ideal choice.
According to Jürgen Schmidhuber, the anthropic principle essentially just says that the conditional probability of finding yourself in a universe compatible with your existence is always 1. It does not allow for any additional nontrivial predictions such as "gravity won't change tomorrow." To gain more predictive power, additional assumptions on the prior distribution of alternative universes are necessary.[21][25]
Playwright and novelist Michael Frayn describes a form of the Strong Anthropic Principle in his 2006 book The Human Touch, which explores what he characterises as "the central oddity of the Universe":
"It's this simple paradox. The Universe is very old and very large. Humankind, by comparison, is only a tiny disturbance in one small corner of it - and a very recent one. Yet the universe is only very large and very old because we are here to say it is... And yet, of course, we all know perfectly well that it is what it is whether we are here or not."
anthropic reasoning
Carter chose to focus on a tautological aspect of his ideas, which has resulted in much confusion. In fact, anthropic reasoning interests scientists because of something that is only implicit in the above formal definitions, namely that we should give serious consideration to there being other universes with different values of the "fundamental parameters" — that is, the dimensionless physical constants and initial conditions for the Big Bang. Carter and others have argued that life as we know it would not be possible in most such universes. In other words, the universe we are in is fine tuned to permit life. Collins & Hawking (1973) characterized Carter's then-unpublished big idea as the postulate that "there is not one universe but a whole infinite ensemble of universes with all possible initial conditions". If this is granted, the anthropic principle provides a plausible explanation for the fine tuning of our universe: the "typical" universe is not fine-tuned, but given enough universes, a small fraction thereof will be capable of supporting intelligent life. Ours must be one of these, and so the observed fine tuning should be no cause for wonder.
But how seriously can we take the multiverse? And which specific multiverse should we assume? — this question must be answered before any quantitative anthropic predictions can be made. Although philosophers have discussed related concepts for centuries, in the early 1970s the only genuine physical theory yielding a multiverse of sorts was the many worlds interpretation of quantum mechanics. This would allow variation in initial conditions, but not in the truly fundamental constants. Since that time a number of mechanisms for producing a multiverse have been suggested: see the review by Max Tegmark. An important development in the 1980s was the combination of inflation theory with the hypothesis that some parameters are determined by symmetry breaking in the early universe, which allows parameters previously thought of as "fundamental constants" to vary over very large distances, thus eroding the distinction between Carter's weak and strong principles. At the beginning of the 21st century, the string landscape emerged as a mechanism for varying essentially all the constants, including the number of spatial dimensions.[29]
The anthropic idea that fundamental parameters are selected from a multitude of different possibilities (each actual in some universe or other) contrasts with the traditional hope of physicists for a theory of everything having no free parameters: as Einstein said, "What really interests me is whether God had any choice in the creation of the world." Quite recently, proponents of the leading candidate for a "theory of everything", string theory, proclaimed "the end of the anthropic principle" since there would be no free parameters to select.
Excerpt from “I Don’t Have Enough Faith to Be an Atheist”
by Norman L. Geisler and Frank Turek.
Scientists are now finding that the universe in which we live is like a diamond studded Rolex, except the universe is even more precisely designed than the watch. In fact, the universe is specifically tweaked to enable life on earth. A Planet with scores of improbable and inter-dependent life- supporting conditions that make it a tiny oasis in a vast and hostile universe. The extent of the universe’s fine-tuning makes the Anthropic Principle perhaps the most powerful argument for the existence of God.
These highly precise and interdependent environmental conditions (which are called “Anthropic Constants” make up what is known as the “Anthropic Principle.” “Anthropic” comes from the Greek word that means “human” or “man”. The “Anthropic Principal” is just a fancy title for the mounting evidence that has many scientists believing that the universe is extremely fine-tuned (designed) to support human life here on earth.
It’s not that there are just a few broadly defined constants that may have resulted by chance. No, there are more than 100 very narrowly defined constants that strongly point to an intelligent Designer.
Astrophysicist Hugh Ross has calculated the probability that these and other constants (122 in all) would exist today for any planet in the universe by chance (I.e., without Divine design). Assuming there are 1022planets in the universe (a very large number: 1 with 22 zeros following it), his answer is shocking; one chance in 10138, that’s one chance in one with 138 zeros after it. There are only about 1070 atoms in the entire universe.
In effect, there zero chance that any planet in the universe would have the life-supporting conditions we have, unless there is an intelligent Designer behind it all.
Here are fifteen of them.
Anthropic Constant 1: Oxygen Level
On earth, oxygen comprises 21 percent of the atmosphere. That precise figure is an Anthropic Constant that makes life on earth possible. If oxygen were 25% fires would erupt spontaneously, if it were 15%, human beings would suffocate.
Anthropic Constant 2: Atmospheric Transparency
If the atmosphere were less transparent, not enough solar radiation would reach the earth’s surface. If it were more transparent we would be bombarded with far roo much solar radiation down here. (In addition to atmospheric transparency, the atmospheric composition of precise levels of nitrogen, oxygen, carbon dioxide and ozone are in themselves Anthropic constants).
Anthropic Constant 3: Moon-Earth Gravitational Interaction
If the interaction were greater than it currently is, tidal effects on the oceans, atmosphere, and rotational period would be too severe. If it were less, orbital changes would cause climatic instabilities. In either event, life on earth would be impossible.
Anthropic Constant 4: Carbon Dioxide level
If the CO2 level were higher than it is now, a runaway greenhouse effect would develop (we’d all burn up). If the level were lower than it is now, plants would not be able to maintain efficient photosynthesis (we’d all suffocate).
Anthropic Constant 5: Gravity
If the gravitational force were altered by 0.00000000000000000000000000000000000001 percent, our sun would not exist, and, therefore neither would we. Talk about precision.
Anthropic Constant 6: Centrifugal Force
If the centrifugal force of planetary movements did not precisely balance the gravitational forces, nothing could be held in orbit around the sun.
Anthropic Constant 7: Rate Of Expansion
If the universe had expanded at a rate one millionth more slowly than it did, expansion would have stopped and the universe would have collapsed on itself before any stars had formed. If it had expanded faster, then no galaxies would have formed.
Anthropic Constant 8: Speed Of Light
Any of the laws of physics can be described as a function of the velocity of light (now defined to be 299,792,458 meters per second). Even a slight variation in the speed of light would alter the other constants and preclude the possibility of life on earth.
Anthropic Constant 9: Water Vapor Levels.
If water vapor levels in the atmosphere were greater than they are now, a runaway greenhouse effect would cause temperatures to rise too high for human life. If they were less, an insufficient greenhouse effect would make the earth to cold to support human life.
Anthropic Constant 10: Jupiter.
If Jupiter were not in it’s current orbit, the earth would be bombarded with space material. Jupiter’s gravitational field acts as a cosmic vacuum cleaner, attracting asteroids and comets that might otherwise strike earth.
Anthropic Constant 11: The Earth’s Crust.
If the thickness of the earth’s crust were greater, too much oxygen would be transferred to the crust to support life. If it were thinner, volcanic and tectonic activity would make life impossible.
Anthropic Constant 12: The Earth’s Rotation.
If the rotation of the earth took longer than 24 hours, temperature differences would be too great between night and day. If the rotation period were shorter, atmospheric wind velocities would be to great.
Anthropic Constant 13: Axis Tilt.
The 23-degree axis tilt of the earth is just right. If the tilt were altered slightly, surface temperatures would be too extreme on earth.
Anthropic Constant 14: Atmospheric Discharge.
If the atmospheric discharge (lightning) rate were greater, there would be too much fire destruction; if it were less there would be little nitrogen fixings in the soil.
Anthropic Constant 15: Seismic Activity.
If there were more seismic activity, much more life would be lost; if there were less, nutrients on the ocean floors and in river runoff would not be cycled back to the continents through tectonic uplift. (yes, even earthquakes are necessary to sustain life as we know it).