The Anthropic Principle

From SkepticWiki

1 Definition
2 Anthropic Coincidences
3 The Anthropic Principle due to Selection Bias
4 The Anthropic Principle as a Tautology
5 Retrospective Determinism
6 Strong and Final Principles
7 The Anthropic Principle and Intelligent Design
8 Quotes
9 References and Further Reading

[edit] Definition

The Anthropic Principle is a principle invoked to explain the seemingly arbitrary values of fundamental natural constants, and to explain the occasional coincidences that led to the origin of Earthly life. It may be briefly stated as:

The properties of the universe are consistent with the existence of intelligent observers.

While all current evidence seems to support this conclusion, the reasons and mechanisms behind this principle are subject to much debate, both scientific and philosophical. Some find the principle simply tautological and meaningless, while others believe that it may be the artifact of a universal selection bias. Paradoxically, it is used both to argue for a universe intelligently designed for human habitation, and to refute such arguments.

[edit] Anthropic Coincidences

Many natural laws discovered by science contain mysterious “fundamental constants”: the universal gravitational constant, the number of spatial dimensions, etc.. The reason why these constants take on certain values is unknown. Thus, it is natural to wonder what the properties of the universe would be if these constants were different.

In many cases, other values for these constants would make the universe a much less interesting place. In particular, it would be unlikely that life would arise in most of these alternative scenarios. It seems then that these constants have been “fine tuned” to allow for intelligent life. To cite two examples:

The rate of expansion of the universe allows several generations of stars to form, which creates the chemistry of life. A faster expansion would have left the universe a thin soup of Hydrogen and Helium, and a slower expansion would have caused the universe to collapse before life could form.
The energy levels of the Carbon nucleus allow the synthesis of Carbon from lighter elements. Different values would have made this impossible, which would have prevented life from occurring.

[edit] The Anthropic Principle due to Selection Bias

One might make the remarkable observation that the Earth’s temperature seems amazingly convenient for life processes. Naively, this observation could be generalized as:

The atmospheric temperature must be compatible with the existence of intelligent observers.

Of course, this appears to be true only when the observation is limited to a single atmosphere. This “law” is based on a single fact. But we now know that there are dozens of totally inhospitable worlds in our own solar system, and suspect that there are billions more throughout the galaxy.

How then can we feel comfortable making statements such as “The energy levels of the carbon nucleus must be compatible with the existence of intelligent observers.” having only observed one universe? It may be possible that the value of this fundamental constant could have been different.

The Anthropic Principle implies that there is an inescapable extreme form of selection bias in all observation, in which our universal special case is assumed to be representative of a larger set. Thus the apparent tendency of the universe to support life is merely an illusion created by our biased view.

[edit] The Anthropic Principle as a Tautology

The following is a valid formal argument:

Given A,

And given B implies ~A,

Therefore ~B.

For example, if predicted energy levels for the carbon nucleus would make lime JELL-O® impossible, then we can conclude, based on the existence of lime JELL-O® that the prediction is incorrect. More generally:

The properties of the universe are consistent with the existence of lime JELL-O®.

Indeed, the universe seems remarkably tailor-made for such phenomena as gelatin dessert. However, the argument says nothing about whether the purpose of the universe is to facilitate this phenomenon, and it is certainly no proof that the need for JELL-O® directly caused the laws of nature to be what they are. This suggests that the Anthropic Principle is a tautology; it is essentially true, but it means nothing and explains nothing.

[edit] Retrospective Determinism

The Anthropic Principle is easily confused with Retrospective Determinism. We are familiar with the reasonable probabilistic argument:

A: What’s the chance of rain today?

B: Well, it’s raining right now, so 100%.

Retrospective Determinism, considered a logical fallacy, makes an argument similar in form:

A: What are the chances that the conditions on Earth would be so favorable to life?

B: Well, it happened, so 100%.

This is a fallacy because the question implicitly considers counterfactual possibilities, which the answer then denies. The implied question is, “Without knowing that it happened, what would the chances be?” or perhaps, “What are the chances it would happen again in similar circumstances?”.

The Anthropic Principle is not Retrospective Determinism, although the two may be easily confused. The Anthropic Principle makes the highly qualified statement:

"Given that we are here, the chances that conditions on Earth are favorable to life are 100%, although without assuming that premise, the probability would be much lower."

[edit] Strong and Final Principles

Since scientists and philosophers commonly assume the Anthropic Principle, it is easy to imagine that the laws of nature we find are somehow “constrained” by it. A phrasing known as the Strong Anthropic Principle, is much more suggestive of a causal relationship:

The eventual existence of intelligent observers constrains the properties of the universe.

When coupled with certain interpretations of Quantum Mechanics that suggest “quantum immortality”, we find the Final Anthropic Principle:

The properties of the universe must permit the rise of intelligent observers, which must then continue forever.

[edit] The Anthropic Principle and Intelligent Design

Many arguments for Intelligent Design focus upon the improbability of various supposed evolutionary steps, and admittedly, not all of these coincidences have yet been explained by scientific theory.

The apparently fine-tuned nature of the universe is a widely used argument for the existence of an intelligent Designer, Who at the very least made some form of life possible even if He did not actually intend the human race specifically. This is essentially the Strong Anthropic Principle, but with deliberateness assumed in the “constraining” of the properties of the universe.

Strangely, the Anthropic Principle is also use to refute the preceding argument, by providing an explanation as to how the fundamental constants of nature came to be what they are by chance. The values of these constants may have occurred at random, or perhaps, they take on other values in other possible universes, but by necessity, we can never be aware of the uncounted universal duds.

If the Anthropic Principle is accepted, then it seems that bizarre coincidences necessary for the development of intelligence need not be explained by science. This is troubling both to scientists who feel that the physical world is deterministic, or at least statistically probable; and to proponents of Intelligent Design, who feel that science has merely applied the free pass of Retrospective Determinism on these questions.