Glossary
The Anthropic Cosmological Principle(s)

The Anthropic Cosmological Principle is an extension of the The Copernican Cosmological Principle and is that not only
on a large scale, the universe is both homogeneous and isotropic (in 3-D space)

but also that
by our very being here, we are viewing "our universe" at a "priveledged" location in spacetime


Rationale/Implications

The rationale behind the first part is as for the Copernican Cosmological Principle. The impliction is that
the same laws of physics hold throughout the universe.

The rationale behind the second part is as an explanation as to why the laws of physics (and the universe itself) are the way they are (at least as seen by us). It is based on some current ideas that the developement of intelligent life on our planet required a series of (apparent) "coincidences" (e.g. see Al Schroeder's links). Thus that the circumstances that permit the developement of intelligent life throughout the universe (or other universes) are rare.

The Anthropic Cosmological Principle comes in two "strengths"
Weak The Weak Anthropic Principle states that the conditions necessary for the development of sentient beings (capable of asking the question why is the universe the way it is ?) will only exist in a universe where the laws of physics are the way they are as seen by us.
i.e. sentient beings can only evolve and exist in a universe that "happens" to have a density close to that observed (by us), that "happens" to be about as old as ours, that the charge of an electron "happens" to have the value observed (by us).. etc.
Strong The Strong Anthropic Principle takes this one step further to state that there could be many different universes (or regions in a single universe) where the laws of physics are different.
There are yet more flavours of the Strong Anthropic Principle dealing with whether of not sentient beings might be able to have evolved in these other "universes". Such metaphysical questions are beyond the scope of the current courses.

As for the the Copernican Cosmological Principle , the Anthropic Cosmological Principle is essentially a metaphysical statement and, it is still unclear (perhaps never knowable) whether it is true


Objections/Arguements Against

Currently few people object to the Weak Anthropic Principle. It offers an explanation (to us only-recently sentient beings) as to why out of the infinite universes that could exist (could have existed/will exist ?!?), we "turned up" in the one we did.

There are, however, a number of objections that can be raised concerning the implications/meaning of the Strong Anthropic Principle (e.g. see A Brief History of Time, p130)

  • Even if there are different universes (or regions in a single universe) where the laws of physics are different, how can any information travel between these entities ? Thus what happens in one universe can have no observational consequences on another, and hence what do we mean when we say other universes "exist" ?
  • Many thinkers struggled for thousands of years trying to determine whether Earth & humans occupied a special place in the universe. From the 1400s onwards the tide (gradually) turned against such a concept. According to Stephen Hawking, p130),
    "the strong anthropic principle would claim that this whole vast construction exists simply for our sake"
    [although, personally I do not agree that the Strong Anthropic Principle actually "claims" this..]

An Analogy

A (small) sentient being living in the center of a "perfect" loaf of bread.
  • There may be obvious structure on small scales (air bubbles etc), but on the large scale the loaf can be considered uniform and isotropic
  • The laws of physics (e.g. which caused the dough to rise) are the same throughout the loaf.
  • The loaf might still be rising - but (in this perfect loaf) this happens uniformly & following the same laws throughout the loaf
  • However, as an extension to the Copernican Cosmological Principle, the sentient being reasons that out of all the possible loaves (ingredients, proportions, open temperatures, baking times etc), they exist in the loaf they do since the conditions were just right, to bake such a loaf. Had they not been (eg. the no yeast added, baking time not long enough etc), then it would not have been possible for them to reach the level they have (if exist at all). This is the Weak flavour of the principle.
    • In the Strong flavour of the principle, the sentient beings go on to allow for the possibility of the existence of other loaves having been in the open (or other ovens), with different mixtures of ingredients etc.
      Whether the conditions may have been right for other sentient beings to evolve in (a very small number of) these other loaves (most likely in a form v.different to themselves) is a matter of debate. However (most of) the sentient beings see no possiblity of loaf-to-loaf travel in any case...

Additional Notes

See also the

For more information see:

  • "The Anthropic Cosmological Principle"
    Author(s): John D. Barrow & Frank J. Tipler
    Publisher: Oxford Univ. Press
    ISBN 0-19-282147-4 (paperback)