The idea behind the anthropic principle is that the basic constants that govern the universe were set so that life would be possible. If the values of these constants were just a fraction out, then it would not be possible for life to exist.

An easy way to demonstrate this is to pretend you are responsible for creating a new universe which will be able to sustain life. The first thing you have to do is to decide on the strength of the explosion of the Big Bang that begins the universe.

You might think that you could have a slightly less powerful Big Bang than we did. If you did this, then the contents of the universe wouldn't be thrown out as fast or far enough, and before long they would slow down and stop expanding, and then start contracting into a Big Crunch, ending the universe. The time in between Big Bang and Big Crunch would be too short for life to exist.

So you start off with a more powerful Big Bang than we had. The contents of the universe fly out with incredible speed, and never slow down enough to start contracting again. In fact, they keep on expanding forever, so much that the matter in the universe doesn't have a chance to accrete and clump together to form stars.

This means that you're stuck with a Big Bang essentially identical to the one we had. Since you don't get to play around with the Big Bang, you decide to move onto gravity. Perhaps you think that life would be much more comfortable with less gravity so it wouldn't be as tiring to walk or run, so you tone down the gravity a little.

Unfortunately, this means that when the hydrogen left over from the Big Bang is lying around, when it began to clump up together to form stars, the gravity isn't strong enough to pull in a large amount of material. As a result, the only stars that form are red giants, which are very small and comparatively cold. The atoms in the core of the star aren't under sufficient pressure to undergo fusion and produce the heavier atoms that are required for life. The stars never explode (undergo supernovae) and even if there were any heavy atoms in their cores, they would never get outside the star.

Perhaps you could change the number of dimensions of the universe; make it a more fun place to live in, if you like

Shrugging your shoulders, you put the gravity above normal - it might mean that it's harder to get out of bed in the mornings, but it's for their own good. Yet again, though, greater gravity is not conducive to life. True, stars would form quicker since the strong gravity causes the hydrogen atoms to clump together very fast. But this strong gravity would also result in the stars literally burning out far too quickly as they collapse in under their own weight. The stars wouldn't last long enough to warm the planets that form around them.

So far, you can't alter the power of the Big Bang, or the gravity in the universe. What else is there to alter, then?

Well, there's the strong nuclear force, the force that binds protons and neutrons together in the nuclei of atoms - you could change the value of that. But you'd better not change it too much - if you increase it by just 13% of its value in our universe, you'd get atoms that were just made of two protons and no neutrons forming. These would quickly decay into atoms made of just two neutrons, leaving no hydrogen, no water and no hydrocarbons in the universe. Which means no life.

Perhaps you could change the number of dimensions of the universe; make it a more fun place to live in, if you like. Why do we only perceive three dimensions in our universe? (string theory postulates that there could be nine or more dimensions in our universe, yet all but three are curled up so tightly you can't notice them).

One consequence of having a three-dimensional space is the inverse square law of forces. This means that only in a three-dimensional space can you have stable planetary orbits: any more or fewer dimensions introduces instability. It can also be shown that stable atoms and chemistry also require three dimensions of space, and the distortion-free propagation of any wave-based signal (e.g. light) also requires exactly three dimensions of space.

What a stroke of luck that our universe has all the constants required for life...

Our universe has exactly the required number of dimensions for stabilty, chemistry and life. If you change the number, it's very possible that you could prevent life from forming.

There are more constants that you could change, but if you look at them closely enough, you'd realise that if you did change them any more than a little, you'd seriously affect the probability of life forming in your 'baby' universe. From all of this, you'd begin to think that it's a remarkable stroke of luck that we live in a universe whose constants are perfect for the formation of life. And you'd not be the first to think of that, either.

The theory that conditions in the universe are tuned exactly for life has been called the Anthropic Principle. The idea was proposed mainly by the physicists Barrow and Tipler, and they split up the anthropic principle into three categories, any one of which could be true, if not necessarily very likely.

	The Anthropic Principle (AP)
	Weak AP			Strong AP			Final AP
	The fact that we can observe physical and cosmological quantities and see that they are apparently 'tuned' for our existence is simply due to the fact that if they weren't, we wouldn't be alive to notice this fact. There should be no deeper meaning read into this fact.			The Universe must have cosmological constanst which allow life to develop within it at some stage in its history. Because: There exists one possible Universe 'designed' with the goal of generating and sustaining life that can observe the universe or... Observers are necessary to bring the Universe into being (Wheeler's Participatory Anthropic Principle - this is related to quantum mechanics) or... There are an infinite number of universes that exist, each with different cosmological constants. It stands to reason, therefore, that at least one will have the right constants for life to exist. This may be related to the 'Many Worlds' interpretation of quantum mechanics.			Intelligent information-processing (life) must come into existence in the Universe, and, once it comes into existence, it will never die out.

Lately, however, there has been growing opposition to the anthropic principle theory. Many claim that the anthropic principle is not a true scientific theory since it is not testable (unlike the theories of the speed of light and relativity).

Also, they feel there may be a fatal flaw with regards to the Grand Unified Theories (GUTs). GUTs attempt to unify the four forces of nature - gravity, electomagnetic and the strong and weak nuclear forces would all be united under GUT. With GUTs, if you vary one force, you vary all the other forces, so they say it is an oversimplication to say that if you increase or decrease gravity the universe will change.

Going even further into physics, string theory is cited as another problem that the anthropic principle faces. With string theory, all matter in the universe is made up of tiny strings vibrating in 9-dimensional space. What's more, there is only one universal constant, from which all other constants (for example, gravity, electromagnetic force, etc) are all derived. So there's no room to manoeuvre and change one force - everything is fixed.

Of course, there are numerous counter-arguments against these, posed by the pro-anthropic principle supporters. As usual, nothing is clear cut, but hopefully this article has given you an understanding of what the anthropic principle means.

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		Life Islands Theory The Selfish Alien					Highly Recommended: Anthropic-Principle.com http://www.anthropic-principle.com/