Today’s IFOD concerns the “Anthropic Principle” which builds on last week’s post on the four  four forces in the Universe and is a philosophical answer to the question “isn’t it really amazing that our Universe is the way that it is?”

If many finely balanced qualities of our Universe weren’t “just so” then our Universe would have grossly different qualities and life as we know it would not exist.  For example:

• Gravity is roughly 10^39 times weaker than electromagnetism. If gravity had been 10^33 times weaker than electromagnetism, stars would be a billion times less massive and would burn a million times faster. That would be bad.
• The nuclear weak force is 10^28 times the strength of gravity. Had the weak force been slightly weaker, all the hydrogen in the universe would have been turned to helium (making water impossible, for example).
• A stronger nuclear strong force (by as little as 2 percent) would have prevented the formation of protons–yielding a universe without atoms. Decreasing it by 5 percent would have given us a universe without stars.
• If the difference in mass between a proton and a neutron were not exactly as it is–roughly twice the mass of an electron–then all neutrons would have become protons or vice versa.
• The very nature of water, which is necessary for life, is something of a mystery. Unique amongst the molecules, water is lighter in its solid than liquid form: ice floats. If it did not, the oceans would freeze from the bottom up and earth would now be covered with solid ice. This property in turn is traceable to the unique properties of the hydrogen atom.
• And so on . . .

Now for the Anthropic Principle. While there are a few forms of the Anthropic Principle, basically what it says is this:  if the properties of our Universe weren’t as they are then the Universe would be so different that it is highly likely that life (as we know it) would not develop and thus nobody would be around to ask the question of why our Universe is so special.  In other words, it is only in universes where the conditions are just right for life that it is possible to pose the questions of being special.   So, there is a selection bias.  We don’t know whether our Universe is special or not.  It is only in a Universe such as ours (fine-tuned for our existence) that we could ask these questions because in other Universes (if they exist) nobody is asking the question because life cannot form in those Universes.  We happen to find ourselves in a Universe where life is conveniently possible because we cannot very well be anywhere else.

The Anthropic Principle naturally leads to consideration  of multiple universes (which is a really interesting topic and now is taken very seriously by physicists)..