The Universe is balanced on a knife-edge of coincidence. It is apparently a coincidence that gravity and the strong force are as strong as they are, or that the Universe happens to be as old as it is. It is also a coincidence that all of these coincidences of physical constants and other phenomena of the Universe have happened together, making the Universe hospitable for intelligent observers like ourselves.
What would happen if one or all of these coincidences didnít occur? Things would be very different indeed. Most likely, life as we know it would be downright impossible, and in some situations, so would anything more complex than a helium atom.
To prove this point, some tinkering with the various coincidences of the Universe is in order. Iíll start with Gravity, a very weak force. Now, letís see what happens when gravity gets slightly stronger. The Big Bang explodes from the void. Only in this Big Bang, gravity slows the expansion of the primordial matter more than it did in the universe we know. More of the first atoms coalesce into bigger and denser stars, whose added gravity clusters them closer together in their galaxies. The result: a smaller universe of larger, brighter, shorter-lived stars, that will eventually collapse in on itself again in a Big Crunch. Most, if not all stars would be binary, trinary, or larger systems. Any planets orbiting these stars would have to go very fast to avoid a fiery demise inside their parent stars, and would be slung around wildly by their multiple suns. Any planetary system in this universe would be devoid of a stable, safe harbor for life, and relative stability is a vital prerequisite for the evolution of complex life forms. Life here would probably get to no more than amino acids, much less true life, before one of the planetís parent stars went nova or the planet was torn apart and swallowed into one of the stars. It would not be a place to develop living creatures as complex as ourselves.
If the strong force were weaker, things would not even get as far as in the previous example. Soon after the Big Bang, as the universe cools, matter coalesces into atoms. The universe cools still further, and atoms themselves clump together due to gravity, forming clouds, which eventually form stars. Up until now, this universe has behaved pretty much like our own. But now it diverges. The repulsive force of the positive protons is more likely to overcome the attraction of the strong force the bigger atoms get, and the atom will undergo spontaneous fission. Since the strong force is weaker in this universe, lighter atoms are likely to be unstable, so stars cannot fuse atoms together enough times to create the heavier elements. This means that rocky planets, of the type that can support life, are impossible to form, because there is nothing from which to form dense enough rock and metal. As living creatures are unlikely to find Saturn-like gas giants habitable, this universe, again, has no safe harbor for life.
The age of the Universe, unlike the aforementioned fundamental forces, does not at first glance seem to have much bearing on whether intelligence can exist in it. However, it is more closely linked than it might appear. As has been said, heavier elements are necessary for life, and elements like carbon, the basis of life as we know it, take a long time to form. In fact, supernovae are the only way that heavy elements can get distributed to places that they can be of use, but supernovae are rare. If we backtracked through the Universeís life, we would soon (soon being a relative term, in comparison to the age of the Universe) come to a time where there are far fewer of the heavier elements than there are now, and then to a time when there are none. It seems, in fact, that it has taken this long (about 15 billion years) for supernovae to distribute enough of the various heavy elements for life to be possible. So we are really living at the earliest point that it would be possible for us to live at. The vast age of the Universe is also necessary for our existence.
All of these various properties of the Universe were discovered separately, and it was only after their respective discoveries that people realized that they had to be so for us to exist. As such, they are not really the best examples of the usefulness of the Weak Anthropic Principle, only of itís validity. There has only been one instance (that I know of) when a discovery was made because the Weak Anthropic Principle was invoked. Without the Weak Anthropic Principle, however, the discovery would never have been made.
Up until the 1950s, one of the most confounding mysteries was how Carbon-12, one of the elements necessary for all life, was manufactured and dispersed into space. To make carbon-12, a helium-4 nucleus must collide with a beryllium-8 nucleus. The problem is, beryllium-8 is highly unstable, breaking up into lighter particles in a matter of only 10(to the negative 17th) seconds. Scientists were stumped as to how carbon-12 could build up in enough quantity to create all the biomass of the Earth, not to mention all the carbon-12 in nonliving matter.
In 1952, Ed Salpeter proposed that carbon-12 might be produced by a very rapid triple collision, with a beryllium-8 nucleus being formed, and then a helium-4 nucleus colliding with it sometime during the 10(to the negative 17th) seconds in which the beryllium-8 nucleus stuck together. Since the addition of the helium-4 nucleus might very easily smash the delicate beryllium-8 nucleus into itís constituent parts, though, this explanation of the creation of carbon-12 still didnít account for the amount of carbon-12 around today.
It was Fred Hoyle who came up with the idea that a resonance involving helium-4, beryllium-8, and carbon-12 might exist which would encourage Salpeterís apparently far-fetched combinations of nuclei. According to ďCosmic CoincidencesĒ by John Gribbin and Martin Rees:
ďResonance works like this. When two nuclei collide and stick together, the new nucleus that is formed carries the combined mass-energy of the two nuclei, plus the combined energy of their motion, their kinetic energy (and minus a small amount of energy from the strong force, the binding energy that holds the new nucleus together). The new nucleus ďwantsĒ to occupy one of the steps on itís own energy ladder, and if this combined energy from the incoming particles is not just right the excess has to be eliminated, in the form of leftover kinetic energy, or as a particle ejected from the nucleus.... If everything meshes perfectly, however, the new nucleus will be created with exactly the energy that corresponds to one of itís natural energy levels.... The matching of energies to one of the levels appropriate for the new nucleus is the effect known as resonance, and it depends crucially on the structure of the nuclei involved in the collisions.Ē
Hoyle knew the mass-energy of each nucleus involved, and could calculate their kinetic energy from what he knew of the average conditions in the interiors of stars. Based on these calculations, and the knowledge that there is more carbon-12 than was accounted for by previous theories, he predicted that there must be a previously unknown energy level in the carbon-12 nucleus, just at the right level to resonate with the combined energies of itís constituent particles. When the theory was tested, the calculations showed that carbon-12 has an energy level just 4% above what Hoyle had calculated, a number so close that the kinetic energy of the respective nuclei involved in the collision could easily supply what was needed.
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