Thursday, November 13, 2008

The Anthropic Principle for Misanthropes, part 1

One of the ways in which contemporary science has appeared to undergird religious belief is the Anthropic Principle. This is the investigation of the necessary conditions for the existence of life. It's an entire field of study, so obviously my treatment here is not even remotely exhaustive. There could easily be a blog entirely devoted to this subject reporting on new discoveries and studies on an almost daily basis.

The Anthropic Principle has its origins in the 1960s with scientists trying to determine the likelihood of other possible life-sites in the universe. It was thought at the time that the universe is so huge, there must be plenty of habitable planets, potentially with life and advanced civilizations already present.

What they have discovered is that the number of conditions that have to be met in order for a planet to be capable of supporting life are so numerous and so unlikely that, even when factoring the size of the universe into the equation, the odds of there being any planet anywhere in the universe that would meet all of the necessary conditions by chance is essentially zero. But this raised an obvious problem: there is a planet that meets all of these conditions. You're sitting on it. Since the Anthropic Principle demonstrates that it's improbable to the point of being impossible for this to have come about by chance, it suggests that these conditions are the way they are because someone intended that Earth should be able to support life.

There is debate as to whether the Anthropic Principle applies to all life or only to complex life. Many scientists argue that simple, unicellular life might be able to survive outside of the severe parameters necessary for advanced life, and could even be widespread. Probably the most popular book arguing this point is Rare Earth: Why Complex Life Is Uncommon in the Universe by Peter Ward and Donald Brownlee. If we grant this for the sake of argument, we're still left with a universe in which advanced life simply shouldn't exist, if left to its own resources. And yet, as you may have noticed, it does.

There are really two levels to the Anthropic Principle. The first is what has already been mentioned: the necessary conditions that must be met within the universe. For example, the planet must be in a particular part of a particular type of galaxy -- in a spiral galaxy and in between spiral arms. It must also have a particular interstellar history, such as nearby white dwarf binary stars that have lost some of their surface material to interstellar space in order to provide flourine. It must orbit a particular type of star with particular types of outer planets. The planet must have a particular axial tilt, a particular magnetic field, have a moon of a particular size and distance, etc., ad infinitum. Again, it's not an issue of individual criteria being met -- the universe is so big that there will be other places that meet even extremely improbable conditions. The point is that it has to meet them all, and when the conditions are combined it shows, even given the unfathomable size of the universe, that the odds are absurdly improbable that there would be a place that would be able to support life.

One of the properties on this level that has impressed me the most involves the Kuiper Belt. This is an asteroid belt outside the orbit of Neptune. A few years ago, scientists decided (ex cathedra) that Pluto isn't actually a planet, but is just a fairly large and fairly close Kuiper Belt Object.

The gravitational effects from the Kuiper Belt stabilize Neptune's orbit. If the Kuiper Belt's mass were any different (either less or greater), it would start a domino effect, throwing off Neptune's orbit, which would in turn throw off Uranus', then Saturn's, and then Jupiter's. Then the orbits of the inner planets, including Earth, would be disrupted to the extent that none of them would have an orbit stable enough to permit life. It just blows me away that life on Earth is dependent on an asteroid belt outside the orbit of Neptune.

Recently, astronomers have found that our dependence on the Kuiper Belt is even greater than was previously thought (see here, here, and here). Using computer modeling of our solar system's development, they discovered that early on, Uranus and Neptune were much closer to the sun (as was the Kuiper Belt) and possessed much more eccentric orbits. The gravitational effects between the Kuiper Belt Objects and Neptune and Uranus had to be very specific in order for all of them to drift further away from the Sun and then establish the stable orbits they have today.

The second level of the Anthropic Principle is the universe as a whole. Scientists have formed mathematical models with the laws of nature slightly tweaked, and used this to investigate what must be necessary for the existence of life. What they've discovered is that if most of the laws were different by very slight amounts -- if gravity was slightly weaker or stronger for example -- it would prevent any kind of life from existing anywhere in the entire history of the universe. This implies, again, that whatever Agency brought the universe into existence did so in such a way that it could support the existence of life.

The best examples of this are the universe's mass density and its space-energy density (or "dark energy"). The former essentially refers to how much matter the universe contains. As the universe expanded outward from the Big Bang, the amount of matter affected the speed, since the more mass there was, the greater gravity would slow it down. If the mass density was any greater by even a tiny amount, it would have been the proverbial straw that broke the camel's back, causing the universe to collapse back on itself. If it was any weaker, then the expansion would overwhelm gravity enough that galaxies would never form, and without galaxies you don't have enough nearby stars to provide the heavier elements on which life depends. Specifically, the mass density has to be exactly what it is to within one part in 1060 in order for life to exist in the universe.

The second factor mentioned above is dark energy. This refers to the "stretchiness" of the space-time fabric. This concept has its origins in Einstein's cosmological constant (symbolized by the Greek letter lambda), a force that counteracts gravity which he posited in order to escape the Big Bang singularity. He suggested that the further away two objects were, the more they would repel each other. However, no such force could be detected, much less at the strength required for Einstein's scenario. In the past several years however, scientists have managed to detect this force. It's far, far too weak to be used in the way Einstein intended -- to avoid a beginning of the universe -- but it does have a positive value. This force accounts for a very unusual phenomenon: that as the universe expands, it actually seems to be speeding up. The further the universe stretches, the more quickly it stretches. The reason this is called "stretching" is because it's not just a matter of stars and galaxies moving away from each other: the fabric of space-time is actually stretching out further. You yourself are getting slightly bigger each year as the universe expands. And you thought it was the donuts.

If the properties of dark energy were slightly different, it would affect the rate at which the universe expands, and this leads to the same problem as the mass density: either the universe would collapse upon itself (if it wasn't stretchy enough) or it would not form stars and galaxies and the heavier elements upon which life depends would not be available (if it was too stretchy). In fact, dark energy has to be fine-tuned to an even greater degree than the mass density is. It has to be exact to within one part in 10120.

As far as I know, the fact that the universe is balanced on a knife's edge -- that if dozens of its properties were different in the slightest degrees, life (or at least advanced life) could never exist at any time and any place in its history -- is recognized by all scientists in the relevant disciplines. Accounting for this is a different matter. As I've suggested above, many scientists have thought that the fine-tuning of the universe demonstrates that, in Fred Hoyle's terms, "a superintellect monkeyed with physics, as well as with chemistry and biology." But of course there have been many objections made against this inference. I'll deal with a few of the more common ones in future installments.

Update (13 Feb): See also part 2, part 3, and part 4.

(cross-posted at Quodlibeta)

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