Citizens of the great void

>> Friday, October 03, 2008

Here's an interesting news item: an alternate hypothesis to the dark-energy hypothesis has turned up to explain the observed acceleration of the universe's expansion. The notion is that our part of the universe might have less matter than other regions; things would appear to be farther away than they are. From the above-linked article:

"If we lived in a very large under-density, then the space-time itself wouldn't be accelerating," said researcher Timothy Clifton of Oxford University in England. "It would just be that the observations, if interpreted in the usual way, would look like they were."

I don't have the math to offer anything more enlightened than, "What an interesting idea!" But this is only a part of what I wanted to devote this entry to. The article goes on to touch on something that sort of bugs me in a lot of ways, the Copernican Principle. Again, quoting the article on

One problem with the [space-time] void idea, though, is that it negates a principle that has reigned in astronomy for more than 450 years: namely, that our place in the universe isn't special. When Nicholas Copernicus argued that it made much more sense for the Earth to be revolving around the sun than vice versa, it revolutionized science. Since then, most theories have to pass the Copernican test. If they require our planet to be unique, or our position to be exalted, the ideas often seem unlikely.

The Copernican Principle is a useful rule of thumb, but it seems to come up periodically as if it were some kind of iron-clad law rather than a reliable guideline. When one looks at discussions of the Drake Equation and whether or not there are other extraterrestrial intelligences, for instance, a common fallback position of those who favor what might be called the pro-ET stance ("pro-life" having been already taken in another context) seems to be that the Copernican Principle makes it statistically likely that we're not only not alone in the universe, but we're not even remarkable. Which could be true, of course, except it might not be.

The SETI context is actually a good one, I think, for showing what's wrong with the Copernican Principle. If we start from the reasonable premise that events occur in order--that one thing must proceed another unless they're simultaneous--than one has to conclude that life must arise on one planet in one star system before it arises on another, that intelligence must evolve in one place prior to another, that civilizations must form on one world before another, that one civilization will invent baseball first, and so on. (Unless it all happens simultaneously, of course, but that really only proves the next point, which is....) Now, while there is no reason to assume we're the first at any of these things, there's equally no reason to assume we're not. In fact, given the sample size (one), we can't even conclude what hard-adherents of the Copernican Principle advocate: that we're in the middle somewhere. The only conclusion we can draw from the list of intelligent, technological species we know of right now is that human beings are the most intelligent, advanced, and indeed awesome living creatures in the entire universe--an assertion that seems improbable but is presently irrefutable.

This is all assuming, of course, that there even are intelligent extraterrestrials out there. Which, once again, seems likely but is presently unproven.

It actually can get worse for the Copernican Principle: there actually are a number of things about Earth that seem a bit improbable or unique. The presence of a double planet within a star's "habitable zone", the larger of the double planets being a rocky, wet world with an atmosphere, said double-planet system largely being protected from catastrophic impacts by a gas giant that's far enough away not to eat the double-planet but close enough to provide a screen--how often, one wonders, does this happen? (A nice list of the improbability of our existence can be found in the Wikipedia entry for the "Rare Earth" hypothesis.

One shouldn't conclude too much from rarity. There's a wonderful quote from mathematician John Allan Paulos as to why improbability alone isn't evidence of anything at all:

When one is dealt a bridge hand of thirteen cards, the probability of being dealt that particular hand is less than one in 600 billion. Still, it would be absurd for someone to be dealt a hand, examine it carefully, calculate that the probability of getting it is less than one in 600 billion, and then conclude that he must not have been dealt that very hand because it is so very improbable.

This can be tied to a form of the Weak Anthropic Principle: the fact that we exist in the universe necessarily implies that events fell into place that allowed our existence to occur; i.e. when we look at the universe, we see the thirteen cards we've been dealt, no matter how unlikely it seems that we got these thirteen particular cards. In a sense, then, the Weak Anthropic Principle can be defined as a kind of tautology.

The paradox of that tautology, in my view, is that it actually embraces the original spirit of the Copernican Principle while rebutting the strong and self-defeating Copernican Principle. That is, the Weak Anthropic Principle can be interpreted as saying we're only as special as our privileged position as observers defines us to be, while the hardcore Copernican Principle manages to say we're especially mediocre. The hardcore Copernican Principle tells us we must be in the middle of the bell curve, even without evidence either way, because we probably are. The Weak Anthropic Principle merely tells us what we already knew: that we're here.

So, going back to the matter of whether we're in a space-time "void"--is there any reason to believe we're not? This is a rhetorical question at this point: the reason depends on evidence that is presumably yet to be gathered. It's an interesting hypothesis that can be tested through observation. That the hypothesis allegedly violates our privileged status of being nobodies in a typical nowhere means precisely nothing.


John the Scientist Saturday, October 4, 2008 at 10:24:00 PM EDT  

I'm having a retard crisis on my blog, but I meant to swing by and say that yes, I agree with you. I call it the Dawkins Syndrome - since religions at one time placed the Earth in a special place, and since all religions are baaaad, we can't ever entertain one of the notions that used to be central to a religious cosmology, even is it is for different reasons. It's a blind spot for a lot of militant scientific atheists.

island Sunday, October 5, 2008 at 12:00:00 PM EDT  

You might find this useful:

Eric Sunday, October 5, 2008 at 12:31:00 PM EDT  

Thanks, Island! The Ryals piece is interesting and provocative, and worth everyone's time--I'd recommend any other readers take a glance at it when they have a chance. I'll need some more time to digest it, myself....

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