Doomsday argument

johnmarkley · Post by **johnmarkley** » 2006-06-13 03:02am

I recently read something about Carter's Doomsday Argument, as described here. It reminds me of the first time I encountered Zeno's paradox or Anselm's ontological proof of God- it seems clear there's something wrong with it, but I can't put my finger on exactly what. Can anyone help me out?

B5B7 · Post by **B5B7** » 2006-06-13 06:31am

It can be summarised easily - as nonsense.
Its a variant of erroneous Anthropic Principles.

It is interesting that you mention Zeno's Paradox (actually there were several different but related ones), as Zeno when he put forward these ideas wasn't advocating them - he knew they were incompatible with reality - he was challenging his readers to figure out why they were wrong - and it needed modern mathematical theory to provide the answer he was looking for.

Wyrm · Post by **Wyrm** » 2006-06-13 11:07am

The whole Doomsday Argument, at its core, is based on the assumption that you, the poser of the Argument, are equally likely to be a caveman at the beginning of time as you are to be a post-civilization human at the end of human history. That you are a person in the modern era, actually posing the argument now, is obviously relevant, as it alters the probability of you existing in other eras, which is the core of the Doomsday Argument. Indeed, the fact that you are proposing the now reduces the probility of you existing at any other era to exactly zero. Therefore, you are not a part of any pool of people that does not include the modern era. Because that you could have been part of such a population pool is a main pillar of the Doomsday Argument, knocking it out brings the entire argument down.

Put another way, the exact same argument can be made at any point in human history. If, for instance, this argument had been made at the point where only 500,000 humans had been born, then we would conclude with 95% probability that the total number of humans to be born is only 1 billion. We in the 21rst century have already reached and surpassed this point by a factor of about six.

The Wikipedia entry states that the argument requires a Bayesian interpretation. While a Bayesian interpretation of probability (probability is a measure of ignorance) is necessary for the argument to make sense, a proper Bayesian treatment brings the argument to its knees: the observation that you make the argument now is obviously relevant to the question of when you could have been born.

Cej4096 · Post by **Cej4096** » 2006-06-13 03:21pm

The problem with the arguement that seems most obvious to me is simply that the time we are born into defines who we are. The arguement seems to say that if humanity is going to continue for any long period period of time humans who live now should instead live in the future. How exactly would that work? According to the arguement, how would the world be different if humanity were to survive?

Even if the total number of humans ever born were to eventually reach a googolplex, people would still exist here and now.

Like the ontological proof of God, I've never understood why people find this arguement convincing.

nasor · Post by **nasor** » 2006-06-14 02:01pm

B5B7 wrote:It is interesting that you mention Zeno's Paradox (actually there were several different but related ones), as Zeno when he put forward these ideas wasn't advocating them - he knew they were incompatible with reality - he was challenging his readers to figure out why they were wrong - and it needed modern mathematical theory to provide the answer he was looking for.

This isn't accurate. Zeno believed in a school of philosophy called Eleaticism, which taught (among other things) that motion was impossible and really just an illusion. His 'paradoxes' were his attempts to provide supporting evidence for this.

http://en.wikipedia.org/wiki/Eleatic
http://www.iep.utm.edu/z/zenoelea.htm

nasor · Post by **nasor** » 2006-06-14 02:08pm

Cej4096 wrote:The problem with the arguement that seems most obvious to me is simply that the time we are born into defines who we are. The arguement seems to say that if humanity is going to continue for any long period period of time humans who live now should instead live in the future. How exactly would that work?

More precisely, the argument states that the people born at the dawn of human civilization would most probably have been born near doomsday, and that the people who were born early just happened to beat the odds.

Suppose you were in a room with nine other people. Someone tells you that nextdoor is another room with an unknown number of people in it, and that one room has twice as many people as the other. Either you are in the larger room and the other room has only one person in it, or you are in the smaller room and the other room has 100 people in it.
If you assume that people were assigned to rooms randomly, then you can conclude that the other room probably only has one person since you were most likely assigned to the room that would have the largest number of people.

drachefly · Post by **drachefly** » 2006-06-14 02:28pm

That situation is totally unlike the doomsday argument, and it is unlike real life. The DA-supporters probably think they're similar, though.

nasor · Post by **nasor** » 2006-06-14 02:41pm

drachefly wrote:That situation is totally unlike the doomsday argument, and it is unlike real life. The DA-supporters probably think they're similar, though.

Indeed. The central premise of the doomsday argument could be summed up as “You are most likely to be born into the time period where the largest number of people exist”. It’s not at all clear that this is a true statement.

B5B7 · Post by **B5B7** » 2006-06-16 02:04am

nasor wrote:
B5B7 wrote:It is interesting that you mention Zeno's Paradox (actually there were several different but related ones), as Zeno when he put forward these ideas wasn't advocating them - he knew they were incompatible with reality - he was challenging his readers to figure out why they were wrong - and it needed modern mathematical theory to provide the answer he was looking for.
This isn't accurate. Zeno believed in a school of philosophy called Eleaticism, which taught (among other things) that motion was impossible and really just an illusion. His 'paradoxes' were his attempts to provide supporting evidence for this.

http://en.wikipedia.org/wiki/Eleatic
http://www.iep.utm.edu/z/zenoelea.htm

Things aren't as clearcut as you claim; for instance this typical opinion from a typical webpage about Zeno's Paradox:
Quote from http://www.mathpages.com/rr/s3-07/3-07.htm

We can't be sure how the historical Zeno intended his arguments to be taken, since none of his writings have survived. We know his ideas only indirectly through the writings of Plato, Aristotle, Simplicus, and Proclus, none of whom was exactly sympathetic to Zeno's philosophical outlook. Furthermore, we're told that Zeno's arguments were a "youthful effort", and that they were made public without his prior knowledge or consent. Also, even if we accept that his purpose was to defend the Eleatic philosophy against charges of logical inconsistency, it doesn't follow that Zeno necessarily regarded his counter-charges as convincing. It's conceivable that he intended them as satires of (what he viewed as) the fallacious arguments that had been made against Parmenides' ideas. In any case, although we cannot know for sure how Zeno himself viewed his "paradoxes", we can nevertheless examine the arguments themselves, as they've come down to us, to see if they contain - or suggest - anything of interest.

Also the second link you provide itself has this paragraph:

Common sense says there is both motion and plurality. This is the Pythagorean notion of reality against which Zeno directed his arguments. Zeno showed that the common sense notion of reality leads to consequences at least as paradoxical as his master's.