The end of the world

Continuing the thread of the End of The World as We Know it (and don't we feel fine?)... I posted this (to great yawns) three years ago in another venue to a different audience. It'll likely fare about the same here, but what the hell...

EDIT: If seriously interested, check a summary of the far more rigorous analysis of this Doomsday Argument with more defensible assumptions and more up-to-date population estimates. I present my simplistic one below mostly just for fun and to show how this sort of anthropic logic goes. You might find that if you read through and understand my simple version, then the more complex analyses with better numbers will be easier to understand.

EDIT #2: The more I think about the conclusions of the Doomsday Argument as presented in the above link, the more I get in a strange philosophical head space. I guess it is a variant of the time-travel paradox. The numbers drop out that if we try to maintain a global population of 10 billion, we only get a few more millenia before doomsday (at best). But if we drop population to a much lower level and try to sustain it there, we can get 100's of millenia before doomsday. And if we maintain a population that declines slowly and exponentially, we might have forever until doomsday. Now, this is just a probability analysis, it's not an ethical lesson or the word of god. But it still rings in the head as though it is a voice from beyond (or the future) telling us what we should do. But it isn't... it's just population stats, probability, and some interesting anthropic assumptions...

Think I'll just go weed the garden.

The end is coming, and I can prove it.

Using a little thing called the anthropic principle.

First we start with what is known: that our species' population is increasing rapidly and has been for many generations. Furthermore, our population doubling time is less than the average lifespan of an individual. This second point leads to the interesting fact that most of the people who have ever lived in the entire existence of our species so far are alive today. Think about that... most of the people who have ever lived are still alive right now. Puts a damper on a lot of past-life theologies, doesn't it? Not enough souls of the departed to go around for all of us to have had even ONE past life, much less dozens...

Let's assume six scenarios for the future of human population:

1. Population growth continues indefinitely.

2. Present rapid growth eventually levels off leading to a large long-term stable population.

3. Present rapid growth reaches a peak, then population declines reaching a smaller long-term stable population.

4. Present rapid growth reaches a peak, then declines to gradual extinction of our species.

5. Present rapid growth ends in a sudden catastrophe and our species goes abruptly extinct.

6. Present rapid growth is a phase in a long term boom and bust process, with peaks and valleys of irregular size and spacing.

These pretty well cover the scenarios for the course of the population of our species, or any other.

Scenario 1 can actually be ruled out as essentially impossible. Population growth that continues forever would result in infinite population. So, there must be some eventual stabilization or crash, even if it is billions of years in the future. Which is really the same as the other scenarios, but with a much longer time frame. So that leaves us with five futures to consider.

OK, now here comes the anthropic principle. This principle, which comes from Cosmology, holds that certain aspects of the observable universe are constrained by the requirement that we must be able to exist in order to observe them. For instance, an "observable" universe (such as our own) must be capable of supporting life and intelligent beings, or there would be no Cosmologists to "observe" and ponder it. Additionally, in its more controversial "strong" form, the anthropic principle holds that some features of the universe can actually be inferred and predicted from our presence. I am using this second form here. So now we will infer something about the likelyhood of each of these future scenarios from the simple fact that we are here now and observing the world as we see it today. No geopolitical or religious or other theories need be involved, except for this:

Let us assume that you are a single random sample of all the people who have ever lived, and who ever will live. Thus, the world that you see is probably (statistically speaking) a world that a single random lifetime is likely to see. This even works if there is in fact reincarnation, because this one lifetime you are experiencing right now is still likely to be a statistically ordinary one. And what do you see in this random lifetime?

You see a world where human population is growing rapidly, and most of the people who have ever lived are alive today with you.

OK, now, what would a random lifetime most likely experience in each of those possible scenarios?

Scenario 2: Growth leading to large stable long-term population. In this case, the vast majority of possible lifetimes fall in the period of long-term large stable population. Not at all what your single random sample actually found. Hence: VERY UNLIKELY

Scenario 3: Growth peaking, then declining to smaller long-term stable population. In this case, most of the people who ever live will live after the boom, again in a period of long-term stable population. Once again, not at all what your sample reveals. VERY UNLIKELY

Scenario 4: Boom and bust, population peaking and then fading out gradually, with eventual extinction. In this case, most of the possible lifetimes should occur around the time of the peak, and your random lifetime is most likely to fall either during the rapid growth just before the peak, the rapid decline after the peak, or (most likely) straddling the peak. This is consistent with what your sample actually observes. QUITE POSSIBLE, and the peak population is probably not very far in the future.

Scenario 5: Boom and catastrophic extinction. Here, the large majority of possible lifetimes occur in the final few generations before the catastrophe; indeed, the majority of all possible lifetimes will in fact extend right up to the catastrophe and be terminated by it. And all possible lifetimes would occur during a time of population growth. This is 100% consistent with the our one sample observation. QUITE POSSIBLE, and the catastrophe is likely to be in the relatively near future (just a few generations at most, more likely sooner than that).

Scenario 6: Cycles of booms and busts continuing for a very long time. In this case, the majority of possible lifetimes would occur around the times of peak population, either during the late boom or the early bust, much like scenario 4. However, here there are many booms, and it is less likely that the single random lifetime would be experiencing the very FIRST of these booms. So we might decide this one is POSSIBLE, BUT LESS LIKELY.

So... it seems that our future is most likely headed to bust, either with a bang or with a whimper. And the end of the boom is likely to happen relatively soon, within just a few generations, probably sooner. The actual extinction date may be far in the future, but only if in the meantime human population drops to levels orders of magnitude below present levels, and never rebounds. There have been much more rigorous and quantitative analyses of this issue published in the academic journals, which have reached the same conclusion: The future prospects of humanity are most likely extinction, and it is not possible to rule out extremely short times between now and the extinction date. Again, this is not based in ANY way on environmental or political or military or technological or theological concerns. Just the philosophical and statistical assumptions layed out here.

One obvious objection always comes up now: Using this logic, wouldn't everyone who has ever lived so far come to this same conclusion? Yes, they would have. And most of them are still alive now. So even though all those medieval doomsayers were wrong, there are FAR more of us around at this very moment who would be right if the world in fact ended tomorrow. So this objection is really irrelevent, in that strange way that Strong Anthopic principle arguments always go.

The second objection is more substantial. We have not just been anthropic, we have been anthropocentric. We have assumed that the random self-aware rational intelligent lifetime that our observer (that's you) experienced HAD to be experienced by a human being. We have neglected the possibility that there are other rational intelligent self-aware beings in the cosmos, past or future. It is possible that our random observer could have also wound up being some other kind of being in some other planet, even in some other galaxy. And this is a faulty presumption. In spite of its name, the anthropic principle assumes the existence of a conscious intelligent observer; it does not demand that that observer be a member of our species (anthropoi)or on this planet. SO... we now need to reanalyze all this using not the population of of Homo sapiens but instead the population of all intelligent life forms in the entire universe. Well, we have no idea about that population. It may be just us, it may be vastly larger than just us. It may be going up, or down, or stable; we have no idea. So... we're stuck, aren't we?

Well, no. If you scratch your head long enough about this you will come to realize that we are not actually off the hook. The logic still applies to the bleak conclusions about the future of this single intelligent species (us), but it does not imply the extinction of intelligent life in general in the universe. A scenario that is very consistent with our observation is that the universe is populated with the recurrent boom and bust model (scenario 6), but that each boom and bust is a different species in a different place. So there is always intelligent life out there. The individual types of beings come and go, but the existence of intelligence continues. There might even be other future intelligent species here on our home planet after we ourselves are gone. This is not a scenario that can be rigorously defended by the statistics, but it is consistent with the overall pattern of evolution and extinction of all kinds of species, intelligent or not, that we do see on our one little planet.

Now, before you jump to the Star Trek model of all these intelligent forms that come and go over evolutionary history banding together in a great space-travelling civilization... there's another really solid anthropic-principle-based "we ARE alone" argument that there is in fact NOT a space-traveling expanding intelligent interstellar civilization in our galaxy. But that is for another time.

Now that I think about it, there is a seventh possible scenario: We achieve immortality and stop reproducing. Sure, baby, Sure. I'll bank on that one, uh huh.