anthropic principle

All posts tagged anthropic principle

Book review: Our Mathematical Universe: My Quest for the Ultimate Nature of Reality, by Max Tegmark.

His most important claim is the radical Platonist view that all well-defined mathematical structures exist, therefore most physics is the study of which of those we inhabit. His arguments are more tempting than any others I’ve seen for this view, but I’m left with plenty of doubt.

He points to ways that we can imagine this hypothesis being testable, such as via the fine-tuning of fundamental constants. But he doesn’t provide a good reason to think that those tests will distinguish his hypothesis from other popular approaches, as it’s easy to imagine that we’ll never find situations where they make different predictions.

The most valuable parts of the book involve the claim that the multiverse is spatially infinite. He mostly talks as if that’s likely to be true, but his explanations caused me to lower my probability estimate for that claim.

He gets that infinity by claiming that inflation continues in places for infinite time, and then claiming there are reference frames for which that infinite time is located in a spatial rather than a time direction. I have a vague intuition why that second step might be right (but I’m fairly sure he left something important out of the explanation).

For the infinite time part, I’m stuck with relying on argument from authority, without much evidence that the relevant authorities have much confidence in the claim.

Toward the end of the book he mentions reasons to doubt infinities in physics theories – it’s easy to find examples where we model substances such as air as infinitely divisible, when we know that at some levels of detail atomic theory is more accurate. The eternal inflation theory depends on an infinitely expandable space which we can easily imagine is only an approximation. Plus, when physicists explicitly ask whether the universe will last forever, they don’t seem very confident. I’m also tempted to say that the measure problem (i.e. the absence of a way to say some events are more likely than others if they all happen an infinite number of times) is a reason to doubt infinities, but I don’t have much confidence that reality obeys my desire for it to be comprehensible.

I’m disappointed by his claim that we can get good evidence that we’re not Boltzmann brains. He wants us to test our memories, because if I am a Boltzmann brain I’ll probably have a bunch of absurd memories. But suppose I remember having done that test in the past few minutes. The Boltzmann brain hypothesis suggests it’s much more likely for me to have randomly acquired the memory of having passed the test than for me to actually be have done the test. Maybe there’s a way to turn Tegmark’s argument into something rigorous, but it isn’t obvious.

He gives a surprising argument that the differences between the Everett and Copenhagen interpretations of quantum mechanics don’t matter much, because unrelated reasons involving multiverses lead us to expect results comparable to the Everett interpretation even if the Copenhagen interpretation is correct.

It’s a bit hard to figure out what the book’s target audience is – he hides the few equations he uses in footnotes to make it look easy for laymen to follow, but he also discusses hard concepts such as universes with more than one time dimension with little attempt to prepare laymen for them.

The first few chapters are intended for readers with little knowledge of physics. One theme is a historical trend which he mostly describes as expanding our estimate of how big reality is. But the evidence he provides only tells us that the lower bounds that people give keep increasing. Looking at the upper bound (typically infinity) makes that trend look less interesting.

The book has many interesting digressions such as a description of how to build Douglas Adams’ infinite improbability drive.

Discussions asking whether “Snowball Earth” triggered animal evolution (see the bottom half of that page) suggest increasing evidence that the Snowball Earth hypothesis may explain an important part of why spacefaring civilizations seem rare.

photosynthetic organisms are limited by nutrients, most often nitrogen or phosphorous

the glaciations led to high phosphorous concentrations, which led to high productivity, which led to high oxygen in the oceans and atmosphere, which allowed for animal evolution to be triggered and thus the rise of the metazoans.

This seems quite speculative, but if true it might mean that our planet needed a snowball earth effect for complex life to evolve, but also needed that snowball earth period to be followed by hundreds of millions of years without another snowball earth period that would wipe out complex life. It’s easy to imagine that the conditions needed to produce one snowball earth effect make it very unusual for the planet to escape repeated snowball earth events for as long as it did, thus explaining more of the Fermi paradox than seemed previously possible.

Book Review: Anthropic Bias: Observation Selections Effects in Science and Philosophy by Nick Bostrom

This book discusses selection effects as they affect reasoning on topics such as the Doomsday Argument, whether you will choose a lane of traffic that is slower than average, and whether we can get evidence for or against the Many Worlds Interpretation of quantum mechanics. Along the way it poses some unusual thought experiments that at first glance seem to prove some absurd conclusions. It then points out the questionable assumptions about what constitute “similar observers” upon which the absurd conclusions depend, and in doing so it convinced me that the Doomsday Argument is weaker than I had previously thought.
It says some interesting things about the implications of a spatially infinite universe, and of the possibility that the number of humans will be infinite.
It is not easy to read, but there’s little reason to expect a book on this subject could be both easy to read and correct.
The author has a web site for the book.

Rare Earth : Why Complex Life Is Uncommon in the Universe provides some fairly strong (and not well known) arguments that animal life on earth has been very lucky, and that planetary surfaces are typically much more hostile to multicellular life than our experience leads us to expect.

The most convincing parts of the book deal with geological and astronomical phenomena that suggest that earth-like conditions are unstable, and that it would have been normal for animal life to have been wiped out by disasters such as asteroids, extreme temperatures, supernovae, etc.

The parts of the book that deal with biology and evolution are disappointing. The “enigma” of the Cambrian explosion seems to have been explained by Andrew Parker (see his book In the Blink of an Eye) in a way that undercuts Rare Earth’s use of it (dramatic changes of this nature seem very likely when eyes first evolve). This theory was apparently first published in a technical journal in 1998 (i.e. before Rare Earth).

They often assume that intelligence could only develop as it has in humans, even suggesting that it couldn’t evolve in the ocean, which is rather odd given how close the octopus is to qualifying. But the various arguments in the book are independent enough that the weak parts don’t have much affect on the rest of the arguments.

I was surprised that they never mentioned the Fermi Paradox, which I consider to be the strongest single argument for their position. Apparently they don’t give it much thought because they don’t expect technological growth to produce effects that encompass more than our planet and are visible at galactic distances.

Their concern over biodiversity seems rather misplaced. I can understand why people who overestimate mother nature’s benevolence think that preserving the status quo is a safe strategy for humanity, but it seems to me that anyone sharing Rare Earth’s belief that nature could wipe us out any time now should tend to prefer a strategy of putting more of our effort into creating technology that will allow us to survive natural disasters.

I am disappointed that they rarely attempt to quantify the range of probabilities they would consider reasonable for the risks they discuss.

Stephen Webb has written a book on roughly the same subject called Where is Everybody? that is more carefully argued, but less entertaining, than Rare Earth.