Boltzmann brains

What if almost everything we thought we knew about our position in the universe was wrong? What if we actually were not members of a species that arose around 200,000 years ago, among life forms that started evolving around 4 billion years ago on a planet that formed 4.5 billion years ago, in a universe that began with a Big Bang around 13 billion years ago? A single cosmological discovery that changed all of that would be an amazingly big deal (at least in terms of scientific knowledge -- it might not change what we actually do with our lives).

That's roughly what's at stake with the question of Boltzmann brains -- whether instead of the picture above, it's more likely that we came into existence a short time ago via random (quantum or thermal) fluctuations during an extremely long quiet period in one of the last ages of the universe. Not only our ideas about our position in the universe are at stake; it's also possible that only my brain arose this way, perhaps a few minutes or seconds ago, meaning that much of what I think is real (other people, places beyond my immediate reach, all of human history, etc.) is not actually physically real.

Now, this sounds suspiciously similar to many radically skeptical arguments, like the brain in a vat thought experiment -- how do you know you're not just a brain in a vat? These arguments are great for an intro-to-Philosophy class, but once the shine wears off, they seem a little thin -- what does it really offer to say "well, you might be a brain in a vat, there might be a deceiving demon, etc.", and what more can we say about these arguments? They might be useful thought experiments for epistemologists who need corner-cases to test their ideas of what "knowledge" really is and what we can really know, but they don't feel productive as a way to think about the world. I think the typical arc is to be surprised by these arguments, live with them for a while, and then forget about them, and I think that's fine.

However, I think the Boltzmann brains (BB) argument is importantly different. The BB argument isn't "how do you know you're not a BB", it's "according to some cosmological theories, many BBs will exist, and using some kinds of anthropic reasoning, it's likely that you're one of those BBs." It's as if scientists pointed their telescopes at the sky and saw vast arrays of brains-in-vats; we have (as far as I know) real reasons to take the BB scenario seriously.

I haven't been able to find a comprehensive survey of argumentation around BBs, or even a very rigorous paper that attempts to thoroughly examine the question; it's usually treated as an example or interesting implication in cosmology or philosophy books and papers, as far as I can tell. It looks like it's only been seriously considered for about 20 years, like so many of the ideas that I think are most important.

To be totally honest, I expect the BB argument to fail. I also don't think it's likely to be importantly action-informing; how would I really make decisions differently if I were a BB? However, it's one of a few really big questions about what the world is actually like that I'd really love to see answered. In fact, I think I'll post again to talk more about those big questions -- stay tuned.

I'm playing with the idea of writing and thinking more about BB -- it's an appealing hobby project. If I do, you'll see it here first!