Confessions of a Boltzmann Brain
If the universe is infinite, either in space or time, or if there is an infinite number of finite universes, or if there is even one actually existing infinite universe sufficiently like our own, then any event of finite probability will occur an infinite number of times (given certain assumptions about cosmic diversity). The spontaneous congealment of a brain from relatively disorganized matter, by sheer quantum or statistical-mechanical chance, is often held to have a very tiny but finite probability. Following recent tradition, we can call this entity a Boltzmann brain.
If the universe is infinite, then, I might expect there to be an infinite number of Boltzmann brains molecule-for-molecule identical to my own, thinking the same thoughts as I do and having the same feelings as I have. If you think, as most philosophers do these days, that some form of externalism is true, then maybe a lone Boltzmann brain can't have exactly my thoughts. It couldn't think about my wife, for example, since it has never met my wife. No matter: Suppose that there is some minimal section of brain-plus-body-plus-environment that is constitutively necessary for a being to have thoughts qualitatively identical to my own. There will be an infinite number of randomly congealed Boltzmann beings that meet those minimal conditions. For convenience, I'll still call them Boltzmann brains.
Now the fate of a Boltzmann brain is very likely to be different from the fate I envision for myself. The odds of simply a brain randomly congealing while the rest of the surrounding matter stays disorganized are massively higher than the odds of an earth-and-sun system congealing full of durable life and society. Setting aside some concerns about probability with infinite cases, it seems reasonable to conclude that the odds are miniscule that a Boltzmann brain will live more than a few minutes, or maybe hours if a fair chunk of environment is assumed. If only a brain is assumed, then virtually right away things will go wrong: My optic nerves won't be stimulated by well-structured light. I won't feel like I'm breathing. If I'm in a churning soup, my brain will start tearing apart. If I'm in a near-vacuum, my brain will do whatever unpleasant thing brains do in near-vacuums. If some environment is assumed, right away it will start to crumble. The oxygen will start to dissipate into space, or gray goo will start to ooze through the walls.
Should I worry that I might be a Boltzmann brain? The odds seem rather hard to assess. By stipulation, the Boltzmann brain differs from the me that I would prefer to think I am, as of right now, not at all in quality of experience, including apparent memories.
The subjective probability judgment seems to depend, first, on the odds that the universe is infinite in the relevant sense. It seems reasonable enough, and common enough among cosmologists (if I'm right that cosmologists exist) to think the universe might be infinite. So let's call it 50-50, for all I can tell. Given the non-trivial possibility of an infinite universe, the issue then seems to turn on the odds that this sort of qualitative experience that I'm having right now (and whatever else is in the relevant, non-question-begging evidence base?) would arise from a non-Boltzmannian process of cosmological formation with an earth and sun, human society, and biological birth, as opposed to arising through Boltzmannian chance. How confidently can I assess those odds? Maybe one way to think of it is this: Given some arbitrary huge-huge-huge but not infinite chunk of the infinite universe, what should I expect the ratio to be of beings qualitatively identical to me that arise through normal cosmological Big-Bang-to-now processes as opposed to Boltzmannian chance? Hm! It doesn't seem to me that I really can in good conscience justifiably say that I know that the ratio whoppingly favors the me I think I am. How many Eric-Schwitzgeblian lives will there be in in that huge-huge-huge arbitrary chunk as opposed to Eric-Schwitzgeblian Boltzmann brains? I really have no idea! I'm tempted again, to call it even odds for all I can tell, but maybe it's better to say I just don't know. It seems, then, that I can't dismiss the possibility that I am a Boltzmann brain.
Sometimes I think about this when I wake in the middle of the night or when I'm driving down the freeway and my mind is wandering. Or at least I am now experiencing apparent memories of that sort.
[slightly revised July 31]