Brain-damaged arguments and Boltzmann Brains

Okay, so I got a question from my friend Tamara, who’s a high school teacher in my hometown of New York City. It concerns a recent article she read on the front page of the New York Times about something funny that us scientists are calling Boltzmann Brains.

I’ve read this article three times since it was featured on the front page of the science section in the NYT and I’m still confused about the Boltzmann brain problem, it’s (non?)validity, the reason it made it’s way onto the front page and whether Emerson’s philosophy about imagined worlds came from this…

There’s a lot of interesting stuff going on in cosmology and astrophysics, and so when I see something that’s, well, illogical (and that’s being nice) getting a lot of attention, I have to take issue with it.

The argument of it all is that the amount of entropy (disorder) in the universe is fairly large. Since there is a lot of entropy, and a lot of random fluctuations in the universe, there’s a low probability that a very unlikely fluctuation has occurred in our universe. That part is fine. The second part, where they say that brains are very unlikely random fluctuations, isn’t a problem either. That’s true as well. The third and final part is the problem, where they draw the conclusion that since conscious brains are unlikely random fluctuations, it’s more likely that we live in a universe with only one conscious brain (presumably, yours, mine, or the theoretical physicist proposing it) than a universe with billions and billions of them. This is wrong, but first let’s make sure you understand the Boltzmann Brain proposal (and for another, less scathing account of it, see Sean Carroll’s post over at cosmic variance):

  1. We live in a universe full of random fluctuations.
  2. We are the result of an unlikely, but not impossible fluctuation.
  3. It is more probable that we are the result of a more likely, rather than a less likely, fluctuation.
  4. A universe with one brain that results from random fluctuations is more likely than a universe with billions and billions of them.

Although there have been a number of scientific papers in the last couple of years on this topic, none of them were written, apparently, by anyone with a very deep understanding of biology. While the first three points in my list are undoubtedly true based on what we know about physics and probability, the last point is not. Why not? Because your brain is not a random fluctuation.

There is not a person alive who knows how the mechanisms of biological evolution are derived from the laws of physics, but even the most elementary student of biology knows that it takes a population of organisms to evolve. While the life that has evolved on our planet is the result of a random fluctuation, the entire 14 billion years of evolution in the universe that has led to our existence at present isn’t a random process; we are the result of a random initial condition undergoing physical, chemical and biological evolution according to intricate natural laws. And since, once self-replicating life exists, sexual reproduction is a more successful template than asexual reproduction, it evolved. One can only assume since human brains evolved, those parts of us are successful templates as well, and are selected for, naturally.

In the absence of any organizing principles or selection laws, yes, the Boltzmann Brain argument is valid. But ignoring biology and biological evolution doesn’t make it go away, and it doesn’t make the Boltzmann Brain question any more interesting than asking how often a box full of deconstructed watch components will form an assembled watch when you shake it like a polaroid picture. Because we do have the laws of evolution in our universe, I’m not losing any sleep over the notion that I might be the only working brain it. Some days are long enough without speculating on the lack of brainpower in others.

3 thoughts on “Brain-damaged arguments and Boltzmann Brains

  1. I’m glad to see a logical refutation of this one. My mom forwarded the article to me, and I recally from one of my philosophy of physics classes (the one that focused on the arrow of time) that the high probability of increasing entropy is as true in the past as it is in the future in some models (those that don’t take into account correlation of particle positions or some such…I’m still trying to figure out what’s meant by “the entropy of a black hole”) meant that our current state is “more likely” to be a random fluctuation than to have actually started at a low entropy state, yadda yadda. Anyway, there’s an awful lot of subtlety in a lot of cutting edge physics that can lead to craziness (not just weirdness, like QM), so it’s good to see it picked apart properly.

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