Boltzmann Brain

I blame the fact I never took high-school physics for my not having heard of the Boltzmann brain until late in life. For those who slept through physics class, the Boltzmann brain is a thought experiment suggested by the 19th-century German physicist Ludwig Boltzmann, best known for his work in thermodynamics. Think of the universe as a big windup toy that is gradually winding down. Due to entropy, it ends up in a state of thermodynamic equilibrium in which everything is uniformly cold and dark. But not quite. Statistically, random fluctuations can occur here and there, so that entropy is temporarily reversed; indeed, the universe that we know might be just such a thermal fluctuation in a larger featureless universe that has undergone heat death. What are the odds? Vanishingly small – in fact, rather than give rise to an entire universe, the odds are better that one such fluctuation produces only a single brain that thinks it is embedded in an entire universe. Hence, the Boltzmann brain.

The Boltzmann brain is a variation on the so-called “brain in a vat” problem. Briefly, how do we know that everything we see, think and feel isn’t fed to us through wires attached to our brains, which are floating in a vat of nutrients? Fans of the Matrix films will be familiar with the premise. Clearly, there is no way to know whether I am actually experiencing what I am experiencing, or that I have been duped by a fiendishly clever computer simulation. The philosopher Rene Descartes anticipated this dilemma in the 17th century, when he wondered whether the evidence of his senses might be the work of an evil demon. Picking up the thread, Oxford philosopher Nick Bostrum speculates that we may already be living in a computer simulation run by some advanced civilization.

It should perhaps come as no surprise that disembodied brains would figure prominently in the speculations of people with big brains. After all, a random thermal fluctuation might just as easily produce a nose or a big toe – statistically, even more so. In any event, it is doubtful that a brain floating in a void would survive long enough to entertain delusions of being embedded in the universe. For obvious reasons, there have been few empirical tests of how long a disembodied human brain can function. Antoine Lavoisier, discoverer of oxygen, was the involuntary subject of one such experiment when he fell victim to the guillotine during the French Revolution. Lavoisier reputedly had his manservant count the number of times he blinked after the blade fell. The answer was 15, although the story is almost certainly apocryphal. Suffice it to say that a human brain can function only a few minutes at most without oxygen, disembodied or not.

Boltzmann’s thought experiment rests on the statistical odds of a brain emerging from random fluctuations in entropy as the universe heads toward oblivion. We are talking about an extremely remote likelihood over trillions of years, sort of like the chances that enough monkeys equipped with enough typewriters will write the works of Shakespeare. Statistically, it may be possible that our simian friends will get inspired. But you don’t necessarily want to hang out forever at the monkey house waiting around for a literary breakthrough.

A brain existing in splendid isolation is exactly the sort of conceit that you might expect a brain to come up with to keep itself entertained. But what does it actually take to produce a functioning human brain, assuming you don’t want to wait around for a random reversal in entropy? For starters, a functioning brain does not spring full-blown from the primordial soup. Some four billion years of trial-and-error were required for those first strings of self-replicating organic molecules to evolve to the point where they can form an organism that amuses itself with highly speculative thought experiments about disembodied brains. But things don’t start there. You need billions more years before that to produce galaxies, stars and rocky planets with water and atmospheres that can support life.

You need to go back to the primordial soup. Start with those first milliseconds after the Big Bang, when there were as yet no galaxies, no stars, no planets, no gases or cosmic dust, not even atoms. When the primordial soup consisted of nothing but superheated plasma made up of elementary particles, and the fundamental forces of nature (gravity, electromagnetism, strong force and weak force) were just beginning to emerge. In those first few fateful moments, the die was cast. To cite just one example among many: had the force of gravity been only slightly stronger, all stars would have been more massive than the sun, which would have made them too short-lived and unstable to sustain life. Had the force of gravity been less than it is, all stars would have been smaller than the sun, and there would have been no heavier elements generated to form rocky earth-like planets.

What does it take to produce a functioning brain? I am reminded of the old saw about what it takes to produce an English garden. Answer: a little watering, a little fertilizer and 600 years of care. No brain exists in isolation, Boltzmann notwithstanding. You need a body to nurture it, a rocky planet with an atmosphere and water, a sun of middling size, fundamental forces of nature that permit such suns to arise – and, of course, 13 billion years or so for that primordial soup to simmer. So what does it take? It takes an entire universe.

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