Invasion of the Boltzmann Brains

Dr. Craig,

I am familiar with the theory of Boltzmann Brains, often given as a response to the atheist multiverse theory. A "Boltzmann Brain" is a universe in which only a single brain exists and nothing else. This universe is supposedly "vastly more likely" to exist than ours. However, I was curious as to how such a brain could form in a world of that size. As I understand the brain, a brain needs a body to survive, and a body needs an external world to interact with, and that external world must be incredibly vast to have produced the body and brain in the first place, and so on and so forth... Doesn't this make the whole Boltzmann Brain theory ridiculous? A brain could not exist in a universe all by itself--at least not a brain as we understand it! The only way to save this theory of a Boltzmann Brain universe seems to be to replace the term "brain" with "mind"--but a mind does not need a universe to exist in, since we have clarified that God is a disembodied mind that exists apart from the universe.

Is the Boltzmann Brain idea of a single-brain universe really as viable as you claim in your popular and scholarly works? Have I misunderstood the theory?


Some serious, scientifically-minded seeker


United States

You just gotta love Boltzmann Brains! What a concept!

For those who lack your background concerning this question, Ben, let’s review. Incredible as it may sound, today the principal--almost the only--alternative to a Cosmic Designer to explain the incomprehensibly precise fine tuning of nature’s constants and fundamental quantities is the postulate of a World Ensemble of (a preferably infinite number of) randomly ordered universes. By thus multiplying one’s probabilistic resources, one ensures that by chance alone somewhere in this infinite ensemble finely tuned universes like ours will appear.

Now comes the key move: since observers can exist only in worlds fine-tuned for their existence, OF COURSE we observe our world to be fine-tuned! The worlds which aren’t finely tuned have no observers in them and so cannot be observed! Hence, our observing the universe to be fine-tuned for our existence is no surprise: if it weren’t, we wouldn’t be here to be surprised. So this explanation of fine tuning relies on (i) the hypothesis of a World Ensemble and (ii) an observer self-selection effect.

Now apart from objections to (i) of a direct sort, this alternative faces a very formidable objection to (ii), namely, if we were just a random member of a World Ensemble, then we ought to be observing a very different universe. Roger Penrose has calculated that the odds of our solar system’s forming instantaneously through the random collision of particles is incomprehensibly more probable that the universe’s being fine-tuned, as it is. So if we were a random member of a World Ensemble, we should be observing a patch of order no larger than our solar system in a sea of chaos. Worlds like that are simply incomprehensibly more plentiful in the World Ensemble than worlds like ours and so ought to be observed by us if we were but a random member of such an ensemble.

Here’s where the Boltzmann Brains come into the picture. In order to be observable the patch of order needn’t be even as large as the solar system. The most probable observable world would be one in which a single brain fluctuates into existence out of the quantum vacuum and observes its otherwise empty world. The idea isn’t that the brain is the whole universe, but just a patch of order in the midst of disorder. Don’t worry that the brain couldn’t persist long: it just has to exist long enough to have an observation, and the improbability of the quantum fluctuations necessary for it to exist that long will be trivial in comparison to the improbability of fine tuning.

In other words, the observer self-selection effect is explanatorily vacuous. It does not suffice to show that only finely tuned worlds are observable. As Robin Collins has noted, what needs to be explained is not just intelligent life, but embodied, interactive, intelligent agents like ourselves. Appeal to an observer self-selection effect accomplishes nothing because there is no reason whatever to think that most observable worlds are worlds in which that kind of observer exists. Indeed, the opposite appears to be true: most observable worlds will be Boltzmann Brain worlds.

It’s amazing that so metaphysical a speculation as the World Ensemble hypothesis should be susceptible to refutation. One might think that there is just a standoff between so speculative a hypothesis and the hypothesis of design. But Penrose’s argument seems to show otherwise. The World Ensemble hypothesis fails as an explanation of the fine tuning of the universe for interactive agents like ourselves.