Our Mathematical Universe: My Quest for the Ultimate Nature of Reality

by Max Tegmark

Before you get too worried about the ontological status of your body, here’s a simple test you can do to determine whether you’re a Boltzmann brain. Pause. Introspect. Examine your memories. In the Boltzmann-brain scenario, it’s indeed more likely that any particular memories that you have are false rather than real. However, for every set of false memories that could pass as having been real, very similar sets of memories with a few random crazy bits tossed in (say, you remembering Beethoven’s Fifth Symphony sounding like pure static) are vastly more likely, because there are vastly more disembodied brains with such memories. This is because there are vastly more ways of getting things almost right than getting them exactly right. Which means that if you really are a Boltzmann brain who at first thinks you’re not, then when you start jogging your memory, you should discover more and more utter absurdities. And after that, you’ll feel your reality dissolving, as your constituent particles drift back into the cold and almost empty space from which they came. In other words, if you’re still reading this, you’re not a Boltzmann brain. This means that something is fundamentally wrong with what we assumed about the future of our Universe, and that there’s a lesson to be learned.