Hyperreal worlds

In a number of papers, I argued against using hyperreal-valued probabilities to account for zero probability but nonetheless possible events, such as a randomly thrown dart hitting the exact center of the target, by assigning such phenomena non-zero but infinitesimal probability.

But it is possible to accept all my critiques, and nonetheless hold that there is room for hyperreal-valued probabilities.

Typically, physicists model our world’s physics with a calculus centered on real numbers. Masses are real numbers, wavefunctions are functions whose values are pairs of real numbers (or, equivalently, complex numbers), and so on. This naturally fits with real-valued probabilities, for instance via the Born rule in quantum mechanics.

However, even if our world is modeled by the real numbers, perhaps there could be a world with similar laws to ours, but where hyperreal numbers figure in place of our world’s real ones. If so, then in such a world, we would expect to have hyperreal-valued probabilities. We could, then, say that whether chances are rightly modeled with real-valued probabilities or hyperreal-valued probabilities depends on the laws of nature.

This doesn’t solve the problems with zero probability issues. In fact, in such a world we would expect to have the same issues coming up for the hyperreal probabilities. In that world, a dartboard would have a richer space of possible places for the dart to hit—a space with a coordinate system defined by pairs of hyperreal numbers instead of pairs of real numbers—and the probability of hitting a single point could still be zero. And in our world, the probabilities would still be real numbers. And my published critiques of hyperreal probabilities would not apply, because they are meant to be critiques of the application of such probabilities to our world.

There is, however, a potential critique available, on the basis of causal finitism. Plausibly, our world has an infinite number of future days, but a finite past, so on any day, our world’s past has only finitely many days. The set of future days in our world can be modeled with the natural numbers. An analogous hyperreal-based world would have a set of future days that would be modeled with the hypernatural numbers. But because the hypernatural numbers include infinite numbers, that world would have days that were preceded by infinitely (though hyperfinitely) many days. And that seems to violate causal finitism. More generally, any hyperreal world will either have a future that includes a finite number of days or one that includes days that have infinitely many days prior to them.

If causal finitism is correct, then “hyperreal worlds”, ones similar to ours but where hyperreals figure where in our our world we have reals, must have a finite future, unlike our world. This is an interesting result, that for worlds like ours, having real numbers as coordinates is required in order to have both causal finitism true and yet an infinite future.