String theory has to be formulated in nine space dimensions or it is not a consistent mathematical theory. There doesn't seem to be a simple way to explain "Why nine?"
Which, rather unexpectedly, is followed immediately by this seemingly simple explanation:
What happens is that if theories to describe nature and to include gravity in the description are formulated in $d$ space dimensions, they lead to results that include terms that are infinite, but the terms that are infinite are multiplied by a factor $(d-9)$, and drop out only for a factor $d=9$.
This leads me to think
- that perhaps this explanation is not as simple as I perceive it to be, or
- that it perhaps misses out on subtleties and therefore perhaps is not quite correct, or
- it's not really considered an explanation by Kane because it doesn't explain why some terms are multiplied by $(d-9)$.
My guess is option 3, but I'd like to be sure. So, my main question is: Is the quoted "explanation" fully correct?