  "Rational bubble", as introduced by the famous paper on money by Samuelson
(1958), means speculation backed by nothing. The large subsequent rational
bubble literature has identified attaching bubbles to dividend-paying assets in
a natural way as an important but challenging question. Miao and Wang (2018)
claim to "provide a theory of rational stock price bubbles". Contrary to their
claim, the present comment proves the nonexistence of rational bubbles in the
model of Miao and Wang (2018). We also clarify the precise mathematical
definition and the economic meaning of "rational bubble" in an accessible way
to the general audience.
