  One would not think that thought experiments could matter to nature, for they
are a humble human device. Yet quantum mechanics very naturally frames thought
experiments (as distinct from precisely defining what exists). They exemplify
the informing powers of radiation. Though based on wave functions that have
time symmetry, these tableaux inevitably tell of irreversible behavior by
nature. The paper sketches how John von Neumann's measurement theory fits into
this and retells N. David Mermin's baseball story.
