but there are indirect possibilities. One involves the radiation predicted by Hawking to be emitted from black holes if they are small enough to have a “Hawking temperature” above that of the cosmic background radiation; in the final stages of black hole evaporation the Hawking radiation should have about the Planck energy. 11 Of course Hawking radiation has not yet been observed, although many theorists believe it must exist. Another possibility of interest is to analyze the radiation from very distant gamma ray bursters, which has been enroute for about 10 $^{10}$yr . 12 Speculations abound on the effect of Planck scale spacetime granularity on propagation of such radiation. 13 Finally we mention that in the earliest stages of cosmological inflation quantum gravity effects might have been large enough to leave an imprint via primordial gravitational radiation on the details of the cosmic microwave background radiation. 14 Lacking real experiments we use thought experiments (Gedankenexperiment) in this note. We give plausible heuristic arguments why the Planck length should be a sort of fundamental minimum - either a minimum physically meaningful length, or the length at which spacetime displays inescapable quantum properties i.e. the classical spacetime continuum concept loses validity. Specifically the six thought experiments involve: (1) viewing a particle with a microscope; (2) measuring a spatial distance with a light pulse; (3) squeezing a system into a very small volume; (4) observing the energy in a small volume; (5) measuring the energy density of the gravitational field; (6) determining the energy at which gravitational forces become comparable to electromagnetic forces. The analyses require a very minimal knowledge of quantum theory and some basic ideas of general relativity and black holes, which we will discuss in section II. Of course some background in elementary classical physics, including special relativity, is also assumed. We rely as little as possible on present theory, both because we desire mathematical and pedagogical simplicity, and because the general principles we use are most likely to survive the vagaries of theoretical fashion. We hope that the discussions are thereby made more accessible to physics undergraduates and nonspecialists. II. COMMENTS ON BASIC IDEAS OF QUANTUM THEORY AND GRAVITY In this section we review the small amount of quantum theory and gravitational theory needed in later sections. Essentially we attempt to compress some root ideas of quantum 3