This same expression for the GUP has been obtained in numerous ways, ranging in sophis- tication from our very naive Newtonian approach to several versions of string theory; 27 it would thus appear to be a rather general result of combining quantum theory with gravity and may indeed be correct. 28 Almost needless to say the GUP should be considered a rough estimate for position uncertainty, with the coefficient of Δ p/$_{ℏ}$ in the second term only being of order of the square of the Planck length, and the whole expression being valid in order of magnitude as we approach the Planck scale from above. For example, we could just as well have factors of 2 π or 10 or 1 /α = 137 etc . multiplying l 2 P in Eq.(16). Moreover we add the uncertainties due to the standard UP and the gravitational interaction linearly; we could equally well have taken the root mean square; this makes little difference in our conclusions, as the reader may verify. From the GUP in Eq.(16) we see that the position uncertainty of a particle has a minimum at $_{ℏ}$/ Δ p = l$_{P}$ and is about as shown in Fig.2. This minimum position uncertainty corresponds to a photon of wavelength about l$_{P}$ and energy E$_{P}$ . Since we cannot measure a particle position more accurately than the Planck length, the above result suggests that from an operational perspective the Planck length may represent a minimum physically meaningful distance. As such one may plausibly question whether theories based on arbitrarily short distances, such as string theory, really make sense from an operational point of view; spacetime at the Planck scale and below might not be a useful concept. 3,29 The next argument uses a thought experiment that is particularly simple conceptu- ally, and has the virtue that length is defined via light travel time, which is the actual present practice: 30 the definition of a meter is “the distance traveled by light in free space in 1/299,792,458 of a second.” Fig.3 shows the experimental arrangement, which we refer to by the generic name light IV. LIGHT RANGING Δ x$_{min}$ ≈ 2 l$_{p}$ (17) 9