It will be argued that compactified spacetime dimensions may lead to potentially observable effects through quantum fluctuations of the lightcone. This talk will begin with a summary of an approach to quantum gravity in which these lightcone fluctuations may be calculated explicitly. It will then be argued that small compactified dimensions can lead to lightcone fluctuations which increase as the compactification scale decreases. This phenomenon is closely related to the Casimir effect. The observational manifestation of the lightcone fluctuations is the broadening of spectral lines from distant sources. Thus the observed widths of spectral lines from quasars lead to lower bounds on compactification length scales.