As an optician, I am uncomfortable with the concept that photons sail through space on geodesics defined by the masses embedded therein. What is the problem with simple refraction, as explained by classical optics? As it turns out, there is absolutely no problem, according to Einstein. In his 1920 book "Relativity - The Special and General Theory" he claimed that gravitational lensing is due to the variable speed of the propagation of light through space. In other words, c must be variable based on location, else "gravitational" lensing cannot occur. The concept of an invariable speed of light in a vacuum is applicable only to the idealized case of Special Relativity. http://www.bartleby.com/173/ This variable c concept is foreign to many people who have not read and reviewed Einstein's later works, but does it have parallels in modern research? Here is a prime example: Fotini Markopoulou-Kalamara of the Perimeter Institute has postulated that GLAST might demonstrate the fine-scale structure of space, by observing wavelength-dependent time delays in the arrival of EM. She believes that higher-energy EM will interact more frequently with the space through which it propagates, and therefore be delayed, compared to low-energy EM. I happen to share her view and eagerly await the science from this instrument. If Einstein were alive and active today, he would be working at Perimeter. They've got his back. Well, GLAST has not yet launched. Is there presently any experimental data to support the concept that the speed of light in a vacuum may be variable and perhaps even frequency-dependent as our PI researcher proposes? As published in August of this year, the MAGIC consortium has observed an apparent delay of 4 minutes in the arrival time of very high energy gamma rays in a burst from LS I +61 303. The object is at redshift z = 0.034 meaning light travel time of 0.46 billion years and there was a delay of 4 minutes in the high-energy EM arrival time. That's a very tiny delay in a very long column of propagation, but what if it is real and repeatable? http://arxiv.org/abs/0708.2889 There may well be a mundane explanation for the observed delay, including unmodeled frequency-dependent evolution of the GRB. If, however, this observation is duplicated with MAGIC or with other instruments, AND (critically!!!!!) if the delay is shown to be proportional to the redshift of the source, then it is time to consider that Zwickey's "tired light" concept has legs. This has global implications for cosmology. I will not elaborate further at this time, except to refer interested readers to Einstein's Leyden address of 1920 and to his essay "On the Ether" of 1924. "On the Ether" is chapter 1 of Saunders and Brown's book "The Philosopy of Vacuum". Einstein believed that space is a local and dynamic player in the emergence of gravitational and inertial effects, and that embedded matter conditions the qualities of space to give rise to the effects we observe regarding the propagation of electromagnetic waves.