I wonder if rainbow Dash got so fast that he could go as fast as the speed of light. If he did , his mass would become infinite. Because according Einsteins M=MC 2 mass and energy have the same physical entity. So as rainbow Dash was going faster and faster he would gain more mass. There is another reason. Rainbow Dash shrinking down to the tiny size. This works with the extreme Doppler effect. So if the light waves or photons were crouded together it would turn red. The faster it would go until it would go so fast that even the spectrum would be completely be shifted out. So you would not see Rainbow Dash's rainbow, of course. Reason three. Time dilation. When objects move at rapid speeds time goes slower. So if rainbow Dash was flying at 90% of the speed of light it would take 20 minutes for her and for the pony watching it , it would be 10 minutes. Reason four. According to the laws of physics . If you traveled at the speed of light you would go back in time ,related to reason no 1. There is a lot of magic and science fiction in Equestria So maybe Rainbow Dash could. Please leave a comment about this subject. Thanks!