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Are space and time the constant and unchanging building blocks of reality (or experience) that Newton (or Kant) thought they were? Experimentally it was discovered that the measured speed of light (C) is a constant no matter how fast the observer is traveling. Einstein realized that if this is indeed true, then our normal idea of the constancy of time and space must be re-evaluated. Because if speed is distance/time and the speed of light cannot change then something about distance and/or time must be changing, to explain the fact that one can never “catch up” to a ray of light (because to catch up to it would mean that its relative speed could change – but this is impossible based on observations). Additionally if nothing can ever catch up to or exceed the speed of light, then gravity thought of as an instantaneously acting force must also be re-evaluated. Gravity cannot be thought of as something that depends only on distance and mass, as Newton had suggested in his famous equation F_G = GmM/r². This is so in part because gravity as an event must be made consistent with the experimentally observed constancy of the speed of light and in part because the distance that Newton put into his equation is no longer the constant he assumed it to be. Moreover another consequence of the constancy of C is that masses also change in response to motion. One of the reasons we can’t ever exceed the speed of light is that as masses accelerate, they become more massive, causing resistance to further acceleration. As a result, one can approach the speed limit, but never achieve it. The consequence of all of Einstein’s ruminations is that we now think of C as constant and space and time as malleable.