One of Einstein's "thought experiments" (I have never liked that term) was to envision a spacecraft which could somehow exceed the speed of light. If an observer were to peer through a hyper-powerful telescope and see the craft at its origination point, then look up and see the very same craft sitting on the ground in front of him (because, during its trip, it had outpaced the image the observer saw in the telescope), it would violate the law that says that matter cannot exist in two places at the same time. Therefore, faster-than-light travel is impossible.
I think that's a dumb example, given that time isn't a fixed flow but an elastic one depending on your distance from a gravity well. If, for example, someone were to be caught inside the event horizon of a black hole, the person's experienced flow of time would remain constant, but any observation of the person from soneone outside the event horizon would show that person slowing down further and further as he or she got closer to the center of gravity within the black hole. So, given that New Krypton has some kind of light-speed or hyperspace drive capability (or they couldn't have come to Earth), we can - as Ann suggested - pretend that Einstein didn't know what he was talking about in this instance.
Besides, who wants to read about a thirty-year-old Clark with a ninety-year-old Lois?
