On the Origin of Time: Stephen Hawking's Final Theory

by Thomas Hertog

Einstein understood that if light is always observed to travel at the same speed, then observers moving relative to each other must have different notions of distance and time. After all, speed is a measure of distance traveled divided by the duration of the journey. According to Einstein, instead of a cosmic Big Ben we all carry our own clock, and though all our clocks are equally accurate, when we move relative to one another, they will tick at slightly different rates, measuring different amounts of time between the same two events. The same is true for distance; one observer’s yardstick can differ slightly from another’s. There just aren’t universally valid measures for duration and distance. This was the crux of Einstein’s 1905 theory of special relativity. The term “relativity” here refers precisely to this revolutionary idea that notions of space, time, and simultaneity aren’t objective facts but always tied to the view of a given observer. One may wonder where the differences in distance measured by one observer relative to another go. Are they simply gone? Not quite. They are transformed into an amount of time. You see, motion through space gets mixed with motion through time in Einstein’s relativistic universe. When I look at my sister’s parked sports car I find that all its motion is through time. But if she speeds away, a tiny bit of its motion through time is channeled into motion through space. My sister’s clock will run a tad more slowly than mine. And while...