Zero - Charles Seife [67]
This assumption challenged everything physicists had assumed about the motion of objects. If the raven were acting like a photon, then an observer on the train and the person standing still would have to agree on the value for the raven’s speed. That would mean that the two observers would disagree about when the raven meets the tree (Figure 50). Einstein realized that there is one way around this: the flow of time changes, depending on an observer’s speed. The clock on the train must tick more slowly than the stationary clock. Ten seconds for the observer on the ground might seem like only five seconds for somebody on the train. It’s the same thing for a person who zooms away at great speed. Every tick of his stopwatch takes more than a second from a stationary observer’s point of view. If an astronaut took a 20-year journey (according to his pocket watch) at nine-tenths of the speed of light, he would come back to Earth having aged 20 years, as expected. But everyone who stayed behind would have aged 46 years.
Figure 50: The raven’s constant speed means that time must be relative.
Not only does time change with speed, so do length and mass. As objects speed up, they get shorter and heavier. At nine-tenths of the speed of light, for instance, a yardstick would only be 0.44 yards long, and a one-pound bag of sugar would weigh nearly 2.3 pounds—from a stationary observer’s point of view. (Of course, this doesn’t mean that you would be able to bake more cookies with the same bag of sugar. From the bag’s point of view, its weight stays the same.)
This variability in the flow of time might be hard to believe, but it has been observed. When a subatomic particle travels very fast, it survives longer than expected before it decays, because its clock is slow. Also, a very precise clock has been observed to slow down ever so slightly when flown in an airplane at great speed. Einstein’s theory works. There was a potential problem though: zero.
When a spaceship approaches the speed of light, time slows down more and more and more. If the ship were to travel at the speed of light, every tick of the clock on board would equal infinite seconds on the ground. In less than a fraction of a second, billions and billions of years would pass; the universe would have already met its ultimate fate and burned itself out. For an astronaut aboard the spaceship, time stops. The flow of time is multiplied by zero.
Luckily, it is not so easy to stop time. As the spaceship goes ever faster, time slows down more and more, but at the same time, the spaceship’s mass gets greater and greater. It is like pushing a baby carriage where the baby grows and grows. Pretty soon you are pushing a sumo wrestler—not so easy. If you manage to push the carriage even faster, the baby becomes as massive as a car…and then a battleship…and then a planet…and then a star…and then a galaxy. As the baby gets more massive, your push has less and less effect. In the same way, you can take a spaceship and accelerate it, getting it closer and closer to the speed of light. But after a while, it gets too massive to push any longer. The spaceship—or for that matter any other object with mass—never quite reaches the speed of light. The speed of light is the ultimate speed limit; you cannot reach it, much less exceed it. Nature has defended itself from an unruly zero.
However, zero is too powerful even for nature. When Einstein extended the theory of relativity to include gravity, he did not suspect that his new equations