We tend to think of time and space as completely separate entities. We move about in the three dimensions of space and all the while time marches inexorably forward. We also assume that space and time are the same to everyone, everywhere. A mile is a mile, and an hour is an hour.
Questioning these basic assumptions is where Einstein’s imagination really kicked in. He realized that in order for a fast-moving observer to measure the same speed for light as a stationary observer, notions of universally agreed-upon space and time go out the window. “There is no audible tick-tock everywhere in the world that could be considered as time,” said Einstein. Two people in relative motion will experience time differently.
From Leo’s perspective on the platform, Gail will experience a slowing of time. Her wristwatch will, to him, run slow. Not that there is anything wrong with Gail’s watch. It is time itself that slows down for her. In fact, any time-keeping device on the train will run slow, even a beating heart. That’s right — Gail will age more slowly than Leo.
Special relativity has passed every experimental test thrown at it since its publication in 1905
And don’t forget the lesson of Galileo: From her seat on the train, Gail can’t tell whether she is in motion or not. That means she is perfectly justified in saying that Leo has zoomed backward past her, and that it is his watch that’s running slow since he’s the one moving. If you insist that Gail is the one in motion, try the thought experiment again in Einstein’s construct, with Gail and Leo floating in empty, featureless space. Either of them can accurately assert that it was the other who drifted past. Gail now claims that Leo ages more slowly, and Leo swears the opposite. This situation — known as the twin paradox — can be resolved when one of the two parties reverses direction in order to reunite and conclusively compare ages.
To wrap your mind around the concept of time slowing down, imagine a specialized clock where a beam of light bounces between two mirrors, one suspended above the other. Each time the beam makes a round trip, the clock “ticks.” We give such a light clock to both Gail and Leo. From Leo’s vantage point on the station platform, Gail’s light beam isn’t tracing a purely up-and-down path. During each journey between the mirrors, the train moves forward a bit. So Leo sees Gail’s light beam tracing out a longer diagonal path to reach the next mirror — in other words, Gail’s clock ticks slower. (And again, Gail would see the same happening to Leo’s clock.)
The weirdness doesn’t end there. Leo will also see that the train, and everything moving along with it, contracts. To him, it becomes shorter. Don’t worry, Gail’s fine. It’s just that space isn’t the immutable, rigid structure that we assume. Unfortunately, there’s no simple way to wrap your mind around this one, but time slowing and length contraction are two sides of the same coin. In order for all observers to get the same answer for the light’s speed — remember, speed is simply distance divided by time — the two effects must coexist.
As outlandish as it seems that Gail’s clock runs slower, or that she and the train are compressed, special relativity has passed every experimental test thrown at it since its publication in 1905. It has become a pillar of physics. The behavior of high-speed particles — whether the result of physicists’ colliders or the sun’s nuclear furnace — only makes sense with special relativity.
It Gets Crazier
Nevertheless, the scope of special relativity was limited, hence the name special relativity — it worked only when objects move at constant speeds. Einstein wasn’t satisfied. He wanted a theory that encompassed all motion, whether the speed is constant or variable.
Just as special relativity was seeded by a simple thought (the light beam race), so too was general relativity. One day in 1907, the story goes, Einstein was working at his job at a patent office in Bern, Switzerland, when he imagined a person in free fall, as if a workman fell off a tall scaffold. The lightbulb went off. What if, while falling, he dropped an object — say, an apple?