It's About Time

Jonathan Strickland

Orin Sealy/The Denver Post via Getty Images

In this week's episode of Fw:Thinking, we tackle the topic of time. What is it? How is it related to relativity? Why is it so hard to synchronize clocks on different moving bodies? And what's up with Daylight Saving Time?

It turns out that time is, as a certain Doctor would say, wibbly-wobbly. Our experience of time is completely subjective. We lose track of time when we focus on something interesting. Time stretches out forever when we're forced to attend to something we hate doing. And then there's the relationship between time, mass and speed.

Think about this: To you, a second is a second. It lasts as long as a second is supposed to last. This is true if you are standing on Earth, zipping around the solar system on a spaceship or setting up camp on an alien planet hundreds of light-years away. When you glance at the second hand on your wristwatch, assuming the watch is in good working order, the second hand will always seem to tick by at the same rate.

But to an independent observer on Earth, let's call him Bob, your watch may behave very strangely. If you and Bob are standing next to one another on terra firma, everything is dandy - as far as Bob can tell, the watch hand moves as it should. But imagine you're on a spaceship moving at near the speed of light. If Bob were somehow able to see your watch from his vantage point back on Earth, it would appear to him that the second hand had slowed down significantly. But to you, the near-light-speed traveler, the watch would seem to behave as normal -- each second would still take just a second to pass. This is all according to Einstein's theory of special relativity.

By the way, you'd have a similar experience if you were to look at Bob's watch back on Earth while you were in the spaceship. Einstein called the whole phenomenon time dilation. To you, Bob would seem to be aging more slowly than you are -- his watch hands would appear to be dragging along. But Bob would say the same thing about you. And if you and Bob met up on Earth after your joyride around the solar system, you'd see that while time seemed to pass normally for each of you individually, Bob actually aged more than you did in that interval. In other words, you traveled into the future - or at least Bob's future. Wacky, huh?

It gets so much wackier. According to Einstein's theory of general relativity, mass also affects time. Essentially, the closer you are to a massive object -- like a planet -- the more slowly time appears to pass for you according to an independent observer. So if you were further out from a very massive planet, you'd have a clock with hands that move so fast they make the wind blow, compared to a clock on the surface of the planet. All of this depends on someone seeing this independently, of course. To you, time would be the same as it ever was.

With all this in mind, if Bob were on Earth and you were in a starship way out in space but otherwise stationary -- something that's really not possible, but let's ignore it for this hypothetical situation -- Bob would think your watch was going bonkers. If you could see Bob's watch, it would seem to be crawling in comparison. It all depends upon your frame of reference -- it's all relative.

This is why synchronizing time among different moving bodies in space is such a challenge -- time itself seems to pass at a different rate depending upon your mass and speed. Satellites orbiting the Earth have this problem. At launch, their clocks match our Earth timepieces precisely. But after spinning around in orbit for a while, the clocks begin to fall out of synch.

That's because of special and general relativity -- the satellites are further out from Earth than terrestrial clocks, which means their clocks tick time faster than Earth clocks. But the satellites are also moving faster than any given reference point on the surface of the planet, which makes clocks aboard satellites tick off time more slowly than timepieces on the planet.

In the end, general relativity wins out. A satellite's speed makes the atomic clock on board tick by about 7 microseconds slower per day than a terrestrial clock. But because the satellite is further out from Earth's mass than a clock on land, the general relativistic effect means the clock on board ticks about 45 microseconds faster than Earth's clocks. Combine the two effects and you come away with atomic clocks that have a surplus of 38 microseconds per day.

You might wonder why that's a big deal. But if we didn't correct for this time dilation, we'd be in a lot of trouble. Some of those satellites are part of global positioning systems. The satellites beam down information to Earth. That information includes the satellite's position along with a timestamp. A GPS receiver -- also known as your navigation system -- picks up signals from several satellites. By calculating how long it took the signal to go from the satellites, whose positions are known, to get to the receiver, the receiver can calculate where on the surface of the Earth it is.

If we didn't correct for time dilation, these timestamps wouldn't be accurate and the navigation system would give you the wrong information. At first, the errors might be relatively tiny. But as the time decay continues, the errors would grow and your navigation system would become useless.

As for Daylight Saving Time, ultimately it was a fuel-conservation effort. Back in World War I, most of the world was lit by coal, an important resource during wartime. To help conserve coal, countries like Germany and the United Kingdom opted during certain parts of the year to shift the clocks back an hour. This would mean people would sleep through more of the darker parts of the day and be awake during the brighter parts, burning less fuel in the process.

I'm really not sure what I find more amazing -- the relativistic nature of time or the fact that many of us still observe Daylight Saving Time. Time for a nap.