### What IS Time, Exactly?

We all seem to know, but ask yourself: can you define it?  I couldn’t even though it is something I have dealt with in many capacities over the years.  In physics, there is a multitude of equations and concepts based on time.  T=D/R or time is the distance traveled divided by the rate of travel is one we learn early and probably still use well past school.  But did you ever notice that getting the rate actually involves a time element as well?  We commonly use MPH or some variant, but that per hour is about time, isn’t it?

### Time Isn’t What It Used to Be

Our daily lives are flooded with clocks of all sorts, and we have become highly dependent on knowing exactly what time it is.  To meet a friend, pick up our kids from school, go to the doctor’s appointment or get our haircut, and countless other daily activities.

We might want to stake a claim on knowing exactly what time it is, but it that realistic?  If we are five minutes late to meet for coffee, fashionably late for a dinner party, or arrive between 4 and 9 for an open house, is knowing the exact time important?  If you are sending a space probe to Saturn and have to fire the rockets at a certain time and for a precise duration, is being fashionably late ok?

Not too many years ago, our choices of clocks was much, much smaller.  We had tall clocks that ran on pendulums and weights, windup alarm clocks, and wrist watches that also required winding, or for the more affluent self-winding ones.  Some kept time very well, but there was also a bit of pleasure in dealing with their individual idiosyncrasy each might demonstrate by running fast or slow and adjusting them, like picking up an old friend who has fallen.  Then came digital clocks that instead of a falling weight or an unwinding mainspring counted something reliable and converted that count into seconds and hence hours and minutes.  Quartz wristwatches counted the vibrations of a quartz crystal, something very predictable.  Some counted the frequency of AC power, usually 6o cycles per second, but not all that predictable nor high enough in count.  With the advent of timer integrated circuits, engineers had a tool that could  be used to generate a wide range of countable frequencies, and the circuits have become sophisticated and expansive to drive clock displays as well, and more.  So we see them in appliances, phones, computers, signs, nearly everywhere.  Any doubts?  What did you do when daylight savings time ended earlier this month?

Jump back in time (pardon me for saying this as we will later explore) a few hundred years, and how we used time or even considered it was important.

### Local City Time

Before clocks were common, time was roughly a sense of solar time.  Morning, mid-day and evening were probably accurate enough for most purposes.  Sun dials may have helped, but they were not that accurate.  Hour glasses could mark time intervals but those also were highly variable and still didn’t give you a specific time.  But along came clocks, and they started appearing on town buildings all over.  However, there was no way to synchronize time among various towns.  So the time of day or night pretty much depended on what town you were in and what their clock showed.

That wasn’t really a serious problem.  People did not travel great distances and could go no faster than a galloping horse.  They didn’t keep schedules or appointments like we are accustomed to doing.  Time might be measured by the passing seasons or progression as we age, if you were lucky enough to do so.

### Then People Began to Take Sea Voyages

Time wasn’t important to schedule your departure or arrival.  But it was critical to use time to know where you were.  Latitude, how far north or south you were on the earth, could be determined by measuring the angle of the sun at its highest point or culmination and using that as a reference to known latitudes on land.  However, longitude, or how far east or west you were, required using a standard time reference to determine when to take a measurement and compare it to the measurement taken at a fixed location. What was missing was an accurate clock that would work at sea so the measurement could be taken at a precise time.  This was so important that in 1714 the British government offered a prize of £10,000 to £20,000 for such a time piece.  Rising to the challenge was John Harrison, a Yorkshire carpenter who between 1730 and 1761 created four versions of a chronometer, each improving on the previous one.

The idea of a standard time originated in Britain and in 1847 adopted use of Greenwich Mean Time, or GMT.  This meant that every railroad schedule in the country would use GMT for uniformity. Other systems and institutions soon followed suit.

In the United States, the problem was more complex because of the size of the country.  Obviously, the sun rose many hours earlier in New York than it did in California, but still, the railroads wanted to publish uniform time schedules for their trains.  A Canadian civil and railway engineer, Sandford Fleming, led the initiative to create time zones, and by 188, the US and Canada adopted them for use in their schedules.  Still, it was not a common practice among ordinary people to even think about them.  But gradually they became part of the times, so to speak.

### Time Is not What We Thought

Along comes Albert Einstein who set time back on its heels.  It turns out that, although synchronizing one or more clocks in our day to day lives seems simple and doable, in general, that is not the case.  Join me in this thought experiment to illustrate.

Suppose you and a friend agree to travel in separate space ships and synchronize your onboard clocks.  To do this, each of you will flash a light towards the other at one second intervals.  You travel away from each other in opposite directions, close to the speed of light.  You dutifully flash your light once a second and expect to see your fellow traveler’s light correspond to yours, but what you will see instead is that his “clock” has slowed down, that the pulses arrive more slowly than what you generate.  Your clock is fine, his is slow.  However, on his space ship, his is fine but yours is slow.  How can that be?  Relativity is the answer, special relativity in particular.  Because the distance between you is increasing rapidly enough to be noticed, light has farther to travel between the two ships but not between either of you and your own clock.

Relative velocities are not the only type of time dilation, as this is called.  Gravity produces a similar effect.  Relativity says that gravity curves the space around mass, the larger the mass the greater the curvature.  Think of a sheet of cloth stretched tight and then drop a bowling ball in the center.  The weight (mass) of the ball causes it to pull the cloth downward – a gravitational well. Now imagine what a black hole would do to the fabric of space near it.  So here is another thought experiment.  You are somewhat near a black hole, and your friend is much close, near the event horizon (the point at which light is pulled into the black hole and cannot escape; you can’t ever see that light, it has disappeared “over the horizon”).  You each flash your lights toward the other at one second intervals.  But his clock seems to run slower because the light from his beacon has a longer path to travel, the curvature of space between you two.  Likewise, your clock will seem slower for the same reason.  Each of your own clocks seem to be normal though.

This inability to synchronize clocks outside a common frame of reference, as physicists like to describe it, seems to not be all that relevant, perhaps – we are unlikely to be in separate space ships traveling near the speed of light.  But we don’t need such thought experiments.  Something use all probably use several times a day is affected by these slower ticking clocks that do need to be synchronized.  GPS or Global Positioning System is used in our smartphones and navigation systems in our cars, and by all the airlines, commercial sea craft and most private craft as well.  The twenty four satellites that make up the GPS system, orbiting high above the earth and moving pretty quickly in orbit have very precise clocks that appear to an observer on the ground to run just a bit slower than a reference clock also on the ground.  GPS uses some very sophisticated instruments to adjust for this subtle difference and keep all of the satellite clocks in sync with a corrected time.  Your receiver does some amazing calculations as well, but that too is another discussion.

### Atomic Clocks

Atomic clocks use the change of energy states of atoms to count up small time intervals and convert them into seconds, minutes, hours and so on.  While various methods were developed over the past century, since 1967 the standard has been to use caesium-133 atoms as an oscillator.  As the atom absorbs energy from lasers or other means, it jumps to a higher state, then drops back to its normal state.  It does this a a precise frequency.  Counting those jumps and knowing how many times per second it will do this can yield a very precise “ticking.” Each tick is  0.0000000000000001 of a second. Just how accurate?  Advanced design atomic clocks of this sort have demonstrated an accuracy of better than one second in 300 million years.  So much for the ritual of adjusting your antique clock.

Atomic clocks not only keep the standard times – think US Naval Observatory for example – but they are critical components for how GPS can work.

Can’t get any better than that, right?  Uh, not so fast.  Or so slow as the time accuracy goes.  There is a new clock in town that uses strontium atoms and has an accuracy of about one second in 5 billion years or about the age of the earth.  It is so sensitive that if you put one on the floor and one on the ceiling it could determine the one on the ceiling is a bit slow – farther away from the center of earth and a tiny, tiny  less pull of gravity.

Don’t look for one of these in your smartphone or on your wrist anytime soon.  In fact, as clocks go, they don’t offer us much advantage over what we have or could have.  These new clocks will probably see use in applications such as probing the earth’s crust detecting changes an inch or less, or they might be used on satellites to detect gravitational changes from distant objects like exploding stars.  Oh something we haven’t yet thought of using them for.

Seems like all we need is a little more time to figure all of this out.

Wait, what is time again?