What, Then, Is Time?

When asked to contribute an essay on some aspect of timekeeping, I had a flashback to the first day of my senior year in high school in Princeton, New Jersey. In AP English, we were assigned to write a single paragraph on something, anything in motion. I made the somewhat curious—and possibly prescient—choice of writing about time. I puzzled over how time moves (what exactly is it that is doing the moving? why does it move in only one direction?) and concluded that, in some sense, time is the ultimate motion, relentless and steady. My English teacher, Mr. Allegretti, said the paragraph was, well, odd, but being a rather odd teacher himself, he found it appealing, and he then even encouraged me to consider expanding it into a longer essay for extra credit.

Well, here I am 37 years later doing just that, and although experimental work toward developing “atomic clocks of the future” has been the focus of my career over the past 30 years, you’ll be happy to know, Mr. Allegretti, that I have spent much of that time writing articles, reports, recommendations, nominations, and now, a personal essay.

I know what it is, but I do not know

From my admittedly biased perspective, time appears to be the most bizarre of our basic units. As St. Augustine remarked, “[w]hat then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know.” Indeed, philosophically, time is a slippery creature. Even Einstein concluded, after much thought, that “[t]ime is what clocks measure.”

Whatever time is, for reasons over which I still puzzle, we can measure the passage of time (insofar as time is the accumulation of identical, repeatable events) nearly a million times more precisely than any other physical quantity. In fact, there is a push to redefine as many of our foundational measurement units as possible in terms of time, or its inverse, frequency, to take advantage of our advanced time-measuring capabilities. This explains the mantra intoned in labs around the world: Try to convert whatever quantity you are trying to measure into a frequency.

The reason I believe that the unit of time—the second—is different from its measurement brethren is that, more than the others, time is a human construct. When I see a room full of scientists doing clock comparisons, it can appear as if they are measuring nothing at all—where is this time they are measuring, and why is it represented by electric pulses?

(To be fair, in the lab we often measure frequency, which allows us to make the intangible appear more tangible, even if it is, at its heart, still an illusion.)

I don’t know if animals have an awareness of time beyond their circadian rhythms, but certainly, they sense mass, light, distance and temperature. We humans, however, have found marking the passage of time to be so useful that most of us structure our lives around our timepieces. To create time signals, we turn to repetitive natural processes: the swing of a pendulum, Earth’s orbit around the Sun or Earth’s rotation on its axis. In fact, it was not so long ago that the second was defined “astronomically,” that is, relative to the length of the solar day or even the solar year.

What is time?

The time of modern times

In more recent times we have exploited the precise oscillation rate of an electron between specific energy levels of an atom. Using atoms as the ultimate pendulum has given us a ticking rate that can be reproduced in principle anywhere in the universe with a precision that is virtually unimaginable on human scales. For instance, the atomic clocks that we use as time standards today do so with such a steadiness that it would not be off by even one second in 300 million years.

(For reference, the average quartz watch might gain or lose about a second every month.)

This year we are celebrating the 50th anniversary of the definition of the standard second as 9,192,631,770 oscillations between the lowest-energy states of atomic cesium. Perhaps coincidentally, in August 2017, the Institute of Electrical and Electronics Engineers (IEEE) installed a plaque in Washington, D.C., and will soon install a copy of that plaque on NIST’s Gaithersburg campus, that commemorates the fact that NIST invented the atomic clock in 1948.

Our next-generation atomic clocks will use visible laser light, which oscillates at approximately 10¹⁵ Hz or 1 million billion times a second. These “optical atomic clocks” now achieve a precision to the 18th decimal place (and soon … the 19th!). At this level, the clocks become very sensitive to small changes in gravity. According to Einstein’s general theory of relativity, clocks closer to objects with a lot of mass, such as Earth, run slower, and these effects can be measured by comparing the ticking rates of clocks located at different altitudes. An optical clock built experiment at  NIST in 2010 succeeded in distinguishing a difference in the speed at which time passes in two side-by-side clocks varying in height by just 33 centimeters. And recent advances could, in principle, show a difference in ticking rates between clocks separated by only 2 centimeters in height!

In fact, daily fluctuations in the height of Earth’s surface due to volcanism, tectonic shifts, and other factors make it hard to differentiate at a level much finer than 1 centimeter. Because even that small fluctuation would cause these very precise clocks to be off, one day soon we may need to locate our best clocks in space. The idea of an atomic clock circling the globe beaming precise time down to us would be a nice callback to the days when time was defined by the rotation of Earth itself.

Time of the future

Those of us in the metrology world are already preparing a roadmap for a redefinition of the second in terms of an optical atomic transition, roughly a decade from now.

As these clocks achieve ever higher performance, they offer new possibilities. One day, optical clocks may be used for deep-space navigation, ultra-high bandwidth communications, finding oil and mineral deposits, detecting (as hinted at above) and studying changes in the Earth’s surface that could indicate when and where earthquakes or volcanic eruptions are likely to occur, or even a greatly enhanced definition of sea level across our planet.

And because we can measure time so much more accurately than other physical quantities, time offers a unique microscope with which to examine the mysteries of the universe. For example, we foresee networks of future clocks on Earth or in space that will search for signs of gravitational waves, dark matter or flaws in our most fundamental physical theories as we attempt to discover the “new physics” that most scientists believe awaits us beyond the standard model.

These possibilities make it seem, in some sense, that maybe I wasn’t too far off in thinking that time might be the ultimate motion. And as this essay circles back on itself (a feat that time is apparently not permitted to perform), I would like to finish by offering it to my high school English teacher:

Mr. Allegretti, here you are, and thanks for the extension!