Space, Time and Motion: Course Preview

What is Time?

by Dr. Charles Liu

This essay was developed for Week 2 of the AMNH online course Space, Time and Motion, part of Seminars on Science, a program of online graduate-level professional development courses for K-12 educators. Explore more sample resources...

Tick. The inexorable passage of a quantity never reclaimable.

Tock. The precious moments flowing away that expose the limits of our existences.

Tick. That which flies when we’re dancing at a party, and crawls when we’re waiting for the bus.

Tock. Such is the dreamer’s view, the poet’s view, the commuter’s view of time.

And the scientist’s view of time? It’s even more wondrous, more amazing than any of these. Time tells us where we were, where we are, and where we’re going. Only when we have time do we then have velocity, momentum, acceleration, force, and motion. The beginning of time marks the beginning of the universe; and the end of time—well, we don’t know if there is an end to time. That’s just as well; it means we have plenty of time to ponder it all.

The Aztec calendar wheel is represented by 13 months of 20 days each, as determined by the movement of the Sun, Moon, and stars. ©Library of Congress

Time Is a Reference…

If I tell you my daughter was born on the second-floor maternity wing of Tucson Medical Center in Tucson, Arizona, you’ll know where in space that blessed event took place. More than likely, though, you’ll consider that information incomplete—until I add that she was born at 11:59 P.M. on July 7, 1994, according to the clock on the delivery room wall. That’s just one example of the importance of time in our lives. It serves as a link to everything in our world; no matter what happens, and where it happens, we can always relate it to anything else by the time at which it occurred.

Since antiquity, humans have understood the link between time and motion. If I walk from one end of the American Museum of Natural History to the other, I know I’m walking faster if I take less time to get there. Humanity’s first efforts to measure and mark time came from motion on a grander scale: that of the Sun, Moon, planets, and stars moving in the sky. With the one observational tool available to them, the unaided eye, our ancient ancestors watched the Sun travel across the sky from east to west each day. They also saw the Sun replaced each night by the stars, which moved in the same direction, all apparently rotating about a single point in the northern sky. By tracing their positions for hundreds, then thousands of days, these earliest astronomers found that the Sun and stars followed predictable paths that change in repeating cycles. When they also noticed that those cycles coincided with the seasons, they realized they could plan their lives around them. When certain patterns of stars appeared exactly at sunset or sunrise, they knew about how long daylight would last, how warm or how cold it would be, and how many more days it would stay that way. With this awareness of the flow of time, the ancients created agricultural societies and communities, which then grew into civilizations. Thus was born the calendar, and the idea of the year. The calendar’s continuing importance in today’s world bears witness to the early sky watchers’ work.

Thanks to modern timekeeping technology, knowledge of seasonal star patterns, once so necessary for human survival, has been relegated to the status of a useful hobby. You may not know, though, that humanity’s ability to keep accurate time hasn’t had a very long legacy. When Isaac Newton worked out his ideas on velocity, momentum, and force—all of which depended on time as a key reference—the first pendulum clocks had only recently been constructed, and were able to keep time to an accuracy of about a minute per day. Portable clocks and watches were 10 times less accurate. Until the middle of the 18th century, when British engineer John Harrison designed a marine chronometer that could keep time with a precision of better than one second per day, clocks weren’t useful at all for navigational purposes. Today, we do much better with atomic clocks; by measuring the resonance frequencies—you can sort of think of them as atomic “vibrations” caused by the absorption and emission of light—of carefully processed cesium, rubidium, or other atoms, we can now measure the passage of time to an accuracy of 0.00000003 second per year!

…But Time Isn’t a Very Good Reference

The difficulty we’ve had historically in accurately measuring time underscores a great irony: for all its importance as a reference in our lives and history, a human being’s perception of time is wildly subjective and shakily inconsistent. Psychologists speak of “phenomenological time,” which passes quickly or slowly based on what we experience, how we’re thinking and feeling as we live through the experience, and what we remember after the experience is over. (So it’s phenomenological time that flies when you’re having fun, and drags when you’re standing in line at the Department of Motor Vehicles.) Other social scientists explain that time is a resource that we all allocate, effectively or not, to the things we care about and want to achieve. Still others interpret time not as a linear series of events (“monochronic”), but as numerous streams of events flowing in parallel (“polychronic”). That’s why, for example, a “New York minute” may go by faster than any other minute—time-wise, you get more for your money. Finally, there are those who synthesize all these ideas and come up with such constructs as “cyclical time,” “helical time,” and many more chronological “topologies.”

Our human experiences imply that the passage of time is an imperfect way to anchor our lives and calibrate our history. On the other hand, the astronomical and physical bases of time measurement—Earth spinning on its axis, the Moon’s orbit, Earth’s orbit around theSun—stay, for any practical human need, constant and unwavering. So to put all of our individual timepieces onto a common reference, we’ve learned to rely on a more or less identical clock, defined by world convention and based on both macroscopic and microscopic motions: Earth’s spins and orbits, subdivided using atomic resonances.

The notion of fixed, or absolute, time is therefore a problem of our own making. The basic challenge of understanding time in the universe stems from humanity’s social need to synchronize the efforts and experiences of individuals, in order to produce results that no single individual can produce alone. This fundamental driving force of society—to be on time!—had been already ingrained in the human psyche for millennia when Albert Einstein proposed his new model for time in 1905. With that in mind, if we find ourselves struggling with the idea that time really isn’t absolute at all, but flexible, malleable and deformable, maybe we shouldn’t be too hard on ourselves.

NIST-F1 is a cesium fountain atomic clock in Boulder, Colorado. Along with other international atomic clocks, NIST-F1 helps define the official world time standard. ©NIST

Time Is a Dimension

When Albert Einstein burst onto the scientific scene in 1905, he completely overturned our concept of time. At the end of the 19th century, one of the most important goals of research in the physical sciences was to understand the interferometric measurements of Michelson and Morley, which we discussed last week. This experiment showed that the motion of light doesn’t follow Newton’s laws of motion—the firmament of all physical sciences of the era. What was wrong with the existing theory that made light behave differently than expected?

One possibility, that objects change their length depending on how they are moving, challenged the very meaning of velocity, or the rate of motion through space. Since velocity is measured as a distance interval divided by a time interval (for example, a mile per hour or a meter per second), the nature of time became part of the discussion. If length could change with motion, could time do the same?

In his first landmark paper on relativity, Einstein explained that any measurement of length or time depends on the motion of both the “measurer” and the “measuree.” Well, if length changes when you move, and time changes when you move, then it’s natural to consider length and time to be the same sort of physical construct. That means that time is a dimension, the way that length, width, and height are dimensions. Space and time are inextricably linked—they’re both relative, not absolute—and when we think about space, we must also think about time. We aren’t three-dimensional creatures that occupy space; we’re four-dimensional creatures that occupy space and time together. Time is the fourth dimension!

Einstein cemented the idea of time as a dimension when he developed the General Theory of Relativity, which we’ll discuss in next week’s essays. Even so, nearly a century after its scientific confirmation, the concept feels foreign. (“The fourth dimension”—every time I say it, I half expect the Twilight Zone theme to start up…) For one thing, since we can change speeds as we travel through space, if time is a dimension, then we should be able to change speeds as we move through time too, right? Well, we can—and as strange as that may seem, it’s the key point to remember as we think about time, motion, and the meaning of relativity.

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