Time As A Reference And A Dimension

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Time, something that google best describes as “the indefinite continued progress of existence and events in the past, present, and future regarded as a whole”. And the scientist’s view of time? It’s even more wondrous, more amazing than any definition found. 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 wonder about that.

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Time is a Reference

Since antiquity, humans have understood the link between time and motion. If I walk from one end of the Moses Mabida stadium to the other, I know I’m walking faster if I take less time to get there or vice versa. Humans’ first efforts to measure and mark time came from motion on a much more magnificent scale: that of the Sun, Moon, planets, and stars moving in the sky. With the one observational tool available to them, the uncovered 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 and moon, 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!

Time is a Dimension

When Albert Einstein burst onto the scientific scene in 1905, he completely overturned our concept of time. n 1908, Hermann Minkowski presented a paper consolidating the role of time as the fourth dimension of spacetime, the basis for Einstein’s theories of special and general relativity. 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!

31 October 2020

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