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Neither the burning of a candle, nor the breaking of a glass: Most of the processes in our world are irreversible. They cannot be undone nor reverted. Why? The reason for this is the nature of time and especially of entropy: Because order always aspires to disorder, time cannot run backwards.

Since Albert Einstein's Relativity Theory we know, that time is not a constant: It changes over the influence of gravity and high speed. But is time reversible? In Science-Fiction movies, time travelling is a popular motivation for action, but physics forbids the roll back of time.

Why this is the case and why thermodynamics and entropy have to do with it, I would like to show you with my upcoming articles about this topic, from which this is the first part.

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Part 1: The size of Time

How we capture time

For all of us, the year 2017 started with a day lasting one second longer than usually - this additional leap second was necessary for world time, which is represented by precise clocks, to bring it into synchronization with earth rotation thus with the change of night and day. The earth is not rotating in a consistent way, because shifting of masses have influence to the rotation speed. Further, day length is increasing constantly, because tidal friction slows down rotation.

While natural daily rhythm is crucial for our time experience, some people's work consists of building more and more accurate clocks. In a pendulum clock the insight is used, that the period of oscillation of a pendulum depends on its length, but not on its amplitude, making division of time into seconds become reality.

References for time measurement

As a unit, seconds were based on the periodic change of day and night and were defined as the 86,400 fraction of a mean solar day. In the year of 1967 it was internationally decided to not define the duration of a second from the day length, but rather through the oscillation of a microwave signal, which is generated by an atomic clock.

Since 1978 the time signal which is relevant for public life is generated by an atomic clock of the Phyiscal-Technical Federal Institute in Braunschweig Germany (PTB) and is sent out in the near of Frankfurt for radio-controlled clocks. But how can someone be sure about which periodic procedure to use, to show the "right" ticking away of time?

The time also goes by without us

Even though the hour glass may break, no more sunlight may fall in the dark, or even when the main spring may run out, making the watch hand stand still - time keeps going on. In the best case the clock shows this process. And because time only knows its own speed, like the heartbeat or the rhythm of tides, clocks do not contain the time. They only try to keep up with it - if they can.

That's how the author Dava Sobel writes about the entity of time in her book "Längengrad" which basically means longitude.

Time takes its path, regardless of somebody missing it or not. At first this is nothing unusual. Similar to space, which was present before we invented units of length. However, our experience of this two occurrences is different, because while space simply is present and contains no direction, time always moves forward into one single direction.

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If you like my article, feel free to read on:
A circular argument? The problem with clocks