SofSW: The Basics (1/2)

We’re waiting for the train to leave the station any minute now. I glance at my watch and notice that we’re three minutes behind schedule. I know that because the display on the platform is clearly visible to me and it is showing the details of my train. Three minutes isn’t much, but when you have a complex network of trains and other forms of traffic, air planes, trams, busses, and in this network you have individual people with their dependencies, it all hangs together. Some of the dependencies are critical and some can be adjusted. There are trains that will wait if a connecting train is arriving slightly late. Some things, like airplane schedules are however not under the control of the railway company. In order to keep everything in sync, everybody wants to stay on schedule. Time passes relentlessly and the train is still standing on the platform. Five minutes late. I see no reason for the delay. Most importantly, there is nothing I can do about it. So, I leave it.

Time is one of the basic properties of the universe we live in. Time is one of the most important concepts there is in physics. You cannot explain time with other means. It just exists and affects the world. Its unit in the International System of Units, Le Système international d’unités, short ‘SI’, is second, abbreviated with the letter ‘s’.

In the beginning of time, or more specifically, in the beginning of time measurement, people used to be concerned about the seasons. It made all the difference for survival whether you sowed in the spring or in the fall. In addition to seasons there were other interesting periods. The period of the moon waxing and waning gave us the month, and the sunset and sunrise define night and day. As science progressed, sundials became all the raze. They measured the apparent time during the cloudless daytime. It took several hundred years before mechanics had advanced far enough to allow for pendulum clocks. It was not until the seventeenth century that a seconds pendulum was first added to a clock. All the time the definition of time depended on Earth’s rotation either around its own axis (day and night) or its rotation around the Sun (seasons). Astronomical observations were the basis of time.

The name ‘second’ comes from ordinal numbers. Already in the thirteenth century a medieval scientist called Roger Bacon expressed times of full moons with hours, minutes, seconds, thirds and fourths. The other fractions have been mostly forgotten, but hours, minutes and seconds stick. Second is so short a time that it is a secondary unit in practical life. It finds use in sports, but even there more accuracy is needed. The second comes second in almost everything these days, only as the official unit of time it gets its worthy first place. Nowadays, the definition of a second has moved from the highly imprecise astronomical observations of large systems to the extremely small world of atomic clocks. A second is defined as a certain number of vibrations of a particular atom. Even that measurement isn’t infinitely accurate, but for all practical purposes the second is defined far too well for its insignificance in use.

Seven minutes late the train leaves the platform. I still don’t have a clue as to the delay. Maybe we were waiting for a connecting train, which is behind another train on this large station. When you have sixteen or more track it’s hard to keep track of each one.

Now that the train is moving, I can feel the acceleration in my back. I sink in my seat as the speed of the train grows from the initial zero to 20, 30, 50, 76, 98, 123 kilometers per hour. The display in the carriage shows the current speed every minute or so. Now that the speed is constant, there is no more pressure on my back, the acceleration is gone. In fact, we have reached quite some conclusions already. We should back up a bit to start from the beginning.

Distance between two points, or the length of an object is another basic property of the universe. We use the meter to measure it in Europe, although the strange island folk in Britain insist on their own peculiar system of inches, feet and yards. They are peculiar because they aren’t decimal. In the metric system, almost everything is divisible by ten. One meter is ten decimeters, one decimeter is ten centimeters and one centimeter is ten millimeters. On the other hand a kilometer is thousand meters and so on. But if you take an inch as your basic measurement of length, you’ll have twelve of them in a foot and thirty six of them in a yard, which gives you three feet in a yard. There are two yards in a fathom, which is equivalent to eighteen hands. As you can see, there are many different names for the units and the conversion factors are all different. It seems totally incomprehensible if you have learned the metric system first.

Measuring length between two points is a one-dimensional measurement. The basic SI unit meter, abbreviated ‘m’. If you want to know the area of your field as a farmer, you’d measure each side separately and multiply the numbers. The unit of area, measuring the size of a plane is m*m or m squared, usually written m2. Measuring the three-dimensional volume of a rectangular object is as easy as measuring its three sides, the width, the height and the depth, individually and then multiplying the result. The unit of volume will be m*m*m* or m cubed, usually written m3.

You can combine the basic measurements to understand more from the universe. There is almost no limit to the combination you can build on top of these basic measurements of distance and time. For example, the average speed of the train is just the distance travelled in the time it took. And the acceleration I felt so clearly a moment ago in my back is just the change in speed over a certain period of time. The faster the speed changes, the higher the acceleration.

Oh, and we shouldn’t forget the third basic property of most objects in the universe: the mass.