Bruce Harvey's Alternative Physics site

The railway carriage fiddle


relativity, anti-relativity, rail car, synchronisation of clocks, Einstein
This derivation of the principles of special relativity is taken from Einstein's own book on the theories of special and general relativity written for the not so technically minded. In this, he considers how two observers, one on a railway embankment and the other in a passing train might view the world.

The basic idea behind his thinking is the need to make measurements of distance and time. To measure the speed of a moving object, we need to time it over a measured distance. To do this properly we need a clock at each end of the measured distance. This is a frame of reference against which we can measure the movement of the object. Einstein poses the question "how do we synchronise the two clocks?" This is not as simple as one might think. Einstein's solution is to measure out the mid point between the two clocks and send out a pulse of light from there setting each clock when the light pulse reaches it. The problem is that when the observer in the railway carriage does this, the clock at the front of the carriage is moving away from the light pulse and the clock at the back of the carriage is moving towards the light pulse. This means that the light has further to travel to the clock at the front of the carriage and it will be set after the other clock is set. The observer on the embankment does not have this problem. Einstein's conclusion is that the two observers have synchronised their clocks to a different standard and this leads to all sorts of problems.

Einstein's logic is based on the assumption that there is no preferred frame of reference. That the observer on the train is justified in believing himself to be stationary and to say that the embankment is moving past him. This assumption may be valid for two passing space craft in the depths of inter-galactic space, but on earth it is not valid. The train moves through the electric fields of all the electrons and quarks which make up the 6,000,000,000,000,000,000,000 tons of the earths mass. Through the presence of these fields, the earth gives space (between the atoms of air) a property of stasis through which light travels and against which the speed of light has a constant value.

The observer on the train has no reason to synchronise his clocks in the way prescribed by Einstein. He can see the motion of the train past the landscape. He knows that the rails are laid in standard lengths and can time the rhythm of the sound of the wheels on the rail joints to measure the speed of the train. All he has to do is to send a pulse of light from one clock to the other and correct for the effects of the speed of light and the speed of the train.

Special Relativity
Relativity and the nature of time
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© Copyright Bruce Harvey 1997.