“Common sense” protests: All this appears obvious to us today and we do not doubt it in the least. Nevertheless, we know from history that it has not been easy for man to realize the relativity of “up” and ”down”. People are inclined to ascribe absolute sense to concepts if their relativity is not evident from everyday experience. Let us recall the absurd objection to the fact that the Earth was round, which came down to us from the middle ages: how can people walk upside-down ….?! If we did not recognize the relativity of the vertical direction and took it to be absolute in Moscow, for example, then, naturally, people in New Zealand would be walking upside-down. But bear in mind that for New Zealanders Muscovites, too, are walking upside-down. There is no contradiction in that at all, since the vertical direction is not really an absolute concept, but a relative one. We begin to feel the true meaning of the relativity of vertical directions only when we consider two points sufficiently far apart on the Earth’s surface-Moscow and New Zealand, for example. But when we take two points close to each other, then we are justified with the absolute concept.
The Tragedy of light: Light does not propagate instantaneously. We have convinced ourselves of the principle of the relativity of motion and of the existence of a countless number of “inertial frames”. However, there exists a kind of motion which, at first glance contradicts the principle we have established above. It is the propagation of light.
Principle of relativity of motion seems to be shaken!!: The colossal but not infinite velocity of light in vacuum brings us into conflict with the principle of relativity of motion. Imagine a train hurtling along a tremendous speed of 240,000 km/sec. We are riding in the head carriage, and an electric bulb is switched on in the tail carriage. Let us see what results we would get if we measured the time necessary for light to travel from one end of the train to the other.
It would seem that this would differ from the one we would obtain if the train were at rest. Indeed, relative to a train moving at 240,000 km/sec the light should travel at a speed of only 300,000 – 240,000=60,000 km/sec. It is as if the light has to catch up with the head carriage. If we place the bulb at the head of the train and measure the time necessary for the light to reach the tail carriage, it would seem that its velocity in the direction opposite to the movement of the train should be 240,000 + 300,000 = 540,000 km/sec. The light and the tail carriage move towards each other.
Thus, it appears that in a moving train light should propagate at different velocities in different directions, while in a train which is at standstill the velocity of light is the same in both directions…
Boarding a train….
Imagine a train 5,400,000 km long moving with a uniform velocity of 240,000 km/sec along a straight line. Suppose a lamp is switched on at some instant of time somewhere in the middle of the train. And suppose the automatic doors in the front and rear of the train carriages open the moment the light of the bulb reaches them . What will the people on board the train and those standing on the station platform see?? The people in the middle of the train will see the following: as light travels relative to the train at the same velocity in all directions 300,000km/sec , it will reach the rear and front carriages simultaneously, 9 seconds later (2,700,000:300,000) and both doors will open at the same time. Relative to the station platform the light also travels at the speed of 300,000 km/sec, but the rear carriage moves to meet the light beam. Therefore, the beam of light will reach rear carriage after ,2,700,000/(300,000+240,000)i.e., 5 seconds. The beam must catch up with the front carriage and, therefore, will reach it 45 seconds later, 2,700,000/ (300,000-240,000). It will seem to the people on the platform that the doors open at different times-the rear door first and the front door 45-5=40 seconds later. Thus, two absolutely identical events-opening of the front and rear doors of the train-will happen at the same time for the people onboard the train and with a 40-second interval for those on the platform……
– Immortal Chiron