For such closed paths, we find that, indeed, light moves with the same constant speed, regardless of the paths’ orientation.
SPACE IN TIME DEFINITION HOW TO
And that’s where we come full circle – after all, we’re still trying to find out how to determine simultaneity in the first place! The only light speed we can measure without that definition is one closed paths, which end at the same location they begin, so we can measure the light’s run-time on a single clock. But for the time difference to make sense, and to tell me how long the light has travelled, these two clocks need to be synchronized in the first place – they need to show the same time, simultaneously. With these clocks, I measure the time when the light has left A, and the time when it has reached B. Not so: In order to measure how much time light takes to travel from a location A to another location B, one needs two clocks, one at A, one at B. How do we know that light speed is the same in all directions? One might think that this should be an easy thing to test by direct measurement.
Or does it? This argument only works if the light travelling from the left clock to the middle was exactly as fast as the light travelling from the right. If the two clocks appear to show the same on the photograph, that means they did show the same at the time the light began carrying that information to the middle. In this situation, the light from both clocks needs exactly the same time to reach the camera in the middle. To compensate for this effect, there is a simple solution: Place a special camera exactly in the middle between the two clocks, and have the camera combine what the two clocks show into a single picture: If it arrives at the camera simultaneously with the light showing “12 o’clock” on the left clock, then it must have left the right clock a bit earlier, in other words: the right clock must have struck noon a bit earlier than the left. After all, the light that carries the information about the clock on the right to the camera on the left needs a tiny amount of time to do so. If, on such a photograph, the two clocks are shown exactly in the 12 o’clock position – does that mean they are perfectly synchonized – that they show the same time, simultaneously? On the contrary, if they look the same on the photograph, I can be sure that they do not run synchronously. For instance, I can attempt to read the time off two clocks with the help of a special camera, fixing what they show onto a photographic plate: Using light to synchronize clocksĪ plausible next step is to use light to transmit information.
Motion influences clocks – and that means that, for a fundamental definition of simultaneity, one should not rely on simply carrying clocks around. The simple fact that I and my watch move around during the day means that an ultra-precise direct comparison of it and the grandfather clock, undertaken when I have returned home for the evening, would show a minute difference – time dilation, a well-known consesquence of special relativity, will have taken its toll. However, if we measured time more precisely (alternatively: if we undertook trips with very fast space-ships) we would realize that clocks are influenced by motion. If I synchronize my watch with my grandfather clock at home, I then simply assume: event A which happens at my home and event B that happens at my current location occur simultaneously, say, at 12 o’clock, if the grandfather clock begins to strike 12 o’clock at the time of event A, while my own watch shows 12:00:00 at event B. In everyday life, our solution is simple: We carry clocks, be they watches or the clocks built into mobile phones. However, thinking matters through, it isn’t obvious at all how one can determine whether or not two events at two different locations happen simultaneously, or not. Simultaneity is something we rarely think about – we just accept it as a fact of life.
0 kommentar(er)
