# Time dilation

I've recently started reading the historical works on relativity.
Although I know the basic concepts behind it, I still think it's
important to visit the source of the theory instead of trusting
re-tellings of it. I started with *Relativity* (1920) and have so far
read up until part 1 chapter 10 so take my notes on it with a grain of
salt.

At the beginning of the book, Einstein spends a lot of time getting through the basics concepts of his theory through examples. He argues for different co-ordinate systems because of the finite speed of light and the principle of relativity. Specifically, in chapter 7 he says that because you are required to measure the speed of light as c, while moving forward in a train from its embankment - where the light came from - and you can't add to or take away from the speed of light the speed of the train. Therefore that breaks the principle of relativity. I don't think that's true though. You can still measure the speed of light as c at that point because it's not the same light that came from the point of embankment that you're measuring. You're measuring light that left a few moments before you embarked. That prior light is still traveling at c. I think this is where a lot of confusion is created. There should be an explicit distinction between the speed of light in general and the speed of light of specific light rays. Although it's the same speed, the photons themselves won't always be the same. What I'm saying is that the problem doesn't lie in light having more than light speed or less so much as it lies in which light ray you're measuring. Because light has a finite speed but it's continuously being emitted, the two are easily confused.

I think it's important that those different co-ordinate systems are specifically labeled different co-ordinate systems of observation because that's what they represent - observation of light from different places.

Going back to the title, I'll now explain time dilation as I understand it. Many people are aware that you can figure out how far away a lighting is by counting the seconds between seeing one and hearing its thunder then multiplying that my the speed of sound. Similarly if you knew the distance, you can figure out the time difference.

Let's apply the same logic to light since it too has a finite speed. Supposing that an absolute position of things exists, you can get the distance between two events and divide it by the speed of light to see the difference in time between their absolute simultaneous happening and when one event observes the other. If you observe the two events at the midpoint of that distance, you'll observe the two events as being simultaneous.

When an event happens at regular intervals, like the ticking of a physical clock, and it moves away in a straight line from you, the distance and therefore the time difference between you and the clock increases. What that means is that if you stick a clock to the back of a car and it drives away from you at constant speed, you'll observe the clock going slower by some factor dependent on the speed of the car. That's simply because the distance and therefore the time light takes to arrive increases. It doesn't mean the clock is moving slower in reality. It's just your observation of it that's moving slower. The effect flips when you go in the opposite direction. That's the twin paradox solution for you.

If you go back to the example at chapter 7 and apply this logic, you'll see that the light you are measuring as having light speed is an older emission of the embankment. The light that leaves at the same absolute time as you won't be observed at the same absolute time when you're moving in a train. It will still have the same speed but it will arrive at a later absolute time because its speed is finite.

There's a problem in my explanation though. It assumes light travels in all directions at the same speed. Or rather, it assumes that the reference to which you measure is truly and absolutely at rest. In reality everything is moving so that simple calculation above won't hold up for long. This absolute position or background that all objects are in reference to doesn't really exists. Or rather it does but not as I described it. The absolute position or background is simply light. When I say absolute, I mean in reference to light. But I think you'll agree that the explanation of light in terms of this faux concept was not as confusing as it would be if it was in terms of itself.