English: (40 sec) Most animations of the train and embankment thought experiment simplify the presented scenario to provide a clear and easily visualized graphic.

The emphasis in most animations is on the first order relativity of simultaneity effect. Second order effects such as length contraction and time dilation are commonly ignored. An interesting question is, "What happens if try to follow what the primed observer sees, with the primed frame stationary and the unprimed frame moving? Wouldn't we witness the light pulses reaching the primed observer simultaneously, and reaching the unprimed observer at different times? How can a simple shift in viewpoint reverse the results? Is this possibly a paradox that invalidates the gedanken?"

The answer is NO, but to explain why this is not an issue, we must consider second order effects.

We start by presenting a variant of the thought experiment. The observers carry poles with electrodes at the ends. When the poles brush against each other, sparks are generated.

Since the sparks are easy to miss, we run the animation twice.

[animated sequence]

(40 sec) The animation shows sparks being simultaneously emitted by the two electrodes as the observers line up. But in the animation, the "speed of light" is 5 pixels per frame, while the speed of the moving observer is one pixel every two frames. In other words, the moving observer is traveling at 0.1 times the speed of light. At this speed, the moving observer's bar is length contracted by about 0.5%. For simultaneous emission of sparks in the unprimed frame, the "proper length" of the primed bar, measured in its own frame, is 402 pixels, compared with 400 pixels for the unprimed bar.

[illustration: proper length comparison]

In the inverse scenario, with the primed bar stationary, the moving unprimed bar will be contracted to a length of 398 pixels

In the primed frame, the right electrodes spark first, followed by the left electrodes.

[animated sequence]