# Relativity of Simultaneity Confusion, yet Another Thought Experiment with a Train

Suppose there is an observer sitting in the middle of a moving train, and another obsever outside on a platform. When this train passes the outside observer, so that the ends of the train are equidistant from the outside observer, a flash of light is emitted from both ends. This can be the commonly used lightning strike or just some light pulse.

I reconstructed this scenario in desmos. The outside observer sees both lights at the same time and upon measuring the distances concludes that they flashed simultaneously. The observer inside the train, however, sees the light on the right flash and then the light on the left and concludes that the lights did not flash simultaneously. This makes sense since Einstein postulates that the speed of light must remain constant in all reference frames and the observer in the train is moving towards and away from the flash on the right and the flash on the left, respectively.

I constructed an alternate scenario in desmos as well. This is essentially the same scneraio, just viewed from the inertial reference frame of the observer sitting inside the train cart. In this case, the observer inside sees the 2 lights flash simultaneously and the observer outside sees them flash in a different order.

Are these scenarios equivalent? If no, why not? If yes, how is it possible that the same observer sees the events play out in a different order? Which is the correct order?

And most importantly, what do each of the observers see?

• If I did not misinterpret what you described, The two physical situations are different. In the first one the flashes are simultaneous on the platform, but not on the train, and vice versa.
– user65081
Commented Oct 29, 2021 at 20:47
• The observers see different things in the two situations.
– user65081
Commented Oct 29, 2021 at 20:55
• Well how are the scenarios any different? The hyperlinks might show seemingly different scenarios, but aren't they the same just from a different frame of reference? Commented Oct 29, 2021 at 21:00
• Did you read my first comment? the simultaneity happens in different reference systems in the two different cases
– user65081
Commented Oct 29, 2021 at 21:01