Simultaneity - Albert Einstein and the Theory of Relativity
Why Physics Stack Exchange recommends it
Physics Stack Exchange users link it as a quick visual of the train and platform simultaneity scenario when asking or explaining why observers in different frames disagree.
Recommended by 2 people across 2 different months, 2014 to 2023.
What it is
A roughly two-minute animated explainer of Einstein's train-and-lightning thought experiment for the relativity of simultaneity. It shows how an observer on the platform and a passenger on the train disagree about whether two lightning strikes were simultaneous, because both must measure light at the same speed. The video comes from the MyEarbot channel.
What people said
Recommends Hypnosifl · 2015-04 · Physics Stack Exchange · on “Is simultaneity testable?”
An observer at any given location can only define events to be simultaneous in retrospect, after there's been time for the light from each event to reach them. Simultaneity in each inertial frame [LINK] is defined based on the assumption that light travels at a uniform speed in that frame (the second postulate of special relativity [LINK] ), so for example if at the stroke of midnight on Jan 1. 2010 I receive light from an event that happened exactly 10 light-years away in the coordinates of my rest frame, and at midnight Jan 1. 2020 I receive light from an event exactly 20 light-years away, I conclude that in my frame both events happened simultaneously at midnight on Jan. 1 2000. The fact
Recommends Patrik Ponjavić · 2017-11 · Physics Stack Exchange · on “If I travel almost side by side with light, how fast does light go near me?”
If speed of light is same in all inertial frames of reference, wouldnt the woman in the train see the lightning at the same time? Since the light from both ends have to travel the same distance. This puzzles me!! The fact that she will run into the front light earlier does not apply to her frame of reference. it is the man's frame of reference. so if we only consider the woman's frame of reference, shdnt she see the lightning at the same time? [this video]
Cites Hypnosifl · 2014-12 · Physics Stack Exchange · on “Why is light different than sound in terms of the assumptions we make regarding causality?”
The problem with FTL and causality has to do with two issues: 1) the relativity of simultaneity [LINK] between inertial frames (not an issue in classical physics with sound waves, since in classical physics all inertial frames agree about simultaneity), which implies that a signal moving FTL but forward in time in one frame is moving backwards in time in other frames (i.e. different frames disagree on the order of the events of the signal being sent and the signal being received), and 2) the fact that any laws governing FTL signals must work the same way in all frames according to the first postulate of special relativity [LINK] , so if it's possible to have an FTL signal go back in time in
Cites Patrik Ponjavić · 2017-11 · Physics Stack Exchange · on “If I travel almost side by side with light, how fast does light go near me?”
[this video] in this video the women in the train see the light infront of her sooner than the light behind her.. how?
Cites PG1995 · 2020-04 · Physics Stack Exchange · on “Sound signals would reach the both cities at the same time”
**Part 2:** Suppose when the sound signals are received in both cities, light pulses are sent to the space guy and also to the tower which is situated in the middle of both cities. The scenario is more like described in this video: [this video] . If the space guy happens to be present right above the tower (just like the guy in the middle of platform), he would receive the light pulses simultaneously therefore he would conclude that sound signals were received simultaneously. Do you agree?
Cites TSR · 2023-09 · Physics Stack Exchange · on “Relativity of Simultaneity - Why doesn't the train/lightning example contradict the absolute speed of light?”
I know similar questions have been asked here before, but none of them seem quite to address my particular confusion. I'm not afraid of math (I did well in calc III last semester, for example) but my familiarity with physics is still primarily conceptual. My basic, high-level understanding of special relativity is that space/time must be malleable in order to preserve the constant speed of light regardless of reference frame. Working from that understanding, the train & platform example where a flash of light originates from the center of the train makes sense to me. I think these two pictures (taken from Wikipedia [LINK] ) make it pretty clear that the two observers will disagree about whet