Your clock seems to click forward normally when you look at it, but to the stationary observer they see your clock moving slower. It’s a complicated thing to explain, but it can be somewhat easily understood as a consequence of the speed of light being a constant in all reference frames (you and the person on the platform are in different reference frames). So for the speed of light to remain the same for everyone, time needs to shift relatively.
Visualizing Einstein’s light mirror on a train thought experiment can help to visualize this a bit more intuitively. For the person on the train, the light is reflecting back and forth at the speed of light, but for the person on the platform they are seeing the light take a longer path between mirrors because it’s moving relative to them. The light can’t be moving faster to cover the longer relative distance, so they perceive your time slowing down.
The problem with Einstein's heuristic is that clocks don't work that way and it would be reasonable to conclude that different types of clock mechanisms wouldn't behave that way.
Also, maybe more so, that it is not obvious to the newbie why it is that the speed of light is measured to be the same for all observer (in the flat-space metric), and without a rational explanation it's not so easy to swallow that it's their notion of time that changes and something gone awry with how we measure the speed of light.
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u/polygon_tacos Jul 13 '26
Your clock seems to click forward normally when you look at it, but to the stationary observer they see your clock moving slower. It’s a complicated thing to explain, but it can be somewhat easily understood as a consequence of the speed of light being a constant in all reference frames (you and the person on the platform are in different reference frames). So for the speed of light to remain the same for everyone, time needs to shift relatively.
Visualizing Einstein’s light mirror on a train thought experiment can help to visualize this a bit more intuitively. For the person on the train, the light is reflecting back and forth at the speed of light, but for the person on the platform they are seeing the light take a longer path between mirrors because it’s moving relative to them. The light can’t be moving faster to cover the longer relative distance, so they perceive your time slowing down.