system
Given the following post, respond as if you're commenting on Reddit:
human
How precisely did Einstein get led to postulate that the speed of light is invariable for all frames of reference?

I understand that the speed of light in a vacuum is constant in all frames of reference.

Further, I understand that due to this one fact, Einstein was able to derive (with his theory of special relativity) many amazing things, such as length contraction of moving objects, time dilation, energy-mass equivalence, and so on. Basically, he started at the invariance of *c* and came up with the rest.

But what led him to figure out that *c* was invariable in the first place? Was he the first to postulate this? I guess I'm asking what was known about light at the time, and what did Einstein come up with, and what led him to his conclusion that *c* was invariable for all frames of reference.
gpt
He looked at Maxwell's equations, the ones that determine the speed of the EM field, and he looked, and he looked...

And then he went "huh, that's funny, the speed of the EM field is this constant divided by that constant, *no matter what*. So the speed never changes. Well, that's bizarre. So, if we assume for an instant that this is *always true*..."

And the rest is history.