system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know light speed is the universal speed limit? For example, if light moves differently in a gravitational field, we'd never be able to gather any data to the contrary on Earth.

This is something I've been wondering for a long time. However, most of what I can understand (which is written for popular audiences, since my grasp of physics is 2 semesters of college intro courses) when I read about it involves discussing the implications of the light speed limit - not necessarily how we know it. Its basic theory stuff - suffice it to say I haven't heard a good explanation of how we know light speed is constant throughout the universe, not just where we can measure it. 

Thanks in advance!
gpt
We only "know" in the sense that it hasn't been proven false yet.

The idea of *c* being constant in all reference frames is a direct result of special relativity, and being constant in all frames implies an absolute speed limit in the universe.  Thus far, no experiment has been performed that suggests special relativity may be incorrect, and until that happens, we take the theory and everything that can be derived from it as true.

As an aside, this is a subtle point about science that some people fail to understand - scientists never produce a theory, sit back, dusts off their hands and say they're done.  All theories are constantly being tested in new ways and with greater precision, looking for holes to be fixed.