system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we know forsure that light has to be the fastest thing in the universe if we haven't observed it all?
gpt
The idea that nothing can be faster than the speed of light is not really an observation about light so much as it as an observation about how speed really works.

The problem is that the normal idea about how we treat speeds, like that you can simply add one speed to another does not actually work.

In school you had problems about one train going 130 km/h one way and another train going 200 km/h the opposite way and you calculated that they are going away from one another at 330 km/h.

It turns out that what you were taught in school was a lie.

Speed does not really work that way at all in real life.

It turns out it only looks that way when you have really slow speeds. If you get faster speeds the error becomes more obvious.

if you add half the speed of light to half the speed of light, you would expect to get out the speed of light if you use the math they taught you in school. It is actually closer to 4/5 of the speed of light.

The math works that no matter how you add stuff up you can never go over the speed of light.

So there really is no way to go faster than light, not because light is special, but because it already moves at the fasted speed there is. The idea that there might be faster speeds is an illusion that comes from being taught about speeds the wrong way in school.

The speed of light is the speed at which the universe works in general.