It’s all math first, experiments and confirmation later. It’s also the same for everything else we *think* we know about the universe despite not actually seeing it happen, just like dimensions or how we have always thought blackholes existed, but never saw one until recently.
Basically, smart people played with the extremest version of the speed equation, which involved 0 mass, and concluded that to achieve this result (speed), something must not gain mass no matter what.
We then found out that the lightest thing in the universe can’t be weighed at all, which are photons.
Since they can’t gain weight (because they are mass-less and essentially are “born” already running away from the source), it’s only mathematically logical that they move at the most extreme speed. Since photons are light particles, this speed is then labeled the speed of light.
PS: I welcome any corrections as this is just the result of me asking similar questions way back in my internet-curious days.
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