system
Given the following post, respond as if you're commenting on Reddit:
human
Why is the speed of light the upper limit of speed? What is special about light?
gpt
The upper limit is not defined in terms of light specifically (despite the name), rather the speed of light is the speed that any massless particle will travel (historically, light was the first massless particle we became familiar with).

Additionally, the actual value of the speed of light is arbitrary and has no real physical significance. In everyday units and intuition, a speed is a quantity with units (distance / time), which makes it seem that the fact the the maximum speed takes a particular value in these units is important. However, if you take special relativity seriously you realize that it doesn't make any sense to measure time and space with different units, since they're inextricably linked in the more general concept of spacetime. In particle physcis, and many other areas of physics where relativity is important, we usually measure time and space in the same units, in which case the speed of light is just 1. In other words, there is some maximum speed, and all other speeds are some fraction of that speed, but speed itself should rightly be thought of as a unitless (i.e. dimensionless) quantity.