1. The fastest speed of light is the basic assumption of special relativity. The reason for this assumption is that the speed of light does not change with the change of reference system, which is proved by Michelson-Morey experiment. It is called a hypothesis because it can't be directly proved, at least the current technology can't. Before Einstein, Lorentz (Lorentz of Lorentz force) deduced the so-called Lorentz transformation from the perspective of pure mathematics to explain the constant speed of light and reconcile the contradiction between Newtonian mechanics and Maxwell equations. This has led to many interesting results. Please refer to /view/33340.htm for details.
As for superluminal things, some scientists believe that there is such a thing as superluminal particles, that is, its lowest speed is the speed of light. I'm sure I haven't observed it experimentally As for whether there is a self-consistent theory to explain it, I'm not sure. This is a very cutting-edge thing.
This conclusion is derived from mathematics. I don't know of any experiment that can directly prove this conclusion, because it hasn't been realized yet. In essence, the faster the movement, the greater the mass, which should be one of the properties of our universe.
In fact, light is electromagnetic wave and energy. As for the deeper, what does it have to do with space? This may have to wait until quantum theory and relativity can be reconciled. Simply put, light reaches the speed of light because its rest mass is zero, and the more essential answer may be related to the nature of space. The great unified field theory that Einstein devoted all his life to but failed to realize may be the answer to this question. Superstring theory is the most promising theory at present, so you can search for it yourself.
When the object moves faster, people on the object will not feel the time difference. Only when compared with another frame of reference can we find out whether time is compressed or expanded. The faster an object moves, the longer it actually stays on it.