[bladedtoys]

If I can take a shot at this, there are broadly two: the quantum mechanic explanation and the relativity explanation. But we seem to only be talking about relativity.

Relativity explains a couple of counter intuitive things that we actually see:

1. Apparently there is no particular velocity that can be called absolutely stopped or special in any way: every velocity is just as valid as every other.

2. Observers moving at different velocities disagree on 3D distances and time intervals between events.

3. But they all agree on the spacetime distance between events. Which is to say, there is a simple formula that resembles a distance formula in 4D where one of the dimensions is time.[0] We know that using this formula, all observers at all velocities get the same result.

gravity is not being discussed but relativity addresses it as well.

So relativity explains actual (but counter intuitive) observations. If we had the simpler Newtonian universe then we could fly around at unlimited relative speeds and time would flow unambiguously at constant rate and be completely unconnected to distance/space. But by direct observation, we see we don't live in such a universe.

If causality could percolate at unlimited speeds then we would not observe any these things and the formulas for relativity would not work but the formulas of Newton would. For one, there would be infinite frames of reference for which you arrive before you depart and these frames of reference could use faster than light travel reverse casualty.

[0] https://en.wikipedia.org/wiki/Spacetime#Spacetime_interval

[criddell]

I know of one other non-mainstream explanation from Max Tegmark: the mathematical universe hypothesis says reality is actually mathematics.

I'm not sure your explanation is an alternative explanation of what reality is. You've laid out some characteristics and consequences, but haven't described what's at the bottom of it.

I've heard that matter is a consequence of quantum field excitations, but what is a field? From the little bit I've read it sounds like mathematics again.

[aeternum]

The concept of a field can be described using mathematics but does that really mean mathematics is actually what the universe is made of?

Suppose for example we found that protons, neutrons, and electrons really were fundamental. Their behavior can be described with mathematics but wouldn't we say it's still the elementary particles that our compose our universe?

[criddell]

Tegmark's theory says exactly that - the particles are ultimately mathematics. Not that they can be modeled with mathematical methods, but that they are actually constructed from mathematical structures.

It's definitely not a mainstream theory.

[goatlover]

What does the mean, though? Mathematics as we understand it is an abstraction. If the universe is fundamentally abstract, what breathes fire into the models? Consciousness?

[wcoenen]

Modal realism. A mathematical structure is "real" to its conscious inhabitants (if any), and merely an abstraction from the perspective of those who are not, i.e. the inhabitants of other mathematical structures.