Correct, the speed of light is actually the speed of causality, we just so named it after light because that was what we first discovered as going at c, but many other things in the universe also due because it's the same underlying principle. That is why gravitational waves also travel at c, ie if you removed the sun instantaneously from the solar system, the Earth will continue to orbit for 8 minutes, as that is how long light (and the gravitational force) takes to get from the sun to the Earth.

Not an expert, but can't resist chiming in anyways... One thing to think about it what exactly is causality? There'll be tons of different definitions, but they'll all have one thing in common, events that cause "later" events, and/or events that depend on "earlier" events.

And in a physics sense what is an event? An interaction between two things, right? And since there doesn't exist any force that can interact instantaneously across distance, the speed limit of causality is equal to the speed of our fastest forces.

If we discovered some scifi-esque Tachyon particle that traveled at 2C, we could no longer say the speed of light is the speed of causality.

I'm not an expert either, but I like to think that the speed of causality is the speed at which a piece of information (e.g. a particle, a gravity wave, etc) is traveling through space.

So the speed of light / gravity is essentially the maximum speed of causality, because nothing can travel faster than that.

EDIT: Or you can think in terms of how information propagates through spacetime. In this point of view, the speed of causality is always the speed of light, for everything, including particles with mass.

Yeah that's what I was trying to work towards. Basically that causality is an abstraction, or at least a "higher level" idea. And if we look at the components of it, we can see that interaction between two things is a central part of it. And an interaction between two things in our universe has a maximum bound of the speed of light (and gravity and so on). The speed of causality is just the speed of the fastest thing.

It's the reverse. Light and gravity travel at c because they (at least light) are mediated by particles / systems that have no mass.

Mass is what slows the speed of causality for certain particles. For example, while the photons I emit may travel at C, the massive particles that make up 'me' cannot.

I'm an expert, and an event is just a point in spacetime.

Sure, but events have a causal future light cone which can have a causal relationship with future events, and a past light cone with the inverse.

They are the same, because there’s no such thing as the “speed of light”.

Theoretically as I understand it, everything moves at exactly the same speed through space-time, whether it is light, the Earth, etc.

At non-relativistic speeds, this means moving along the time axis at approximately one second per second, with the rest of the movement in space. At relativistic speeds, higher proportions of the “speed” of an object are along the time axis.

As the energy requirements for moving massive objects through space at relativistic speeds are huge, we can only really observe this phenomenon with light, which has no mass, and therefore does not need huge amounts of energy to move through space.

As a result, we call 186k miles per second the “speed of light” when actually it is just the maximum speed anything can travel through space, and due to light being massless, it happens to be the speed that light travels through space too.

This is my understanding as well. All massless particles must travel at the speed of light (they cannot be slowed down) and moreover, they must travel at the speed of light in all reference frames.

Whereas massive particles can remain at rest. There's no such thing as an unmoving photon.

My point is subtly different, I think. The point I’m trying to make is that if you’re “at rest” in space, all of your movement is through time. If you are “at rest” in time, all of your movement is through space.

As far as I understand it, having mass is basically a result of moving more slowly through space.

I'm not a physicist, but AFAIU the speed of travel/causality for light is only the maximum speed of causality because photons have a mass of literally zero.

If photons had non-zero mass they could only travel slower than the maximum speed of causality (which would probably be called speed of gravity rather than speed of light, in this alternative universe).