Thanks, I’ve never heard this and it’s quite profound. It’s always bothered me that there even is a top speed, and further that mass becomes infinite as it’s approached. But “speed of causality” makes these less strange.
The fact that time even exists is implied by / a result of causal actions having some finite propagation time.
Imagine a universe simulated in a computer with “ticks” where the entire state is updated.
It would be different to ours, but it would work just fine.
The fact that causal effects happen in the next tick means some minimal time has passed. So in your definition causality can cover the entirety of a finite universe in an incredibly short amount of time (one tick). But it seems like that's not the same as covering the entirety of a finite universe in zero time. In that case, every result would happen within the same tick as its cause.
A simulation could use particles with attributes like position, velocity, and also a "sum of forces". Then each update has the following steps:
1. Reset the sum of forces for all particles to zero.
2. For all particles, add the contribution to the forces on it from all other particles.
3. For all particles, update the next particle position based on the collected forces.
In the above, there's no intermediate state between updates, everything moves to the next position synchronously from the perspective of in-universe observers. (The external simulation can update particles one-at-a-time, but this is not an "observable" inside the simulation.)
I think I know what you're getting at, but somehow the phrasing bothers me, as if there is meta-time or as if cause and effect have time between them... for the photon at light speed, time isn't passing, it's emitted and then zero "time" later it hits something very far away.
It's more like we somehow need to think of cause and effect chains that have orderings without time.
I wonder if future generations will ever look back and casually quip something about "well they believed X existed, that was their problem, it all makes intuitive sense if you just..."
So what exactly are you talking about?
he’s talking about the photons frame of reference
However, you have to be careful with terminology. There is no inertial frame co-moving with the photon. All we can say is: as a massive particle gets faster relative to an observer's frame, the time it experiences relative to the observer's frame becomes shorter, and in the limit, as it approaches the speed of light (but never reaches c), the experienced relative time approaches zero (but never reaches 0).
This is well explained by Don Lincoln on the Fermilab YT channel:
Do photons experience time?
If there’s some universal limit for causality itself, then light just happens to be the fastest thing among all the other things subject to causality.
Other similar particles, like the W and Z bosons, are manifestations of the weak field. Since that field breaks the symmetry, those particles have mass and move slower.
BTW, that symmetry breaking is the very same one that physicists talk about when we discuss the Higgs boson.