Writing tests to demonstrate actor re-entrance bugs

And what if you studied computer hardware and discovered that multi-core machines work by the clock crystal sending an electromagnetic pulse through the conductors that moves through each core, one at a time, so that all N cores perform a clock cycle's worth of work on each clock cycle, but very zoomed in analysis of the timing reveals that there is serial execution, where core 1 performs on the first 1/N slice of the clock cycle, then the second core performs on the second 1/N slice of the clock cycle, etc.?

Would that realization that computer hardware actually works that way on a physical level change everything and reveal that no computers ever perform "parallel" execution?

I understand the difference. I'm saying it doesn't matter and might even be an artificial fabrication of our minds. The stipulation that parallel execution means "truly" simultaneous in some absolute sense requires us to make assertions about physics I'm not even sure we ever could. What if the universe is a single core simulation that executes each "tick" of time by nudging each physical object forward one at a time, but the time unit is a tiny fraction of the Planck time unit, so we'll certainly never be able to watch this serial execution? The only thing that matters is that all objects make forward progress "together". That's what it means that all objects move "in parallel".

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