It's the ultimate form of determinism: not only is everything predetermined but it already happened and we're just experiencing it as though it hasn't yet.
I'm not being glib or contrary when I ask this question: in what ways does this matter beyond the philosophical implications?
But did you know that GR predicts we can build a sub light star drive that shields passengers from acceleration, just by using laser beams to hold together a Jupiter sized shell of matter. The matter shell could be accelerated at 100 Gs by massive star scaled lasers. But so long as it respects certain warp equations and energy conditions, the passengers would feel no acceleration.
That's possible with just physical matter, but GR also predicts negative energy. That allows us to theoretically build things like portals to far away portions of space, just by spinning a ring of matter and zapping it with negative energy.
The weird possibilities only seem to multiply if you add quantum states to the mix, and the potential for a larger quantum universe. Think of that a giving us many extra toe holds into the kind of spacetime fuckery that GM enables. Plus new kinds, like the idea that different realities or nonlocal regions of space could share quantum variables and allow universal travel and communication that way. Which is just amazing , and also terrifying because of the possibility of encountering very strange and dangerous places!
The strange part is not that there is some outer spacetime view where things are static and crystalline. It's that the bulk crystalline structure seems weirdly malleable and reconfigurable by subjective agents, and likely inhabited by other minds that live in portions of that crystal that are alien, and maybe even fundamentally dangerous to us. That makes these hyper determinist models a real mind trip! Okay, the structure is determined. But why is it determined in such a way that it causes such weird and disruptive potential experiences for subjective beings? And how would we cope if the structure let us build hyper technologies to, eg, contact aliens, travel to other universes, or change the laws of space and time?
When you model reality as some static crystal you observe from the outside, your intuition helpfully adds things like 1) some outside where there is time (otherwise “static” has no meaning, it’s defined in terms of time) and 2) you (the conscious observer with free will to inspect the crystal). Those things create the tension of respectively “everything already happened” and “no free will in there, surely”, but very counter-intuitively they don’t belong in that model in the first place.
That is, just like insisting determinism must eliminate free will is begging the question, simply insisting that determinism does not eliminate free will is also insufficient.
We also need a theory of free will that is compatible with determinism. In another subthread to OP's question, I detail my idea of a protected computational substructure as the best version of free will a determinist can expect, and suggest that's enough to explain the survival benefits of what we call free will. If you are in need of a model I propose that one.
But I am also curious about your thoughts, especially if you have a very different model in mind! What kind of model do you have in mind?
Also no free will sounds so horrible yet so liberating.
>> free will can't exist because everything already happened at once
No, it’s definitely not.
Those say that free will is possible if a being establishes a computationally independent substrate and uses that to compute optimal choices for a self model.
This relative computational independence is the best we can hope for, and I believe the resulting freedom is strong enough to explain our subjective experiences of feeling like we're freely choosing. All we're doing is scoping out our possible actions, and picking one that lines up with our self models. And that matters because it's as free as we can be, and it improves our survival ability beyond than of a being with no protected internal model.
My real concern, if reality is just a bundle of all possible existence states threaded together, is that there might easily exist very scary and unpleasant states of existence, and it might be possible to subjectively encounter those states in a way that is terrifying and causes huge amounts of suffering.
The universe where things happen in different times and for clear physical reasons seems less likely to expose us to terrible irrational cosmic horror, compared to the random bundle of all possible chaotic states.
In the universe I'm proposing you might accidentally create a portal to a sub universe of hellish demonic entities. Those things exist already right now in parallel with you and they want to be where you are, like on a Rick and Morty episode!
I would honestly prefer to be in a less cosmically terrifying and alienating universe, if possible! Perhaps the universe is just very very big, but not a jumbled multiverse where everything is happening all at once.