It is and physicists have proven that. It's predetermined. No free will.
Time is an illusion. Free will is an illusion. Neither one exists
I don't think physicists have proven this. But I can see what you're saying. You're right in the sense that time has been shown not to be a Newtonian absolute, like a line that the whole universe is moving across at the same time.
But time is not an 'illusion' in physics, it is part of the four-dimensional manifold of spacetime in relativity theory, and has definite properties and measurable effects (time dilation, curvature of time etc). What relativity did was to show that 'time' is not absolute across the universe as the Newtonian model posited, but depends on frames of reference and specifying sets of coordinates (making simultaneous events relative to an observer's reference frame). So 'past' and 'future' have no meaning for the universe as a whole, but depend on specifying a frame of reference.
It is also possible to distinguish the objective 'time' of physics, from the subjective experience of time. The philosopher Bergson had the concept of 'duration'. It is the qualitative flow of consciousness as it is experienced from the first person, and it cannot be measured or dissected like the space of physics, and neither can causation be applied to it. Within duration a subject experiences a feeling of freedom, since the causation and determinism of physics are absent (the flow of consciousness cannot be measured or quantified).
But this feeling of freedom is not an illusion, any more than consciousness itself can be an illusion (if you have a conscious experience, you cannot doubt that you are having that experience, though you can have doubts about what causes that experience). It is situated at a metaphysical level, and can only be grasped by intuition. However, the qualitative experience of time (and freedom) could be called a kind of subjective illusion if you contrast it with the measurable spacetime of physics and assume that only objective measurable things are ultimately real, so you'd be right in that sense. But it's not an illusion in the sense of being a mistake or of being deceived. The inner experience of time and freedom is just as valid and real as the objective spacetime of physics (going by Bergson's philosophy). I think Bergson even had an ongoing debate with Einstein about this, through letters and correspondence.
From another angle, although you are right that on the macro scale of relativity theory determinism seems true, this no longer applies in quantum mechanics. At the quantum level nothing is 'predetermined'; quantum systems are actually in indeterminate states and outcomes can only be predicted using probability theory, until observations/measurements are made (this is the copenhagen interpretation of quantum mechanics) which make wave functions collapse, and so the system then takes on definite properties (e.g. an electron can be precisely located or given a definite speed).
It would seem that free will could still somehow fit in at the quantum level, or at an even deeper level (string theory?), although no one knows exactly how yet, and it would still come up against the problem of randomness (it would seem that the will is no more free if its decisions are random than if its decision are predetermined).
But the claim 'free will is an illusion' is just an assertion, and not an established fact. Philosophers are still debating and writing papers on the problem of free will and even on the problem of time.
Sorry for the long post, and if I've said things that you already know or if I've misinterpreted what you were trying to say.
Just thought I'd use this as an excuse to talk about philosophy lol
