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Thanks for your replies. Wow! This gets way beyond me..."spin" is just "bolted on" to the "Hilbert space"...! Don't have a clue what that's all about. Anyway, the consensus seems to be-- No, relativity is not logically necessary, at least as far as anyone can tell. I guess I was thinking that if you can think through the implications of relativity in this universe, then you could also do the same for the opposite case--the case of a universe that is exactly the same as this one--except--without the principle of relativity. But I guess you can't do that, it would be too difficult. I gather that what you can say is just that without relativity, the universe would be very different from how it is.

hi everyone I don't have an aptitude for physics so I can't answer this for myself but I was curious about something-- Is relativity logically necessary? I mean, would it be possible for a God to create a universe with just Newtonian, and not relativistic, physics? If not, why? And if so, in what way would it be different from this universe? thanks