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An observer-independent global presentism conflicts with the absence of a distinguished simultaneity relation in bare special relativity. It is not a formal contradiction to add a preferred foliation. The issue is what the proposed present contains, and whether that extra structure is part of the physical theory or an additional metaphysical commitment. Presentism must be distinguished from the harmless indexical fact that an observer calls their own current event “now”.

Let two spacetime events have coordinate differences $\Delta t=0$, $\Delta x=L\ne 0$ in an inertial frame. A relatively moving frame assignsThey are simultaneous in one frame and not in another. For a fixed event $P$, each inertial four-velocity $u$ defines a different simultaneous hyperplane through $P$ by $\eta (u,Q-P)=0$. No particular $u$ is selected by the Minkowski metric alone. Thus an extended objective present cannot both be the equal-time slice of one ordinary inertial frame and be invariant under all Lorentz transformations.

An equivocation can make the incompatibility seem stronger than it is. If one defines “real together” using one observer's simultaneity, then changes observers in the next inference, transitivity across the mixed relations has not been justified. It is the failure of transitivity for mixed-frame simultaneity. The physics supplies the frame-indexed relations; it does not supply a single absolute co-existence relation formed by silently joining all of them. A sound argument against relativistic presentism challenges the physical basis of its preferred relation, rather than proving all events real by that equivocation.

There are several coherent responses, with different costs. A preferred foliation can be appended to Minkowski spacetime, declaring one of its slices the objectively existing present. If it affects no physical process, it has no support from the relativistic laws and breaks the symmetry of the total ontology without changing observable predictions. If it affects matter, new laws must explain why ordinary tests display Lorentz invariance. This is additional structure, not a logical impossibility. Eternalism, by contrast, treats the complete spacetime history as real and makes simultaneity relational, but the relativity of coordinate time by itself does not deductively establish that metaphysics.

A local present containing only $P$ is invariant once $P$ is given, yet it abandons a spatially extended world existing now. The past light cone is invariant too, but contains events of different ages and is not an instantaneous spatial present. An observer-relative present is meaningful, but relativizes the existence claim rather than defending a unique global present. The defensible conclusion is therefore an incompatibility among bare relativistic structure, an objective extended present, and full symmetry of existence claims; it is not an unconditional proof that every formulation of presentism is incoherent.

The relativity of simultaneity creates a serious difficulty for an observer-independent global present, but a conclusion about what exists requires an additional metaphysical premise. Eternalism regards past, present and future events as equally real. Presentism restricts existence to the present. These are ontological claims; the coordinate dependence of distant simultaneity is a physical claim.

A useful argument can be made mathematically explicit. In units $c=1$, consider spacetime events $O=(0,0)$, $C=(1,2)$ and $D=(1,0)$ in coordinates $(t,x)$. The Lorentz boost of velocity $v=1/2$ makes $O,C$ simultaneous, since $t_C^{\prime }=\gamma (1-\frac{1}{2}\cdot 2)=0$. The original inertial frame makes $C,D$ simultaneous. Yet $D$ is in the timelike future of $O$. Suppose one assumes that simultaneity in every inertial frame equally warrants co-reality, and that this co-reality is transitive. From the reality of $O$ one then obtains the reality of the future event $D$. Applying the construction repeatedly puts pressure on a presentist ontology.

The extra premise does real work. There is a failure of transitivity for mixed-frame simultaneity: $O$ is simultaneous with $C$ in one inertial frame, and $C$ with $D$ in another, but $O,D$ are not simultaneous in any inertial frame. The physical relation therefore cannot itself be identified with a transitive relation of co-reality. Special relativity supplies the geometry in this argument, not the ontological bridge. Writing an entire history as a four-dimensional mathematical solution also does not, by itself, prove that every represented event exists in the same sense.

A presentist could postulate an additional preferred foliation. That recovers an absolute present but adds background structure absent from ordinary Minkowski spacetime, and must explain its physical role or its empirical unobservability. Alternatively, one might give a local account of becoming along a world line, rather than a single simultaneous becoming throughout the universe. The latter does not automatically vindicate global presentism, but shows why lack of a universal present is not identical to a denial of all temporal passage.

General relativity further weakens any universal identification of time with a single inertial foliation. Some cosmological solutions have a useful matter-selected time slicing; this is a property of those solutions and matter distributions. It is not an absolute simultaneity structure in every relativistic spacetime. The defensible conclusion is that relativity gives strong reasons against a fundamental observer-independent global present in its minimal structure. It does not entail eternalism without further premises about existence and the interpretation of that structure.