 Solution

There is an important structural difference, but the issue needs two distinctions. The freedom to change distant clock labels is not the same as the absence of a physically preferred simultaneity relation; and an inability to measure a one-way signal speed without synchronization is not by itself a proof that all proposed temporal structures are mere conventions.

In a standard Einstein synchronisation procedure, a clock at A sends a light signal at $t_1$, receives its return at $t_3$, and assigns the reflection event at B the time $(t_1+t_3)/2$. More generally one could assignFor a fixed spatial distance $d$ with round-trip duration $2d/c$, the two assigned one-way speeds become $c/(2ϵ)$ and $c/[2(1-ϵ)]$. Their reciprocal travel times still add to $2d/c$. Thus the two-way measurement alone does not establish the midpoint rule. Einstein presents synchronization as a definition supported by consistency conditions, rather than as a direct comparison of already synchronized distant clocks.^[ref]

Both Newtonian mechanics and special relativity allow coordinated relabellings such as $t_{\kappa }=t+\kappa x/c$. They can be used as nonstandard coordinates if the equations and interpretations of the transformed quantities are changed consistently. A mere relabelling cannot change a reunion of clocks or another local coincidence. Conversely, insisting that the familiar equations must retain their usual simple component form while changing only clock labels is not a neutral test of conventionality.

The difference is in the invariant structures of the standard theories. Newtonian spacetime includes an absolute time, whose equal-time slices are preserved by Galilean transformations. It therefore contains one simultaneity structure shared by its inertial observers. Its instantaneous interaction laws, where adopted, are formulated relative to that time. Ordinary relativistic spacetime contains no analogous observer-independent foliation. Einstein synchronisation defines simultaneity relative to an inertial observer; different inertial observers generally assign different distant events to the same time. This difference survives the freedom to rewrite either theory in awkward coordinates.

It does not follow that any arbitrary synchronization is equally natural relative to the full structure of special relativity. An inertial observer together with the Minkowski metric picks out the usual orthogonal simultaneity hyperplanes, and ideal slow clock transport supplies a physical comparison under assumptions about clock behaviour. A conventionalist must say which such structures and operational assumptions are being held fixed. Likewise, a Newtonian theorist must not confuse positing absolute time with having a theory-independent operational measurement of it.

The best answer is therefore qualified: both theories admit synchronization conventions at the level of coordinate assignment, but their standard physical accounts privilege simultaneity differently. Newtonian absolute simultaneity and the relativistic absence of a shared inertial foliation are genuine theoretical differences. Whether an alternative clock assignment is merely a convention or a rival physical claim depends on whether it only relabels the same structure or adds, removes or changes physical structure.