= Solution
Historically, <electromagnetism> was central to <Albert Einstein>'s 1905 argument. His opening example compares a moving magnet and a moving conductor. The same relative <motion> produces the observed induction, but a description using <electric fields> in one frame and the magnetic part of the <Lorentz force> in another seemed to divide the explanation according to which object was called stationary. The unsuccessful search for an observable preferred state of rest also motivated treating the laws of <mechanics>, <optics> and <electromagnetism> on a common footing. That establishes a strong historical connection, not a deduction that relativity applies only to electromagnetic systems.<On the Electrodynamics of Moving Bodies by Albert Einstein>[opening magnet–conductor example and statement of the two postulates]{ref}
The logical starting points of the kinematical argument are <Einstein's principle of relativity> and an invariant <speed of light> in vacuum, together with the assumptions needed to compare homogeneous <inertial frames> and physical measuring standards. <Einstein synchronisation> assigns distant <clock> readings by light exchange. In standard coordinates, the resulting <Lorentz transformation> for a boost along $x$ is
$$
t'=\gamma\left(t-\frac{vx}{c^2}\right),\qquad
x'=\gamma(x-vt),\qquad
\gamma=(1-v^2/c^2)^{-1/2}.
$$
Neither the derivation of this transformation nor the deductions of <relativity of simultaneity> and <time dilation> require solving <Maxwell equations> for every particular material system. They constrain the relation between measurements made in different <inertial frames>.<On the Electrodynamics of Moving Bodies by Albert Einstein>[sections 1–4: synchronization, the transformation, and its kinematical consequences]{ref}
There is nevertheless an empirical commitment in identifying $c$ with the speed of vacuum <light>. It is not a freely chosen numerical label: the theory says different inertial observers, using the prescribed standards, obtain the same measured value. More generally, $c$ specifies the <causal structure> of <Minkowski spacetime>. A theory could contain a massive <scalar field> with a <Lorentz invariant> equation and no electromagnetic field at all. Its particles, <proper times> and causal propagation would still be governed by relativistic geometry. This gives a direct sense in which the framework is logically more general than <electromagnetism>.
Conversely, the presence of <Maxwell equations> alone would not establish the full relativity principle for all matter. One could append a law involving a physically preferred <timelike vector field>, giving a nonelectromagnetic sector a preferred frame while retaining the usual electromagnetic equations. The <covariant equation with a preferred timelike vector> is an explicit model of this distinction. Requiring every relevant interaction to respect <Lorentz invariance> adds content; writing its components in four-dimensional notation does not supply that content automatically.
The magnet–conductor puzzle is resolved because <electric fields> and <magnetic fields> are related parts of one <electromagnetic field>, with the <Lorentz force> transforming consistently. The measured current must be compared for the same physical arrangement, including the conductor's state of motion, rather than by demanding the same electric and magnetic components separately. This is an application of the general frame transformations to a particular interaction, not the definition of every possible relativistic interaction.
<Albert Einstein>'s later distinction between a <principle theory> and a <constructive theory> clarifies what the 1905 argument provides. It places universal restrictions on admissible laws without constructing every rod and <clock> from a particular microscopic model. A constructive explanation can additionally investigate why actual material standards approximate the required behavior. <Harvey R. Brown> and <Oliver Pooley> emphasize this dynamical question; it does not make the kinematical transformations meaningless or confine them to electromagnetic phenomena.<What Is the Theory of Relativity? by Albert Einstein>[1919 article: distinction between constructive theories and principle theories]{ref}<Minkowski space-time: a glorious non-entity by Harvey R. Brown and Oliver Pooley>[sections 3–5: rods, clocks, and dynamical explanation]{ref}
The strongest account therefore distinguishes historical motivation, the operational use of light, and the theory's universal scope. The 1905 paper addresses an electromagnetic problem through a framework whose restrictions extend to every physical process claimed to obey <special relativity>.
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