Law (concise statement):
Under the laws of physics as presently established and by the best observational tests to date, no material, structure, or conventional physical medium in this universe can stop, absorb, or measurably decelerate the propagation of gravitational waves. Gravitational waves travel at the invariant speed c and pass through ordinary and extreme forms of matter with effectively negligible interaction.

Scientific basis:
Gravitational waves are perturbations of spacetime itself. General Relativity predicts that they propagate at the speed of light and couple extremely weakly to matter. Direct detections and multi-messenger observations confirm these predictions and show that any interaction with intervening matter is vanishingly small for all practical purposes.

Scope and caveats:
• “As presently established” means this law is grounded in current theory (General Relativity) and the best available empirical evidence.
• The law rules out any material that would stop or produce a measurable slowdown of gravitational waves within the domain constrained by current observations.
• Some speculative or beyond-standard theories of gravity (e.g., theories with a massive graviton or exotic new fields) could, in principle, modify propagation; however, existing observations place strong limits on such deviations. This formulation uses the physically meaningful standard of “measurable” interaction.

Implications for artificial generation and emulation:
• Producing gravitational waves with amplitudes comparable to those detected from astrophysical events requires enormous masses and energies; human-scale laboratory devices remain far below those levels and would therefore be harmlessly weak.
• Because gravitational waves couple so weakly to matter, any artificially generated waves of achievable amplitude would not be effectively blocked or contained by ordinary materials under current physics.
• If future physics discovers mechanisms that enable strong matter–gravitational-wave coupling, this law would need to be reassessed in light of new empirical evidence.

Recommended short wording for display:
No material, structure, or conventional physical medium in this universe can stop, absorb, or measurably decelerate gravitational waves; they propagate at c and traverse matter with effectively negligible interaction.

Epistemic note:
This statement reflects the highest-confidence scientific understanding available today. Science remains open to new data or theories, and the law should be read as a robust scientific principle that is subject to revision if warranted by future empirical discovery.