The following is a paper by H. Aspden delivered at a Conference on 'Physical Interpretations of Relativity Theory', held at the Imperial College in London in September 1988 under the auspices of the British Society for the Philosophy of Science.
Abstract: The physical reality underlying the experimental support for relativity poses certain unanswered questions of particular significance to the role of the 'observer' versus the role of a physical electromagnetic reference frame. These questions, which concern electrodynbamic interactions, energy transfer, and standing waves as forcing 'influences, causal to invariance, are discussed. It is concluded that a physical insight into the meaning of the theory of relativity (RT) has no certain basis until further experiments resolve some of the open issues of which the most important is whether the field energy entrained by standing waves in test apparatus causes the vacuum coextensive with that apparatus to have an electromagnetic reference frame seated, and comoving with the apparatus, even though that same vacuum region presents a different frame of reference to freely moving electromagnetic waves.