Concepts of the external world have evolved in the history of Western thought, from a naïve realism toward an increasing recognition of the role of the subject in all forms of cognition, including science. The two conceptual revolutions of modern physics both acknowledge the role of the observer in descriptions of phenomena observed. That is significant, because science traditionally brackets the role of the observer for the sake of a purely objective description of the world. The desirability of an objective description is self-evident, whether to facilitate control through technology or to achieve a possibly disinterested understanding. Yet the object cannot be truly separated from the subject, even in science.
Knowledge of the object tacitly refers back to the participation of the observer as a physical organism, motivated by a biologically-based need to monitor the world and regulate experience. On the other hand, knowledge may seem to be a mental property of the subject, disembodied as “information.” However, the subject is necessarily also an object: there are no disembodied observers. Information, too, is necessarily embodied in physical signals.
A characteristic of all physical processes, including the conveyance of signals, seems to be that they take time and involve transfers of energy. These facts could long be conveniently ignored in the case of information conveyed by means of light, which for most of human history seemed instantaneous and with negligible physical effect. Eventually, it was realized through observation (Eotvos), in experiment (Fizeau), and in theory (Maxwell) that the speed of light is finite and definite, though very large. Since that was true all along, it could have posed a conceptual dilemma for physicists long before the late 19th century, since the foundation of Newtonian physics was instantaneous action-at-distance. Even for Einstein and his contemporaries, however, the approach to problems resulting from the finite speed of light was less about incorporating the subject into an objective worldview than to compensate the subject’s involvement in order to preserve that worldview. Einstein’s initial motivation for relativity theory lay less in the observational consequences of the finite speed of light signals than in resolving conceptual inconsistencies in Maxwell’s electrodynamics.
Nevertheless, perhaps for heuristic reasons, Einstein began his 1905 paper with an argument about light signals, in which the signal was defined to travel with the same finite speed for all observers. This, of course, violated the foundational principle of the addition of velocities. It skirted the issue of the physical nature of the signal (particle or wave?), since some observations seemed to defy either the wave theory or the emission theory of light. Something had to give, and Einstein decided it was the concept of time. What remained implicit was the fact that non-local measurement of events in time or space must be made via intervening light signals.
When the distant system being measured is in motion with respect to the observer, the latter’s measurement will differ from the local measurement by an observer at rest in the distant system. The difference will be proportional to their relative speed compared to the speed of light. By definition, these are line of sight effects. By the relativity postulate, the effects must be reciprocal, so that whether the observers are approaching each other or receding, each would perceive the other’s ruler to have contracted and clock to have slowed! Such a conclusion could not be more contrary to common sense. But that meant simply that common sense is based on assumptions that may hold true only in limited circumstances (namely, when the observation is presumed instantaneous). In other words, circumstances that are non-physical.
The challenge embraced by Einstein was to achieve coherence within the framework of physics as a logical system, which is a human construct, a product of definitions. Physics may aim to reflect the structure of the real world, but invokes the freedom of the human agent to define its axioms and elements. Einstein postulated two axioms in his famous paper: the laws of physics are the same for observers in uniform relative motion; and the speed of light does not depend on the motion of its source. From these it follows that simultaneity can have no absolute meaning and that measurements involving time and space depend on the observers’ relative state of motion. In other words, the fact that the subject does not stand outside the system, but is a physical part of it, affects how the object is perceived or measured. Yet, a contrary meta-truth is paradoxically also insinuated: to the degree that the system is conceptual and not physical, the theorist does stand outside the system. Einstein’s freedom to choose the axioms he thought fundamental to a consistent physics implied the four-dimensional space-time continuum (the so-called block universe), which consists of objective events, not acts of observation.
Could other axioms have been chosen—alternatives to his postulates? Indeed, they had been. The problem was in the air in the late 19th century. In effect, Lorentz and FitzGerald had proposed that movement through the ether somehow causes a change in intermolecular forces, so that apparently rigid bodies in motion literally change shape in such a way that rulers “really” contract in length in the direction of motion. This was an ontological (electrodynamic) explanation of the null result of the crucial Michelson-Morley experiment. (Poincaré was also working on an ontological solution.) That approach made sense, since the space between atoms in solid bodies depends on electrical forces. Though Einstein knew about the Michelson-Morley experiment, his epistemic (kinematic) approach did not focus on that experiment, but originated with his reflections in a youthful thought experiment concerning what it would be like to travel along with a light beam. It continued with reflections on apparent contradictions in Maxwell’s electrodynamics. Yet, it returned to focus on the physical nature of light, which bore fruit in the equivalence of matter and energy and in General Relativity as a theory of gravitation.
Despite his early positivism, it was Einstein’s lifelong concern to preserve the objectivity, rationality and consistency of physics, the principle challenges to which were the dilemmas that gave birth to the two great modern revolutions, relativity and quantum theory. His solutions involved taking the observer into account, but with an aim to preserve an essentially observer-independent worldview—the fundamental stance of classical physics. While he chose an epistemic over an ontological analysis, he was deeply committed to realism. There were real, potentially observable, consequences to his theories, which have since been confirmed in many experiments. Yet alternative interpretations are conceivable, formulated on the basis of different axioms, to account for the same—mostly subtle—effects. While relativity theory renders appearances a function of the observer’s state of motion, it is really about preserving the form of physical laws for all observers—reasserting the possibility of objective truth.
One ironic consequence is that space and time are no longer considered from the point of view of the observer but are objectified in a god’s-eye view. The four-dimensional manifold is mathematically convenient; yet it also makes a difference in how we understand reality. As a theory of gravitation, General Relativity asserts the substantial existence of a real entity called spacetime. Space and time are no longer functions of the observer and of the means of observation (light); now they have an existence independent of the observer—ironically, much as Newton had asserted. What was grasped as a relationship returned to being a thing.
Even in the Special theory, there is confusion over the interpretation of time dilation. In SR, time dilation was initially a mutually perceived phenomenon, which makes sense as a line-of-site effect. In modern expositions, however, mechanical clocks are replaced by “light clocks,” and the explanation of time dilation refers to the lengthened path of light in the moving clock. This is no longer a line-of-site or mutual effect, since the light path is no longer in the direction of motion relative to the observer. Instead, it substitutes a definition of time that circularly depends on light. While “objective” in the sense that it is not mutual, the explanation for the gravitational time dilation of General Relativity rests on an incoherent interpretation of time dilation in SR.
Einstein derived both the famous matter-energy equivalence and General Relativity using arguments based on Special Relativity. These arguments slide inconsistently from an epistemic to an ontological interpretation. While the predictions of GR and E=mc2 may be accurate, their theoretical dependence on SR remains unfounded if the effects are purely epistemic: that is, if they do not invoke a physical interaction of things with an ether, when they accelerate with respect to it (the so-called clock hypothesis). Or, to put it the other way around, GR and the mass-energy equivalence actually imply such an interaction.
The Lorentz transformation could as well be interpreted in purely epistemic terms, of observers’ mutually relative state of motion, given the finite intermediary of light. Spacetime need not be treated as an object if the subject’s role is fully taken into account. The invariance of the speed of light could have a different interpretation, not as a cosmic speed limit but as a side-effect of light’s unique role as signal between frames of reference. Time dilation could have a different explanation, as a function of moving things physically interacting with an ether.