Vesselin Petkov's Relativity and the Nature of Spacetime (2005) promises to specifically address "conceptual questions" relating to special relativity, such as "the physical meaning of the relativity of simultaneity" (p. 1, Petkov's emphasis), so this book should be right up my street.
As it turns out, it is anything but. The reason is that, far from probing the foundations of special relativity, Petkov takes them for granted. What is more, he takes a literal interpretation of those foundations for granted, no questions asked.
More specifically, of the three questions I have raised in previous posts:
Petkov answers none. In fact, he doesn't even mention Einstein's clock adjustment procedure or the issue of clock synchronization, so in that sense he has even less to say about these issues than Leo Sartori in Understanding Relativity (1996), if that's possible.
How then does Petkov explain the principle of the constancy of the speed of light? He says it's a consequence of Galileo's principle of relativity, which according to him states that "by performing mechanical experiments, the uniform motion of a body cannot be detected" (p. 26). Petkov later makes it clear that by "motion" he means "absolute motion" relative to "some kind of medium" that pervades all of space (pp. 30-32).
Petkov seems to take the empirical finding that the two-way speed of light is always c for granted since he moves straight to the issue of the one-way speed of light. He argues that observers who "determine" or "see" or "observe" that the speed of light is not the same in every direction "would discover their absolute uniform motion", and since that's not possible according to the relativity principle they cannot observe any such thing (pp. 40-41).
What Petkov fails to realize, or to mention, is that in the first instance the constancy of the speed of light in every direction is not a matter that can be "determined" or "seen" or "observed" because before we can make such an observation we first need to adjust distant clocks. To do that, we need to decide on a clock adjustment procedure. We could, for example, try to adjust our clocks such that those clocks are synchronized, in other words such that equal time coordinates define a relationship of simultaneity.
As Einstein observed, two clocks may be synchronized by setting them to zero when a signal sent in all directions from the mid-point between the two clocks arrives there - provided the conditions of signal emission and propagation are symmetrical in all directions. Concepts such as one-way speed, simultaneous existence and causality are interwoven with and depend on this notion of simultaneity.
But, as Max Born suggested, it is difficult to be sure whether the conditions in which light or anything else propagates are symmetrical in every direction because we don't know the full list of parameters on which such symmetry might depend. I have proposed that local acceleration history might be one such parameter that is missing from current models, but this is just an idea at this stage, an idea which I will have to explore in much greater detail in the months and years to come.
Einstein adopted a different approach: he effectively proposed that we should dispense with the requirement of synchronization and instead adjust our clocks such that the laws of physics, in particular those of electrodynamics, take on a particularly simple form. Out of that, the elegant theory of special relativity was born. Elegant certainly, but with the drawback that the concepts of one-way speed, simultaneous existence and causality are no longer fully applicable in that theory.
There is thus more to special relativity than Petkov's formulation of Galileo's principle of relativity. In particular, special relativity is partly based on Einstein's choice of clock adjustment procedure with all its conceptual consequences. This issue is completely lost to Petkov. Instead, he takes Einstein's clock adjustment procedure and the idea that it represents a synchronization procedure for granted. Literally. For he concludes that according to relativity every event is simultaneous with every other in this world and that we therefore live in a block universe in which the flow of time is just an illusion - a typical example of the simultaneity syndrome in modern physics.
Over my last few blog entries, I have looked at the way in which three contemporary specialists on special relativity discuss the role of decision-making or convention in the principle of the constancy of c, in textbooks or monographs that set out to clarify the main conceptual issues relating to special relativity. What I have found is that neither Sartori (1996) nor Rindler (2001) nor Petkov (2005) even mention, let alone discuss, the issue of whether or in what sense Einstein's clock adjustment procedure is a synchronization procedure, and only Rindler suggests an answer to the question of why it may be useful or advisable to adopt Einstein's procedure.
I think the time has now come for me to turn my attention to some other texts in the literature which I hope and believe offer deeper answers to my questions. The first of these is an article published in 1977 by Reza Mansouri and Roman U. Sexl, followed finally by Kevin Brown's very important book, Reflections on Relativity (2010).
As it turns out, it is anything but. The reason is that, far from probing the foundations of special relativity, Petkov takes them for granted. What is more, he takes a literal interpretation of those foundations for granted, no questions asked.
More specifically, of the three questions I have raised in previous posts:
- how decision-making contributes to the principle of the constancy of the speed of light;
- why it may be useful or advisable to adjust clocks in line with Einstein's procedure;
- and whether or in what sense Einstein's clock adjustment procedure can be regarded as a synchronization procedure and whether, therefore, it leads to meaningful statements about one-way speeds, simultaneous existence and causality
Petkov answers none. In fact, he doesn't even mention Einstein's clock adjustment procedure or the issue of clock synchronization, so in that sense he has even less to say about these issues than Leo Sartori in Understanding Relativity (1996), if that's possible.
How then does Petkov explain the principle of the constancy of the speed of light? He says it's a consequence of Galileo's principle of relativity, which according to him states that "by performing mechanical experiments, the uniform motion of a body cannot be detected" (p. 26). Petkov later makes it clear that by "motion" he means "absolute motion" relative to "some kind of medium" that pervades all of space (pp. 30-32).
Petkov seems to take the empirical finding that the two-way speed of light is always c for granted since he moves straight to the issue of the one-way speed of light. He argues that observers who "determine" or "see" or "observe" that the speed of light is not the same in every direction "would discover their absolute uniform motion", and since that's not possible according to the relativity principle they cannot observe any such thing (pp. 40-41).
What Petkov fails to realize, or to mention, is that in the first instance the constancy of the speed of light in every direction is not a matter that can be "determined" or "seen" or "observed" because before we can make such an observation we first need to adjust distant clocks. To do that, we need to decide on a clock adjustment procedure. We could, for example, try to adjust our clocks such that those clocks are synchronized, in other words such that equal time coordinates define a relationship of simultaneity.
As Einstein observed, two clocks may be synchronized by setting them to zero when a signal sent in all directions from the mid-point between the two clocks arrives there - provided the conditions of signal emission and propagation are symmetrical in all directions. Concepts such as one-way speed, simultaneous existence and causality are interwoven with and depend on this notion of simultaneity.
But, as Max Born suggested, it is difficult to be sure whether the conditions in which light or anything else propagates are symmetrical in every direction because we don't know the full list of parameters on which such symmetry might depend. I have proposed that local acceleration history might be one such parameter that is missing from current models, but this is just an idea at this stage, an idea which I will have to explore in much greater detail in the months and years to come.
Einstein adopted a different approach: he effectively proposed that we should dispense with the requirement of synchronization and instead adjust our clocks such that the laws of physics, in particular those of electrodynamics, take on a particularly simple form. Out of that, the elegant theory of special relativity was born. Elegant certainly, but with the drawback that the concepts of one-way speed, simultaneous existence and causality are no longer fully applicable in that theory.
There is thus more to special relativity than Petkov's formulation of Galileo's principle of relativity. In particular, special relativity is partly based on Einstein's choice of clock adjustment procedure with all its conceptual consequences. This issue is completely lost to Petkov. Instead, he takes Einstein's clock adjustment procedure and the idea that it represents a synchronization procedure for granted. Literally. For he concludes that according to relativity every event is simultaneous with every other in this world and that we therefore live in a block universe in which the flow of time is just an illusion - a typical example of the simultaneity syndrome in modern physics.
Over my last few blog entries, I have looked at the way in which three contemporary specialists on special relativity discuss the role of decision-making or convention in the principle of the constancy of c, in textbooks or monographs that set out to clarify the main conceptual issues relating to special relativity. What I have found is that neither Sartori (1996) nor Rindler (2001) nor Petkov (2005) even mention, let alone discuss, the issue of whether or in what sense Einstein's clock adjustment procedure is a synchronization procedure, and only Rindler suggests an answer to the question of why it may be useful or advisable to adopt Einstein's procedure.
I think the time has now come for me to turn my attention to some other texts in the literature which I hope and believe offer deeper answers to my questions. The first of these is an article published in 1977 by Reza Mansouri and Roman U. Sexl, followed finally by Kevin Brown's very important book, Reflections on Relativity (2010).
