We have studied the elements of Einstein’s special theory of relativity, and can now put them together into a more panoramic view. He began by assuming the truth of two principles, both drawn from experience and experiments: the principle of relativity; and the constant speed of light. From these two central principles, Einstein and his followers deduced a series of stunning consequences, of which we have met several in turn during the previous chapters:
• time dilation
• relativity of simultaneity
• length contraction
• symmetry of effects
• relativistic mass increase
• energy–mass conversion
• celestial speed limit
• invariance of the spacetime interval.
These are predictions about what observation and measurement will discover, that is, about phenomena and appearances. We have not explored the details of the arguments Einstein gave for deducing these effects from his principles. It is enough here to state that they are con- sequences of the principles and have been confirmed by experiments.
These two principles and their predicted consequences together form the theory: Einstein’s special theory of relativity. Note that this deals only with measurements made by equipment moving inertially (say, carried by a coasting spaceship). Einstein removed this restriction in his general theory, which we will examine in Chapter 13. Clearly, there is much that is puzzling and mysterious about the special theory of relativity. Why is the speed of light, unlike all other moving things, constant? Why are steady speeds undetectable? Why are physical laws the same regardless of speed? Why do distances and durations and masses depend on speed? Einstein’s theory does not answer these questions. At best it explains one mystery only by postulating another. It is content to assume its principles, make predictions and subject those to experimental test. This has frustrated many physicists and philosophers, who have therefore gone beyond the bare bones of the theory by interpreting it, and saying what it implies about the reality beneath appearances. We can now compare the two interpretations we have studied.
The mainstream interpretation:
• was originated by Einstein in 1905 and Minkovski in 1908
• asserts that distances and durations are not real properties – they are relations
• and therefore asserts that there is no objective present
• and therefore asserts that we live in a four-dimensional universe
• and therefore favours the block universe view – the past and future exist
• and therefore favours an event ontology without real change or movement.
As emphasized earlier, not all of those who defend the mainstream view accept all these points. Most physicists probably accept that distances and durations are not real properties, that there is no objective present and that we live in a four-dimensional universe.
However, most do not speculate about the existence of the past or the future. As the arguments above showed, though, if simultaneity is really relative, the block universe view may be unavoidable – as several prominent physicists have thought.
The mainstream interpretation seems to adhere cautiously to Einstein’s theory. For example, since the theory predicts that times depend on who measures them, it concedes that these are not real and objective. But any attempt to spell out what this implies about change and the reality of a four-dimensional spacetime soon encounters unpleasant implications. What begins as a minimalist interpretation seems, by the end, implausible.
It should be noted that Einstein’s own views were always complicated and shifted considerably during his long career. It is best to be careful and not assume that he finally favoured any single interpretation of relativity.
The minority interpretation:
• was developed by Lorentz and defended with variations by others
• asserts that distances and durations are real properties, but vary with speed (relative to the ether, etc.)
• asserts that we live in a three-dimensional space with time flowing
• asserts that there is an objective present
• and therefore is compatible with presentism – only the present exists
• and therefore is compatible with an ontology of persistent, changing objects.
The minority interpretation proposes an elaborate ontology that leads to many satisfying explanations, but also creates new puzzles without leading to any new predictions and experimental support.
In 1913, after years of struggle with special relativity, Lorentz rather wistfully summarized the debate between the two interpreta- tions – a debate he was losing:
According to Einstein, it has no meaning to speak of [the true] motion relative to the ether. He likewise denies the existence of [invariant and] absolute simultaneity. It is certainly remarkable that these relativity concepts, also those concerning time, have found such a rapid acceptance.
The acceptance of these concepts belongs mainly to epistemology [i.e. to philosophy, since no experiment yet compels us to adopt one view or the other]. It is certain, how- ever, that it depends to a large extent on the way one is accustomed to think whether one is most attracted to one or another interpretation. As far as this lecturer is concerned, he finds a certain satisfaction in the older interpretations, according to which the ether possesses at least some substantiality, space and time can be sharply separated, and simultaneity is not relative.
Finally, it should be noted that the daring assertion that one can never observe velocities larger than the velocity of light contains a hypothetical restriction on what is accessible to us, a restriction which cannot be accepted without some reservation. (Lorentz, 1913) This last point shows extraordinary foresight. Lorentz did not know that, some 80 years later, new experiments would hint at the existence of faster-than-light effects and revive his interpretation of relativity in some quarters. These historic experiments have weakened the dominance of the mainstream interpretation and have renewed hopes for the minority interpretation, as we investigate below.
The debate over relativity theory is very much alive, and perhaps will only be settled by readers of this book and the coming generation. This is a time of great progress, and of deepening mysteries.