Once Before Time - Martin Bojowald [10]
Einstein had to struggle for a long time before he understood the right principles and the required mathematics, but his work was eventually crowned with immense success. Not only did the theory satisfy the highest demands of mathematics, a field in which it continues to provide important stimulus for further research, but later it was also able to explain many observations that Newton’s theory could not.
Such long-lasting implications surely justify the great interest in Einstein’s work; but in the last decades, alas, this success has often turned into a curse. In wide circles of physicists, the prevailing opinion often seems to be that general relativity is already completely understood and experimentally fully verified. Sometimes such a view is even used to justify cutting back research, and with it jobs, in this area. A complete verification of a theory is in any case never possible, and for this reason alone we should never forgo new experiments that might provide independent comparisons of theory and observations—especially with such an important and fundamental idea as general relativity. The set of experiments testing a theory can, at any given time, cover only a limited range of phenomena. An experimentally tested theory may be successful to a certain degree, but we can never be certain that it correctly describes all processes to which it can in principle be applied. Just as Newton’s theory was consistent with observations for a long time, until it was recognized as a special case of general relativity with a limited range of validity, general relativity, too, could turn out to be a limiting case of an unknown theory yet to be found. Even on a purely theoretical basis, relativity remains incompletely understood; there are many unanswered questions, of direct importance in particular for cosmology, and thus an acute need for research remains. There is indeed mounting evidence that general relativity itself needs to be extended.
Most physical theories are established through a long and tedious process starting from a creative idea, or in other cases from an observation not explicable by available knowledge. An idea may be followed up on because it might appear attractive from an aesthetic or mathematical perspective; a new, unexplained experimental result might force us to change current theories so that they agree with the new observation. Such a process can go on for decades, and it keeps scores of physicists busy—theoretical as well as experimental ones. Many currently hot theories, such as those of particle physics or quantum gravity, are still subjected to this process. The development of quantum mechanics also followed this course for a long time, until it was cast in its currently accepted form. (Even here, many foundational questions remain open; from the perspective of physical applications, however, quantum mechanics can be said to be understood.) The end result, as it enters textbooks later on, is often hardly recognizable compared with the early formulations; many a historical contribution has turned out to be unimportant, too complicated, or just plain wrong. For theories still in development, it is not even clear whether they will ever become a solid part of our worldview; entire branches of physics can come to a dead end, even though research always provides lessons that then become important elsewhere.
The situation was completely different when Einstein devised general relativity. Einstein alone, supported only by a few friends such as Marcel Grossmann and in a certain competition with the mathematician David Hilbert, provided the decisive work. Not all contributions followed a direct step-by-step route, and some of the published articles did turn out to be fruitless. But in a relatively short amount of time he completed his work, which soon proved successful in confrontations with observations. One may easily