Once Before Time - Martin Bojowald [39]
It is initially mathematics that raises the concept of theory in physics to a much higher status than is granted colloquially. Theories are based on simple principles readily accepted by experienced physicists, but they imply much less obvious statements, often following only after long calculations.4 Such implications, in contrast to underlying principles, can be subjected to experimental tests. If these are passed successfully, the theory is established; otherwise it will soon be forgotten. In this sense, special and general relativity are indeed theories.
Hypotheses, by contrast, are much less secured and often remain to be completed by a systematic theory—unless they are disproved in the process. A rather influential hypothesis, held throughout the nineteenth century but eventually disproved, was the postulate of the ether, a pervading medium thought to support the wavelike excitations that we perceive as light. As a consequence of this hypothesis, light rays moving in various directions on Earth—all aimed differently with respect to Earth’s motion through the space-filling ether—should move at different speeds. Several experiments were undertaken to detect such velocity changes, and thereby possibly provide insights into the nature of the ether. Finally, Albert Michelson and Edward Morley were able to conclude in 1887 that light moves with the same speed in all directions, disproving the concept of the ether. A short while later, this observation found a natural explanation within special relativity. A successful example is the light particle hypothesis, referred to thus even by its originator, Einstein. In those days, around 1905, the atomic composition of matter was not accepted at all; assuming the existence of photons as particles of light must have appeared objectionable to many. Even though Einstein was able to derive Planck’s formula for black body radiation from this hypothesis—a considerable success—the assumption itself remained to be secured further. Only much later did this hypothesis mature into a theory, culminating in modern quantum electrodynamics as the theory of photons and their interactions with electrically charged matter.
What is nowadays referred to as quantum gravity is, strictly speaking, not yet a theory even though many independent (but only mathematical) tests have successfully been done. The final demonstration of complete consistency is missing, and so far not a single supporting observation is available (though no observations clearly contradicting such theories exist either). Even so, these edified thoughts are more than just hypotheses, and for simplicity’s sake they are often called theories—or sometimes, a little less boldly, called frameworks: The frame is clear, but much of the interior remains to be filled in. All the work done so far in quantum gravity, including what is discussed in this book, is, as a scientific formulation, situated between a hypothesis and a full-fledged theory; the field must still be considered speculative. Only observations, possibly of a kind described later, will be able to change this status.
4. QUANTUM GRAVITY
COMBINING EVERYTHING
When Paul Dirac was working on a combination of special relativity and quantum mechanics in the years leading up to 1928, he was driven neither by serious conceptual