Superstring theory, perhaps the most notorious attempt to unify general relativity with quantum mechanics, was once described as a piece of twenty-first-century physics that fell into the twentieth century. Certainly, physicists struggling to marry these reluctant partners have long sensed that the crucial piece of the puzzle has yet to be invented. But a theory of quantum gravity remains more a question of tidiness than anything else — quantum effects on space-time become important only on time and length scales well out of experimental reach. A quantum field theory of electromagnetism has plenty of practical relevance for applied physics, and such a field theory for the nuclear forces generates no shortage of testable predictions. But gravity yields to the quantum rod only at the so-called Planck scale: over distances of about 10−33 cm and times of 10−43 s.
A physics to match the Planck timescale is the biggest challenge to physicists in the coming century. This is the timescale relevant to the graviton, the putative carrier of the gravitational force. And naturally enough, only such a theory can extend our understanding of the Big Bang inside the first 10−43 seconds of existence.
What will a theory of quantum gravity do to space-time? Will it be like John Wheeler's ‘quantum foam’, furiously alive with ephemeral black holes and worm holes? Or will it be more like Abhay Ashtekar's fabric of woven loops? Will particles become strings, and will the strings be supersymmetric? How many extra dimensions will turn up, curled out of sight over the Planck distance?
And will we ever be able to test such a theory? One quantum-gravity researcher has confessed that “it seems highly unlikely that a machine will ever be built with which these minute distances can be studied directly”. For guidance, it seems that physicists may have to fall back on the old stand-bys: elegance and beauty.
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Ball, P. Physics at the Planck time. Nature 402, C61 (1999). https://doi.org/10.1038/35011550