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In this closing chapter we thought it only fitting to reintroduce Thanu Padmanabhan's Hypothetically Alert Relativist Open to Logical Discussions (Harold) who, having engaged much with the popular media and with his classical relativist background, has some probing questions about string theory. Making her debut here as his correspondent is Steph, a ‘String Theorist of Endless Patience and some Humility’.
Harold: It seems to me that there has been an enormous amount of resources spent on the various quantum gravity programs. Is there an actual proof that gravity has to be quantized at all?
Steph: Well, that depends on what you think would constitute a proof. Does classical mechanics have to be quantized? Apparently. Do we have a proof to that effect? No. We have a theory, it makes predictions and seems to agree with nature so we accept it. By the nature of the whole enterprise though, if we encounter a prediction that is wrong then we have to give up the theory. So, does gravity need to be quantized? I don't know. What I do know is that it is a fundamental interaction. The other three fundamental interactions all seem to have consistent quantum descriptions and, personally, I find that three out of four having quantum descriptions and only one being completely classical is unappealing. But maybe this is just the way nature is.
Harold: Where does the need to quantize gravity come from except for a belief in unification which may or may not be satisfied in reality?
Steph: I would say that it comes from a belief that at sufficiently small scales all the interactions exhibit quantum behaviour.
After almost a century, the field of quantum gravity remains as difficult and inspiring as ever. Today, it finds itself a field divided, with two major contenders dominating: string theory, the leading exemplification of the covariant quantization program; and loop quantum gravity, the canonical scheme based on Dirac's constrained Hamiltonian quantization. However, there are now a number of other innovative schemes providing promising new avenues. Encapsulating the latest debates on this topic, this book details the different approaches to understanding the very nature of space and time. It brings together leading researchers in each of these approaches to quantum gravity to explore these competing possibilities in an open way. Its comprehensive coverage explores all the current approaches to solving the problem of quantum gravity, addressing the strengths and weaknesses of each approach, to give researchers and graduate students an up-to-date view of the field.
“The effort to understand the Universe is one of the very few things that lifts human life a little above the level of farce, and gives it some of the grace of tragedy.”
Steven Weinberg, The First Three Minutes, 1997
After almost a century, the field of quantum gravity remains as difficult, frustrating, inspiring, and alluring as ever. Built on answering just one question – How can quantum mechanics be merged with gravity? – it has developed into the modern muse of theoretical physics.
Things were not always this way. Indeed, inspired by the monumental victory against the laws of Nature that was quantum electrodynamics (QED), the 1950s saw the frontiers of quantum physics push to the new and unchartered territory of gravity with a remarkable sense of optimism. After all, if nothing else, gravity is orders of magnitude weaker than the electromagnetic interaction; surely it would succumb more easily. Nature, it would seem, is not without a sense of irony. For an appreciation of how this optimism eroded over the next 30 years, there is perhaps no better account than Feynman's Lectures on Gravitation. Contemporary with his epic Feynman Lectures on Physics, these lectures document Feynman's program of quantizing gravity “like a field theorist.” In it he sets out to reverse-engineer a theory of gravity starting from the purely phenomenological observations that gravity is a long-range, static interaction that couples to the energy content of matter with universal attraction.