Forget Stardust: You are made of spacetime

The most accurate theories ever invented by humankind are quantum mechanics and the general theory of relativity. Both were—and are— somewhat contradictory to common sense and hence widely disputed. Both have been tested extensively and their predictions, common sense aside, have proven accurate in both cases out to the twelfth decimal place or better. Quantum mechanics deals with the physics of the very small and general relativity with the very large, and so neither is particularly observable in everyday life. Unfortunately, each of these theories retains older assumptions about the nature of reality which the other contradicts. Quantum mechanics retains a Newtonian view of space and time; general relativity a Newtonian view of the relation between observer and observed. They can't both be right. Reconciling these views has been the principal unfinished task of physics for at least half a century.

For a while, all theories attempting reconciliation rested on extensions of the huge unification achieved with the introduction of quarks around 1964, that theory now being called the "Standard Model". Unfortunately, several reasons have been found to make it necessary to try to replace the Standard Model with a better theory.

At least three possibilities have been mooted in the last quarter century, each of them having partially slipped into the public consciousness at least peripherally: string theory, supersymmetry, and quantum gravity. None of the three are yet subject to experimental verification or denial and hence all three could on legitimate grounds be spurned as non-scientific in nature. But obviously science, particularly hard science, involves a theoretical stage as well as a verification stage and it is probably quite reasonable to view these as scientific theories which happen to remain in the theoretical stage at the moment. Other theories including relativity and some predictions of quantum mechanics have historically required a long hiatus between theory and empirical verification.

Another related puzzle which has long tormented contemporary physics is the nature of the "zoo" of fundamental particles discovered rather haphazardly during the Twentieth Century. While the Standard Model has allowed us to discover much about these particles and their relations, there is no a priori reason known why the mass of the electron, say, should be exactly what it is, or how many other fundamental electron-like particles may exist. Such very basic questions remain completely unanswered. In addition to trying to reconcile relativity with quantum mechanics, all three of the newer theories mentioned attempt to explain the nature of the "fundamental" particles currently known empirically, using even simpler notions as the building-blocks. The well-known string theory, for example, simply rests on the assumption that particles are not 0-dimensional points but rather are the oscillations of little tiny "fundamental" 1-dimensional curves, called "strings".

Lately, despite the popularity of Brian Greene's The Elegant Universe, physicists have started to abandon string theory for other more promising pastures. The alternative theory of quantum gravity has begun to attract more adherents as some of its ideas begin to be fleshed out and some surprising results obtained. This theory posits the existence of "fundamental" particles called "preons", which can be conceived of as ribbons that can somehow interact by wrapping around each other. It is supposed that these ribbons can cross over and under each other to form a braid when three preons come together to make a particle. Individual ribbons can also twist clockwise or counter-clockwise along their length. The theories predict that such structures, essentially individual building blocks in the fabric of spacetime, are so ephemeral as to disappear after 10^{-44} seconds, yet electrons for example are much longer-lived.

In a radical reworking of the theory, quantum loop gravity, these braids of particles can be viewed as qubits (quantum bits), the entire universe of spacetime being thought of as one huge connected network of isolated units, functioning in essence as a single enormous quantum computer. It turns out that such a conception allows the braided preons to have a much longer-lasting lifespan than individual preons, essentially because of mutual harmonic resonance when they are together in their braided state. The leading theorists in this field have lately been able to show that this model can produce the lightest particles in the standard model—the electron, the "up" and "down" quarks, the electron neutrino and their antimatter partners. That is not "proof" but it is a huge hint from the universe that they might be on the right track. The theory has some startling implications.

Meanwhile, Markopoulou's vision of the universe as a giant quantum computer might be more than a useful analogy: it might be true, according to some theorists. If so, there is one startling consequence: space itself might not exist. By replacing loop quantum gravity's chunks of space with qubits, what used to be a frame of reference - space itself - becomes just a web of information. If the notion of space ceases to have meaning at the smallest scale, Markopoulou says, some of the consequences of that could have been magnified by the expansion that followed the big bang. "My guess is that the non-existence of space has effects that are measurable, if you can only see it right." Because it's pretty hard to wrap your mind around what it means for there to be no space, she adds.

Another clue that this exciting new theory may be on the right track is its ability to predict Newton's laws, essentially from nothing at all.

For loop quantum gravity to succeed as a fundamental theory of gravity, it should at the very least predict that apples fall to Earth. In other words, Newton's law of gravity should naturally arise from it. It is a tall order for a theory that generates space and time from scratch to describe what happens in the everyday world, but Carlo Rovelli at the University of the Mediterranean in Marseille, France, and his team have succeeded in doing just that. "Essentially we have calculated Newton's law starting from a world with no space and no time," he says.

To summarize, the new theory of loop quantum gravity imagines a universe which is little more than a huge quantum computer, whose individual computing elements form the basis of spacetime, and when they become wrapped up together into mathematically precise braids whose symmetry groups are the well-known "braid groups", they form what we have come to know as matter. If you are interested in further reading, you might find Roger Penrose's new book The Road to Reality useful, as it provides a good explanation of some parts of the new theory, based on spinors.