This weeks New Scientist features four articles by Mike Duff on M-Theory in which he explains the motivations behind it and answers his critics. It is worthy that New Scientist has allowed him to attack some earlier articles in the magazine that attempted to compare cosmic strings with pseudoscience, and M-Theory with religion. My impression is that more people are beginning to realize that there are good reasons why many of the best theorists are not giving up on string theory just because a few people use such rhetoric to try to discredit its successes.
- Theory of everything: The big questions in physics
- Theory of everything: The road to unification
- Theory of everything: Have we now got one?
- Theory of everything: Answering the critics
M-Theory came to prominence in 1995 when Ed Witten started to take the idea of supermemberane theories in 11 dimensions seriously, but its history goes back to at least 1987 when Mike Duff and others classified the possibilities for membrane theories in various dimensions. They showed that the recently discovered superstring theories might emerge from dimensional reductions with the membranes wrapped round to form the strings. Physicists still don’t have a full description of the dynamics of these membranes but a partial solution is provided by Matrix Models.
In his New Scientist article, Mike Duff explains how M-Theory came about. It is important to appreciate that it is not just a wild idea that someone came up with at random. It follows from a need to bring together the standard model of particle physics with general relativity in a way free of the infinities that plague some approaches. The five Superstring theories in 10 dimensions are the only obvious solutions to this problem and they can all be unified into a unique framework using M-Theory. No other approach answers the same questions.
But M-theory is not without its problems. There is an embarrassment of choice when you look at ways to reduce it to 4 spacetime dimensions in order to match it to physics accessible to experiment. It is hoped that the Large Hadron Collider will discover supersymmetry bringing some hope that a connection between string theories and physics at reachable energies is possible. The trouble is that string theory does not make a definitive prediction that supersymmetry will be observed, and conversely the existence of supersymmetry does not necessarily imply string theory. At best we can say is that there is a correlation between these two ideas so the discovery or not of supersymmetry in the Higgs sector will have a strong influence on the acceptability of string theory.
A second unresolved problem with M-Theory is the absence of a full non-perturbative formulation that is required to make possible any analysis of its phenomenology at the Planck scale. These shortcomings have been explored in a paper on the arXiv last week by Steve Giddings. Mike Duff has identified some relationships between string theories and the information theory of qubits that might just be the first signs of where to look for such a formulation. In work with Borsten, Dahanayake, Ebrahim Marrani and Rubens, Duff has explored a subtle relationship between the classification of STU black holes and 4 qubit entanglement. He takes pains to stress that for the moment at least they “are only claiming that it is useful, not deep.”
The idea that the laws of physics emerges from the dynamics of information has been around for some time and has been boosted in recent years by the theoretical success of the holographic principle and entropic gravity. Whether or not this is a way to understand the fundamentals of M-theory is unclear. It’s a hard problem but not without hope.
Having been lucky enough to meet Mike Duff and some of his students, I know that he remains committed to his work on M-theory and the search for a deeper understanding of its principles. He is unusually open to new ideas but is quick to get to the mathematical details and dismiss anything that simply does not work out. It is not so hard to invent ideas using some persuasive numerology that sound good through the written word, but nature prefers the sound logic of equations.