From Anil Ananthaswamy:
One of the joys of writing a book on modern physics, especially cosmology, is that you get to tackle some mind-expanding concepts, such as string theory and the multiverse. But when the book is about extreme experiments and how theory and experiment need to get back in lockstep if physics is to move forward, then string theory presents a peculiar challenge. On one hand, it remains far removed from experimental verification. On the other, it constitutes a significant chunk of physics theory these days, making it hard to ignore.
This was the challenge I faced while writing The Edge of Physics. How could I write about experiments and yet address string theory? I chose to focus on two ideas that are experimentally relevant, and have some connection to string theory.
The first is the possible discovery of supersymmetry at the Large Hadron Collider. Supersymmetry (SUSY) is an extension to the standard model of particle physics, and it posits that every particle we know of has a partner particle. The symmetry is between fermions and bosons, so that every known fermion has a supersymmetric bosonic partner, and every known boson has a supersymmetric fermionic partner. These supersymmetric particles would have existed in the early universe, and would have decayed soon after the symmetry was broken. Since we haven't seen any in particle colliders yet, such particles, if they exist, must be very heavy. The LHC is hoping to create supersymmetric particles, if supersymmetry exists at the teraelectronvolt (TeV) energy scale.
What's this got to do with string theory? Well, most string theory models require the universe to be supersymmetric. So, if the LHC finds SUSY, it would be a boost for string theory. But the universe can be supersymmetric without being stringy - so finding supersymmetry is not a proof of string theory.
Also, if the LHC does not find SUSY, it does not mean the universe is not supersymmetric - it could exist at energy scales beyond the LHC's reach. So, not discovering supersymmetry at the LHC does not disprove string theory.
The other experiment that is of some relevance to string theory is the precise measurement of the curvature of the universe. For all practical purposes, spacetime seems to be flat, the curvature equal to zero. But the error bars on existing measurements are enough to allow for either a slightly positive or negative curvature. And experiments like the European Space Agency's PLANCK satellite and the proposed Square Kilometre Array (the world's largest radio telescope) will measure the curvature of spacetime with great precision. (The photo at the top of this post shows a prototype dish for the Karoo Array Telescope near Johannesburg; the SKA might have 3000 such dishes...)
Again, what's this got to do with string theory? There's a controversial prediction from string theory that the curvature of spacetime should be ever-so-slightly negative (an open universe). This comes from the idea of the landscape of string theory, a collection of 10500 or more vacua of spacetime. Our universe would have emerged through a series of tunneling events, as one vacuum morphed into another, eventually ending up with ours. (Check out this week's classic article, "The Universe's Odyssey?" for more about that.) This process predicts a slightly negative curvature. A measurement of a positive curvature, however small, would cause considerable consternation for this model of how our universe emerged.
Given the preoccupation with string theory (among groups both for and against it), it's no wonder that almost each and every time I have talked about The Edge of Physics, someone in the audience has invariably asked: What can these experiments tell us about string theory? It's clear that string theory is not just a significant chunk of physics research, it has also grabbed people's imagination. Whether we'll have any answers soon is open to debate.
