October 30, 2007 feature
Brane trust: tunneling and stringy physics
“As is often the case in science, everybody contributes their piece, forming a complete picture only after years hard work,” Amanda Weltman tells PhysOrg.com. Weltman, a scientist at the University of Cape Town and at Cambridge University, believes that she and her collaborators have found another piece of that puzzle, especially with regard to string theory. “We took a tool developed in quantum field theory and adapted it to study stringy physics.”
Weltman and her colleagues, Adam Brown and Saswat Sarangi at Columbia University, and Benjamin Shlaer at the University of Colorado, explain their work in a piece titled “Enhanced Brane Tunneling and Instanton Wrinkles,” published in Physical Review Letters.
Weltman says she is most interested in understanding dynamics in what is known in string theory as the landscape. “In the early years of string theory, people hoped that there would be a unique ground state describing our universe.” She describes a valley between two hills, and then points out that there are other valleys with similar low points, or vacua.
“There may be a landscape of such vacua with many different ways that each can be reached,” she continues. “This realization of a whole landscape of possible values has opened new questions for the field.”
Weltman says that she and her collaborators “asked a very basic question: If the universe were in one of the myriad of such vacua then how long would it stay there?” To answer this question they “studied tunneling between different vacua, including stringy degrees of freedom.”
“The concept of tunneling has been around in quantum theory for a while,” she points out. “The notion is that if you have a quantum particle hitting a wall, the probability of it appearing on the other side of the wall is non-zero, unlike its classical counterpart. We are now studying such tunneling in the context of strings and branes in string theory rather than just particles in quantum theory.”
Keeping with the idea of the valley, Weltman explains that a field in a valley would try to get to the other side of the hill – and into another valley. “You would think that the higher the barrier, the harder it would be, much as if you tried to cycle over a hill.” She pauses. “It didn’t work that way. The higher the barrier, the easier the tunneling was.”
Weltman says that using the old quantum techniques to study the more complicated landscape of string theory brought out quite a different answer than many would expect. She speaks of tunneling in terms of D-branes, which are located at the ends of strings. These branes represent the boundaries of the strings in string theory. Weltman visualizes it as one brane at either end of a length of licorice.
In an email, Weltman expounds on how the brane tunneling works: “Our interpretation of this result is that rather than the field tunneling through, a new brane-antibrane pair is created on the other side, and the antibrane tunnels back and annihilates the original brane. By raising the barrier height, the nucleation of such pairs, and consequently tunneling, is enhanced. You get faster tunneling than you would naively expect.”
Right now, like much of string theory, this brane tunneling concept is in its early stages. “The next thing is to study concrete models.” Weltman says it is already being worked on. “We want to be as specific as possible, and see how it changes what people have looked at before.”
She continues: “In other areas of research, people are studying these vacua and looking for classes of vacua that would give us the features we expect of the standard model – the particle masses, their couplings and the correct number of generations. To explain why we are in such a vacuum state, we must understand the dynamics of string theory.
The difficulty lies in getting it right. “It’s not easy getting everything at the same time, especially when we include the cosmological constant,” Weltman admits. “Any mechanism which drives us to small values of the cosmological constant, but keeps us from zero, would be compelling.”
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