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2Physics Quote:
"The exchange character of identical particles plays an important role in physics. For bosons, such an exchange leaves their quantum state the same, while a single exchange between two fermions gives a minus sign multiplying their wave function. A single exchange between two Abelian anyons gives rise to a phase factor that can be different than 1 or -1, that corresponds to bosons or fermions, respectively. More exotic exchanging character are possible, namely non-Abelian anyons. These particles have their quantum state change more dramatically, when an exchange between them takes place, to a possibly different state." -- Jin-Shi Xu, Kai Sun, Yong-Jian Han, Chuan-Feng Li, Jiannis K. Pachos, Guang-Can Guo
(Read Full Article: "Experimental Simulation of the Exchange of Majorana Zero Modes"

Sunday, April 22, 2007

Superstring Theory: 5 Needed Breakthroughs
-- John H. Schwarz

John H. Schwarz (photo credit: Patricia Schwarz)

[In 2001, Prof. John Preskill of Caltech wrote a poem "To John Schwarz" (full text here) which started as ..

"Thirty years ago or more
John saw what physics had in store.
He had a vision of a string
And focused on that one big thing."

The name 'Schwarz' is intimately associated with the origin and evolution of Superstring theory. In 1971 John Schwarz and André Neveu developed an early version of superstring theory, which led among other things to the discovery of supersymmetry. In 1974 Joël Scherk and he proposed that string theory should be used to construct a unified quantum theory containing gravitation.

In 1984 Michael Green and he discovered an anomaly cancellation mechanism, which resulted in string theory becoming one of the hottest areas in theoretical physics. As Prof. Preskill's poem describes:

If you weren't there you couldn't know
The impact of that mightly blow:
"The Green-Schwarz theory could be true ---
It works for S-O-thirty-two!"

John Schwarz is the Harold Brown Professor of Theoretical Physics at California Institute of Technology (Caltech) where he taught and conducted research since 1972. He received the Dirac Medal of the International Centre for Theoretical Physics, Trieste in 1989, as well as the Dannie Heineman Prize for Mathematical Physics of the American Physical Society in 2002. He was a fellow of the MacArthur Foundation in 1987 and in 1997 he was elected to the National Academy of Sciences.

Prof. Schwarz coauthored a new string theory textbook entitled `String Theory and M-Theory: A Modern Introduction,' which was published earlier this year by Cambridge University Press.

We can't resist ending this note quoting again from John Preskill's poem:

Because he never would give in,
Pursued his dream with discipline,
John Schwarz has been a hero to me.
So please, don't spell it with a "t"!

Ladies and Gentlemen, it's our honor and privilege to share with you the excitement of superstring theory by presenting this list of 5 breakthroughs that John Schwarz would like to see.
-- 2Physics.com Team]

5 most important breakthroughs that I would like to see in

by John H. Schwarz

(1) Discovery of supersymmetry at the Large Hadron Collider (LHC):

Supersymmetry is an intrinsic feature of superstring theory, and therefore I am convinced that it exists at a fundamental level. The big question is whether it is broken at a sufficiently low energy (the TeV scale) that supersymmetry partner particles can be discovered at the LHC. There are several well-known arguments for why this is likely. Discovery of supersymmetry would not prove that superstring theory is the correct fundamental theory and nondiscovery would not prove that it is wrong. Still, if it is discovered, string theory would deserve credit for spawning the study of supersymmetry in the first place.

The experimental discovery of superpartner particles (and hence supersymmetry)would be very exciting for several reasons: It would set the agenda for experimental particle physics for decades to come ensuring the vitality of high-energy physics research. It would be enormously informative, leading eventually to the formulation of a "supersymmetric standard model'' extending the current standard model to much higher energies. Such a supersymmetric standard model would provide a much better target for string theorists to try to relate to Planck scale physics, where string theory is most directly applicable, by "top-down reasoning". String theorists would like to predict all of this in advance, of course,but that does not seem to be possible.

(2) Other experimental evidence for string theory:

Aside from supersymmetry, there are a number of other possible experimental signals for string theory that have been considered, and there may be others that nobody has thought of yet. In my opinion, all of the following are unlikely to be observed, because the Planck scale (the natural energy scale of quantum gravity) is so far beyond what is experimentally accessible. However, there are scenarios in which quantum gravity phenomena can extend to much lower energies, and thereby possibly become observable, which certainly are worth exploring. The methodologies for making such a discovery fall into two broad categories: astronomical/cosmological observations and accelerator experiments. The first category can look for cosmic strings, primordial gravity waves, and certain subtle features of the cosmic microwave background. Accelerators, such as the LHC, can look for signals indicating the presence of extra dimensions, black holes, gravitons, or fundamental strings.

(3) More fundamental formulation of string theory/M-theory; emergent spacetime :

The current understanding of string theory is based on perturbation theory expansions of various symmetrical limits supplemented by a beautiful web of conjectured duality relations. What is missing is a single complete formulation of the theory that accounts for these various symmetrical limits and dualities. Such a formulation is likely to implement some deep principle that has not yet been recognized. It is also likely to be completely unique without any adjustable parameters or other features that can be altered.

There are various reasons to believe that the existence of space and time is not something built into the theory itself, but rather emerges as a property of certain classes of solutions. If this is correct, the theory will be radically different from any previous physical theory all of which describe what happens in a given spacetime. Even Einstein's theory of gravity (the general theory of relativity), in which the geometry of spacetime is determined dynamically,assumes the prior existence of a spacetime manifold.

(4) Determine whether time is emergent and clarify the status of quantum mechanics:

The previous item suggested that space and time are emergent properties of solutions to string theory rather than intrinsic features of the underlying theory. There is considerable evidence for the emergence of spatial dimensions in various settings, but there is no compelling evidence for the emergence of time. Experience with relativity makes it hard to imagine that space and time could be radically different in this regard. On the other hand, the notion of time is central in quantum mechanics, which is formulated as unitary time evolution. If time is emergent, some extension of the rules of quantum mechanics would seem to be required. The consistency of string theory requires that quantum mechanics is exactly correct. I am not questioning that this will continue to be the case in the future, only that quantum mechanics may need to be generalized somewhat to extend its domain of applicability.

(5) Determine the correct solution of the theory:

A unique equation can have many different solutions. By the same token, string theory can describe a rich variety of physical realities. We are still in the early stages of mapping out the possibilities, but the indications are that the number of possibilities is enormous. The picture that has been proposed, whose validity is not completely evident, is that there is an energy function that is a complicated function of many variables (called moduli) and that each of the minima of this function corresponds to a different solution of the theory. Assuming its validity, this picture raises a lot of questions: How is the "correct'' solution (i.e., the one that describes the Universe that we observe) determined? Is it a cosmological accident or is there some other principle? How can we determine the correct solution? How much empirical information needs to be input in order to determine it uniquely and make everything else computable (in principle)? These types of questions are very important to explore. They are stimulating a lot of serious research, as well as some spirited debate that is even spilling over into the public domain.

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