Fifty Orders of Magnitude Stretch Physics to the Limits

Fifty Orders of Magnitude Stretch Physics to the Limits

by Dr. Simon Mitton

WHAT'S HOT IN PHYSICS...

Rank	Paper	Citations This Period Nov- Dec 98	Rank Last Period Sep- Oct 98
1	T. Banks, et al., "M theory as a matrix model: a conjecture," Phys. Rev. D, 55(8):5112-28, 15 April 1997. [Rutgers U., Piscataway, NJ; U. Texas, Austin; Stanford U., CA] *WV250	42	1
2	H.L. Lai, et al., "Improved parton distributions from global analysis of recent deep inelastic scattering and inclusive jet data," Phys. Rev. D, 55(3):1280-96, 1 February 1997. [Michigan St. U., Lansing; Argonne Natl. Lab., IL; Southern Methodist U., Dallas, TX; Florida St. U., Tallahassee; U. Oregon, Eugene] *WF297	23	6
3	M.R. Douglas, et al., "D-branes and short distances in string theory," Nucl. Phys. B, 485(1,2):85-127, 3 February 1997. [Rutgers U., Piscataway, NJ] *WH433	19	†
4	A. Hanany, E. Witten, "Type IIB superstrings, BPS monopoles, and three-dimensional gauge dynamics," Nucl. Phys. B, 492(1,2):152-90, 12 May 1997. [Inst. Advanced Study, Princeton, NJ] *WZ677	19	†
5	C.C. Bradley, C.A. Sackett, R.G. Hulet, "Bose-Einstein condensation of lithium: Observation of limited condensate number," Phys. Rev. Lett., 78(6):985-9, 10 February 1997. *WG888	18	†
6	S.S. Gubser, I.R. Klebanov, "Absorption by branes and Schwinger terms in world volume theory," Phys. Lett. B, 413(1,2):41-8, 6 November 1997. [Princeton U., NJ] *YH890	17	†
7	E. Witten, "Solutions of four-dimensional field theories via M-theory," Nucl. Phys. B, 500(1-3):3-42, 1 September 1997. [Inst. Advanced Study, Princeton, NJ] *XW954	16	†
8	P. Madau, et al., *"High-redshift galaxies in the Hubble Deep Field: Colour selection and star formation theory* Z ~ 4," Mon. Not. Roy. Astron. Soc., 283(4):1388-1404, 15 December 1996. [Space Telescope Sci. Inst., Baltimore, MD; Carnegie Observ., Pasadena, CA; Palomar Observ., Pasadena, CA] *WA235	14	†
9	I.R. Klebanov, "World-volume approach to absorption by non-dilatonic branes," Nucl. Phys. B, 496(1,2):231-42, 7 July 1997. [Princeton U., NJ] *XJ490	14	†
10	N. Seiberg, "Why is the matrix model correct?" Phys. Rev. Lett., 79(19):3577-80, 10 November 1997. [Inst. Advanced Study, Princeton, NJ] *YF186	14	†

SOURCE: ISI's Hot Papers Database. the full legend.

Once again the Physics Top Ten shows the extraordinary confrontation between the smallest and largest entities that researchers study. The reductionists are convoked in #6, 7, 9 and 10 with the latest complexities of superstring theory. Meanwhile, the big picture is confronted in #8, which addresses star and element formation in the universe.

What is superstring theory trying to do? It is an attempt at a theory of quantum gravity, and it follows an enduring theme of fundamental physics, the unification of the forces. Time and again in the history of physics, the intellectual puzzles of diverse phenomena have been solved by a small number of underlying principles and building blocks. This is what the Hot Papers are all about: finding the new principles and building blocks.

Quantum field theory, in the form of the Standard Model, is incredibly powerful for explaining many observables. But it has limitations which signal the road ahead has principles that unify the fields of the Standard Model in a simpler structure, and do away with the horrendous divergence problems. Newtonian physics is still highly applicable in the real world, but relativistic physics is better. So too with string theory: as it is unravelled it will supersede quantum field theory.

We start with a tool kit of strings of Planck length, which is 10^-35m. Strings can be open or closed loops, and the resulting quantum physics depends on the vibrations of strings and their interactions with the topology of the space where they exist. The next question is whether one is looking at a string theory for bosons (the particles that carry forces) or fermions (the particles that make matter).

Bosonic string theory has 26 spacetime dimensions and a particle with imaginary mass, the tachyon. Worse still, the theory has no matter. So we need matter. That means an equal matching of mass particles (fermions) and force particles (bosons), which is achieved by invoking symmetries that are realized in superstring theory.

There are five kinds of superstring theories, all with 10 spacetime dimensions. Their field equations have 4 large flat dimensions and 6 small curved dimensions. Quantum mechanics in 10 dimensions, does it work? Yes, because its 4-dimensional physics converges to the Standard Model. But how we descend from superstrings to the four spacetime dimensions we observe in our world is tricky. There are many possible ways to go from 10 dimensions to 4. The process of compactification of the unwanted dimensions leads to much interesting physics.

The Top Ten shows that theorists are now embroiled in a new superstring revolution. For example, #6 and #9 both deal with Dirichlet brane (D-brane) descriptions of black holes. "Superstring theory contains soliton-like membranes called D-branes," Igor Klebanov of Princeton University tells Science Watch. "A D-brane is like a topological defect: when a closed string touches it, it can open up and turn into an open string whose ends are free to move along the D-brane." The theoretical confrontation revolves around two very different descriptions of how D-branes stack up, and #6 and #9 are among the first papers to explore the relation between the two approaches. Klebanov (#6) finds "precise agreement between the two in the low energy limit, which is an important step towards showing that the two are different descriptions of the same object."

Chugging in at #10 is Nathan Seiberg, Institute for Advanced Study, Princeton, on M-theory (Matrix Model). The number of string theories has been shrinking in recent years because theorists are discovering that what they once thought were completely different theories are in fact different ways of looking at the same theory! The biggest rush is to collapse all into one: M-theory, for it is the Mother of all theories. Seiberg tells Science Watch: "The matrix model had been conjectured to be a non-perturbative exact definition of string theory. The derivation in #10 involves a careful definition of compactification light-like direction as a limit of a compactification space-like direction, and shows that under such a compactification string theory becomes equivalent to the Matrix Model."

If you can't get your head around 10^-35m try 10⁺²⁶m instead! This is the scale of modern physics as measured by the Top Ten! Fifty orders of magnitude majestically sweep us from strings to #8 and the Hubble Deep Field, a deep core sample of the universe grabbed by the Hubble Space Telescope in a 10-day exposure. First author Piero Madau explains to Science Watch: "Our work shows that a substantial population of star forming galaxies were already present when the universe was only 10% of its present age. We provide a first sketch of the history of star formation and metal enrichment throughout cosmic time, starting with the first 10⁺⁹ years." Paper #8 is being cited because the star-formation history it describes confronts many theories of galaxy formation.

Dr Simon Mitton directs science and professional publishing
for Cambridge University Press, Cambridge, UK