Physics News Update
Number 745, September 15, 2005
by Phil Schewe and Ben Stein

Why Do We Reside in a Three-Dimensional Universe?
Andreas Karch (University of Washington) and Lisa Randall (Harvard) propose
to explain why we live in three dimensions and not some other number.
Currently, the popular string theory of matter holds that our universe is
actually ten-dimensional, including, first of all, the dimension of time,
then the three "large" dimensions we perceive as
"space," plus six more dimensions that are difficult to see,
perhaps because they are hidden in some way. There is reason to believe,
therefore, that our common 3D space is but a portion of some membrane or
"brane" within a much more complicated higher-dimensional
reality. Specifically, Karch and Randall address themselves to the behavior
of three-dimensional force laws, including the force of gravity. Having
several dimensions rolled up is one way to explain why gravity if so weak.

Another view, pioneered by Randall and Raman Sundrum, holds that if gravity
is localized on a 3D defect in the larger multi-dimensional universe and if
spacetime is sufficiently warped, then the other spatial dimensions might
be large after all. But why is our "local gravity" apparently a
3D defect in a 10D universe? Why not a 4D defect or some other
dimensionality?

In the present paper, Karch (karch{at}feynman.phys.washington.edu) and Randall
show that the cosmic evolution of the 10D universe, involving a steady
dilution of matter, results in spacetime being populated chiefly by 3D and
7D branes. Several versions of string theories require the existence of 3D
and 7D branes; indeed, the particles that constitute matter---such as
quarks and electrons---can be considered open strings with one end planted
on a 3D brane and the other end planted on a 7D brane.

Karch and Randall, Physical Review Letters, upcoming article


The "Cheerios" Effect
The tendency for certain floating things to clump under the action of
surface tension---things such as Cheerios cereal bits in your breakfast
bowl, bubbles in a glass of beer, pepper flakes on water, even strands of
hair up against a washbasin---has important potential engineering
implications, such as for the design of self-assembling circuits and
devices.

Study of the clumping phenomenon has a long history. For example, an
excellent summary was prepared by no less than James Clerk Maxwell for the
Encyclopedia Britannica as long ago as 1875. Now a Harvard professor,
Lakshminarayanan Mahadevan, and an undergraduate student, Dominic Vella
(now a graduate student at Cambridge University), have taken up the subject
and written a pedagogical review, hoping to rescue the subject from the
obscuring algebraic complexity that has settled around it (as Mahadevan
argues) and concentrate on the pertinent relatively simple physics
principles. They emphasize that contrary to general belief, chemical
interactions are oftentimes not paramount in determining whether clumping
occurs; instead a simple equilibrium of forces and torques---including
things such as buoyancy and surface tension---are the deciding factors.

Even objects denser than water can float if the geometry is right: See this
picture of a floating thumbtack. Even more interestingly, one can control
the strength and sign of this interaction; indeed, there are indications
that insects that live on the air-water interface might even use this
effect to great advantage.

Vella and Mahadevan, American Journal of Physics, September 2005
Contact: lm{at}deas.harvard.edu
Also see lab's website

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