Text 637, 66 rader
Skriven 2005-10-18 07:04:35 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 745
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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@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@deas.harvard.edu
Also see lab's website

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 * Origin: Big Bang (1:106/2000.7)