Also available as http://math.ucr.edu/home/baez/week225.html
December 24, 2005
This Week's Finds in Mathematical Physics - Week 225
John Baez
Happy holidays! I'll start with some gift suggestions for people
who put off their Christmas shopping a bit too late, before moving
on to this week's astronomy pictures and then some mathematical
physics: minimal surfaces.
Back in 2000 I listed some gift ideas in "week162". I decided to do
it again this year. After all, where else can you read about quantum
gravity, nonabelian cohomology, higher categories... and also get
helpful shopping tips? But, I put off writing this Week's Finds a
bit too late. Oh well.
I just saw this book in a local store, and it's GREAT:
1) Robert Dinwiddie, Philip Eales, David Hughes, Ian Nicholson, Ian Ridpath,
Giles Sparrow, Pam Spence, Carole Stott, Kevin Tildsley, and Martin Rees,
Universe, DK PUblishing, New York, 2005.
If you like the astronomy pictures you've seen here lately, you'll love
this book, because it's *full* of them - all as part of a well-organized,
clearly written, information-packed but nontechnical introduction to
astronomy. It starts with the Solar System and sails out through the
Oort Cloud to the Milky Way to the Local Group to the Virgo Supercluster
... and all the way out and back to the Big Bang!
The only thing this book seems to lack - though I could have missed it -
is a 3d map showing the relative scales of our Solar System, Galaxy, and
so on. I recommended a wall chart like this back in "week162", and my
friend Danny Stevenson just bought me one. I'll probably put it up
near my office in the math department... gotta keep the kids thinking big!
You don't really need to buy a chart like this. You can just look at
this website:
2) Richard Powell, An Atlas of the Universe,
http://www.anzwers.org/free/universe/
It has nine maps, starting with the stars within 12.5 light years and
zooming out repeatedly by factors of 10 until it reaches the limits of
the observable universe, roughly 14 billion light years away. Or more
precisely, 14 billion years ago!
(The farther we look, the older things we see, since light takes time to
travel. The most distant thing we see is light released when hot gas
from the Big Bang cooled down just enough to let light through! If we
calculate how far this gas would be *now*, thanks to the expansion of the
universe, we get a figure of roughly 78 billion light years. But of course
we can't see what that gas looks like *now* unless we wait a lot longer.
It's a bit confusing until you think about it for a while.)
I especially like the map of the galaxies within 100 light years of
us. The biggest of these is the Virgo cluster, which I discussed in
"week224". This contains about 2000 galaxies. The second biggest
is the Fornax cluster. The whole agglomeration is called the Virgo
Supercluster. Superclusters are among the biggest structures in the
universe.
This atlas is fun to browse when you're at your computer. But, if
someone you know wants to contemplate the universe in a more relaxing
way, try getting them one of these:
3) Bathsheba Grossman, Crystal model of a typical 100-megaparsec cube
of the universe, http://www.bathsheba.com/crystal/largescale/
Crystal model of the Milky Way, http://www.bathsheba.com/crystal/galaxy/
I found out about these from David Scharffenberg, who owns the Riverside
Computer Center nearby - a cool little shop that's decorated with archaic
technology ranging from a mammoth slide rule to a gizmo that computes
square roots using air pressure. He gave me the 100-megaparsec cube as
a present, and it's great! It's lit up from below, and it shows the
filaments, sheets and superclusters of galaxies that reign supreme
at this distance scale.
David says Grossman's model of the Milky Way is also nice: it takes into
account the latest research, which shows our galaxy is a "barred" spiral!
You can see the bar in the middle here:
4) R. Hurt, NASA/JPL-Caltech, Milky Way Bar,
http://www.spitzer.caltech.edu/Media/mediaimages/sig/sig05-010.shtml
If you really have money to burn, Grossman has also made nice sculptures
of mathematical objects like the 24-cell, the 600-cell and Schoen's
gyroid - a triply periodic minimal surface that chops 3-space into two parts:
5) Bathsheba Grossman, Mathematical models, http://www.bathsheba.com/math/
However, the great thing about the web is that lots of beautiful stuff
is free - like these *pictures* of the gyroid.
I explained the 24-cell and 600-cell in "week155". So, let me explain
the gyroid - then I need to start cooking up a Christmas eve dinner!
A "minimal surface" is a surface in ordinary 3d space that can't reduce
its area by changing shape slightly. You can create a minimal surface
by building a wire frame and then creating a soap film on it. As long
as the soap film doesn't actually enclose any air, it will try to minimize
its area - so it will end up being a minimal surface.
If you make a minimal surface this way, it will have edges along the wire
frame. A minimal surface without edges is called "complete". For about
200 years, the only known complete minimal surfaces that didn't intersect
themselves were the plane, the catenoid, and the helicoid. You get a
"catenoid" by taking an infinitely long chain and letting it hang to form
a curve called a "catenary"; then you use this curve to form a surface of
revolution, which is the catenoid:
6) Eric Weisstein, Catenoid, from Mathworld,
http://mathworld.wolfram.com/Catenoid.html
In cylindrical coordinates the catenoid is given by the
equation
r = c cosh(z/c)
for your favorite constant c.
A "helicoid" is like a spiral staircase; in cylindrical coordinates it's
given by the equation
z = c theta
for some constant c. You can see a helicoid here - and see how it
can continuously deform into a catenoid:
7) Eric Weisstein, Helicoid, from Mathworld,
http://mathworld.wolfram.com/Helicoid.html
In 1987 a fellow named Hoffman discovered a bunch more complete
non-self-intersecting minimal surfaces with the help of a computer:
8) D. Hoffman, The computer-aided discovery of new embedded minimal
surfaces, Mathematical Intelligencer 9 (1987), 8-21.
Since then people have gotten good at inventing minimal surfaces.
You can see a bunch here:
9) GRAPE (Graphics Programming Environment), Surface overview,
http://www-sfb256.iam.uni-bonn.de/grape/EXAMPLES/AMANDUS/bmandus.html
10) GANG (Geometry Analysis Numerics Graphics), Gallery of minimal
surfaces, http://www.gang.umass.edu/gallery/min/
As you can see, people who work on mininal surfaces like goofy acronyms.
If you look at the pictures, you can also see that a minimal surface
needs to be locally saddle-shaped. More precisely, it has "zero mean
curvature": at any point, if it curves one way along one principal
axis of curvature, it has to curve an equal and opposite amount along
the perpendicular axis. Supposedly this was proved by Euler.
If we write this requirement as an equation, we get a second-order nonlinear
differential equation called "Lagrange's equation" - a special case of
the Euler-Lagrange equation we get from any problem in the variational
calculus. So, finding new minimal surfaces amounts to finding new
solutions of this equation. Soap films solve this equation automatically,
but only with the help of a wire frame; it's a lot more work to find
minimal surfaces that are complete.
For the theoretical physicist, minimal surfaces also go by another
name: strings! The "worldsheet" of a bosonic string is just a 2-dimensional
surface in spacetime. The equation governing the string's motion just
says that the area of this surface can't be reduced by wiggling it
slightly. In other words, it's just the "Lagrange's equation"
I mentioned before. There's a big difference between string theory
and the theory of minimal surfaces, though: in string theory we need
to take quantum mechanics into account! (Another big difference is
that spacetime is a Lorentzian rather than Riemannian manifold, unless
we do a trick called "Wick rotation".)
So, bosonic string theory is about the quantum version of soap films -
and "D-branes" serve as the wire frames. If this reminds you of "spin
foams", there are a few big differences. The main thing is that
spin foams are background-free: they don't live in spacetime, they
*are* spacetime. So, it doesn't make any obvious sense for them to
minimize area, though Smolin has suggested it might make an *unobvious*
kind of sense. All the fun must happen when the "bubbles" of a spin
foam meet along edges... but we don't really know how this should work,
to create a foam with the right consistency at large scales.
Anyway....
There are a lot of minimal surfaces that have periodic symmetry in
3 directions, like a crystal lattice. You can learn about them here:
11) Elke Koch, 3-periodic minimal surfaces,
http://staff-www.uni-marburg.de/~kochelke/minsurfs.htm
In fact, they have interesting relations to crystallography:
12) Elke Koch and Werner Fischer, Mathematical crystallography
http://www.staff.uni-marburg.de/~kochelke/mathcryst.htm#minsurf
I guess people can figure out which of the 230 crystal symmetry groups
(or "space groups") can arise as symmetries of triply periodic minimal
surfaces, and use this to help classify these rascals. A cool mixture
of group theory and differential geometry! I don't get the impression
that they have completed the classification, though.
Anyway, Schoen's "gyroid" is one of these triply periodic minimal
surfaces. Schoen discovered it before the computer revolution kicked
in. He was working for NASA, and his idea was to use it for building
ultra-light, super-strong structures:
13) A. H. Schoen, Infinite periodic minimal surfaces without
selfintersections, NASA Tech. Note No. D-5541, Washington, DC, 1970.
You can learn more about the gyroid here:
14) Eric Weisstein, Gyroid, from Mathworld,
http://mathworld.wolfram.com/Gyroid.html
Apparently the gyroid is the only triply periodic non-self-intersecting
minimal surface with "triple junctions". I'm not quite sure what that
means mathematically, but I can see them in the picture!
I said that soap films weren't good at creating *complete* minimal
surfaces. But actually, people have seen at least approximate gyroids
in nature, made from soap-like films:
15) P. Garstecki and R. Holyst, Scattering patterns of self-assembled
gyroid cubic phases in amphiphilic systems, J. Chem. Phys. 115 (2001),
1095-1099.
An "amphiphilic" molecule is one that's attracted by water at one end
and repelled by water at the other. For example, the stuff in soap.
Mixed with water and oil, such molecules form membranes, and really
complicated patterns can emerge, verging on the biological. Sometimes
the membranes make a gyroid pattern, with oil on one side and water on
other! It's a great example of how any sufficiently beautiful mathematical
pattern tends to show up in nature somewhere... as Plato hinted in his
theory of "forms".
People have fun simulating these "ternary amphiphilic fluids" on computers:
16) Nelido Gonzalez-Segredo and Peter V. Coveney, Coarsening dynamics of
ternary amphiphilic fluids and the self-assembly of the gyroid and
sponge mesophases: lattice-Boltzmann simulations, available as
cond-mat/0311002.
17) Pittsburgh Supercomputing Center, Ketchup on the grid with joysticks,
http://www.psc.edu/science/2004/teragyroid/
The second site above describes the "TeraGyroid Project", in which
people used 17 teraflops of computing power at 6 different facilities
to simulate the gyroidal phase of oil/water/amphiphile mixtures and
study how "defects" move around in what's otherwise a regular pattern.
The reference to ketchup comes from some supposed relationship between
these ternary amphiphilic fluids and how ketchup gets stuck in
the bottle. I'm not sure ketchup actually *is* a ternary amphiphilic
fluid, though!
Hmm. I just noticed a pattern to the websites I've been referring
to: first one about a "Milky Way bar", then one about a "GRAPE", and
now one about ketchup! I think it's time to cook that dinner.
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