From: baez@math.removethis.ucr.andthis.edu (John Baez)
Subject: This Week's Finds in Mathematical Physics (Week 290)
Organization: University of California, Riverside
Sender: baez@math.removethis.ucr.andthis.edu (John Baez)
Newsgroups: sci.math.research
Also available at http://math.ucr.edu/home/baez/week290.html
January 15, 2010
This Week's Finds in Mathematical Physics (Week 290)
John Baez
This week we'll start with a math puzzle, then a paper about
categorification in analysis. Then we'll continue learning about
electrical circuits and their analogues in other branches of physics.
We'll wrap up with a bit more rational homotopy theory.
But first: here's an image that's been making the rounds lately.
What's going on here?
http://math.ucr.edu/home/baez/007962_2635.jpg
Next: a math puzzle! This was created by a correspondent who wishes
to remain anonymous. Here are some numbers. Each one is the number
of elements in some famous mathematical gadget. What are these
numbers  and more importantly, what are these gadgets?
How many minutes are in an hour?
How many hours are in a week?
How many hours are in 3 weeks?
How many feet are in 1.5 miles?
How many minutes are in 2 weeks?
How many inches are in 1.5 miles?
How many seconds are in a week?
How many seconds are in 3 weeks?
The answers are at the end.
The wave of categorification overtaking mathematics is finally hitting
analysis! I spoke a tiny bit about this in "week274", right after I'd
finished a paper with Baratin, Freidel and Wise on infinite
dimensional representations of 2groups. I thought it would take a
long time for more people to get interested in the blend of
2categories and measure theory that we were exploring. After all,
there's a common stereotype that says mathematicians who like
categories hate analysis, and vice versa. But I was wrong:
1) Goncalo Rodrigues, Categorifying measure theory: a roadmap,
available as arXiv:0912.4914.
Read both papers together and you'll get a sense of how much there is
to do in this area! A lot of basic definitions remain up for grabs.
For example, Rodrigues' paper defines "2Banach spaces", but will his
definition catch on? It's too soon to tell. There are already lots
of theorems. And there's no shortage of interesting examples and
applications to guide us. But finding the best framework will take a
while. I urge anyone who likes analysis and category theory to
jump into this game while it's still fresh.
But my own work is taking me towards mathematics of a more applied
sort. My excuse is that I'll be spending a year in Singapore at the
Centre for Quantum Technologies, starting in July. This will give me
a chance to think about computation, and condensed matter physics, and
quantum information processing, and diagrams for physical systems
built from pieces. Such systems range from the humble electrical
circuits that I built as a kid, to integrated circuits, to fancy
quantum versions of these things.
So, lately I've been talking about a set of analogies relating various
types of physical systems. I listed 6 cases where the analogies
are quite precise:
displacement flow momentum effort
q q' p p'
Mechanics position velocity momentum force
(translation)
Mechanics angle angular angular torque
(rotation) velocity momentum
Electronics charge current flux voltage
linkage
Hydraulics volume flow pressure pressure
momentum
Thermodynamics entropy entropy temperature temperature
flow momentum
Chemistry moles molar chemical chemical
flow momentum potential
This week I'd like to talk about five circuit elements that we can use
to build more complicated electrical circuits: resistors, inductors,
capacitors, voltage sources, and current sources. I'll tell you the
basic equations they obey, and say a bit about their analogues in the
mechanics of systems with translational degrees of freedom. They also
have analogues in the other rows.
Engineers call these five circuit elements "1ports". A 1port can be
visualized as a black box with 2 places where you can stick in a wire:



 
 



More generally, an "nport" has 2n places where you can attach a wire.
This numbering system may seem peculiar. Indeed, it overlooks
circuits that have an odd number of wires coming out, like this one
made of just wires:
\ /
\ /
\ /



You can use gizmos like this to stick together 1ports "in parallel":



/ \
/ \
/ \
 
   
 
\ /
\ /
\ /



However, if you've ever looked at the back of a TV or stereo, you'll
see that place where you can plug in cables tend to come in pairs!
Each pair is called a "port". So, electrical engineers often  though
not always  focus on nports, where the wires coming out are grouped
in pairs. out are grouped in pairs. And there's probably even a good
*mathematical* reason for paying special attention to these  something
related to symplectic geometry. That's one of the things I'm trying
to understand better.
Later I'll tell you about some famous 2ports and 3ports, but today
let's do 1ports. If we have a 1port with wires coming out of it, we
can arbitrarily choose one wire and call it the the "input", with
the other being the "output":

V


 
 


V

If you know a little category theory, this should seem suspiciously
similar to a "morphism". And if you know a bit more, this should
remind you of other situations where it takes an arbitrary choice to
distinguish between the "input" and the "output" of a morphism.
Any 1port has a "flow through it" and an "effort across it", which
which are functions of time. Remember, "flow" is the general concept
that reduces to current in the special case of electronics. "Effort"
is the concept that reduces to voltage in the special case of
electronics.
The time integral of flow is called the "displacement" and denoted q,
and the time integral of effort is called the "momentum" and denoted
p. So, flow is q' = dq/dt and effort is p' = dp/dt.
To mathematically specify a 1port, we give one equation involving p,
q, p', q', and the time variable t. Here's how it works for the five
most popular types of 1ports:
1. A "resistance". This is the general term for what we call a
"resistor" in the case of electrical circuits, and "friction" in
mechanics. In hydraulics, you can make a resistance using a
narrowed pipe.
In all cases, the effort is some function of the flow:
p' = f(q')
An easy special case is a linear resistance, for which the effort
is proportional to the flow:
p' = R q'
Here R is some constant, also called the "resistance". In electric
circuit theory this equation is called Ohm's law, and people write
it using different symbols. Note we need to be careful about our
sign conventions: in mechanics we usually think of friction as
giving force = R velocity with R *negative*, while in electric
circuit theory we usually think of an ordinary resistor as giving
voltage = R current with R *positive*. The two cases are not
fundamentally different: it's just an artifact of differing sign
conventions!
2. A "capacitance". This is the general term for what we call a
"capacitor" in the case of electrical circuits, or a "spring" in
mechanics. In hydraulics, you can make a capacitance out of a tank
with pipes coming in from both ends and a rubber sheet dividing it
in two.
In all cases, the displacement is some function of effort:
q = f(p')
An easy special case is a linear capacitance, for which the
displacement is proportional to the effort:
q = C p'
Here C is some constant, also called the "capacitance". Again
we need to be careful with our conventions: in mechanics we
usually think of a spring as being stretched by an amount
equal to 1/k times the force applied. Here k, the *reciprocal*
of C, is called the spring constant. But some engineers work
with C and call it the "compliance" of the spring. An easily
stretched spring has big C, small k.
3. An "inertance". This is the general term for what we call an
"inductor" in the case of electrical circuits, or a "mass" in
mechanics. The weird word "inertance" hints at how mass gives
a particle inertia. In hydraulics, you can build an inertance
by putting a heavy turbine inside a pipe: this makes the water
want to keep flowing at the same rate.
In all cases, the momentum is some function of flow:
p = f(q')
An easy special case is a linear inertance, for which the
momentum is proportional to the flow:
p = L q'
Here L is some constant, also called the "inertance". In the
case of mechanics, this would be the mass.
4. An "effort source". This is the general term for what we call
a "voltage source" in the case of electrical circuits, or an
"external force" in mechanics. In hydraulics, an effort source
is a compressor set up to maintain a specified pressure difference
between the input and output.
Here the equation is of different type than before! It can
involve the time variable t:
p' = f(t)
5. A "flow source". This is the general term for what we call
a "current source" in the case of electrical circuits.
In hydraulics, an flow source is a pump set up to maintain a
specified flow.
Here the equation is
q' = f(t)
It's interesting to ponder these five 1ports and how they form
families.
The voltage and current sources form a family, since only these
involve the variable t in an explicit way. Also, only these can be
used to add energy to a circuit. So, these two are called "active"
circuit elements.
The other three are called "passive". Among these, the capacitance
and inertance form a family because they both conserve energy. The
resistance is different: it dissipates energy  or more precisely,
turns it into heat energy, which is not part of our simple model. If
you're more used to mechanics than electrical circuits, let me
translate what I'm saying into the language of mechanics: a machine
made out of masses and springs will conserve energy, but friction
dissipates energy.
Let's try to make this "energy conservation" idea a bit more precise.
I've already said that p'q', effort times flow, has dimensions of
power  that is, energy per time. Indeed, for any 1port, the
physical meaning of p'q' is the rate at which energy is being put in.
So, in electrical circuit theory, people sometimes say energy is
"conserved" if we can find some function H(p,q) with the property that
dH(p,q)/dt = p'q'
This function H, called the "Hamiltonian", describes the energy stored
in the 1port. And this equation says that the energy stored in the
system changes at a rate equal to the rate at which energy is put in!
So energy doesn't get lost, or appear out of nowhere.
Now, when I said "energy conservation", you may have been expecting
something like dH/dt = 0. But we only get that kind of energy
conservation for "closed" systems  systems that aren't interacting
with the outside world. We'll indeed get dH/dt = 0 when we build a
big circuit with no inputs and no outputs out of circuit elements that
conserve energy in the above sense. The energy of the overall system
will be conserved, but of course it can flow in and out of the various
parts.
But of course it's really important to think about circuits with
inputs and outputs  the kind of gizmo you actually plug into the
wall, or hook up to other gizmos! So we need to generalize classical
mechanics to "open" systems: systems that can interact with their
environment. This will let us study how big systems are made of
parts.
But right now we're just studying the building blocks  and only the
simplest ones, the 1ports.
Let's see how energy conservation works for all five 1ports. For
simplicity I'll only do the linear 1ports when those are available,
but the results generalize to the nonlinear case:
1. The "resistance". For a linear resistance we have
p' = R q'
so the power is
p'q' = R (q')^2
In the physically realistic case R > 0 so this is nonnegative,
meaning that we can only put energy *into* the resistor.
And note that p'q' is *not* the time derivative of some
function of p and q, so energy is not conserved. We say the
resistance "dissipates" energy.
2. The "capacitance". For a linear capacitance we have
q = C p'
so the power is
p'q' = qq' / C
Note that unlike the resistor this can take either sign, even
in the physically realistic case C > 0. More importantly, in
this case p'q' is the time derivative of a function of p and q,
namely
H(p,q) = q^2 / 2C
So in this case energy is conserved. If you're comfortable
with mechanics you'll remember that a spring is an example of
a capacitance, and H(p,q) is the usual "potential energy" of a
spring when C is the reciprocal of the spring constant.
3. The "inertance". For a linear inertance
p = L q'
so the power is
p'q' = pp' / L
Again this can take either sign, even in the physically realistic
case L > 0. And again, p'q' is the time derivative of a function
of p and q, namely
H(p,q) = p^2 / 2L
So energy is also conserved in this case. If you're comfortable
with mechanics you'll remember that a mass is an example of
a inertance, and H(p,q) is the usual "kinetic energy" of a mass
when L equals the mass.
4. The "effort source". For an effort source
p' = f(t)
for some function f, so the power is
p'q' = f(t) q'
This is typically not the time derivative of some function of
p and q, so energy is not usually conserved. I leave it
as a puzzle to give the correct explanation of what's going on
when f(t) is a constant.
5. The "flow source". For a flow source
q' = f(t)
for some function f, so the power is
p'q' = f(t) p'
This is typically not the time derivative of some function of
p and q, so energy is not usually conserved. Again, I
leave it as a puzzle to understand what's going on when f(t)
is constant.
So, everything works as promised. But if your background in classical
mechanics is anything like mine, you should still be puzzled by the
equation
dH(p,q)/dt = p'q'
This is sometimes called the "power balance equation". But you mainly
see it in books on electrical engineering, not classical mechanics.
And I think there's a reason. I don't see how to derive it from a
general formalism for classical mechanics, the way I can derive dH/dt
= 0 in Hamiltonian mechanics. At least, I don't see how when we write
the equation this way. I think we need to write it a bit differently!
In fact, I was quite confused until Tim van Beek pointed me to a
nice discussion of this issue here:
2) Bernard Brogliato, Rogelio Lozano, Bernhard Maschke and Olav
Egeland, Dissipative Systems Analysis and Control: Theory and
Applications, 2nd edition, Springer, Berlin, 2007.
I'll say more about this later. For now let me just explain two
buzzwords here: "control theory" and "dissipative systems".
Traditional physics books focus on closed systems. "Control theory"
is the branch of physics that focuses on open systems  and how to
make them do what you want!
For example, suppose you want to balance a pole on your finger. How
should you move your finger to keep the pole from falling over?
That's a control theory problem. You probably don't need to read a
book to solve this particular problem: we're pretty good at learning
to do tricks like this without thinking about math. But if you wanted
to build a robot that could do this  or do just about anything 
control theory might help.
What about "dissipative systems"? I already gave an example: a
circuit containing a resistor. I talked about another in "week288": a
mass on a spring with friction. In general, a dissipative system is
one that loses energy, or more precisely converts it to heat. We
often don't want to model the molecular wiggling that describes heat.
If we leave this out, dissipative systems are not covered by ordinary
Hamiltonian mechanics  since that framework has energy conservation
built in. But there are generalizations of Hamiltonian mechanics
that include dissipation! And these are pretty important in practical
subjects like control theory... since life is full of friction, as
you've probably noticed.
So, this book covers everything *my* classical mechanics education
downplayed or left out: open systems, dissipation and control theory!
And in the chapter on "dissipative physical systems", it derives power
balance equations for "inputoutput Hamiltonian systems" and
"portcontrolled Hamiltonian systems". Apparently it's the latter
that describes physical systems built from nports.
For more on portcontrolled Hamiltonian systems, this book recommends:
3) B. M. Maschke and A. J. van der Schaft, Port controlled Hamiltonian
systems: modeling origins and system theoretic properties, in
Proceedings of the 2nd IFAC Symp. on Nonlinear Control Systems Design,
NOLCOS'92 (1992), pp. 282288,
4) B. M. Maschke and A. J. van der Schaft, The Hamiltonian formulation
of energy conserving physical systems with ports, Archiv fur
Elektronik und Ubertragungstechnik 49 (1995), 362371.
5) A. J. van der Schaft, L^2gain and Passivity Techniques in
Nonlinear Control, 2nd edition, Springer, Berlin, 2000.
So, I need I learn more about this stuff, and then explain it to you.
But let's stop here for now, and turn to... rational homotopy theory!
Nothing big this week: I just want to take stock of where we are.
I've been trying to explain a triangle of concepts:
RATIONAL SPACES
/ \
/ \
/ \
/ \
/ \
DIFFERENTIAL GRADED  DIFFERENTIAL GRADED
COMMUTATIVE ALGEBRAS LIE ALGEBRAS
In "week287" I explained a functor going down the left side of this
triangle. In fact I explained how we can get a differential graded
commutative algebra, or DGCA, from *any* topological space. This
involved a grand generalization of differential forms.
In "week289" I explained a functor going down the right side. In fact
I explained how we can get a differential graded Lie algebra, or DGLA,
from *any* space with a chosen baseopint. This involved a grand
generalization of Lie groups, and their Lie algebras.
Today I'd like to explain a sense in which all three concepts in this
triangle are "the same". I won't give you the best possible theorem
along these lines  just Quillen's original result, which is pretty
easy to understand. It says that three categories are equivalent: one
for each corner of our triangle!
I explained the first category back in "week286". I called it the
"rational homotopy category", and I described it in several ways.
Here's one. Start with the category where:
the objects are 1connected pointed spaces;
the morphisms are basepointpreserving maps.
Then, throw in formal inverses to all "rational homotopy equivalences"
 that is, maps
f: X > X'
that give isomorphisms between rational homotopy groups:
Q tensor pi_n(f): Q tensor pi_n(X) > Q tensor pi_n(X')
This gives the rational homotopy category.
The second category involves DGCAs. Well  actually not. To get the
nicest results, it seems we should work dually and use differential
graded *co*commutative *co*algebras, or DGCCs. I'm sorry to switch
gears on you like this, but that's life. The difference is "purely
technical", but I want to state a theorem that I'm sure is true!
In "week287" we saw how Sullivan took any space and built a DGCA whose
cohomology was the rational cohomology of that space. But today
let's follow Quillen and instead work with a DGCC whose homology is
the rational homology of our space.
So, let's start with the category of DGCC's over the rational
numbers  but not *all* of them, only those that are trivial in
the bottom two dimensions:
d d d
0 < 0 < C_2 < C_3 <
Why? Because our spaces are 1connected, so their bottom two
homology groups are boring. Then, let's throw in formal inverses to
"quasiisomorphisms"  that is, maps between DGCCs
f: C > C'
that give isomorphisms between homology groups:
H_n(f): H_n(C) > H_n(C')
The resulting category is *equivalent* to the rational
homotopy category!
The third category involves DGLAs. We start with the category of
DGLAs over the rational numbers  but not *all* of them, only
those that are trivial in the bottom dimension:
d d d
0 < L_1 < L_2 < L_3 <
Just the very bottom dimension, not the bottom two! Why? Because we
get a DGLA from the group of *loops* in our rational space, and
looping pushes down dimensions by one. Then, we throw in formal
inverses to "quasiisomorphisms"  that is, maps between DGLAs:
f: L > L'
that give isomorphisms between homology groups:
H_n(f): H_n(L) > H_n(L')
Again, the resulting category is *equivalent* to the rational homotopy
category!
So, we have a nice unified picture. We could certainly improve it in
various ways. For example, I haven't discussed the bottom edge of the
triangle. Doing this quickly brings in Linfinity algebras, which are
like DGLAs where all the laws hold only "up to chain homotopy". It
also brings in gadgets that are like DGCAs or DGCCs, but where all the
laws hold only up to chain homotopy. This outlook eventually leads us
to realize that we have something much better than three equivalent
categories. We have three equivalent (infinity,1)categories!
But there's also the question of what we can *do* with this triangle
of concepts. There are lots of classic applications to topology, and
lots of new applications to mathematical physics.
So, there's more to come.
As for the number puzzle at the beginning, all the numbers I listed
are the sizes of various "finite simple groups". These
are the building blocks from which all finite groups can be built.
You can see a list of them here:
6) Wikipedia, Finite simple groups,
http://en.wikipedia.org/wiki/List_of_finite_simple_groups
There are 16 infinite families and 26 exceptions, called "sporadic"
finite simple groups. Anyway, here we go:
* How many minutes are in an hour?
60, which is the number of elements in the smallest nonabelian finite
simple group, namely A_5. Here A_n is an "alternating group": the
group of even permutations of the set with n elements. By some
wonderful freak of nature, A_5 is isomorphic to both PSL(2,4) and
PSL(2,5). Here PSL(n,q) is a "projective special linear group": the
group of determinant1 linear transformations of an ndimensional
vector space over a field with q elements, modulo its center.
* How many hours are in a week?
168, which is the number of elements  or "order"  of the second
smallest nonabelian finite simple group, namely PSL(2,7). Thanks to
another marvelous coincidence, this is isomorphic to PSL(3,2). See
"week214" for a lot more about this group and its relation to Klein's
quartic curve and the Fano plane.
* How many hours are in 3 weeks?
504, which is the order of the finite simple group PSL(2,8).
* How many feet are in 1.5 miles?
7,920, which is the order of the finite simple group M_{11}  the
smallest of the finite simple groups called Mathieu groups. See
"week234" for more about this.
* How many minutes are in 2 weeks?
20,160, which is the order of the finite simple group A_8. Thanks to
another marvelous coincidence, this is isomorphic to PSL(4,2). And
there's also another nonisomorphic finite simple group of the same
size, namely PSL(3,4)!
* How many inches are in 1.5 miles?
95,040, which is the order of the finite simple group M_{12}  the
second smallest of the Mathieu groops. See "week234" for more about
this one, too.
* How many seconds are in a week?
604,800, which is the order of the finite simple group J_2  the
second Janko group, also called the HallJanko group. I don't know
anything about the Janko groups. They don't seem to have much in
common except being sporadic finite simple groups that were discovered
by Janko.
I like what the Wikipedia says about the third Janko group: it
"seems unrelated to any other sporadic groups (or to anything else)".
Unrelated to anything else? Zounds!
* How many seconds are in 3 weeks?
1,814,400, which is the order of the finite simple group A_{10}.
If you like this sort of stuff, you might enjoy this essay:
7) John Baez, Why there are 63360 inches in a mile?,
http://math.ucr.edu/home/baez/inches.html
It's a curious number:
63360 = 2^7 x 3^2 x 5 x 11
It's very unusual for the number 11 to show up in the British system
of units. Find out why it does here!
Finally, what about that image? Unsurprisingly, it's from Mars.
It shows a dune field less than 400 kilometers from the north pole,
bordered on both sides by more barren regions:
8) HiRISE (High Resolution Imaging Science Experiments),
Falling material kicks up cloud of dust on dunes,
http://hirise.lpl.arizona.edu/PSP_007962_2635
Some streaks on the dunes look like stands of trees lined up on
hilltops. It would be great if there were trees on Mars, but it's not
true. In fact what you're seeing are steep slopes with dark stuff
slowly sliding down them! Here's a description written by Candy
Hansen, a member of NASA's Mars Reconnaissance Orbiter team at the
University of Arizona:
There is a vast region of sand dunes at high northern latitudes on
Mars. In the winter, a layer of carbon dioxide ice covers the
dunes, and in the spring as the sun warms the ice it
evaporates. This is a very active process, and sand dislodged from
the crests of the dunes cascades down, forming dark streaks.
In the subimage falling material has kicked up a small cloud of
dust. The color of the ice surrounding adjacent streaks of material
suggests that dust has settled on the ice at the bottom after
similar events.
Also discernible in this subimage are polygonal cracks in the ice
on the dunes (the cracks disappear when the ice is gone).

Quote of the Week:
"The most important thing is to keep the most important thing the most
important thing."  Donald P. Coduto

Addenda: I thank Toby Bartels, Bruce Smith, and Don Davis of Boston for
some corrections.
The number of inches in a mile is divisible by 11 because there are
33/2 feet in a rod. For the explanation of *that*, see my webpage
cited above. But Don Davis pointed out that this is not the only
reason why the number 11 appears in the American system of units. A
US liquid gallon is 231 = 3 x 7 x 11 cubic inches! Does anyone know
why? Is this logically independent from the number of feet in a rod?
Why? According to Don Davis and the Wikipedia article on gallons, the
reason is that once upon a time, a British wine gallon was 7 inches
across and 6 inches deep  for some untold reason that deserves
further investigation. If we approximate pi by 22/7, the volume then
comes out to 3 x 7 x 11 cubic inches!
This 11ness of the gallon then infects other units of volume. For
example, a US liquid ounce is
3 x 7 x 11 / 2^7
cubic inches!
My friend Bruce Smith says that his young son Peter offered the
following correction to the Quote of the Week: it should really be
"The most important thing is to keep the 2nd most important thing
the 2nd most important thing"  because the first most important
thing is the topic of the sentence!
John McKay writes:
You say you don't know anything about the Janko groups. Let me
help you...
The first Janko group is a subgroup of G2(11). It is called J1 and
has order = 11.(11+1).(11^31) suggesting incorrectly it may be one
of a family. This is the first of the modern sporadics. Then came
J2 and J3 both having isomorphic involution centralizers. The first
was constructed by Marshall Hall and the second by Graham Higman
and me.
David Wales and I decided on the names so that J_k has a Schur
multiplier (=second cohomology group) of order k. J2 is the
HallJanko group. Janko finally produced his fourth group J4
(which unfortunately does not have a Schur multiplier of order 4)!
J1,J3, and J4 are among the Pariahs (as are O'Nan, Rud, LySims).
They are those sporadics that have no involvement with M =
the Monster group (see Mark Ronan's book).
This group, M, appears to have incredible connections with many
areas of mathematics and of physics. Its real nature has yet to be
revealed.
Best,
John
Here G2(11) is like the exceptional Lie group G2 except it's defined
over the field with 11 elements. So, the number 11 raises its ugly
head yet again!
For more discussion, visit the nCategory Cafe at:
http://golem.ph.utexas.edu/category/2010/01/this_weeks_finds_in_mathematic_51.html

Previous issues of "This Week's Finds" and other expository articles on
mathematics and physics, as well as some of my research papers, can be
obtained at
http://math.ucr.edu/home/baez/
For a table of contents of all the issues of This Week's Finds, try
http://math.ucr.edu/home/baez/twfcontents.html
A simple jumpingoff point to the old issues is available at
http://math.ucr.edu/home/baez/twfshort.html
If you just want the latest issue, go to
http://math.ucr.edu/home/baez/this.week.html