# April 20, 1996 {#week80}

There are a number of interesting books I want to mention.

Huw Price's book on the arrow of time is finally out! It's good to see
a philosopher of science who not only understands what modern physicists
are up to, but can occaisionally beat them at their own game.

Why is the future different from the past? This has been vexing people
for a long time, and the stakes went up considerably when Boltzmann
proved his "H-theorem", which seems at first to show that the entropy
of a gas always increases, despite the time-reversibility of the laws of
classical mechanics. However, to prove the H-theorem he needed an
assumption, the "assumption of molecular chaos". It says roughly that
the positions and velocities of the molecules in a gas are uncorrelated
before they collide. This seems so plausible that one can easily
overlook that it has a time-asymmetry built into it --- visible in the
word "before". In fact, we aren't getting something for nothing in
the H-theorem; we are making a time-asymmetric assumption in order to
conclude that entropy increases with time!

The "independence of incoming causes" is very intuitive: if we do an
experiment on an electron, we almost always assume our choice of how to
set the dials is not correlated to the state of the electron. If we drop
this time-asymmetric assumption, the world looks rather different...
but I'll let Price explain that to you.

Anyway, Price is an expert at spotting covertly time-asymmetric
assumptions. you may remember from ["Week 26"](#week26) that he
even got into a nice argument with Stephen Hawking about the arrow of
time, thanks to this habit of his. you can read more about it in:

1) Huw Price, _Time's Arrow and Archimedes' Point: New Directions for a Physics of Time_, Oxford University Press, 1996.

Also, there is a new book out by Hawking and Roger Penrose on quantum
gravity. First they each present their own ideas, and then they duke it
out in a debate in the final chapter. This book is an excellent place to
get an overview of some of the main ideas in quantum gravity. It helps
if you have a little familiarity with general relativity, or
differential geometry, or are willing to fake it.

There is even some stuff here about the arrow of time! Hawking has a
theory of how it arose, starting from his marvelous "no-boundary
boundary conditions", which say that the wavefunction of the universe
is full of quantum fluctuations corresponding to big bangs which erupt
and then recollapse in big crunches. The wavefunction itself has no
obvious "time-asymmetry", indeed, time as we know it only makes sense
*within* any one of the quantum fluctuations, one of which is presumably
the world we know! But Hawking thinks that each of these quantum
fluctuations, or at least most of them, should have an arrow of time.
This is what Price raised some objections to. Hawking seems to argue
that each quantum fluctuation should "start out" rather smooth near
its big bang and develop more inhomogeneities as time passes, "winding
up" quite wrinkly near its big crunch. But it's not at all clear what
this "starting out" and "winding up" means. Possibly he is simply
speaking vaguely, and all or most of the quantum fluctuations can be
shown to have one smooth end and wrinkly at the other. That would be an
adequate resolution to the arrow of time problem. But it's not clear,
at least not to me, that Hawking really showed this.

Penrose, on the other hand, has some closely related ideas. His "Weyl
curvature hypothesis" says that the Weyl curvature of spacetime goes to
zero at initial singularities (e.g. the big bang) and infinity at final
ones (e.g. black holes). The Weyl curvature can be regarded as a measure
of the presence of inhomogeneity --- the "wrinkliness" I alluded to
above. The Weyl curvature hypothesis can be regarded as a
time-asymmetric law built into physics from the very start.

To see them argue it out, read

2) Stephen Hawking and Roger Penrose, _The Nature of Space and Time_, Princeton University Press, 1996.

There are also a couple of more technical books on general relativity
that I'd been meaning to get ahold of for a long time. They both
feature authors of that famous book,

3) Charles Misner, Kip Thorne and John Wheeler, _Gravitation_, Freeman Press, 1973,

which was actually the book that made me decide to work on quantum
gravity, back at the end of my undergraduate days. They are:

4) Ignazio Ciufolini and John Archibald Wheeler, _Gravitation and Inertia_, Princeton University Press, 1995.

and

5) Kip Thorne, Richard Price and Douglas Macdonald, eds., _Black Holes: The Membrane Paradigm_, 1986.

The book by Ciufolini and Wheeler is full of interesting stuff, but it
concentrates on "gravitomagnetism": the tendency, predicted by general
relativity, for a massive spinning body to apply a torque to nearby
objects. This is related to Mach's old idea that just as spinning a
bucket pulls the water in it up to the edges, thanks to the centrifugal
force, the same thing should happen if instead we make lots of stars
rotate around the bucket! Einstein's theory of general relativity was
inspired by Mach, but there has been a long-running debate over whether
general relativity is "truly Machian" --- in part because nobody
knows what "truly Machian" means. In any event, Ciufolini and Wheeler
argue that gravitomagnetism exhibits the Machian nature of general
relativity, and they give a very nice tour of gravitomagnetic effects.

That is fine in theory. However, the gravitomagnetic effect has never
yet been observed! It was supposed to be tested by Gravity Probe B, a
satellite flying at an altitude of about 650 kilometers, containing a
superconducting gyroscope that should precess at a rate of 42
milliarcseconds per year thanks to gravitomagnetism. I don't know what
ever happened with this, though: the following web page says "Gravity
Probe B is expected to fly in 1995", but now it's 1996, right? Maybe
someone can clue me in to the latest news.... I seem to remember some
arguments about funding the program.

6) Gravity Probe B, `http://stugyro.stanford.edu/RELATIVITy/GPB/`

(Note added in 2002: now this webpage is gone; see `http://einstein.stanford.edu/` for the latest story.)

Kip Thorne's name comes up a lot in conjuction with black holes and the
LIGO --- or Laser-Interferometer Gravitational-Wave Observatory ---
project. As pairs of black holes or neutron stars spiral emit
gravitational radiation, they should spiral in towards each other. In
their final moments, as they merge, they should emit a "chirp" of
gravitational radiation, increasing in frequency and amplitude until
their ecstatic union is complete. The LIGO project aims to observe these
chirps, and any other sufficiently strong gravitational radiation that
happens to be passing by our way. LIGO aims to do this by using laser
interferometry to measure the distance between two points about 4
kilometers apart to an accuracy of about $10^{-18}$ meters, thus
detecting tiny ripples in the spaceteim metric. For more on LIGO, try

7) LIGO project home page, `http://www.ligo.caltech.edu/`

Thorne helped develop a nice way to think of black holes by envisioning
their event horizon as a kind of "membrane" with well-defined
mechanical, electrical and magnetic properties. This is called the
membrane paradigm, and is useful for calculations and understanding what
black holes are really like. The book "Black Holes: The Membrane
Paradigm" is a good place to learn about this.

------------------------------------------------------------------------

Now let me return to the tale of $2$-categories. So far I've said only
that a $2$-category is some sort of structure with objects, morphisms
between objects, and $2$-morphisms between morphisms. But I have been
attempting to develop your intuition for $\mathsf{Cat}$, the primordial example of
a $2$-category. Remember, $\mathsf{Cat}$ is the $2$-category of all categories! Its
objects are categories, its morphisms are functors, and its $2$-morphisms
are natural transformations --- these being defined in
["Week 73"](#week73) and again in
["Week 75"](#week75).

How can you learn more about $2$-categories? Well, a really good place is
the following article by Ross Street, who is one of the great gurus of
$n$-category theory. For example, he was the one who invented
$\omega$-categories!

8) Ross Street, "Categorical structures", in _Handbook of Algebra_, vol. **1**, ed. M. Hazewinkel, Elsevier, 1996.

Physicists should note his explanation of the yang-Baxter and
Zamolodchikov equations in terms of category theory. If you have trouble
finding this, you might try

9) G. Maxwell Kelly and Ross Street, _Review of the elements of $2$-categories_, Springer Lecture Notes in Mathematics **420**, Berlin, 1974, pp. 75--103.

I can't really compete with these for thoroughness, but at least let me
give the definition of a $2$-category. I'll give a pretty nuts-and-bolts
definition; later I'll give a more elegant and abstract one. Readers
who are familiar with $\mathsf{Cat}$ should keep this example in mind at all times!

This definition is sort of long, so if you get tired of it, concentrate
on the pictures! They convey the basic idea. Also, keep in mind is that
this is going to be sort of like the definition of a category, but with
an extra level on top, the $2$-morphisms.

So: first of all, a $2$-category consists of a collection of "objects"
and a collection of "morphisms". Every morphism $f$ has a "source"
object and a "target" object. If the source of $f$ is $x$ and its target
is y, we write $f\colon x\to y$. In addition, we have:

1) Given a morphism $f\colon x\to y$ and a morphism $g\colon y\to Z$, there is a
morphism $fg\colon x\to Z$, which we call the "composite" of $f$ and $g$.

2) Composition is associative: $(fg)h = f(gh)$.

3) For each object $x$ there is a morphism $1_x\colon x \to x$, called the
"identity" of x. For any $f\colon x\to y$ we have $1_x f = f 1_y = f$.

you should visualize the composite of $f\colon x\to y$ and $g\colon y\to Z$ as follows:
$$x\xrightarrow{f}y\xrightarrow{g}Z.$$
So far this is exactly the definition of a category! But a $2$-category
ALSO consists of a collection of "2-morphisms". Every $2$-morphism $T$ has
a "source" morphism $f$ and a target morphism $g$. If the source of $T$ is $f$
and its target is $g$, we write $T\colon f\Rightarrow g$. If $T\colon f\Rightarrow g$, we require that
$f$ and $g$ have the same source and the same target; for example, $f\colon x\to y$
and $g\colon x\to y$. you should visualize $T$ as follows:
$$\includegraphics[scale=0.3]{../images/Tnatftog.pdf}$$
In addition, we have:

1') Given a $2$-morphism $S\colon f\Rightarrow g$ and a $2$-morphism $T\colon g\Rightarrow h$, there is
a $2$-morphism $ST\colon f\Rightarrow h$, which we call the "vertical composite" of $S$
and $T$.

2') Vertical composition is associative: $(ST)U = S(TU)$.

3') For each morphism $f$ there is a $2$-morphism $1_f\colon f\Rightarrow f$, called the
"identity" of $f$. For any $T\colon f\Rightarrow g$ we have $1_f T = T 1_g = T$.

Note that these are just like the previous 3 rules. We draw the vertical
composite of $S\colon f\Rightarrow g$ and $T\colon g\Rightarrow h$ like this:
$$\includegraphics[scale=0.3]{../images/STvertical.pdf}$$
Now for a twist. We also require that we can "horizontally" compose
2-morphisms as follows:
$$\includegraphics[scale=0.3]{../images/SThorizontal.pdf}$$
So we also demand:

1'') Given morphisms $f,g\colon x\to y$ and $f',g'\colon y\to z$, and $2$-morphisms $S\colon f\Rightarrow g$
and $T\colon f'\Rightarrow g'$, there is a $2$-morphism $S\cdot T\colon ff' \Rightarrow gg'$,
which we call the "horizontal composite" of $S$ and $T$.

2'') Horizontal composition is associative: $(S\cdot T)\cdot U = S\cdot (T\cdot U)$.

3'') The identities for vertical composition are also the identities
for horizontal composition. That is, given $f,g\colon x\to y$ and $T\colon f\Rightarrow g$ we
have $1_{1_x}\cdot T = T\cdot 1_{1_y} = T$.

Finally, we demand the "exchange law" relating horizontal and vertical
composition:
$$(ST)\cdot (S'T') = (S\cdot S')(T\cdot T')$$
This makes the following $2$-morphism unambiguous:
$$\includegraphics[scale=0.3]{../images/STS'T'.pdf}$$
We can think of it either as the result of first doing two vertical
composites, and then one horizontal composite, or as the result of first
doing two horizontal composites, and then one vertical composite!

Here we can really see why higher-dimensional algebra deserves its name.
Unlike category theory, where we can visualize morphisms as
1-dimensional arrows, here we have $2$-morphisms which are intrinsically
2-dimensional, and can be composed both vertically and horizontally.

Now if you are familiar with $\mathsf{Cat}$, you may be wondering how we vertically
and horizontally compose natural transformations, which are the
2-morphisms in $\mathsf{Cat}$. Let me leave this as an exercise for now...
there's a nice way to do it that makes $\mathsf{Cat}$ into a $2$-category. This
exercise is a good one to build up your higher-dimensional algebra
muscles.

In fact, we could have invented the above definition of $2$-category
simply by thinking a lot about $\mathsf{Cat}$ and what you can do with categories,
functors, and natural transformations. I'm pretty sure that's more or
less what happened, historically! Thinking hard enough about nCat leads
us on to the definition of $(n+1)$-categories....

But that's enough for now. Typing those diagrams is hard work.

To continue reading the "Tale of $n$-Categories", see ["Week 83"](#week83).

------------------------------------------------------------------------

I thank Keith Harbaugh for catching lots of typos and other mistakes in
["Week 73"](#week73) -- ["Week 80"](#week80).