# December 29, 1999 {#week143}

Since this is the last Week of the millennium, I'll make sure to pack
it full of retrospectives and prognostications. But I'd like to start
with an update on something I discussed a while back.

By the way, please don't give me flak about how the millennium starts
in 2001. I use the CE or Common Era system, which starts counting at the
year zero, not the AD system, which starts at the year one because it
was invented in 526 CE by Dennis the Diminutive, long before the number
zero caught on. In my opinion, the real "millennium bug" is that
anyone is still using the antiquated AD system!

Anyway....

In ["Week 73"](#week73), I mentioned a theory about why molecules
important in biology tend to come in a consistent chirality, or
handedness. For example, there's lots of dextrose in nature --- this
being the right-handed form of the sugar sucrose --- but not much of its
left-handed counterpart, levulose. It's no surprise that *one or the
other* would dominate, but you might guess that *which one* was just an
accident of history. After all, there's no fundamental difference
between right and left, right?

Or is there? Actually there is: the weak nuclear force distinguishes
between the two! So some people theorized that very slight differences
in energy levels, due to the weak force, favor the formation of
left-handed amino acids and right-handed sugars --- which is what we see
in nature.

Recently people have found evidence for a somewhat different version of
this theory:

1) Robert F. Service, "Does life's handedness come from within?", _Science_ **286** (November 12, 1999), 1282--1283.

When radioactive atoms decay via the weak force, the electrons they
shoot off tend to have a left-handed spin. Could this affect the
handedness of molecules or crystals that happen to be forming in the
vicinity? Sodium chlorate is a chemical that can form both left-handed
and right-handed crystals, so researchers took some solutions of the
stuff and let them crystallize while blasting them with electrons formed
by the decay of radioactive strontium. Sure enough, this biased the
handedness of the crystals! Blasting the stuff with right-handed
positrons favored formation of crystals of the opposite handedness. The
strangest part was that the effect was even bigger than expected.

I still think the whole business is pretty iffy --- after all, the flux of
radiation in this experiment was a lot bigger than what we normally see
on earth. But it would sure be neat if the origin of chirality in
biology was related to the deeper mystery of chirality in particle
physics.

Okay, now for a little retrospective. Don't worry --- I won't list the
top ten developments in mathematical physics of the last millennium!
Instead, I just want to recommend two papers. First, an old paper by
Poincare:

2) Henri Poincare, "The present and future of mathematical physics", _Bull. Amer. Math. Soc._ **12** (1906), 240--260. Reprinted as part of a retrospective issue of the _Bull. of the Amer. Math. Soc_., **37** (2000), 25--38, available at `http://www.ams.org/bull/`

This article is based on a speech he gave in 1904. After a fascinating
review of the development of mathematical physics, he makes some
accurate predictions about quantum mechanics and special relativity ---
but closes on a conservative note:

> In what direction we are going to expand we are unable to foresee.
> Perhaps it is the kinetic theory of gases that will forge ahead and
> serve as a model for the others. In that case, the facts that appeared
> simple to us at first will be nothing more than the resultants of a
> very large number of elementary facts which the laws of probability
> alone would induce to work toward the same end. A physical law would
> then assume an entirely new aspect; it would no longer be merely a
> differential equation, it would assume the character of a statistical
> law.
>
> Perhaps too we shall have to construct an entirely new mechanics,
> which we can only just get a glimpse of, where, the inertia increasing
> with the velocity, the velocity of light would be a limit beyond which
> it would be impossible to go. The ordinary, simpler mechanics would
> remain a first approximation since it would be valid for velocities
> that are not too great, so that the old dynamics would be found in the
> new. We should have no reason to regret that we believed in the older
> principles, and indeed since the velocities that are too great for the
> old formulas will always be exceptional, the safest thing to do in
> practice would be to act as though we continued to believe in them.
> They are so useful that a place should be saved for them. To wish to
> banish them altogether would be to deprive oneself of a valuable
> weapon. I hasten to say, in closing, that we are not yet at that pass,
> and that nothing proves as yet that they will not come out of the fray
> victorious and intact.

The same issue of the AMS Bulletin also has a lot of other interesting
papers and book reviews from the last century, by folks like Birkhoff,
Einstein and Weyl.

The second paper I recommend is a new one by Rovelli:

3) Carlo Rovelli, "The century of the incomplete revolution: searching for general relativistic quantum field theory", to appear in the _Journal of Mathematical Physics_ 2000 Special Issue, preprint available as [`hep-th/9910131`](https://arxiv.org/abs/hep-th/9910131).

Let me just quote the abstract:

> In fundamental physics, this has been the century of quantum mechanics
> and general relativity. It has also been the century of the long
> search for a conceptual framework capable of embracing the astonishing
> features of the world that have been revealed by these two ''first
> pieces of a conceptual revolution''. I discuss the general
> requirements on the mathematics and some specific developments towards
> the construction of such a framework. Examples of covariant
> constructions of (simple) generally relativistic quantum field
> theories have been obtained as topological quantum field theories, in
> nonperturbative zero-dimensional string theory and its higher
> dimensional generalizations, and as spin foam models. A canonical
> construction of a general relativistic quantum field theory is
> provided by loop quantum gravity. Remarkably, all these diverse
> approaches have turn out to be related, suggesting an intriguing
> general picture of general relativistic quantum physics.

Now for the prognostications. Since we should never forget that the
towering abstractions of mathematical physics are ultimately tested by
experiment, I'd like to talk about some interesting physics
*experiments* that are coming up in the next millennium. These days more
and more interesting information about physics is coming from astronomy,
so I'll concentrate on work that lies on this interface.

In ["Week 80"](#week80) I talked about how Gravity Probe B will try
to detect an effect of general relativity called "frame-dragging"
caused by the earth's rotation. I also talked about how LIGO --- the
Laser Interferometric Gravitational Wave Observatory --- will try to
detect gravitational waves:

4) LIGO homepage, `http://www.ligo.caltech.edu/`

If all works as planned, LIGO should be great for studying the final
death spirals of binary black holes and/or neutron stars. When it starts
taking data sometime around 2002, it should be able to detect the final
"chirp" of gravitational radiation produced a pair of inspiralling
neutron stars in the Virgo Cluster, a cluster of galaxies about 15
megaparsecs away. Such an event would distort the spacetime metric
*here* by only about $1$ part in $10^{21}$. This is why LIGO needs to
compare oscillations in the lengths of two arms of an interferometer,
each $4$ kilometers long, with an accuracy of $10^{-16}$ centimeters: about
one hundred-millionth of the diameter of a hydrogen atom. To do this
will require some *very* clever tricks to reduce noise.

As the experiment continues, they intend to improve the sensitivity
until it can detect distortions in the metric of only $1$ part in
$10^{22}$, and second-generation detectors should get to $1$ part in
$10^{23}$. At that point, we should be able to detect neutron star
"chirps" from a distance of $200$ megaparsecs. Events of this sort
should happen once or twice a year.

Since it's crucial to rule out spurious signals, LIGO will have two
detectors, one in Livingston, Louisiana and one in Hanford, Washington.
This should also allow us to tell where the gravitational waves are
coming from. And there are other gravitational wave detection projects
underway too! France and Germany are collaborating on a laser
interferometer called VIRGO, with arms 3 kilometers long, to be built in
Cascina, Italy:

5) VIRGO homepage, `http://www.pi.infn.it/virgo/`

Germany and Great Britain are collaborating on a 600-meter-long one
called GEO 600, to be built south of Hannover:

6) GEO 600 homepage, `http://www.geo600.uni-hannover.de/`

The Japanese are working on one called TAMA 300, which is a 300-
meter-long warmup for a planned kilometer-long interferometer:

7) TAMA 300 homepage, `http://tamago.mtk.nao.ac.jp/`

In addition, the Brazilian GRAVITON project is building something called
the Einstein Antenna, which uses mechanical resonance rather than
interferometry. The basic principle goes back to Joseph Weber's
original bar detectors, which tried to sense the vibrations of a
2-meter-long aluminum cylinder induced by gravitational waves. But the
design involves lots of hot new technology: SQUIDS, buckyballs, and the
like:

8) GRAVITON homepage, `http://www.das.inpe.br/graviton/project.html`

There are also other gravitational wave detectors being built... but
ultimately, the really best ones will probably be built in outer space.
There are two good reasons for this. First, outer space is big: when
you're trying to detect very small distortions of the geometry of
spacetime, it helps to measure the distance between quite distant
points. Second, outer space is free of seismic noise and most other
sources of vibration. This is why people are working on the LISA project
--- the Laser Interferometric Space Antenna:

9) European Space Agency's homepage on the LISA project,
`http://www.estec.esa.nl/spdwww/future/html/lisa.htm`

NASA's homepage on the LISA project: `http://lisa.jpl.nasa.gov/`

The idea is to orbit 3 satellites in an equilateral triangle with sides
5 million kilometers long, and constantly measure the distance between
them to an accuracy of a tenth of an angstrom --- $10^{-11}$ meters --- using
laser interferometry. (A modified version of the plan would use 6
satellites.) The big distances would make it possible to detect
gravitational waves with frequencies of 0.0001 to 0.1 hertz, much lower
than the frequencies for which the ground-based detectors are optimized.
The plan involves a really cool technical trick to keep the satellites
from being pushed around by solar wind and the like: each satellite will
have a free-falling metal cube floating inside it, and if the satellite
gets pushed to one side relative to this mass, sensors will detect this
and thrusters will push the satellite back on course.

I don't think LISA has been funded yet, but if all goes well, it may
fly within 10 years or so. Eventually, a project called LISA 2 might be
sensitive enough to detect gravitational waves left over from the early
universe --- the gravitational analogue of the cosmic microwave background
radiation!

The microwave background radiation tells us about the universe when it
was roughly 10^5 years old, since that's when things cooled down
enough for most of the hydrogen to stop being ionized, making it
transparent to electromagnetic radiation. In physics jargon, that's
when electromagnetic radiation "decoupled". But the gravitational
background radiation would tell us about the universe when it was
roughly $10^{-38}$ seconds old, since that's when gravitational
radiation decoupled. This figure could be way off due to physics we
don't understand yet, but anyway, we're talking about a window into
the *really* early universe.

Actually, Mark Kamionkowski of Caltech has theorized that the European
Space Agency's "Planck" satellite may detect subtle hints of the
gravitational background radiation through its tendency to polarize the
microwave background radiation. You probably heard how COBE, the Cosmic
Background Explorer, detected slight anisotropies in the microwave
background radiation. Now people are going to redo this with much more
precision: while COBE had an angular resolution of 7 degrees, Planck
will have a resolution of 4 arcminutes. They hope to launch it in 2007:

10) Planck homepage, `http://astro.estec.esa.nl/SA-general/Projects/Planck/planck.html`

What else is coming up? Well, gravity people should be happy about the
new satellite-based X-ray telescopes, since these should be great for
looking at black holes. In July 1999, NASA launched one called
"Chandra". (This is the nickname of Subrahmanyan Chandrasekhar, who
won the Nobel prize in 1983 for his work on stellar evolution, neutron
stars, black holes, and closed-form solutions of general relativity.)
The first pictures from Chandra are already coming out --- check out this
website:

11) Chandra homepage, `http://chandra.harvard.edu/`

On December 10th, the Europeans launched XMM, the "X-ray Multi-Mirror
Mission":

12) XMM homepage, `http://sci.esa.int/xmm/`

This is a set of three X-ray telescopes that will have lower angular
resolution than Chandra, but 5--15 times more sensitivity. It'll also be
able to study X-ray spectra, thanks to a diffraction grating that
spreads the X-rays out by wavelength. And in January, the Japanese plan
to launch Astro-E, designed to look at shorter wavelength X-rays:

13) MIT's Astro-E homepage, `http://acis.mit.edu/syseng/astroe/xis_home.html`

Taken together, this new generation of X-ray telescopes should tell us a
lot about the dynamics of the rapidly changing accretion disks of black
holes, where infalling gas and dust spirals in and heats up to the point
of emitting X-rays. They may also help us better understand the X-ray
afterglow of $\gamma$-ray bursters. As you probably have heard, these rascals
make ordinary supernovae look like wet firecrackers! Some folks think
they're caused when a supernova creates a black hole. But nobody is
sure.

Peering further into the future, here's a nice article about new
projects people are dreaming up to study physics using astronomy:

14) Robert Irion, Space becomes a physics lab, Science 286 (1999),
2060-2062.

In 2005 folks plan to launch GLAST, the Gamma-Ray Large Area Space
Telescope, designed to study $\gamma$-ray bursters and the like, and also the
Alpha Magnetic Spectrometer, designed to search for antimatter in space.
But there are also a bunch of interesting projects that are still
basically just a twinkle in someone's eye....

For example: OWL, the Orbiting Wide-Angle Light Collector, a pair of
satellites that would trace the paths of super-high-energy cosmic rays
through the earth's atmosphere. As I explained in
["Week 81"](#week81), people have seen cosmic rays with
ridiculously high energies, like 320 Eev --- the energy of a 1-kilogram
rock moving at 10 meters per second, all packed into one particle. OWL
would orbit the earth, watch these things, and figure out where the heck
they're coming from.

Or how about this: The Dark Matter Telescope! This would use
gravitational lensing to chart the "dark matter" which seems to
account for a good percentage of the mass in the universe --- if, of
course, dark matter really exists.

15) Dark Matter Telescope homepage, `http://dmtelescope.org`

Anyway, there should be a lot of exciting experiments coming up. But as
usual, the really exciting stuff will be the stuff we can't predict.