ANN ARBOR, Mich. - Scientists
don't know everything. Even physicists admit that there's a lot about the
universe they would like to know.
Many physicists are betting,
though, that someday they'll have a way to explain everything, with the
help of a theory about strings. About 300 of those physicists gathered at
the University of Michigan this month to trade tales of their latest
mathematical forays into the underworld of existence, the realm where time
and space dissolve and matter and energy merge in the form of supertiny
strings - superstrings for short.
Participants at the conference,
Strings 2000, submitted their favorite question to a panel of judges,
three leading string theorists. The goal was an end-of-the-millennium
compendium of the deepest mysteries facing physics on a fundamental level.
"Fundamental" for this purpose means important to string theorists, said
one judge, David Gross of the University of California, Santa Barbara.
"We've excluded many important fundamental questions in other
fields of physics, such as condensed matter physics, astrophysics and
biophysics," he said in announcing the Top 10 questions.
"You
know," he acknowledged, "there are interesting questions in those fields."
But those questions are the province of other conferences. At this one,
the winning questions reflected the dreams of people desiring a theory
that ties all of nature's particles and forces, and maybe space and time
too, in one neat mathematical package.
First, a warning. These
questions have been translated from the original physics. And the order of
presentation has been altered for dramatic effect. For the actual
phrasing, you may venture to
feynman.physics.lsa.umich.edu/strings2000/millennium.html on the World
Wide Web.
And now, the Top 10 Questions, Strings 2000 edition:
*10. Precisely how do quarks stick together?
One of
quarks' most charming features is the way they bond inside an atom's
nucleus; if you try to pull them apart, they get even stickier. Physicists
understand the basics, but calculating the forces between quarks precisely
awaits a deeper understanding.
*9. How long do protons live?
Neutrons can come and go, but protons remain stable, for something
like a trillion trillion trillion years or so. Theories have predicted
that protons should decay sooner than that, but efforts to find broken
protons have failed.
*8. Why are elementary particles so massive
compared to other basic quantities in physics?
Good question.
*7. Are the numbers describing nature something physicists can
calculate?
Maybe.
*6. Does supersymmetry describe nature?
"Many of us believe so, but it's up to experiment to decide," said
Dr. Gross. Supersymmetry is a mathematical notion which, if true, requires
that every basic type of particle should have a partner with a funny name
- for the electron, a selectron; for quarks, squarks; and for the photon,
a photino. No such sparticles have been discovered. But if scientists do
find one, it would be super.
*5. Why does the universe seem to
have one dimension of time and three of space?
Actually, string
theory suggests that there are additional dimensions of space, hidden from
human view. Some physicists even think there might be more than one
dimension of time.
*4. How big is the cosmological constant, and
why?
The cosmological constant was Einstein's idea - a strange
energy pervading space that counters gravity, thereby preventing all
matter from collapsing into one big blob. But Einstein discarded his idea
when he learned the universe was expanding fast enough to keep matter from
collapsing. Now, though, the universe appears to be expanding too fast to
explain without some sort of repulsive energy. No theory has accurately
predicted how much of this funny energy occupies space, however.
*3. How can a quantum theory of gravity explain the origin of the
universe?
"We're not sure that there is a theory of the origin of
the universe," said Dr. Gross. But superstring theory is, after all, a way
to merge quantum mechanics and Einstein's theory of gravity. If it's the
theory of everything, maybe it can explain how the universe came to be.
*2. What is the resolution of the black hole information paradox?
Black holes swallow everything, with no apparent means for
anything to escape. Yet quantum mechanics requires an exact accounting of
all the information in the universe, including information about objects
that black holes swallow. If black holes hide information forever, quantum
mechanics faces a problem. Resolving this paradox may help in finding an
ultimate theory that successfully combines gravity and quantum physics.
*1. Does M theory describe nature? And what are its fundamental
objects?
It now appears that superstring theory isn't the ultimate
theory of everything, after all. Physicists have figured out that M
theory, encompassing 11 dimensions of space and time, can reproduce the
features of several superstring theories. M theory thus can be thought of
as the mother of all theories.
But nobody knows exactly how to
write down the complete math of M theory or what nature's most basic
objects will turn out to be if the theory is correct. Nevertheless, it
still seems like the best bet to unify gravity and quantum physics and
perhaps in so doing answer the rest of the millennium's Top 10 questions.
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