From: GBurch1@aol.com
Date: Sun Sep 03 2000 - 10:07:46 MDT
http://www.nytimes.com/library/national/science/081500sci-physics-questions.
html
10 Physics Questions to Ponder for a Millennium or Two
By GEORGE JOHNSON
"Who of us would not be glad to lift the veil behind which the future lies
hidden; to cast a glance at the next advances of our science and at the
secrets of its development during future centuries?"
One hundred years ago, with those inviting thoughts, the German
mathematician David Hilbert opened his landmark address to the
International Congress of Mathematicians in Paris, laying out 23 of the
great unsolved problems of the day. "For the close of a great epoch,"
Hilbert declared, "not only invites us to look back into the past but also
directs our thoughts to the unknown future."
With another century ending -- a whole millennium in fact -- the pressure
is all the greater to tabulate human ignorance with lists of the most
enticing cosmic mysteries.
In May, the Clay Mathematics Institute of Cambridge, Mass., emulated
Hilbert, announcing (in Paris, for full effect) seven "Millennium Prize
Problems," each with a bounty of $1 million.
The list is at: www.claymath.org/prize_problems/.
And last month physicists, with a typically lighter touch, ended a
conference on superstring theory at the University of Michigan with a
session called "Millennium Madness," choosing 10 of the most perplexing
problems in their field. It was like a desert island game, involving some
of science's smartest people.
"The way I thought about this challenge was to imagine what question I
would ask if I woke up from a coma 100 years from now," said Dr. David
Gross, a theoretical physicist at the University of California at Santa
Barbara, as he unveiled the winners. He and the other judges made the
selection, he noted, "in the middle and after this party in which we were
sufficiently drunk."
After weeding out unanswerable questions (like "How do you get tenure?"),
the judges came up with enough puzzles to occupy physicists for the next
century or so. There are no monetary prizes, though solving any one of
these would almost guarantee a trip to Stockholm.
1. Are all the (measurable) dimensionless parameters that characterize
the physical universe calculable in principle or are some merely determined
by historical or quantum mechanical accident and uncalculable? Einstein put
it more crisply: did God have a choice in creating the universe? Imagine
the Old One sitting at his control console, preparing to set off the Big
Bang. "How fast should I set the speed of light?" "How much charge should I
give this little speck called an electron?" "What value should I give to
Planck's constant, the parameter that determines the size of the tiny
packets -- the quanta -- in which energy shall be parceled?" Was he
randomly dashing off numbers to meet a deadline? Or do the values have to
be what they are because of a deep, hidden logic?
These kinds of questions come to a point with a conundrum involving a
mysterious number called alpha. If you square the charge of the electron
and then divide it by the speed of light times Planck's constant, all the
dimensions (mass, time and distance) cancel out, yielding a so-called "pure
number" -- alpha, which is just slightly over 1/137. But why is it not
precisely 1/137 or some other value entirely? Physicists and even mystics
have tried in vain to explain why.
2. How can quantum gravity help explain the origin of the universe? Two
of the great theories of modern physics are the standard model, which uses
quantum mechanics to describe the subatomic particles and the forces they
obey, and general relativity, the theory of gravity. Physicists have long
hoped that merging the two into a "theory of everything" -- quantum gravity
-- would yield a deeper understanding of the universe, including how it
spontaneously popped into existence with the Big Bang. The leading
candidate for this merger is superstring theory, or M theory, as the
latest, souped-up version is called (with the M standing for "magic,"
"mystery," or "mother of all theories").
3. What is the lifetime of the proton and how do we understand it? It
used to be considered gospel that protons, unlike, say, neutrons, live
forever, never decaying into smaller pieces. Then in the 1970's, theorists
realized that their candidates for a grand unified theory, merging all the
forces except gravity, implied that protons must be unstable. Wait long
enough and, very occasionally, one should break down.
The trick is to catch it in the act. Sitting in underground laboratories,
shielded from cosmic rays and other disturbances, experimenters have whiled
away the years watching large tanks of water, waiting for a proton inside
one of the atoms to give up the ghost. So far the fatality rate is zero,
meaning that either protons are perfectly stable or their lifetime is
enormous -- an estimated billion trillion trillion years or more.
4. Is nature supersymmetric, and if so, how is supersymmetry broken? Many
physicists believe that unifying all the forces, including gravity, into a
single theory would require showing that two very different kinds of
particles are actually intimately related, a phenomenon called supersymmetry.
The first, fermions, are loosely described as the building blocks of
matter, like protons, electrons and neutrons. They clump together to make
stuff. The others, the bosons, are the particles that carry forces, like
photons, conveyors of light. With supersymmetry, every fermion would have a
boson twin, and vice versa.
Physicists, with their compulsion for coining funny names, call the
so-called superpartners "sparticles": For the electron, there would be the
selectron; for the photon, the photino. But since the sparticles have not
been observed in nature, physicists would also have to explain why, in the
jargon, the symmetry is "broken": the mathematical perfection that existed
at the moment of creation was knocked out of kilter as the universe cooled
and congealed into its present lopsided state.
5. Why does the universe appear to have one time and three space
dimensions? "Just because" is not considered an acceptable answer. And just
because people can't imagine moving in extra directions, beyond
up-and-down, left-and-right, and back-and-forth, doesn't mean that the
universe had to be designed that way. According to superstring theory, in
fact, there must be six more spatial dimensions, each one curled up too
tiny to detect. If the theory is right, then why did only three of them
unfurl, leaving us with this comparatively claustrophobic dominion?
6. Why does the cosmological constant have the value that it has? Is it
zero and is it really constant? Until recently cosmologists thought the
universe was expanding at a steady clip. But recent observations indicate
that the expansion may be getting faster and faster. This slight
acceleration is described by a number called the cosmological constant.
Whether the constant turns out to be zero, as earlier believed, or some
very tiny number, physicists are at a loss to explain why.
According to some fundamental calculations, it should be huge -- some 10
to 122 times as big as has been observed.
The universe, in other words, should be ballooning in leaps and bounds.
Since it is not, there must be some mechanism suppressing the effect. If
the universe were perfectly supersymmetric, the cosmological constant would
become canceled out entirely. But since the symmetry, if it exists at all,
appears to be broken, the constant would still remain far too large. Things
would get even more confusing if the constant turned out to vary over time.
7. What are the fundamental degrees of freedom of M-theory (the theory
whose low-energy limit is eleven-dimensional supergravity and that subsumes
the five consistent superstring theories) and does the theory describe
nature? For years, one big strike against superstring theory was that there
were five versions. Which, if any, described the universe? The rivals have
been recently reconciled into an overarching 11-dimensional framework
called M theory, but only by introducing complications.
Before M theory, all the subatomic particles were said to be made from
tiny superstrings. M theory adds to the subatomic mix even weirder objects
called "branes" -- like membranes but with as many as nine dimensions. The
question now is, Which is more fundamental -- are strings made from branes
or vice versa? Or is there something else even more basic that no one has
thought of yet? Finally, is any of this real, or is M theory just a
fascinating mind game?
8. What is the resolution of the black hole information paradox?
According to quantum theory, information -- whether it describes the
velocity of a particle or the precise manner in which ink marks or pixels
are arranged on a document -- cannot disappear from the universe.
But the physicists Kip Thorne, John Preskill and Stephen Hawking have a
standing bet: what would happen if you dropped a copy of the Encyclopaedia
Britannica down a black hole? It does not matter whether there are other
identical copies elsewhere in the cosmos. As defined in physics,
information is not the same as meaning, but simply refers to the binary
digits, or some other code, used to precisely describe an object or
pattern. So it seems that the information in those particular books would
be swallowed up and gone forever. And that is supposed to be impossible.
Dr. Hawking and Dr. Thorne believe the information would indeed disappear
and that quantum mechanics will just have to deal with it. Dr. Preskill
speculates that the information doesn't really vanish: it may be displayed
somehow on the surface of the black hole, as on a cosmic movie screen.
9. What physics explains the enormous disparity between the gravitational
scale and the typical mass scale of the elementary particles? In other
words, why is gravity so much weaker than the other forces, like
electromagnetism? A magnet can pick up a paper clip even though the gravity
of the whole earth is pulling back on the other end.
According to one recent proposal, gravity is actually much stronger. It
just seems weak because most of it is trapped in one of those extra
dimensions. If its full force could be tapped using high-powered particle
accelerators, it might be possible to create miniature black holes. Though
seemingly of interest to the solid waste disposal industry, the black holes
would probably evaporate almost as soon as they were formed.
10. Can we quantitatively understand quark and gluon confinement in
quantum chromodynamics and the existence of a mass gap? Quantum
chromodynamics, or QCD, is the theory describing the strong nuclear force.
Carried by gluons, it binds quarks into particles like protons and
neutrons. According to the theory, the tiny subparticles are permanently
confined. You can't pull a quark or a gluon from a proton because the
strong force gets stronger with distance and snaps them right back inside.
But physicists have yet to prove conclusively that quarks and gluons can
never escape. When they try to do so, the calculations go haywire. And they
cannot explain why all particles that feel the strong force must have at
least a tiny amount of mass, why it cannot be zero. Some hope to find an
answer in M theory, maybe one that would also throw more light on the
nature of gravity.
11. (Question added in translation). Why is any of this important? In
presenting his own list of mysteries, Hilbert put it this way: "It is by
the solution of problems that the investigator tests the temper of his
steel; he finds new methods and new outlooks, and gains a wider and freer
horizon."
And in physics, the horizon is no less than a theory that finally makes
sense of the universe.
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