- 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. - 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").
- 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. - 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. - 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? - 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 1010 to 10122 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. - 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? - 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. - 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. - 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. - (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.
EDIT: I guess I should have put this in Tech & Science
