[escepticos] String theory - articulo interesante y bien escrito

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Encontre esto por algun luger de la net - Mig
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Source:Los Angeles Times
Copyright 1999 

Time, Space, Obsolete in New View of Universe
By K.C. COLE
LA Times Science Writer

Many physicists are embracing a revolutionary, still mysterious idea
called string theory. The concept rejects several familiar notions and
includes the existence of 11 dimensions.

Ever since early astronomers yanked Earth from center stage in the solar
system some 500 years ago, scientists have been pulling the rug out from
under people's basic beliefs.

"The history of physics," says Harvard physicist Andrew Strominger, "is
the history of giving up cherished ideas."

No idea has been harder to give up, however--for physicists and
laypeople alike--than everyday notions of space and time, the
fundamental "where" and "when" of the universe and everything in it.
Einstein's unsettling insights more than 80 years ago showed that static
space and fixed time were flimsy facades, thinly veiling a cosmos where
seconds and meters ooze like mud and the rubbery fabric of space-time
warps into an unseen fourth dimension. About the same time, the new
"quantum mechanical" understanding of the atom revealed that space and
time are inherently jittery and uncertain.

Now, some physicists are taking this revolutionary line of thinking one
step further: If their theories are right, in the words of Edward Witten
of the Institute for Advanced Study in Princeton, space and time may be
"doomed."

Concurs physicist Nathan Seiberg, also of the institute: "I am almost
certain that space and time are illusions. These are primitive notions
that will be replaced by something more sophisticated."

That conclusion may not affect anyone's morning commute. But it is
rocking the foundations of physics--as well as causing metaphysical
reverberations that inevitably follow major changes in our fundamental
understanding of how the universe works.

The impetus behind this tumult is an idea that has become increasingly
dominant in modern physics: string theory. According to string theory,
the most basic igredients in the universe are no longer point-like
particles, the familiar electrons and quarks. Instead, they are
unimaginably small vibrating strings of some unknown undamental stuff.

String theory suggests that different configurations of strings produce
different harmonic chords--just as a piano produces a sound different
from that of a flute. The vibrating string gives rise to the particles,
and the way the string vibrates determines each particle's properties.
This all takes place in a convoluted landscape of 11-dimensional space.
It is a concept so strange that even theoretical physicists struggle to
understand it. String theory offers a universe bizarre beyond imagining:
Under powerful enough magnification, every known particle in the
universe would resemble a complex origami folded out of sheets or
strings of the three familiar spatial dimensions, plus one dimension of
time, plus seven extra dimensions of space.

While string theory is far from proven, or even well formulated, its
consequences would be enormous. Among other things, it would:

* Reshape fundamental notions of space and time, energy and matter,
expanding the number of dimensions to 11.

* Give the first comprehensive list of all the ingredients that make up
the universe.

* Reveal that every tick of a clock, every barking dog, every dying
star, can be described by one master mathematical equation.

Being Involved in a 'Scientific Revolution'

Which practical fruits will flow from the new view of the universe
remain unknown. But in the past, fundamental revolutions in physics
have--against everyone's wildest expectations--flowered into everything
from cell phones to brain scans.

"I've been in physics for 35 years, and this is the first time I've felt
I'm involved in a scientific revolution," said Stanford physicist
Leonard Susskind. "In the last five or six years, I really have the
feeling we're doing something as crazy, as interesting, as new as the
revolution that Einstein wrought."

Perhaps most revolutionary of all, it appears that space and time aren't
essential ingredients of a universe ruled by strings. To grasp the
extent of the current upheaval in physics, consider what has happened to
our basic understanding of space and time over the past hundred years.

Until the early 20th century, scientists, like laypeople, assumed that
space and time were fixed--like huge, metaphysical clocks and rulers in
the firmament. Objects that moved in this unchanging background could be
pinned down to definite positions.

"Everything was where it was when it was supposed to be, and that was
all there was to it," said Strominger. "Space-time was out there. You
could count on it."

Then, Einstein revealed that space and time were woven into a single
fabric that deforms like so much Silly Putty; indeed, it is the warping
of the fabric of space-time by massive objects that produces the force
of gravity. We perceive gravity as a "force" only because we can't
directly perceive the fourth dimension.

Because gravity affects everything, everything gets warped by its
pervasive influence--including the clocks and rulers we use to measure
time and space.

Even more unsettling, Einstein's now well-proven theories showed that
the fabric of space-time, with its three dimensions of space and one of
time, is not a passive backdrop for the events and objects in the
universe. Space-time also creates objects and events.

Imagine the universe as a performance on a stage. The stage of
space-time does not act like a static floor. It also pulls and pushes
the actors around.

Quantum mechanics introduced even more uncertainty. In the subatomic
realm, the entire concept of fixed particles in time and space fuzzes
out into an ever-shifting haze of probabilities. Trying to pin down a
subatomic particle's location or motion is like trying to put your
finger on a snowflake; the very act of measurement destroys the thing
being measured.

"That means . . . space-time is an uncertain concept, so you've lost
your firm footing," said Strominger. "And that is a deep conceptual
issue we have not yet come to grips with."

Now string theory appears to be propelling this evolution one drastic,
perhaps inevitable, step further.

Certain approaches to string theory dispense with the notion of
space-time completely. Yet, they seem to produce the same set of results
as string theories with normal space and time.

To some theorists, this strongly suggests that space and time are
superfluous. Space and time as fundamental concepts may be about to
disappear altogether--literally pulling the floor out from under
physics.

"The notion of space-time is something we've cherished for thousands of
years, and it's clearly something we're going to have to give up," said
Strominger.

Even before string theory enjoyed its recent successes, physicists knew
they would have to grapple again with the inadequacy of our
understanding of space and time. The reason is a glaring mismatch
between gravity, which rules large-scale events in the cosmos, and
quantum mechanics, which rules small-scale happenings.

Both gravity and quantum theory are well understood and have survived
decades of experimental tests. Quantum mechanics gave rise to lasers and
computers; Einstein's theory of gravity predicted everything from black
holes to the bending of light by stars, insights since proved by
observations. The problem is, the two theories are mutually exclusive.
The space and time of quantum theory don't mesh with the space and time
of Einstein's theory of gravity, or General Relativity. In the language
of gravity, the quantum mechanical aspects of the universe turn into
gobbledygook. And vice versa.

"We can describe the world that we see and experience completely," said
UC Santa Barbara physicist Sean Carroll, "but the explanations are
internally inconsistent."

Some Things Don't Affect Everyday Life

Until recently, physicists found it easy to sweep this unpleasantness
under the rug--in part because they didn't know how to deal with it, in
part because it doesn't make a difference in our everyday lives.

The inherently uncertain behavior of subatomic particles affects only
things as small as atoms, not everyday objects like chairs; the warping
of space and time shapes the orbits of planets, but is too diluted to
make itself felt on the scale of our own backyards.

Where the large-scale fabric of space-time gets tangled in the inner
lives of atoms, however, chaos erupts; space and time fail to make
sense. And increasingly, physicists find themselves face to face with
situations where quantum mechanics and the extreme warping of space-time
collide.

For example, physicists won't be able to understand either the innards
of black holes or the origins of the universe until they come to grips
with how gravity behaves at extremely small scales. Indeed, the ultimate
laboratory for studying the collision of these two opposing realms is
the infinitely compressed dollop of space-time that gave rise to the Big
Bang.

That cataclysmic speck, physicists believe, contained everything now in
our universe, so it would have packed a huge gravitational wallop. At
the same time, it would have been small enough to behave according to
quantum mechanical laws. Because physicists can't study the Big Bang
directly, they wind back the clock with equations and thought
experiments--imagining what might happen, for example, if time really
reversed.

The results are disturbing: As the universe gets smaller and smaller,
the warping of space-time gets stronger and quantum uncertainties get
progressively larger.

Finally, the uncertainty becomes larger than any time interval that
could possibly be measured. Measurement becomes meaningless.

Time at the first moment dissolves into nonsense. 

"If you ask questions about what happened at very early times," said
Harvard physicist Sidney Coleman, "and you compute the answer, the
[real] answer is: Time doesn't mean anything."

Or consider what happens inside a black hole--a region where gravity is
so strong that space-time curls in on itself, in effect, shutting out
the rest of the universe. Black holes are swirling pits of pure
space-time. And according to Einstein's theory, their enormous gravity
causes them to collapse to an infinite point of zero size--what
physicists call singularity.

Is there such a nonsensical thing as infinite density packed into zero
size?

"I remember puzzling about that when I was a kid," said Gary Horowitz of
the Institute for Theoretical Physics at UC Santa Barbara. "I thought
when I went to college I would find out the answer . . . I'm still
waiting."

In the pinched-off centers of black holes, space-time appears to simply
stop. "The singularity acts like an edge," said Horowitz. "You run into
it, and it's the end. There's no time after that; there's no space after
that. But we don't think physics should end [there]. That's why we're
trying to" find new laws of physics, which will describe what happens
beyond that edge.

Black holes, said Princeton physicist John Archibald Wheeler, "[teach]
us that space can be crumpled like a piece of paper into an
infinitesimal dot, that time can be extinguished like a blown-out flame,
and that the laws of physics that we regard as 'sacred,' as immutable,
are anything but."

Space, Time May Be Doomed as Concepts

String theory has emerged as the only viable candidate to reconcile the
differences between gravity and quantum mechanics. It does so by
eliminating the notion of infinitely small particles. The loop of string
is the smallest allowable size.

"You never get to the point where the disasters happen," said Seiberg of
the Institute for Advanced Study. "String theory prevents it." But
rather than rescuing space and time, string theory only seems to make
their doom as fundamental concepts more imminent.

When they are incorporated into string theory, "space and time get
confused," said Seiberg. "It's telling us that the traditional
understanding of space and time will evaporate and there will be a more
interesting and subtle result."

Some string theorists believe that space and time somehow emerged in the
early universe out of the  disorganized, 11-dimensional strings. The
strings are "shards" of space and time, said physicist Brian
Greene of Columbia University.

Imagine grains of sand on the surface of a drum. If you tap the surface
over and over at the same place, the sand falls into patterns--like iron
filings around a magnet.

Did space and time emerge in the same way, as resonant patterns of
vibrating strings?

Trying to make sense of such an idea is a struggle even for theorists.
"String theory has been giving us a lot of clues," said Strominger, "but
we haven't been able to put them together into a unified picture."

Even philosophically, the challenge of replacing space and time is
daunting. What does it mean to inhabit a spaceless, timeless universe?
Clocks and rulers not only measure hours and inches; they tell us where
we've been and where we're going.

"When we talk about space and time, we think there is something there,
and we live in it," said David Gross, director of the Institute for
Theoretical Physics. The idea that space and time might be illusions, he
said, "is very disturbing. Where are we? When are we?"

The almost unfathomable scenario of a universe without space and time in
turn calls into question the very connection between cause and effect.
If time can break down, how can one event be placed clearly "before" or
"after" another?

Hypothetically, if there is no clear difference between now and the
instant after, how can we say whether the gunshot caused death--or death
caused the gunshot? "We normally think of causality as a basic
property," said Horowitz.

"Something effects something else. But when you're getting rid of space
and time . . . are we sure that causality is going to be preserved?"

New views of time could lead to even more bizarre consequences--for
instance, more than two dimensions of time, a theory being worked on by
USC physicist Itzhak Bars, among others.

Whatever the outcome of these efforts, it's clear, said Greene, that
"space is undergoing a drastic  rearrangement of its basic pieces; we
will not understand string theory until we make a major breakthrough
in notions of space and time."

If Greene and his colleagues are right, expanding the universe into 11
dimensions and looping it into strings are only the beginning. On the
horizon looms a new kind of physics, where space and time melt down
completely.

"The real change that's around the corner [is] in the way we think about
space and time," said Gross. "We haven't come to grips with what
Einstein taught us. But that's coming. And that will make the world
around us seem much stranger than any of us can imagine."

The Fabric of Space-Time

As the eye sees them, space and time are woven into a smooth "fabric" of
four-dimensional space-time. At close magnification, however, the
inherent uncertainty of the subatomic realm (quantum mechanics) disrupts
this smooth landscape, creating submicroscopic chaos.

Source: The Elegant Universe by Brian Greene 

String Theory

According to string theory, unimaginably small vibrating strings form
the building blocks of everything in the cosmos. This differs from
prevailing theories of subatomic physics, in which particles meet and
exchange energy at specific points in space and time. Under string
theory, strings spread out these interactions over space and time. This
"spreading out" eliminates the mathematical problems that crop up when
infinitely small points collide. 

Researched by NONA YATES/Los Angeles Times

Adding Dimensions

In string theory, each point in everyday space-time also contains an
extra seven dimensions curled up so small that we can never perceive
them. The exact forms of these curled-up dimensions determine all the
particles and forces in the universe. One possibility for the geometry
of the curled-up dimensions is shown here; because only two dimensions
can be shown on a flat piece of paper, this only hints at its
complexity. If string theory is right, each movement of your finger
travels not only through the familiar three dimensions of space and
one of time, but also through seven curled-up dimensions.

-Mark-