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THE NIGHT SKY,
when you think about it, is one of the strangest sights imaginable.
The pinpoint stars that catch your eye are all but swallowed
up by the black nothingness of space - an entity billions
of light-years deep with which we here on Earth have no direct
ex- perience.
What is empty space, really? At first the question seems silly.
There's nothing to it! But look again in light of what modern
physics knows and suspects, and the nature of space emerges
as one of the most important "sleeper" issues growing
for the last 50 years. "Nature abhors a vacuum,"
proclaimed Aristotle more than 2,300 years ago. Today physicists
are discovering that this is true in ways the ancient Greeks
could never have imagined.
True, the cosmos
consists overwhelmingly of vacuum. Yet vacuum itself is proving
not to be empty at all. It is much more complex than most
people would guess. "But surely," you might ask,
"if you take a container and remove everything from inside
it - every atom, every photon - there will be nothing left?"
Not by a long shot. Since the 1920s physicists have recognized
that on a microscopic scale, the vacuum itself is alive with
activity. Moreover, this network of activity may extend right
down to include the very structure of space-time itself. The
fine structure of the vacuum may ultimately hold the keys
to some of the deepest questions facing physics - from why
elementary particles have the properties they do, to the cause
of the Big Bang and the likelihood of other universes outside
our own.
THINGS
THAT GO BUMP IN THE DARK
The state of
the art in physics - our deepest current understanding of
the world - is embodied in the so-called Standard Model, in
which all matter and forces are accounted for by an astonishingly
few types of particles (see Sky & Telescope - December
1987, page 582). Six quarks and six leptons make up all possible
forms of matter. In practice just two of the quarks (the up
and down) and one lepton (the electron) account for everything
in the world except for a few whiffs of exotica known only
to high-energy physicists. The 12 particles of matter (and
their 12 corresponding particles of antimatter, or antiparticles)
are acted upon by "messenger particles" that carry
all the known forces. The photon mediates the electromagnetic
force, including all the familiar chemical and structural
forces around us on Earth. The members of the gluon family
carry the strong force that binds neutrons and protons together
in atomic nuclei. The W', W-, and Zo mediate the weak nuclear
force, and the as-yet-undiscovered graviton is believed to
carry the force of gravity.
Every possible
event involving the 12 matter particles can be completely
explained as an exchange of messenger particles. During some
of these events, for example when electrons accelerate in
a radio-transmitter antenna, messenger particles (in this
case photons) materialize and travel through space. At other
times, however, the messengers remain almost entirely hidden
within the interacting system. When the messengers exist in
this hidden form, they are called "virtual particles."
Virtual particles may seem ghostly and unreal by everyday
standards. But real they are. Moreover, they are not limited
to their role of mediating interactions. Virtual particles
can also pop in and out of empty space all by themselves.
Quantum mechanics,
the rulebook of the Standard Model, states as a bedrock principle
that you need a certain length of time to measure a particle's
energy or mass to a given degree of accuracy. The shorter
the observation time, the more uncertain the measurement.
If the time is very brief, the uncertainty becomes larger
than the particie's entire mass, and you cannot say whether
or not the particle is there at all. The lighter the particle,
the longer its uncertainty time. In the case of an electron-positron
pair, the uncertainty time scale is about 10^-21" seconds.
On time scales
shorter than this, virtual electrons and positrons can, and
do, pop in and out of nothingness like peas in a shell game.
It's as if, just because you can't say a particle doesn't
exist when you look very briefly, then in a sense it does.
This is not mere theorizing. In 1958 a tabletop experiment
demonstrated the "Casimir effect," measuring the
force caused by virtual particles appearing and vanishing
in total vacuum through the attraction they caused between
two parallel metal plates. If the vacuum were truly empty
the plates should not have attracted, but the incessant dance
of virtual particles in the space between them produces a
detectable effect.
Every particle
- matter as well as messenger - seems to display a virtual
form, each seething in greater or lesser abundances in what
physicists call the "physical vacuum." When it comes
to affecting the ordinary world, moreover, virtual particles
may do much more than just mediate forces. Some, in fact,
may cause matter to have the property we call mass. The electron
is the simplest of matter particles. Our knowledge of the
physical world rests upon a solid understanding of its properties.
Yet despite its abundance in the circuitry around us, the
electron harbors an enigma. The fact that it has mass cannot
be explained in the Standard Model, at least the parts of
it that have been experimentally verified. More than 30 years
ago particle physicist Peter Higgs suggested that the existence
of mass has to do with a new ingredient of nature that is
now called the Higgs field, which provides a new type of messenger
particle that interacts with the electron to make it "weigh."
The Higgs field
has yet to be discovered, but many physicists expect it to
exist everywhere in the physical vacuum, ensuring through
its interactions with electrons and other particles that they
will display mass. Even now, particle accelerators at CERN
in Switzerland and at Fermilab near Chicago are straining
at their maximum capabilities to cause just one "Higgs
boson," the presumed messenger particle for this field,
to break loose from the vacuum and leave a detectable trace.
Success would provide a triumphant completion of the Standard
Model.
So to answer
our question about whether a container of empty space is truly
empty, the best anyone can do is remove the normal, physical
particles that nature allows us to see and manipulate. The
virtual particles can never be evicted. And in addition there
may exist the ever-present Higgs field.
QUANTUM
GRAVITY
For most of
this century, physicists have struggled to bring gravity into
the scheme of forces that are mediated by virtual messenger
particles. To put this another way, the theory of general
relativity, which shows the force of gravity to be a curvature
of space-time, needs to be integrated with quantum mechanics,
which shows forces to be virtual particle exchanges. Working
on the assumption that such a marriage is possible, physicists
named gravity's messenger particle the graviton. But general
relativity requires that gravitons be more than just quanta
of gravity. In essence, gravitons define the structure of
space-time itself.
The reconciliation
of quantum mechanics and general relativity may lead us to
dramatically new notions of the nature of space and time.
Some theorists have suggested that points in space-time become
defined only when a particle (such as a graviton or photon)
interacts with other particles. In this view, what they are
doing between interactions is a nonphysical question, since
only an interaction defines a measurable time and place. Gravitational
forces (and thus gravitons) exert an influence at distances
much larger than the subatomic realm, as anyone who has fallen
down a flight of stairs can attest. But only at an extremely
small scale - the Planck length of 10^-33 centimeters - does
the quantum nature of gravity become important.
Suppose you
could magically look through a microscope that magnified an
atomic nucleus to be some 10 light-years across. Under this
magnification the smallest gravitons - that is, the most energetic
and massive ones - would be about a millimeter in size. Here
we might see a strange world in which space-time itself was
defined by gravitons intersecting and looping around each
other. In a similar vein, Roger Penrose has suggested that
the gravitational field and space-time are built up from still
more primitive mathematical entities called twistors, and
that "ultimately the [space-time] concept may possibly
be eliminated from the basis of physical theory altogether."
In essence, space and time become factored out as less- than-fundamental
parts of the physical world.
In such a view,
only the interactions between twistors, or perhaps gravitons,
define when and where space-time is and is not. Are there
gaps in the physical vacuum, voids of true and absolute nothing
where space and time themselves do not exist?
Another viewpoint
on the structure of space-time is offered by "superstring
theory." String theories posit that the fundamental objects
of nature are one-dimensional lines rather than points; the
"elementary" particles we measure are only oscillations
of these strings. Superstring theory only seems to work, however,
if space-time has not just four dimensions (three of space
and one of time), but 10 dimensions. This hardly seems like
the world we live in. To hide the extra six dimensions, mathematicians
roll them up into conceptual corners that go by such cryptic
names as "Calabi-Yau manifolds" and "orbifold
space." A recent textbook on the subject concludes on
a wistful note that "if the string idea is correct, we
may never catch more than a glimpse of the full ex- tent of
reality."
More recently,
theorists Carlo Rovelli (University of Pittsburgh) and Lee
Smolin (Pennsylvania State University) completed their analysis
of a quantum gravity model developed by Abhay Ashtekar at
Syracuse University in 1985. Unlike string theory, Ashtekar's
work applies only to gravity. However, it posits that at the
Planck scale, space-time dissolves into a network of "loops"
that are held together by knots. Somewhat like a chain-mail
coat used by knights of yore, space-time resembles a fabric
fashioned in four dimensions from these tiny one-dimensional
loops and knots of energy.
Is this the
way the world really is on its most fundamental level, or
have mathematicians become detached from reality? Superstring
theory has enticed physicists for over a decade now because
it hints at a super unification of all four fundamental forces
of nature. But it remains frustratingly hard to plant anchors
down from these cloud castles into the real world of observation
and experiment. The famous remark that superstring theory
is "a piece of 21st- century physics that accidentally
fell into the 20th century" captures both the excitement
and frustration of workers stuck with 20th-century tools.
Surprisingly,
string theory, Ashtekar's loopy space-time, and twistors are
not entirely independent ways of looking at space-time. In
1986 theorists discovered that superstrings have some things
in common with twistors. A deep connection had been uncovered
between two very different, independent theories. Like two
teams of tunnelers starting on opposite sides of a mountain,
they had met at the middle - a sign, perhaps, that they are
dealing with a single real mountain, not separate ones in
their own imaginations. And in 1995 Rovelli and Smolin also
found that their graviton loops are very closely related to
both the twistors and superstrings, though not identical in
all respects.
THE
COSMIC CONNECTION
Space-time could
be strange in other ways too. Theorist John A. Wheeler (In-
stitute for Advanced Study) has long advocated that at the
Planck scale, space-time has a complex shape that changes
from instant to instant. Wheeler called his picture "space-time
foam" - a sea of quantum black holes and worm holes appearing
and vanishing on a time scale of about 10^-43" seconds.
This is the Planck time, the time it takes light to cross
the Planck length. Shorter than that, time, like space, presumably
cannot exist - or, at least, our everyday notions of them
cease to be valid.
Wheeler's idea
of space-time foam is a natural extrapolation from the idea
of virtual particles. According to quantum mechanics, the
higher the energy and mass of a particle, the smaller it must
appear. A virtual particle as small as 10^-33" cm, lasting
only 10^-43 second, has so great a mass (10^-5 gram) in such
a tiny volume that its own surface gravity would give it an
escape velocity greater than the speed of light. In other
words, it is a tiny black hole. But a black hole is not an
ordinary object sitting in space- time like a particle; it
is a structure of distorted, convoluted space-time itself.
Although the consequences of such phe- nomena are not understood,
it is rea- sonable to assume that these virtual par- ticles
dramatically distort all space-time at the Planck scale.
If we take this
reasoning at face value, and consider the decades-old experiments
proving that the virtual particle phenomenon in a vacuum is
real, it is hard to believe that space-time is smooth at or
below the Planck scale. Space must be broken up and quantized.
The only question is how. Wheeler's original idea of space-time
foam is especially potent because according to recent proposals
by Sidney Coleman (Harvard) and Stephen Hawking (Cambridge
University), its worm holes not only connect different points
very close together within our space-time, but connect our
space-time to other universes that, as far as we are concerned,
exist only as ghostly probabilities. These connections to
other universes cause the so-called cosmological constant
- an annoying intrusion into the equations of cosmology ever
since Einstein (see Sky & Telescope- April 1991, page
362) - to neatly vanish within our own universe.
Space-time foam
has also been implicated as the spawning ground for baby universes.
In several theories explaining the cause of the Big Bang and
what came before, big bangs can bud off from a previously
existing space-time, break away completely while still microscopic,
and inflate with matter to become new universes of their own,
completely disconnected ("disjoint") from their
space- time of origin. This process, proposed by Alan Guth
(MIT) and others, gives a handle on what many expect to be
another key issue of 21st-century physics: was our Big Bang
unique? Or was it just a routine spinoff of natural processes
happening all the time in some larger, outside realm? (see
Sky & Telescope- September 1988, page 253).
Yet there are
problems. The amount of latent energy in the quantum fluctuations
of space-time foam is staggering: 10^105 ergs per cubic centimeter.
This amounts to 10 billion billion times the mass of all the
galaxies in the observ- able universe - packed into every
cubic centimeter! Fortunately, Mother Nature seems to have
devised some means of exactly canceling out this phenomenon
to an accuracy of about 120 decimal places. The problem is
that we haven't a clue how.
It's unnerving
to think that in the 16 inches separating this page from your
eyes, new big bangs are perhaps being spawned out and away
from our quiet space-time every instant. By comparison, it
seems positively dull that the photons by which you see this
page might be playing a hop-scotch game to avoid gaps where
space-time doesn't exist.
REALITY
CHECK
Some physicists
have begun to throw cold water on these fantastic ideas. For
instance, in 1993 Matt Visser (Washington University) studied
the mathematical properties of quantum worm holes and discovered
that, once they are formed, they become stable: they can't
foam at all. Kazuo Ghoroku (Fukuoka Institute of Technology,
Japan) also found that quantum worm holes become stable even
when their interactions with other fields are considered.
What Wheeler called space-time foam may be something else
entirely.
Among the unresolved
problems facing theorists is the nature of time, which has
been recognized as inextricably bound up with space ever since
Einstein posited a constant speed for light. In general relativity,
it isn't always obvious how to define what we mean by time,
especially at the Planck scale where time seems to lose its
conventional meaning. Central to any quantum theory is the
concept of measurement, but what does this imply for physics
at the Planck scale, which sets an ultimate limit to the possibility
of measurement? How any of these ideas about space- time can
be tested is currently unknown. Some physicists believe this
makes these ideas not real scientific inquiry at all. And
it's worth remembering that mathematics can sometimes introduce
concepts that are only a means to an end and have no independent
reality.
In the abstract
world of mathematical symbolism, it isn't always clear what
is real and what's not. For example, when we do long division
on paper to divide 54,162 by 2 to get 27,081, we generate
the intermediate numbers 14, 16, and 2, which we then just
throw away. Are virtual particles, compact 6-dimensional manifolds,
and twistors simply nonphysical means to an end - mere artifacts
of how we humans do our mathematics? Particle physicists often
have to deal with "ghost fields" that are simply
the temporary scaffolding used for calculations, and that
vanish when the calculations are complete. Nonphysical devices
such as negative probability and faster- than-light tachyon
particles are grudgingly tolerated so long as they disappear
before the final answers. Even in super- string theory, recent
work suggests that it may be possible to build consistent
models entirely within ordinary four-dimensional space-time,
without recourse to higher dimensions.
ANGEL
FOOD CAKE
So, how should
we think of the great, dark void that we gaze into at night?
All clues point to space-time being a kind of layer cake of
busy phenomena on the submicroscopic scale. The topmost layer
contains the quarks and electrons comprising ordinary matter,
scattered here and there like raisins in the frosting. These
raisins can be plucked away to make a region of space appear
empty. The frosting itself consists of virtual particles,
primarily those carrying the electromagnetic, weak, and strong
forces, filling the vacuum with incessant activity that can
never be switched off. Their quantum comings and goings may
completely fill space-time so that no points are ever really
missing. This layer of the cake of "empty space"
seems pretty well established by laboratory experiment.
Beneath this
layer we have the domain of the putative Higgs field. No matter
where the electron and quark "raisins" go, in this
view, there is always a piece of the Higgs field nearby to
affect them and give them mass. Below the Higgs layer there
may exist other layers, representing fields we have yet to
discover. But eventually we arrive at the lowest stratum,
that of the gravitational field. There is more of this field
wherever mass is present in the layers above it, but there
is no place where it is entirely absent. This layer recalls
the Babylonian Great Turtle that carried the universe on its
back. Without it, all the other layers above would vanish
into nothingness.
We know that
space-time is quite smooth down to at least the scale of the
electron, 10^-20 cm - 10 million times smaller than an atomic
nucleus. This is the size limit set for any internal component
of the electron, based on careful comparisons between experiment
and the predictions of quantum electrodynamics. But near the
Planck horizon of 10^-33 cm, space-time must change its structure
drastically. It may be a world in which conventional notions
of dimensionality, time, and space need to be redefined and
possibly eliminated altogether.
The conceit
of our universe's uniqueness may disappear, with big bangs
becoming viewed as run-of-the-mill events in some much larger
outside realm, and with physical constants being attributed
to causes in space-times forever beyond human experience.
There is much
that's spooky about the physical vacuum. This spookiness may
be rooted more in the way our brains work than in some objective
aspect of nature. Einstein stressed, "Space and time
are not conditions in which we live, but modes in which we
think." Our understanding of space remains in its infancy.
With Aristotle smiling at us down the centuries, we now see
the vacuum as much more than a vacancy. It will take many
decades, if not centuries, before a complete understanding
of it is fashioned. In the meantime, enjoy the nighttime view!
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