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!