June 6, 2000
ESSAY
Celebrating the Poetry of Imagination Without Boundaries
By LAWRENCE M. KRAUSS
n the bitter cold of Antarctica and
the blistering heat of Texas, scientists recently coaxed the universe to
display its hidden structure. In the
past weeks, two different high altitude balloon experiments studying
the microwave radiation left over
from the Big Bang independently reported that the universe is very close
to being "flat."
This is a geometric concept. Einstein's general theory of relativity
tells us that light rays bend in a
gravitational field because space is
"curved" in the presence of such a
field, meaning that the standard Euclidean rules of geometry taught in
high school need not always apply.
The recent observations, however,
imply that when crossing the entire
visible universe, light seems to travel in straight lines.
The Antarctic announcement, first
in print of the two, was impressive
enough to be reported in local and
national newspapers, but it did not, to
my knowledge, produce even a slight
blip in the stock market.
Nor should
it have.
Nothing in the entire course
of human history would have been
noticeably different if the universe
were not flat.
Yet precisely for this reason there
is poetry in the recent discoveries
that I believe is worth celebrating,
even if one is not a cosmologist.
They
represent on the one hand a triumph
of the human imagination, and on the
other a valuable reminder that science can never be purely cerebral.
The universe always seems to surprise us.
In a century full of scientific revolutions, Einstein's discovery of general relativity stands as a monument
to the power of human reasoning. In
1915, not a single direct piece of
evidence existed that clearly suggested that space might be curved in
the presence of massive bodies.
However, indirect reasoning, based on
careful observations of the nature of
light and the motion of massive bodies, led to the realization that the
Newtonian gravity theory would
have to be revised.
Empirically, the observable effects of general relativity were minuscule. Light, for example, was predicted to bend around the Sun by
about five 10-thousandths of a degree.
But small as these effects were,
they changed the way we think about
space and time.
Einstein's theory paved the way
for interpreting Edwin Hubble's remarkable discovery that the universe is expanding.
For it is only
within the context of general relativity that such a global expansion can
be consistently described. But, there
remained a problem: if the universe
is currently expanding, will the expansion stop, or continue unabated
forever?
The answer to this question appeared to depend upon the specific
geometry of our universe, and could
not be determined within the context
of the theory alone, without further
observations. Moreover, it seemed to
depend on a very careful tuning of
the initial conditions in the expanding universe.
Twenty years ago, when I was a
graduate student beginning to get
interested in cosmology, a poll of
astronomers and physicists would
have yielded almost no support for
the possibility that we live in a flat
universe.
In the first place, none of the data
then seemed to point in such a direction.
In the second, a flat universe
seemed very special, merely the
boundary between two more generic
geometric possibilities, the so-called
open or closed universes.
In the former case, light rays would diverge
as they traveled across the universe
and in the latter case they would
converge together.
But there was a problem, first
voiced by the remarkable experimental physicist Dr. Robert H. Dicke
and his colleague, the distinguished
cosmological theorist Dr. P. James
E. Peebles. They pointed out that
living in a flat universe was like
sitting atop a very steep mountain.
If
one moved slightly away from the
top in either direction, one would
very quickly come tumbling down.
But our universe is over 10 billion
years old.
Surely if the universe were
not essentially flat, it would long ago
either have collapsed back upon itself, or have expanded so fast that
matter would have long ago been
diluted to irrelevancy on a cosmic
scale.
This theoretical argument led Dr.
Alan Guth, a physicist now at Massachusetts Institute of Technology, to
develop a new cosmological model,
called inflation, which would naturally ensure that the early universe
would be driven to being so close to
being flat that it should remain indistinguishable from being flat for virtually an eternity thereafter. Dr.
Guth's theory was itself so compelling that within a decade, again without any direct evidence for a flat
universe, many cosmologists had
been converted.
I remember progressing from
graduate student to professor during
this time, as we in the "flat-universe
trenches" began to win the day, and
others in the field and out began to
come around.
The argument seemed
so compelling that it appeared that
nature would have had no choice but
to adopt it, even if there was always
a slight equivocation that what
seemed natural to us might not be so
natural for the universe.
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| `The universe
always seems to surprise us.' | |
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So had the discoveries in Antarctica taken place 20 years ago, most
physicists would have been shocked,
and perhaps dubious. Now, however,
many in the cosmological community can be seen patting themselves on
the back for their foresight.
When I ponder these developments, I remain amazed that we
have come to understand the universe so intimately that we may
have suspected in advance that it
should be flat.
We are, after all, confined to the immediate proximity of
our spinning globe on the outskirts of
the Milky Way galaxy.
No one could guess by simply peering with a naked eye at the night sky
that our galaxy is one of 100 billion
or so in the visible universe, or that
the universe is expanding, much less
that space on large scales might be
curved.
Fortune indeed favors the
prepared mind, and it is hard to over-emphasize the intellectual journey
required before the very question of
the naturalness of a flat universe
could even be discussed.
But this essay is written in praise
of cosmology, not cosmologists. I
have made it sound as if the universe
willingly followed the demands of
human reason.
Nothing could be further from the truth.
The models we
invented in the 1980's to try to bring a
flat universe into accord with the
observations at the time are clearly
wrong.
Much to our surprise the energy that dominates in the expanding
universe is not associated matter of
any sort, pedestrian or exotic.
Rather, it appears that our universe is only flat because empty
space is endowed with some sort of
funny energy, whose origin we can
only begin to imagine at the present
time. Although Einstein had been the
first to speculate about such a possibility by introducing what became
known as a cosmological constant
(in order to explain why an apparently static universe might not collapse
of its own gravity) he later found it
so abhorrent that he dismissed it as
his greatest blunder.
It is fair to say that if observations
had not driven us to this precipice, no
one would have traveled there in
advance or later revisited it.
Indeed,
theoretical a priori arguments suggest that a cosmological constant
tuned to produce a flat universe today is as unnatural on fundamental
grounds as a flat universe now
seems natural.
It gets worse.
We were driven to
determine whether the universe is
flat or not with one main goal in
mind: to constrain eternity.
The
classical arguments in pre-1995 cosmology books, mine included,
stressed that if we could determine
the geometry of the universe, we
would know its ultimate destiny.
How
very marvelous to imagine that in
our lifetimes we might determine,
for certain, whether the universe
would end with a bang or a whimper.
But the universe has outsmarted
us once again.
The realization that
empty space might in fact provide
the dominant source of energy in the
universe has again changed everything.
We now recognize that any
universe, open, closed or flat, can
collapse or expand forever, depending upon the magnitude of this new
form of energy.
Geometry and destiny have been disentangled.
Indeed, my colleague Michael
Turner and I have argued that there
will never be any finite set of astronomical measurements made over a
finite time that will allow us to determine the ultimate fate of our universe. Some things, it seems, may be
forever shrouded in mystery.
So we find ourselves at the dawn of
the 21st century strangely self-satisfied and at the same time confused.
A flat universe doesn't matter a
tinker's damn, an expression my
wife likes to use, in the everyday
course of human events.
Yet the lessons that arise from these discoveries can leave us with a completely
new perspective of our place in the
cosmos. That is what the progress of
culture is all about.
Ultimately this,
not technology, may be the greatest
legacy of science.
Finally, we learn that the universe
is a stranger and more interesting
place than human imagination alone
can ever foretell.
If we stop looking
outward, we are likely to end up
going nowhere.
Dr. Lawrence M. Krauss is chairman of the physics department at
Case Western Reserve University. His book "Odyssey: The Lives of
an Atom" will appear early next year.
