By Chet
Raymo'58, '64Ph.D.
"One
can't believe impossible things," says Alice in Through the
Looking-Glass. "I daresay you haven't had much practice,"
replies the Red Queen. "When I was your age I always did it for
a half an hour a day. Why, sometimes I've believed as many as
six impossible things before breakfast."
Well, I haven't had breakfast yet this morning, but I've been
reading physicist Brian Greene's best-selling book The Fabric
of the Cosmos, and here are a few impossible things he asks
us to believe:
• Fourteen billion years ago the entire universe we observe
today -- containing upward of 100 billion galaxies -- was contained
in a speck much smaller than the period at the end of this sentence.
• The speck suddenly expanded, in an inflation that lasted for
a billion billion billion billionth of a second, bringing the
observable universe of matter and energy into existence.
• In some versions of the inflation scenario, the part of the
universe we observe is just a tiny fraction of what exists. Vast
realms of galaxies are too far away for their light to have yet
reached us.
• The visible stars and galaxies are just the tip of the cosmic
iceberg. Space is also filled with mysterious dark matter and
dark energy that make up most of what exists. We know this stuff
is out there, by its effect on the luminous galaxies, but so far
no one knows what it is.
• The universe -- including you and me -- is made of string.
A special kind of string, to be sure, vibrating linear mathematical
entities in 10 spatial dimensions that give rise in their various
excitations to all the known elementary particles.
• And why, if space has 10 spatial dimensions, do we observe
only three? Because the extra dimensions are curled up too small
-- a hundred-billion-billion times smaller than an atomic nucleus
-- to be presently observable with even the most powerful particle
accelerators.
I could go on, listing not just six impossible things but dozens.
One puts down a book like Greene's -- and there are many others
in which the new cosmologists try to explain what they are up
to -- wishing one had spent more time practicing with the Red
Queen.
* * *
These Alice-in-Wonderland stories are not just idle
speculations. The fundamentals of big-bang cosmology have been
put to the test, and so far they pass with flying colors. Let
me mention just two stunning confirmations.
First, if the universe began with a big bang, as physicists
say, then the flash of that primeval event should still fill the
entire universe with a diffuse microwave radiation. Inflationary
theory predicts exquisitely detailed variations in the temperature
of the radiation across the dome of the sky. The radiation has
now been mapped with special satellite telescopes -- first in
1992 with COBE (the Cosmic Background Explorer) and more recently
with WMAP (the Wilkinson Microwave Anisotropy Probe) -- and the
agreement of theory and observation is so close as to take one's
breath away.
Second, just this past year a group of European theoretical
physicists, after 20 years of preparation, modeled the big bang
in a month-long run on one of the most powerful supercomputers
in the world, tracking trillions of simulated particles. They
plugged in what we think we know about the relevant physics and
let the computer spin out a universe from the first moments of
creation to the present day. And -- this is the kicker -- the
simulated universe looks stunningly like the one we live in, the
same wisps and streamers of galaxies. Crucially, the computer
model seems to confirm the existence of the so-called dark matter
and dark energy that supposedly account for most of "what's there."
Humans have had creation stories since the dawn of time, and
until recently the stories belonged mostly to the theologians.
But in recent decades something remarkable has happened. A fleet
of extraordinary space telescopes has revealed details of the
universe that were previously invisible. Giant particle-accelerating
machines on Earth reproduce ever more closely conditions that
existed in the earliest universe. Supercomputers make it possible
to mathematically model the creation itself, at least back to
the first trillion trillion trillionth of a second. Cosmology
-- the story of the origin and evolution of the universe -- has
become an experimental science.
What the physicists are telling us may seem impossible to believe,
but it becomes increasingly apparent that they are doing something
right. Albert Einstein's wife, Elsa, was once asked if she understood
her famous husband's theory of relativity. She replied, "Oh, no,
although he has explained it to me many times -- but it is not
necessary to my happiness." Big-bang cosmology, with its strange
subatomic world of multidimensional vibrating strings, may not
be necessary to our happiness, but it is certainly essential information
if we are to understand where we came from and who we are.
* * *
It has been just over a century since Einstein's anno mirabilis,
his year of miracles. During 1905, the relatively unknown Swiss
patent clerk published four astonishingly original papers in the
prestigious journal Annalen der Physik, any one of which
might have won him a Nobel prize. One of those papers established
the theory of special relativity, linking space and time into
a seamless space-time fabric and asserting the equivalence of
matter and energy. Another showed that light consists of small
indivisible packets of energy called quanta. Later, Einstein expanded
his special theory of relativity to include gravity, showing that
this fundamental force of the universe can be understood as a
curvature of four-dimensional space-time. It is perhaps not too
much to say that Einstein single-handedly established the agenda
for the two great pillars of 20th-century physics: general relativity
(gravitation) and quantum physics, the sciences of the very large
and the very small.
One prediction of Einstein's gravity theory was that the universe
could not be stable; it must expand or contract. Einstein was
so skeptical of this possibility that he added a fudge factor
to his theory -- a "cosmological constant," a sort of negative
gravity, a push to balance the pull -- to allow for a universe
that didn't stretch itself thin or collapse into a heap.
Then, in the late 1920s, Edwin Hubble and Milton Humason, working
at the Mount Wilson Observatory in California, discovered that
the galaxies are in fact racing away from one another. Einstein
took the train from Princeton to the West Coast to have a look
at the astronomers' data. Yes, the universe was expanding. The
"cosmological constant" was, he said, the biggest mistake he ever
made.
If the galaxies are moving apart, then they must have been closer
together in the past. It is mathematically possible to run the
movie in reverse, so to speak, and show that billions of years
ago all the matter and energy of the universe was contained within
a seed of infinite density. The big-bang theory was born.
Still, astronomers resisted. Few scientists liked the idea of
a beginning. Even fewer liked the idea of a violent beginning.
Theoreticians struggled to explain the outward flight of the galaxies
without invoking a special moment of creation.
When I was a student at Notre Dame in the 1950s, two stories
of the universe were in contention: the big bang and the steady
state. In the latter theory, the galaxies move apart, as observation
insists, but new matter continuously appears in the space between
the galaxies, eventually bringing new stars and galaxies into
existence. A steady-state universe has no beginning and no end,
and looks pretty much the same from every time and place. It is
the sort of universe a physicist can love.
But observation has not been kind to the steady-state universe.
In the 1960s astronomers observed the flash of the big bang --
the blaze of radiation that accompanied the universe's birth --
now much diluted by the subsequent expansion, an invisible microwave
energy that uniformly fills the entire universe with precisely
the spectrum predicted by big-bang theory. Most scientists were
convinced.
Other evidence for a big bang soon fell into place. The relative
abundance of elements in the observable universe -- hydrogen,
helium and so on -- is exactly as predicted by big-bang cosmology.
Also, since light takes time to reach us, looking at distant objects
is equivalent to looking back in time. And what we see far away
-- most notably, the quasars -- is different from what we see
nearby. The universe has a demonstrable history.
So far, the big bang has survived every observational test.
What once seemed unimaginable -- space and time emerging from
a tiny mathematical point -- now seems almost commonplace.
Meanwhile, physicists were using high-energy accelerating machines
to smash apart atomic nuclei. In the resulting debris they found
whole families of exotic particles, with matter and energy flickering
back and forth one to the other as Einstein had predicted. Quantum
theory describes this subatomic realm with impressive precision.
* * *
But one very big problem still looms for theoretical cosmologists,
the very problem that defeated Einstein, though he devoted much
of his later life to its solution: The unification of the two
great theories of the 20th century, general relativity (gravitation)
and quantum physics. Each theory reigns supreme in its domain
of application, gravitation on the cosmic scale, quantum theory
on the scale of elementary particles. Only occasionally, as in
discussions of black holes or the big bang, where cosmic quantities
of matter are contained in subatomic-sized spaces, do the two
theories rub against each other. The rubbing can be abrasive,
and the race is on to find a unified theory of "quantum gravity,"
sometimes called a Theory of Everything. The invention and verification
of such a theory is the central problem of physics today. Only
when such a theory is in hand will we understand the evolution
of the universe in the first infinitesimal fraction of a second
as it emerged from the unknowable.
Run the movie of the expanding universe in reverse. As we approach
time zero, the size of the known universe becomes vastly smaller
than the nucleus of an atom, the realm of quantum physics. But
the universe also becomes exceedingly dense, and therefore gravity
becomes exceedingly strong. With no unified theory to work with,
that first trillionth trillionth trillionth of a second of the
universe's history remains uncharted territory. The true beginning
-- the singular instant of creation -- lies tantalizingly out
of reach.
Currently, the most promising approach to quantum gravity is
string theory, which proposes that the ultimate elements of reality
are vibrating filaments of energy. Electrons and quarks, the fundamental
components of ordinary matter, and gravitons, the constituents
of the gravitational field, are not points, as previously imagined,
but instead have a linear dimension: metaphorically speaking,
not tiny billiard balls but string. In the most popular version
of the theory, the strings are very small, a hundred-billion-billion
times smaller than an atomic nucleus, but it is precisely these
little snips of energy -- that can't be snipped any smaller --
that make possible a unification of quantum physics and general
relativity.
All versions of quantum gravity, when they have battered their
way to a common vision, will probably suggest that space and time,
like matter and energy, come in quantized (indivisible) units,
and that relationships, a sort of mathematical music,
rather than things, are the fundamental elements of reality.
Even those particles of matter we thought of as hard, solid "stuff"
dissolve into pure song.
And it just gets weirder and weirder. Some physicists think
of the universe as a hologram, a kind of grand illusion. Others
talk of myriad universes that bubble from the void like bubbles
from champagne, or a mirror universe on the other side of the
big bang where time runs backward. Many cosmologists believe that
the universe is truly infinite in extent and contains an infinitude
of galaxies, including some that are exactly like the one we live
in, down to the very color of the socks you are wearing at this
instant.
Ordinary folks like you or me might reasonably ask, "What does
any of this have to do with me?" And indeed, like Elsa Einstein,
we manage to live out our lives in a universe of three space dimensions
and one time dimension with no consciousness of those proposed
ultra-small vibrating strings and multidimensional spaces. We
have, in spite of ourselves, become more or less accustomed to
the big bang, but the wildly abstract ruminations of the quantum
gravity physicists seem rather like those of medieval theologians
who supposedly debated how many angels can dance on the head of
a pin.
* * *
There is a difference between strings and angels. No
matter how weird the speculations of the physicists, at least
potentially it is possible to put their ideas to an observational
test. Without a test, yes, they might as well be talking angels
and pins, and, for the moment, strings, multiple universes, and
11-dimensional space-time are beyond experimental verification.
But that might change in the decade to come.
In 2007, the European Space Agency's Planck satellite will test
theories of the early universe by looking more precisely at the
radiation left over from the big bang. In 2008 the first results
from the Large Hadron Collider, the world's most powerful particle
accelerator, will be available from CERN, the European high-energy
physics laboratory near Geneva, Switzerland, recreating even more
closely those first moments of the big bang when quantum gravity
presumably ruled. And sometime after 2011, NASA's Laser Interferometer
Space Antenna, spread out across a huge space of the solar system,
could reveal the effects of quantum gravity in the early universe
as ripples echoing in space-time.
Meanwhile, astronomers use the instruments we currently have
on Earth and in space to map the universe in ever greater detail.
Of particular interest is the precise rate of expansion of the
universe. Astronomers measure the expansion rate by observing
supernovas -- exploding stars -- in distant galaxies, and using
the apparent brightness of the supernovas as distance indicators.
The speed of the galaxies is measured as a shift in the wavelength
of their light, in much the same way as a radar gun measures the
speed of a moving car. Put the two observations together and the
conclusion is inescapable: The expansion of the galaxies is accelerating,
not slowing down as would be the case if only gravity were
at work. Something is pushing the universe toward a fate of infinite
dispersal, cold and dark. That mysterious "something," spread
out uniformly through the universe, is called dark energy -- an
unexpected re-incarnation of Einstein's cosmological constant.
Dark energy is not to be confused with dark matter, an equally
mysterious massy, non-luminous substance detectable by its gravitational
effect on the clustering and motion of galaxies, almost certainly
some hitherto undiscovered kind of elementary particle. It is
humbling to realize that most of what exists is invisible to us
and -- so far -- unknown.
Perhaps after all it is not so absurd to think of theoretical
and experimental cosmologists as the new theologians. They are,
after all, working at the very limits of the knowable -- what
the physicist John Wheeler has called "the flaming ramparts of
the world" -- asking the biggest questions of all: Why is there
something rather than nothing? Whence the circumstances of life
and consciousness? What will be the fate of the universe? Many
centuries ago Saint Columbanus asked in a sermon: "Who shall examine
the secret depths of God? Who shall dare to treat of the eternal
source of the universe? Who shall boast of knowing the infinite
God, who fills all and surrounds all, who enters into all and
passes beyond all, who occupies all and escapes all?" Those who
wish to know God, he said, must first review the natural world.
* * *
All cultures, everywhere on Earth, have stories, passed down
in sacred writings or tribal myths that answer the questions:
Where did the world come from? What is our place in it? What is
the source of order and disorder? What will be the fate of the
world and of ourselves? The new story, the scientific story, is
the product of thousands of years of human curiosity, observation,
experimentation and creativity. It is an evolving story, not yet
finished. Perhaps it will never be finished. It is a story that
began about 14 billion years ago (13.7 billion years, to give
the most up-to-date estimate) with an explosion from an infinitely
small, infinitely hot seed of energy, and that has provided, in
this tiny corner of the universe at least, conscious creatures
who match their brains against the ultimate mysteries.
The new scientific cosmology has important advantages over all
the stories that have gone before. For one thing, it works. Scientists
test the story in every way they can devise, in its particulars
and in its totality. They build giant particle-accelerating machines
to see what happened in the first hot moments of the big bang.
They put telescopes into space to look for the radiation of the
primeval explosion. They model the formation of stars and galaxies
with supercomputers. With spectroscopes and radiation detectors
they analyze the composition of stars and galaxies and compare
them to our theories for the origin of the world. Always and in
every way they try to prove the story wrong. And when the story
fails, they change it.
The new cosmology places us squarely in a cosmic unfolding of
space and time, and affirms our biological affinity to all humanity
and other creatures. We are inextricably related to all of life,
to the planet itself, and even to the lives of stars.
The microbiologist Ursula Goodenough, in her book The Sacred
Depths of Nature, reminds us that the word religion derives
from the Latin religio, to bind together again. She writes:
"We have throughout the ages sought connection with higher powers
in the sky or beneath the earth, or with ancestors living in some
other realm. We have also sought, and found, religious fellowship
with one another. And now we realize that we are connected to
all creatures. Not just in food chains or ecological equilibria.
We share a common ancestor . . . We share evolutionary constraints
and possibilities. We are connected all the way down." All the
way down, perhaps, even to those mysteriously vibrating strings
that bind together the very large and the very small. And in a
universe of a hundred billion galaxies -- at least! -- we have
only begun to understand our place in the fabric of creation.
We treasure the ancient creation stories for the wisdom and
values they teach us. But only the new story has the universal
authority to help us navigate the future. It may not be the "true"
story, but for the time being it is certainly the truest. Of all
the stories, it is the only one that has had its feet held to
the fire of exacting empirical experience.
* * *
Chet Raymo's newest book is Walking Zero: Discovering
Cosmic Space and Time Along the Prime Meridian. He resides
on the web at www.sciencemusings.com.
(April 2006)
