Hiding in the Mirror (29 page)

All of this metaphysical speculation began to
take on greater significance in the latter part of the twentieth
century as new ideas in physics spawned new ideas in cosmology. For
example, once inflationary theory became widely accepted as a
wonderful candidate idea to resolve various puzzles in the nature
of observational cosmology, it was quickly recognized and stressed
by the physicist Andrei Linde—one of the most inventive of the
inflationary pioneers—that its principles would in general imply
that the entire visible universe is likely to be merely a part of
an incredibly complicated “metaverse” of causally disconnected
universes. Some of these may be collapsing, others expanding, some
may only now be experiencing a big bang expansion, and others may
have long ago ended inside of cosmic black holes. The possibility
that many different universes might exist even in our mere
three-dimensional space became compounded by the possibility that a
higher ten-or eleven-dimensional space might settle into one of a
virtually uncountable total number of possible ground states. The
natural question then becomes: Did a single universe settle into a
single ground state, or could it be that there are a host of
different universes in a kind of “metaverse,” each of which could
settle into a different possible ground-state configuration?

For the most part, except for a cadre of
philosophically minded theorists, no one much worried about this
issue for a long time. Physicists are trained to calculate things
from first principles, and moreover the remarkable successes of
particle physics in the 1970s had demonstrated that it should be
possible to explain all of the observed phenomena at subatomic
scales using three simple and elegant theories. It is true that in
order for objects such as stars to operate and to be able to cook
light elements such as hydrogen and helium into heavier elements
such as carbon, oxygen, nitrogen, and all the other substances so
vital to life on Earth, some remarkable coincidences seemed to be
required at the level of nuclear physics. But, coincidences happen
all the time, and indeed without knowing all of the spectra of
possibilities, the fact that the observed route to stellar burning
seemed to depend on some numerical fine-tuning was not particularly
extraordinary, even if some intrepid speculators did write articles
and books on the subject.

Then along came dark energy. Suddenly there was
a parameter in nature that was so strange that no sensible
explanation of its existence seemed within sight. Physicists began
to explore possibilities that had otherwise seemed perhaps too
distasteful, and Weinberg and his collaborators asked themselves
the question: If there are possibly an infinite number of different
universes, and if each universe could have a different value of the
energy of empty space, what value might we expect to measure in a
universe full of stars and galaxies that is over ten billion years
old?

The somewhat surprising answer to this question
is that one would expect, without knowing the details of what might
be the a priori probability of having a universe with a given
vacuum energy, that a universe in which galaxies could form after
billions of years and astronomers could measure their properties
would seem to require that this energy not be much larger than
about five to ten times the vacuum energy we currently infer. Given
that naive estimates based on quantum mechanics and relativity
would suggest a value that is 10120 times larger, the anthropic
argument provides an estimate that is far closer to the value we
apparently live with in our universe.

At present, it is fair to say that this
anthropic “explanation” of a vacuum energy that is comparable to
the value we actually measure is one of the few viable proposals on
the table. Having said that, however, it is important to realize
that at this stage it is virtually impossible to know if this
explanation really is an explanation at all. For example, while
Weinberg and company did a calculation to show that if the vacuum
energy alone were freely varying among all possible universes, one
might expect a value comparable to what we see in our universe.
Without a fundamental theory that tells us which fundamental free
parameters are variable, and which are fixed by fundamental laws, it
is hard to know how seriously to take this simple first guess.

This, ultimately, is the fundamental problem in
my mind with anthropic arguments. They may seem suggestive, but
without a fundamental theory they can never be more than this.
Indeed, as I have said on at least one public occasion, the
anthropic principle is something that physicists play around with
when they don’t have any fundamental theory to work with, and they
drop it like a hot potato if they find one. Nevertheless, while my
own biases about this notion are clear, it is fair to say that the
moment one recognizes the possibility that multiple separated
universes might exist, due either to separate inflationary phases in
an otherwise infinite volume, or to the existence of higher
dimensions, an anthropic explanation of fundamental parameters in
our universe becomes at least a reasonable logical possibility. It
is for this reason that a variety of sensible and distinguished
individuals had begun to advocate this idea, and why it is at least
worth examining further, even before string theory adds its two
cents.

This finally brings us back to M-theory. Faced
with the prospect that this theory may ultimately predict a
virtually uncountable set of possible universes, some string
theorists did a 180-degree about-face. Instead of heralding a
unique Theory of Everything that could produce calculable
predictions, they are now resorting to what even a decade ago they
may have called the last refuge of scoundrels.

But, when string theorists take a position,
they do it with flair. In attempting to graphically explore the
different ground states of a subset of the set of all string vacua,
some theorists realized that the diagrams looked like complicated
landscapes, with billions and billions of sharp mountains and deep
valleys. Physicists Joe Polchinski, Raphael Bousso, and Leonard
Susskind felt that the images were so striking that they
capitalized on the description, and invented what they called “the
landscape.”

You can guess the argument by now. String
theory/M-theory predicts more than 10100 possible configurations in
which a three-dimensional universe might arise from a
higher-dimensional framework (even though no one quite knows how
many dimensions are truly fundamental). So, among all these vacua
there are likely to be some with extremely small values for the
vacuum energy, comparable in fact to what we measure today. These
would be anything but generic universes, and would certainly not be
what an otherwise unbiased observer would predict to find in a
random universe. But, perhaps there are no unbiased observers! If
observers like ourselves can exist only in universes that have at
most an extremely small cosmological constant, then as long as the
M-theory landscape provides that possibility somewhere, then that
is where we will find ourselves. What is perhaps most amazing about
this is the degree to which this new reliance on postdiction is
being adopted in parts of the community. In the end, it may be
correct. It may be that string theory cannot predict from first
principles a parameter as fundamental as the ground state energy of
our universe. It may merely be an environmental accident, after
all. Still, this is a far cry from the excitement about a Theory of
Everything raised twenty years ago during the first flush of
enthusiasm associated with string theory, extra dimensions, and the
new potential for unifying quantum mechanics and general
relativity. Indeed, after the incredible journey of physics during
the past century, after all the remarkable discoveries, theoretical
and experimental, discussed in this book, this proposal seems
rather like an anticlimax. As Edward Witten has commented,
politely, about this approach: “I’d be happy if it is not right. I
would be happy to have a more unique understanding of the
universe.”

His point is well taken. A cynical individual
might suggest that some string theorists have embraced landscapes
because since the theory cannot apparently predict anything anyway,
it is gratifying to find a quantity that reinforces the notion that
ultimately no fundamental constant in our universe is predictable.
Nevertheless, as Witten’s remark underscores, if the landscape
turns out to be the main physical implication of the grand edifice
of string theory or M-theory, then instead of precise predictions
about why the observable universe of three large and expanding
spatial dimensions must be the way it is, we might be left with the
mere suggestion that anything goes. What was touted twenty years
ago as a Theory of Everything would then instead have turned quite
literally into a Theory of Nothing. But the good news is that we
don’t yet know. The more we explore the ideas of string theory,
M-theory, and Braneworlds, the more it becomes clear that we
understand far less than we thought about what might be possible in
nature. Even the fundamental concepts of strings and
dimensions—which lay at the heart of the original 1984
revolution—may now be beginning to melt away.

Will whatever physical theory results in the
aftermath of all this, following whatever discoveries are made by
experimentalists in the coming decades and by theorists in the
coming centuries, resemble any of the speculative, if beautiful,
mathematical notions at the heart of the current focus of research?
That, I believe, is anyone’s guess. I have recently discussed this
question with two active string theorists, John Schwarz and Nati
Seiberg, and perhaps not surprisingly both still feel that the
mathematical insights already gleaned from string theory are so
powerful that whatever ultimate theory we may derive for the
workings of nature at fundamental scales, it will contain at least
the germ of present string theory ideas. I admit that, during the
course of thinking about these issues as I have written this book,
I myself have run hot and cold. There have been moments when the
remarkable depth of the mathematical insights being explored in the
course of recent years has left me awed, and there have been times
when the sheer hubris of the claims, and the lack of associated
results has left me shaking my head in disbelief. But I want to
make it clear that while I think it is certainly possible and,
given historical perspective, perhaps even likely that all of the
formalism currently being explored is a mere house of cards, and
that it might tumble as soon as the force of some new experiment or
observation overwhelms it, this does not mean the effort is not
worthwhile. If the joy of the search exceeds the pleasure of the
finding, then we continue to be joyfully engaged in an intellectual
struggle that shows no signs of ending and in which hidden
universes have always been a part. To make progress in our attempt
to understand the universe at its most fundamental level, we need
to fearlessly open up new paths into otherwise unexplored places,
and we must not be afraid of wrong turns and dead ends, even if,
like the ether squirts of the nineteenth century, that is what
ideas such as grand unification or string theory ultimately turn out
to be. No doubt we are hardwired to believe that the universe of
our experience cannot be all that there is. This would certainly
explain the persistence of religious faith in an apparently unfair
world of toil and struggle without obvious purpose. Perhaps that,
too, is why we keep returning to the notion that just beyond our
reach, just behind the mirror, lies the key to knowledge.

But even if in the end this longstanding
pursuit of extra dimensions proves to have been a grand illusion,
generations of dreamers have been inspired by it to keep on
dreaming, and generations of seekers to keep on seeking. We have
learned and will in the process continue to learn more about nature
and our own place within the cosmos. And I believe one could make a
good argument that such efforts make life worthwhile. For those who
may be less romantic, there is another plus. In our continued and
possibly flawed search for hidden universes and extra dimensions, we
are certain to stumble upon unexpected and undoubtedly unrelated
natural wonders that are currently beyond our wildest imagination,
and that may have a direct impact upon our own future. If the past
is any guide, one thing seems certain: The universe always seems to
come up with new ways of surprising us.

In . . . Philosophical
Theories as well as in persons, success discloses
faults and infirmities which failure might have concealed
from obser-
vation.

—John Stuart Mill,
On
Liberty

O
n January 30, 1991,
the physicist John Bardeen died. An obituary appeared in various
major papers around the country, but most people then, like most
people now, would hardly recognize the name—in spite of the fact
that it is arguable that Bardeen changed the face of the twentieth
century as much as any other scientist of his era. He was the only
physicist ever to win two Nobel Prizes in physics. The first was for
the invention of the transistor, which, as I have mentioned
already, is at the very basis of almost all of modern technology.
The second was for the explanation of superconductivity, the
remarkable property of some materials to allow currents to flow
without resistance of any kind below a certain temperature, a
phenomenon whose technological impact will most surely grow in this
century.

Yet, even among lay people with an interest in
science, I would venture to suggest that there is more interest in
string theory than superconductivity, in spite of the fact that the
former has yet to have any clear impact on our understanding of the
physical universe, much less our daily lives. This is not meant to
be judgmental. Rather, it simply reflects something that I think is
deeply ingrained in the human psyche. “Space” and “time” are among
the very first concepts that are framed as our own consciousness
emerges shortly after the fog of birth. So it is not surprising
that considerations of the ultimate nature of space and time may
continue to appear more interesting than the things that merely
happen within space and time.

I began this book wondering about what drove an
ancient ancestor to leave an imprint of his or her child’s hand on
a cave wall. I suggested that it was to create a measure of
permanence, something that might live on, as it in fact did, long
after the participants in this artistic enterprise were gone. Time
is our ultimate enemy, and to conquer time means first trying to
understand it. Time is a subtler concept than one might imagine.
Both future and past are not directly experienced, but must be
intellectualized. Space, on the other hand, while immediate and
visceral, nevertheless taunts us with its mysteries every time we
do something as simple as gazing out at the horizon. Recall that
for early European sailors the horizon represented the end of a
world that we now know has no end. If we can be so easily fooled
here on Earth, what do the more exotic mysteries that lie out in
the darkness of the night sky hold for us?

Yet recall that I also ended the first chapter
of this book with a warning from the famous French chemist Antoine
Lavoisier about guarding against flights of the imagination
regarding things one can neither see nor feel. His warning, of
course, continues to go unheeded. Indeed, this book pays homage to
the history of the remarkably constant human impetus, both
scientific and artistic, to first imagine and then explore the
reality that exists beyond our direct sensory experience.
Nevertheless, in spite of all the excitement regarding the possible
existence of extra dimensions, I confess yet again to being an
agnostic. Perhaps it is more appropriate to call myself a skeptic.
This position sometimes gets me into trouble, especially in public
debates, but I am nevertheless proud to be part of a noble
tradition in science. I earlier referred to Richard Feynman’s
statement that science is “imagination in a strait-jacket.” Most
good ideas are wrong, in that nature does not choose to exploit
them. If that were not the case, doing science would be far easier.
I do remain fascinated with the myriad possibilities for new and
hidden realities afforded by extra dimensions, but I try to temper
my enthusiasm with the realization that, like Fox Mulder, I “want
to believe.” Large, hidden extra dimensions are seductive, and I
wish that they were true in the same sense that I wish I could use
a warp drive to travel to distant stars, to go where no man or
woman has gone before. We may indeed be on the threshold of
discoveries that will truly change everything, that will further
inspire a generation of artists and writers, and vindicate once
again the wildest imaginings of science fiction writers. But there
is no evidence at this time that any such imminent breakthrough is
likely or inevitable. There are beautiful theoretical arguments
that are strongly seductive, as I have tried to describe, but there
were beautiful theoretical arguments in 1970 that were also
strongly suggestive—but also wrong—that string theory might provide
a fundamental theory of the strong interaction. Equally beautiful
theoretical arguments prompted Kaluza and Klein to make their bold
proposals, but we now understand those elegant concepts were
introduced before their proper time. Kaluza and Klein could never
have known that the theory they were exploring was missing key
features of reality, including two of the strongest forces in
nature. Perhaps we are in the same boat today.

Today’s confused and tentative explorations of
possibly infinite extra dimensions and infinite landscapes of
extra-dimensional worlds must be seen as simply the most recent
expression of a longstanding scientific and cultural tradition. One
can marvel, for example, at the remarkable resemblance between the
claim that elementary charges in our space are merely the ends of
fundamental strings that may stretch out into higher dimensions,
and the nineteenth-century claim that these charges were “ether
squirts”—places where a four-dimensional ether flowed into our
threedimensional world. Such eerie resemblances imply neither that
current science is pure fiction, nor that the ill-founded
speculations of the 1870s bore some hidden truth. To make such
arguments would be just as misplaced a notion as subscribing to the
claims that a resemblance between ancient Eastern mystical writings
and some of the tenets of quantum mechanics implies the ancient
writers had any idea of even what hydrogen was, much less how to
calculate the spectrum of light emitted by it. Similarly, it has
been stated many times since 1984 that the remarkable discovery of
string theory in the 1970s and its rediscovery in the 1980s was a
unique situation in the history of physics: We were living in the
twentieth century, having accidentally discovered the physics of
the twenty-first or twenty-second century. That could, in fact, be
true. But we have no proof that it is or was. It is just as likely
to be true that we are instead reliving the delusional enthusiasm
for the extra dimensions of the nineteenth century. That is also
cause for neither despair nor hope, in my opinion. It is simply an
inevitable product of living in confusing times. But being confused
is
cause for hope. Perhaps there is no
state more desired by theoretical physicists than being confused,
for it is confusion that compels us to seek out new knowledge and
the opportunities for breakthroughs.

As we thus celebrate the remarkable ideas that
have emerged from the solid scientific progress of the past century,
we must be careful to keep things in perspective. I can think of no
better way to do this than to relate the intertwined discussions of
three of the most accomplished theoretical physicists of my own
generation: David Gross, Frank Wilczek, and Edward Witten. All
three have played important roles in the stories related in the
preceding pages.

When I first told Wilczek that I was writing
this book, he related a somewhat disconcerting story to me about a
time when he tried to explain the remarkable aspects of the strong
interaction between quarks to a public audience (before the Nobel
committee anointed this work as being important). After the talk, a
member of the audience raised his hand and asked: “Why should I
care about all of this? Isn’t it just the fourdimensional
manifestation of the far more fundamental predictions made by
string theory in ten dimensions?”

This reminded both of us of an earlier time
when we were working together to advise the Smithsonian Institution
on several projects it was sponsoring, supported by the Defense
Advanced Research Projects Agency (DARPA, a national security
funding group), on the detection of neutrinos. DARPA was interested
in detecting neutrinos because they are emitted by nuclear
reactors, and nuclear reactors are on submarines, and detecting
submarines is of vital strategic importance. Thus, even far-out
schemes seemed to DARPA to be worth throwing a bit of money at,
because if any of them worked, it could easily have tipped the Cold
War strategic balance in our favor.

Of the projects we examined, all were rather
fanciful, but one was at least marginally plausible. It was a
proposal to detect neutrinos from possible nearby nuclear weapons
tests using a large ton-sized detector. However, when we informed
DARPA of our choice, we were told that they had already been
supporting the work of a well-known (but misguided) scientist, who
claimed he had a bread box–sized device that could detect neutrinos
from every nuclear reactor and nuclear weapon on Earth. How could
DARPA therefore justify funding a ton-sized detector near a nuclear
weapons test when it was spending millions on a far smaller
detector that was argued to be far more sensitive?

This is the problem that often arises when
speculative science is valued more than the remarkable achievements
of empirically tested science. The moral for our present
discussions is, I hope, clear. The tremendous intellectual efforts
over the past century to formulate a candidate theory that might
unify quantum mechanics and gravity in a higher-dimensional
framework should not be minimized. The theoretical and mathematical
results that have been developed are fascinating. But neither
should they be celebrated for more than they yet are.

It does a disservice to the most remarkable
century in the history of human intellectual investigation to
diminish the profound theoretical and experimental discoveries we
have made in favor of what is at the present time essentially
well-motivated, educated speculation. It is also simply
disingenuous to claim that there is any definitive evidence that any
of the ideas associated with string theory yet bear a clear
connection to reality, or that they will even survive in their
present form for very much longer. Perhaps more to the point, the
deeper we probe these theories, the hazier they seem to have
become.

Which brings us to Edward Witten, who has been
the leading force driving string theory since the mid 1980s. Ed is
not only an incredible intellect, but he is also a refreshingly
honest one. He says what he means, and he always has a sound reason
for saying what he does. Edward is also the attributed author of
the infamous statement regarding twenty-first-century physics in the
twentieth century, which is probably one reason it is so often
repeated. But one should not read more into that observation than I
believe Ed intended. Ed may be a “true believer” in string theory,
but that simply reflects the very nature of his position on the
theoretical forefront. It is, as I have stressed, very difficult to
devote the incredible intellectual energy and focus that are
required over long periods of time in the attempt to unravel the
hidden realities of nature if one does not have great personal
conviction that one has a good chance of being on the right track.
As Edward said succinctly at a recent meeting on the future of
physics, regarding why one should study string theory: “I don’t
consider it plausible that a completely wrong theory would generate
so many good ideas.”

The same level of personal conviction is
required of artists and writers, as well. But what makes science
somewhat different, I believe, is that great scientists are
prepared to follow an idea for as long as decades, but at the same
time are equally prepared to dispense with all of this effort in a
New York minute if a better idea or a contradictory experimental
result comes along.

With this in mind, a number of other statements
that Edward made at this recent meeting are quite telling and, I
believe, validate the gestalt I have tried to characterize here.
Summarizing the essential progress of the theory he has devoted
much of the past two decades to studying, he said:

“It [string theory] is a remarkably simple way
of getting a rough draft of particle physics unified with gravity.
There are, however, uncomfortably many ways to reach such a rough
draft, and it is frustratingly difficult to get a second draft.” He
next reiterated that while we lack any understanding of the core
idea—equivalent to the Equivalence Principle (between gravity and
acceleration) that was at the heart of general relativity—behind
string theory, at its heart is the notion that space-time is an
“emergent” and not a fundamental concept. Thus, the whole notion of
what an extra spatial dimension may mean within the context of
string theory is not clear. More interesting still, he argued that
even strings themselves are not likely to be fundamental, but that
they, too, would prove to be an emergent concept based on something
more fundamental. Finally, Witten stressed what I believe, given
the current situation in string theory after more than twenty years
of research, is an eminently reasonable position: That it is at
best plausible that we will manage to ever understand what string
theory is all about, and, whether or not we do, that it is not at
all clear whether we will be able to use it to understand nature.
This will depend upon factors beyond our control, including how
complex the ultimate answer may be, and what clues we might be
lucky enough to derive from experiment. I reiterate that these were
statements made not by a skeptic, but by someone who passionately
believes that string theory contains a germ of truth.