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AI and genetic engineering and cryonics are cool, sure –
not to mention the reconfiguration of matter in to arbitrary
structures -- but what about the staples that every sci-fi-loving
youth grew up on? What about time travel, what about entering
into alternative universes through naked singularities,
what about colonizing the solar system, the galaxy, the
universe?
Personally, I grew up wanting to be a combination astronomer/astronaut,
and then reached a point at age 16 or 17 where I realized
the big show was really inside – inside my skull,
inside everybody’s skull, inside the collective cultural
mind. I became more interested in the process by which our
minds and our culture constructed a consensus reality containing
notions like outer space and naked singularities, than in
these physical entities themselves.
Now I find myself entranced by the feedback involved here:
physics gives rise to chemistry, which gives rise to biology,
which gives rise to psychology and sociology … and
mind and culture then turn around and create our subjective
perceptual worlds and belief systems, which (in the case
of modern Western culture) include such things as physics
and chemistry…. I am not a relativist: I don’t
believe that “everything is just in the mind,”
with physical reality having its reality only because we
believe it’s there. On the other hand I’m not
an objectivist either: I don’t believe that there
is an absolutely real world independent of all minds perceiving
it. I suspect reality is far richer than any of these terms
and concepts capture. Perhaps the philosophy of superintelligent
post-Singularity AI’s will do a better job.
Among hyperoptimistic Singularity theorists (a category
into which I must place myself), one sometimes sees disagreements
about how physical law will hold up after the advent of
superintelligent AI beings. Will these superminds be able
to find a way around the “cosmic speed limit”
Einstein identified (the principle that nothing can travel
faster than light speed, which means that getting around
the universe is always going to be a damn slow process)?
One side argues that physical law cannot be circumvented
by any amount of intelligence. Another argues that the “physical
laws” we’ve inferred are just crude patterns
that our limited minds have recognized in the data we’ve
gathered using our limited sensory organs. My own sympathy
tends to be about 80% with the latter view. For what little
my measly human intuition is worth, my feeling is that there
will be limits to what superminds can do, but these limits
will bear only subtle and indirect connections to the limits
that our current physical “laws” identify.
Indeed, the word “law” in this context is in
my view a little odd and misleading, with unnecessary religious
overtones. Physical laws, as we call them, are just particularly
powerful observed patterns. So far, as we investigate the
physical universe more broadly and more precisely, we seem
to consistently observe new patterns, refining the old though
not totally invalidating them – and it seems reasonable
that this process will continue in even fuller force when
there are superhuman minds carrying it out.
And, in support of this view, far from being in a condition
of stasis, modern physics theories are all over the place,
postulating all sorts of mad things, most famously (as we’ll
discuss shortly) that the entire universe is a kind of music
created by the vibrations of hyperdimensional strings. String
theory is deep and fascinating, and may well turn out to
be true … at least for a while (a few decades? A century?)
The aspects of contemporary physical theory that interest
me the most, however, are the ones that hint at future physics,
far beyond anything theorists are explicitly calculating
or experimentalists are explicitly measuring today. There
is a subterranean line of thinking, not embraced by the
mainstream of contemporary physicists but pursued by a handful
of brilliant mavericks, including some very well-known ones,
which suggests that rather than being composed of hyperdimensional
strings, the universe is in some sense a cross between a
hyperdimensional life form and a hyperdimensional computer.
The image of the universe as a self-organizing, evolving,
self-creating biocomputing system is an exciting one. But
the mathematics required to flesh out such an idea is forbidding,
and may even be beyond the capability of the human brain.
Only time will tell – or it may not: we may not get
the chance to find out whether human brains are capable
of doing such calculations, because our AI creations may
get there before us.
Perhaps
the most amazing feature of 20’th century science,
overall, is the huge kick in the ass it has given to objectivist
philosophy. At the end of the 19’th century, it looked
like science was going to give us a detailed portrayal of
the universe as a giant machine, a clockwork to use the
common metaphor. Instead it did something vastly more complicated,
showing us that our petty human concepts of “objectivism”,
“relativism”, “mechanical” and “deterministic”
are far from subtle enough to cope with the world as it
is.
Plato, who knew little science but had philosophy down fairly
well, seems to have been onto something rather profound
with his parable of the cave. Science has proved this story
true in more ways, and in richer detail, than its author
could have imagined. Today, as in ancient Greece, with all
our powerful scientific machinery and technological achievements,
we still must view ourselves sitting here in a cave, our
backs toward the cave mouth, watching the shadows of trees
and birds and bears dance on the cave’s back wall,
taking the shadows for the reality because we’re unable
to see the real world outside. This is the lot of all finite
minds, and always will be – thus was Plato’s
intuition. And how amused and intrigued he would be to see
the complex combination of experimental tools and mathematical
and conceptual theories we have conceived, describing one
after another aspect of this real world we cannot see –
never perfectly, but better and better as time goes on,
converging toward the infinite limit of real understanding,
a limit that will never be reached.
Cognitive neuroscience shows us that the cave extends even
into ourselves. The mind we perceive is not the actual mind
that controls us. We believe our conscious decisions are
controlling our actions, but underneath, most decisions
arise prior to the conscious process that believes it originates
them, as a result of the self-organizing combination of
millions of microscopic neural events.
Quantum physics shows us that even the most apparently solid
and simple phenomenon of physical reality is just a shadow
dancing on the wall. All of us, all the objects around us,
are really far more than 99.99% empty space. The idea that
anything has a definite position, mass or speed is an illusion:
at bottom everything is made of particles whose state is
fundamentally indeterminate. The shadow world looks determinate
and solid, until you look at it closely, then you see that
the real world reflected in the shadows is indeterminate
and fuzzy, and this indeterminacy is important for understanding
some aspects of everyday things. The genes and proteins
that guide our body rely on quantum mechanical phenomena
in their every interaction. You can’t understand them
by thinking about the immediately perceptible world, the
shadows on the wall – rather, you have to use what
we’ve learned about the world behind the shadows,
as bizarre and counterintuitive as this world has come out
to be.
And our investigations into the world behind the shadows
gets stranger and stranger as time goes on. Indeed our minds,
adapted to the phenomenal world, stretch to grasp these
new insights. It is quite remarkable how science, itself
a channeling of the human mind in particular directions,
can lead to conclusions that rattle the mind so thoroughly,
down to its foundations.
Some of the most dramatic revelations of this kind, in recent
years, have come from physics. Quantum theory shows us that
the everyday world with its solid objects and definite events
is just a shadow world, that the real world underneath is
quite different -- and much more confusing, because our
brains aren’t adapted to it. The particles we’re
made of don’t have definite states, they’re
suspended between different conditions of being, and it’s
this suspension that, by complex chains of causation, makes
it possible for proteins to bind together creating organisms
from genes, and for electricity to zip between neurons creating
emergent thought-patterns. But quantum theory itself is
not complete: it only explains a piece of the shadow-world
we see around us. Gravity, which holds us down on the Earth
and keeps us from flying into space, is a rather obvious
feature of the world around us, yet so far no one has a
really good understanding of how quantum theory is consistent
with gravity.
To put it another way: Among the shadows we observe on the
wall we call the world, are proteins and electrical fields,
planets and gravity. We have a theory about how the real
world casting the shadows works that explains proteins and
electrical fields – this is called quantum physics.
We have a theory about how the real world casting the shadows
works that explains gravitation – Einstein’s
General Theory of Relativity. But these theories, when you
get down into the nitty-gritty of them, appear to contradict
each other.
And in their efforts to resolve this contradiction, scientists
are coming up with yet more complex and peculiar hypotheses
about the world behind the shadows. The real universe, hundreds
of physicists at esteemed institutions now believe, is a
ten-dimensional world made of vibrating strings resonating
at different frequencies. But this hypothesis isn’t
yet as convincingly proven as quantum physics or gravity.
There are alternative theories. Some scientists believe
that the real hidden universe isn’t a multidimensional
string symphony, but rather a huge invisible computer. We’re
all patterns in the universe-computer’s memory, produced
by a software program called physical law.
Who’s right? Building multibillion dollar particle
accelerators may help us find out. Or it may not. At the
moment the string theory’s most interesting concrete
predictions involve the behaviors of certain types of black
holes – interesting predictions, but hard to test
given the current state of practical astronomy. These lines
of scientific research may lead to the technology of the
next century: superpowerful atomic-scale computers, even
faster-than-light travel or time travel. Or they may just
lead to a lot of difficult and expensive head-banging against
the wall of unsolvable mathematical equations and experimental
predictions unverifiable in practice.
From a very general perspective, though, one thing seems
clear. Some journalists and a handful of scientists have
referred to string theory and other advances in modern physics
as the search for a “Theory of Everything.”
But this phrase is misleading in many ways. Progress in
these areas of physics may lead to a lot of exciting things,
but a universal understanding of everything is not likely
to be one of them! For a number of particular reasons that
I’ll discuss a little later, even a universal unified
physics theory is likely to leave a huge number of gaps
in our understanding of the cosmos. The quest to map out
the real world behind our backs, by inferring patterns in
the dance of the shadows, is an ongoing one, with periods
of apparently stable understanding, periods of confusion,
and periods of revolutionary conceptual progress. Modern
physics is just another chapter, albeit a fascinating one.
The more things change, the more they stay the same.
The
history of physics can be understood as a series of successful
attempts to use mathematics to come to grips with the intuitively
incomprehensible aspects of the world. Human common sense
is tuned for explaining human-scale earthly physical events,
but as experimental science progressed, we discovered more
and more about the physical world in other places and on
other scales. The further our discoveries got from the everyday
world, the less useful our common sense was for explaining
them. Mathematics, however, is not restricted to describing
structures and processes that agree with our common sense.
We are able to set up equations and models based on our
common sense understanding, and then derive highly counterintuitive
conclusions from them. This is the power of mathematics.
Newton, in the 1600’s, explained why the moon didn’t
fall down on the Earth, by positing the laws of motion spanning
both earthly and cosmic motions. The explanation of the
moon’s continued elevation drops right out when one
solves the differential equations for the motions of heavenly
bodies. The contradiction between outer space and the earthly
world, so vivid for earlier thinkers, falls away as if it
had never existed.
Maxwell, in the 1800’s, resolved a number of peculiarities
to do with electricity and magnetism, building on the conceptual
and experimental work of Faraday and others, and positing
a set of equations nearly as fundamental as Newton’s.
But this leap in understanding lead to its own share of
contradictions. Maxwell’s equations demonstrate that
radio waves and all sorts of other waves are really just
light waves at different frequencies, and that they all
travel at the same speed – the speed of light –
regardless of how fast the observer is moving. But this
constancy of speed leads to mathematical contradictions
in terms of Newtonian physics. Einstein’s theory of
Special Relativity arose to deal with this contradiction,
explaining how indeed it’s possible for all these
waves to travel at the speed of light – by throwing
out the commonsensical notion that the length, mass and
time of objects are the same no matter how fast the person
observing them is moving.
And Maxwell’s equations led to other problems as well,
in the theory of heat. A hot object lets off electromagnetic
radiation of many different frequencies, but if one adds
up the energy of the radiation let off at different frequencies,
one arrives at an infinite sum. Very bad. Max Planck, around
the start of the 20’th century, had the answer: the
energy is let off only in discrete chunks or “quanta,”
not in a continuous range of values. With this humble act
of contradiction-resolution quantum physics began. The simple
idea of quantized energy led to a rapid-fire series of experimental
and theoretical breakthroughs, similar to what we see today
in molecular biology, and revealing the existence of a strange
and unfamiliar world underlying the world observed every
day, a world in which positions, momentums, energies and
times are shifting and indeterminate rather than definite,
in which particles can leap through solid barriers as long
as they’re not being observed, in which particles
can travel back through time.
While quantum theory was developing, Einstein was not only
helping it along but also pushing in another direction.
His Special Relativity theory frustrated him, because it
directly contradicted Newton's theory of gravitation. Newtonian
gravitational theory explained why the moon fails to fall
in the sea, and how the planes move, but it came with a
price: the unrealistic, clearly false assumption that grativational
force moves from one object to another at infinite speed
across great distances. Special Relativity says that nothing
can move faster than the speed of light. To resolve this
contradiction, the theory of General Relativity was conceived.
The notion of spacetime, a 4-dimensional surface including
three dimensions of space and one dimension of time was
introduced, and in this context, gravity was explained as
the curvature of spacetime. Massive objects curve spacetime,
and then they move along paths in the spacetime continuum
they have curved. This delicate feedback between objects
and spacetime is captured in Einstein’s elegant but
fabulously complex mathematics.
Gravity is a relatively weak force – you and I have
only a very weak gravitational attraction between each other,
for example. It only really kicks in for objects of large
mass. This means that the difference between Newton’s
and Einstein’s approaches to gravity isn’t really
observable on the Earth in any simple way. But it is observable
in outer space. General Relativity explained a previously
baffling eccentricity in Mercury’s orbit. And it predicted
that light coming from a star behind the sun, but near the
edge of the sun from our perspective, would be slightly
bent as it voyated toward us – a prediction that was
validated by measurements done during a solar eclipse.
Making the Special Theory of Relativity consistent with
quantum physics was not easy; this unification led to the
theory of quantum electrodynamics (QED), a theory of the
mid-20’th century that in many ways is the crown jewel
of physics. QED is a complex and beautiful theory, commonsensically
counterintuitive and yet fantastically pragmatic, predicting
the observed mass of the electron, for example, to well
over a dozen decimal places. There was no single superhero
of QED: it was created by an international group of brilliant
scientists such as Feynman, Tomonaga, Schwinger and Dirac,
building on the mathematical quantum physics of earlier
minds like Pauli, Schrodinger and Heisenberg. Laser physics
is but a single example of the very numerous practical applications
of QED.
QED grew into quantum field theory, which extends basic
QED to give a fuller explanation of what happens inside
the nuclei of particles. This is where the fascinating objects
called “quarks” (a word drawn from Finnegan’s
Wake) come into play. Quarks can never be observed in isolation,
but in combinations they create particles like protons and
neutrons. Understanding quarks helped us understand nuclei,
but it required the use of whole new fields of mathematics,
different from anything used for physics before. A class
of theories called gauge theories or Yang-Mills theories
was created, integrating all aspects of physical law except
gravity. There are infinitely many Yang-Mills theories,
but one has emerged as the best explanation of observed
data; this is what’s called “the standard model.”
Each different gauge theory potentially hypothesizes a different
set of particles. The standard model postulates three families
of quarks and leptons (e.g. the electron is a lepton), and
also other mysterious entities called Higgs particles.
The standard model is not a simple thing. There are more
than 20 parameters, whose values aren’t predicted
by the theory. But by setting these 20 parameters, one gets
an immense number of practical predictions. The theory is
really quite a good one. The only problem is this little
tiny thorn in its side called gravity. Einstein’s
grand and beautiful vision of nonlinear feedback between
matter and the spacetime continuum, doesn’t fit into
the standard model picture of the cosmos at all. These two
different theories explain different aspects of the shadows
we see dancing on the wall, by making radically different
postulates about the real world underlying the shadows.
The two theories have to be brought together. That tall
shadow can’t be both a tree blowing in the wind, and
a tall creature moving about – there has to be a common
explanation encompassing the data favoring each explanation.
And so the big question confronting theoretical physics
these days is: How do you make gravity and the standard
model play together? It’s thought that if we can answer
this, we should be able to answer a bunch of other related
questions. Simple things like: Where do the four forces
we see come from? Why do we have the particles and waves
that we have, instead of other ones? Why is space three
dimensional and time one dimensional?
Physicists are a creative lot, and many potential answers
to these questions have been proposed. Here I’ll discuss
only two of them: superstring theory, which is perhaps the
leading candidates in the physics establishment; and the
universal computation approach, which is the domain of a
handful of mavericks, but has an impressive philosophical
simplicity and conceptual power. I have to admit that the
universal computation approach has more appeal to me personally,
because it highlights the similarities between the universe
as a whole and complex self-organizing systems like minds,
bodies and ecosystems. But right now, no one knows which
approach is right. At the moment, progress in creating interesting
new theories is fast, and progress in testing these theories
is far slower. Getting concrete predictions out of theories
based on such outlandishly complex mathematics is not easy;
and doing the very-high-energy experiments needed to most
cleanly differentiate between the theories’ predictions
is not cheap. I have my own radical ideas about how AI might
be used to vastly accelerate this process, which I’ll
mention briefly a little later on.
The
standard model of physics treats particles as points, as
zero-dimensional objects. The particles may be blurred out
over spacetime in that peculiar quantum way, but they’re
still essentially points. This seems a reasonable approximation.
But in the period 1968—1973, a number of physicists
doing complex mathematical calculations regarding some advanced
physics theories realized that, in fact, the math they were
playing with implied a somewhat different model. What their
physics equations were telling them was that particles aren’t
points, they’re one-dimensional objects – strings,
like violin strings.
Like QED, this work didn’t come out of any single
genius scientist – there is no Einstein of strings.
Edward Witten is most often mentioned as the leader of the
string theory movement, but John Schwarz, Michael Green
and many others have also made huge contributions. It has
been observed that, in the early days, string theory was
developed by middle-aged scientists because, according to
the sociology of the physics community at the time, young
scientists were too precarious in their careers to risk
working on something as speculative as ten-dimensional harmonics.
Nowadays of course, string theory is fairly mainstream,
and it’s working on radical alternatives to string
theory that’s more likely to get you denied tenure.
The strings these scientists found their equations were
describing were very short strings – short as in 10-33
centimeters. They’re so short that for almost all
practical purposes, you can consider them as point particles.
But for some things, like the unification of gravity and
quantum physics, the stringiness of these little strings
seemed to be important.
It’s important to understand the course that development
took here. It wasn’t at all a matter of some kooky
scientists sitting around and brainstorming about what the
universe might be made of, and coming up with the idea that
maybe particles are teeny-tiny violin strings. Rather, the
mathematics of previous physics theories seemed to extend
in a certain direction, and the most intuitive interpretation
of this mathematics was in terms of vibrating strings. Once
again, as in the birth of quantum physics itself, mathematics
lead where common sense knew not how to tread.
Strings can be “open” like a violin string or
“closed” like a rubber band. They move through
spacetime they sweep out an imaginary surface called a “worldsheet.”
And
the strings don’t just move – they vibrate.
They vibrate in different modes, loosely similar to the
harmonics or notes of the strings of a musical instrument.
Physical parameters like mass, spin, and so forth, are mathematically
derived from the vibrational modes of these tiny strings.
Ultimately, in this view, there’s only one kind of
object underlying every particle – the string. Different
particles are just different modes of vibration of strings.
A graviton, a gravity particle, is a particular kind of
vibration of a closed string, and so on.
Strings interact with each other by splitting and joining,
as in the following example:
They
can also be pinned to various objects, just as a violin
string is pinned to the violin at both ends. Unlike violin
strings, these tiny little physics strings can be pinned
to objects of various dimensions, called D-branes. Much
of string theory has to do with the study of D-branes, as
well as strings themselves.
These are strings, but what about superstrings? What so
super about them? To understand superstrings, you must understand
that there are two basic types of particles in nature, named
fermions and bosons (after 20’th century physicists
Fermi and Bose). Fermions are “matter-ish” particles,
like electrons, protons, neutrons, and quarks. Bosons are
wispier things, like photons, gravitons, and W and Z particles.
The standard model deals with both. In the mathematics of
string theory, we find a certain kind of symmetry naturally
arises: a symmetry between fermions and bosons. Fermions
and bosons are groups together into collections called “supermultiplets”
(hence the “super” in “supersymmetry”).
And here the mathematics yields the next big surprise. The
only way to explain both fermions and bosons in string theory
is to introduce supersymmetry, and the only way to make
supersymmetry work in a logically consistent way, is to
assume that the little strings exist in a 10 dimensional
world. So, boom!, all of a sudden the real world producing
the cave-wall shadows we see is ten dimensional, unlike
the shadow-world we observe, which has three space dimensions
and one time dimension. The idea of a higher-dimensional
underlying spacetime wasn’t new to string theory –
it was invented by Kaluza and Klein back in the early days
of General Relativity. But back then it was one interesting
speculation among others – now it was being pushed
on physicists by complex mathematics that they barely understood.
How can it be that the real world is 10 dimensional whereas
the observed world has three space dimensions and one time
dimension? The standard explanation is a simple one: the
extra six dimensions are curled up very small. Just as a
two-dimensional piece of paper, rolled up small, looks like
a one-dimensional line; so a ten-dimensional universe, rolled
up small, can look like a four-dimensional universe. If
the size of the rolled-up shape is around the same as the
size of the strings themselves, then the extra 6 dimensions
are almost impossible to observe, but the mathematics may
still tell us they’re there.
It’s a fantastically surreal picture of the real world.
In a very abstract, mathematical way, it seems to explain
all four forces. Gravity, electromagnetism, and the weak
and strong nuclear forces all come out of the same unifying
equations – if you follow the mathematics and are
willing to go with equations that render particles as vibrations
of ten-dimensional strings.
But where’s the evidence that this isn’t all
some kind of fabulous mathematical hallucination? Who says
the universe really works this way? Where’s the proof?
There really isn’t any yet. Perhaps the most interesting
work drawing practical applications out of string theory
involves black holes – regions of space with gravity
so strong that any object that enters them, including light,
can never escape. Black holes have mass, charge, and spin;
and Stephen Hawking showed that they also radiate particles,
because of complex effects in the vacuum around their boundaries.
String theory has recently proved successful in explaining
results in black hole theory, previously understood only
in much more ad hoc, inelegant ways. Strominger and Vafa
derived important equations about a certain class of black
holes by describing them as 5-branes, 1-branes and open
strings traveling down the 1-brane all wrapped on a 5-dimensional
donut shape.
Of course, this branch of physics isn’t something
that can very easily be experimented with – black
holes are far away and are still mostly the domain of theory,
although astronomers have identified a few. For more concrete
implications of superstrings, we need to look elsewhere,
perhaps in experiments physicists run in particle accelerators.
Remember, gravity is relevant for massive objects; but remember
also Einstein’s discovery that mass and energy are
two aspects of the same thing …
E=m c 2
When small particles are accelerated fast enough, they achieve
very high energies, hence gravity applies to them significantly.
The standard model integrates three forces: electromagnetism,
and the weak and strong nuclear forces (two aspects of the
physics that holds nuclei of atoms together). All these
forces have about the same strength at energies of about
1016 gigavolts (GeV, a billion volts). It is estimated than
when we get up to about 1019 GeV, gravity will join the
party too, and all the forces will be about equally important.
No existing particle accelerator can generate energies this
high, but we’re creeping closer and closer. Unfortunately,
the American superconducting supercollider project (SSC),
which would have yielded about 20,000 GeV, was cancelled
in favor of other scientific projects after being partially
constructed. But it’s expected that around 2005, a
European accelerator called the large hadron collider (LHC)
will begin to operate at 8000 GeV per beam. We’re
getting there.
Particle accelerator experiments should let us look for
the supermultiplets, the fermion-boson groupings, that supersymmetry
theories predict. Specifically, supersymmetry implies that
every known elementary particle must have a "superpartner"
particle. It is clear that no known particles are superpartners
for each other. Where are the squarks corresponding to quarks,
the selectrons corresponding to electrons, the gluinos corresponding
to gluons, and so forth. It is hypothesized that the superpartners
are so heavy you can only see them at very high energies.
While this is quite reasonable and possible, one can understand
why detractors call this a convenient excuse for maintaining
a theory that postulates a huge number of particles no one
has ever seen. Even if the LHC doesn’t reveal these
particles, this won’t prove for sure that they don’t
exist – they could just be even more massive than
this machine would reveal. The problem is current mathematics
of superstring theory doesn’t let us exactly predict
superpartner mass, so we don’t know how big the accelerator
would have to be to reveal the existence or otherwise of
all these particles that the mathematics tells us should
be there.
Although the creation of billion-dollar accelerators to
test theories that are still not entirely quantitatively
clear, is naturally a slow business, theoretical progress
on superstrings progresses at a rapid pace. The period since
1994 has ground a second superstring theory revolution,
in which new mathematical techniques have shown that what
used to look like five different superstring theories, were
actually just one mega-theory viewed from different perspectives.
The march to unification moves on – on a mathematical
level, with very little feedback from experimental data.
The mathematical theories underlying different aspects of
physics appear to unify more and more elegantly, if one’s
willing to accept abstract mathematical structures like
ten-dimensional vibrating strings and superpartner particles.
But whether the universe agrees with this mathematics, or
is stranger yet, remains to be seen. There is much history
in physics to show that elegant mathematics can lead to
empirically valid results. But this time, perhaps, have
the mathematicians gone too far?
Superstring
theory is popular at the moment, but it’s really just
one among many approaches to unifying gravity and the standard
model. It does have a lot going for it. The mathematics
is beautiful, and one result after another comes out, providing
more and more evidence that the equations are sensible,
yet providing no real quantitative predictions or solid
empirical validation. It’s not hard to see, however,
why not all physicists accept the viability of the superstring
approach. The problem isn’t so much that a universe
of ten-dimensional vibrating strings is an absurd idea.
Physicists are pragmatic as well as aesthetic, and will
accept any hypothesis that seems to predict observed empirical
data. The problem is that superstring theory doesn’t
make any useful empirical predictions, and on the other
hand, it postulates a huge number of particles that have
never been seen. To go back to the Plato metaphor, it doesn’t
tell us anything about how the shadows we see on the cave
wall move. It predicts a whole lot of shadows we’ve
never seen, though of course it’s possible we’ll
see these shadows under some strange conditions that we
haven’t encountered yet. What is does is to unify,
conceptually and mostly mathematically, the most powerful
and useful existing theories for explaining how shadows
move.
Some of the alternatives, like loop theory, are worked out
in almost as much detail as string theory, and by scientists
with equally impressive mainstream-physics pedigrees. And
some of them are a bit more speculative and fringe-y. In
the latter camp we find one of the more fascinating directions
–the envisioning of the universe as a computer.
Unlike the string theorists, who have followed their mathematics
wherever it leads and bent their intuitions to follow their
math, the universe-as-computer folks are beginning with
an intuition, and seeking to build appropriate mathematics
around their intuition. Their intuition is a simple one:
perhaps the universe is a giant computer. Computer programs,
we have seen, can lead to fabulously complex behaviors.
In fact there is mathematics suggesting that any system
whatsoever can be represented as a computer program. So
why not the universe itself?
One of the more vocal advocates of this point of view is
Ed Fredkin, an eccentric multimillionaire who owns his own
island and has been responsible for several innovations
in computer science. Fredkin has been working for years
on a model of the physical universe as a special kind of
computer program called a “cellular automaton.”
This is a reasonable choice, as cellular automata have been
used to model all sorts of physical phenomena, from weather
systems to immune systems to water flow in the ocean and
neural networks in the brain. A cellular automaton consists
of a collection of cells, each with a very simple program
in it, which changes the cell’s state based on the
state of other nearby cells.
Fredkin has gone a long way toward resolving various conceptual
problems associated with the “universe as computer”
idea. On the face of it, it’s not clear how quantum
randomness and indeterminacy could come out of a computer
program, or how the time-reversal that we see on the microscopic
scale (particles can go back in time) could either. But
Fredkin invented a whole new research area of reversible
computing – it turns out that with enough memory available,
computer programs can run both backwards and forwards. And
he solved the quantum indeterminacy problem by introducing
the seemingly oxymoronic notion of "unknowable determinism."
Basically, he notes, if the universe is a computer that’s
running at full blast and using all its resources to compute
its own future as fast as possible, then there is no shortcut
to predicting the future of this machine – there’s
no way to build a faster computer than the universe itself.
So if the universe is a computer, there’s no way to
predict the future course of evolution of this computer,
you just have to wait and see – in effect, from the
point of view of particular observers like ourselves, the
universal computer may be indeterminate.
But Fredkin, although he’s made a lot of progress,
has not yet released his complete equations for the universe
as a computer. One suspects there are difficult problems
to crack here. Philosophically this approach is more appealing
than string theory, but the mathematics doesn’t seem
to roll out nearly as elegantly. And there are other competitors
in this particular space who seem to be running up against
similar walls. For instance, Stephen Wolfram, noted scientist
and entrepreneur and founder of Mathematica Inc., has been
working for years on a book describing a new theory of physics
based on cellular automata. This has to be taken somewhat
seriously since Wolfram himself set the direction for much
of modern cellular automata research. But his book has been
in preparation for years, and no one has seen exactly what
he’s up to; how relevant his work will be to the dilemmas
of quantum gravity remains to be seen.
Another advocate of this sort of work – and another
possessor of a fascinating, diverse history and a super-brilliant
brain -- is Stephan Wolfram. Although best-known these days
at the CEO and founder of Mathematica, the leading maker
of software that does advanced mathematics, he started out,
like so many other technology pioneers, as a theoretical
physicist. In May 2002 he launched his book A New Kind of
Science, which does not give a detailed theory of unified
physics or any other aspect of physical science, but lays
out a fascinating new approach to dealing with scientific
issues, with notions of self-organizing computation at the
center. Unlike Fredkin, who is big on concept and vision
but skimpy on details (though in other domains he has proved
himself quite capable of supplying formal details as needed),
Wolfram’s book unleashed on the world a barrage of
details, together with a vision that, while not quite as
powerful as the more delightful of his detailed examples,
is nothing to be scoffed at.
Wolfram published his first scientific paper at the age
of 15, and got his CalTech physics Ph.D. at age 20. He began
his career studying quantum field theory, cosmology, and
other aspects of advanced physics, but by the late 70’s,
as he approached the old age of 30, his attention had shifted
to computing, and in 1981 the first commercial version of
what was to become Mathematica was released. Mathematica
now has many imitators, but when it first came out it was
revolutionary in its impact. It occupies a space somewhere
inbetween calculator-style math and AI. It doesn’t
do mathematical thinking, but it carries out complex algorithmic
operations in areas like algebra, calculus and geometry
that, much like chess playing, would be thought by a non-expert
to be impossible without deep thought. Wolfram did not invent
the type of algorithmics on which Mathematica is based,
but he was the first one to tie it together in a handy and
usable package.
Around the same time that Mathematica came out, Wolfram’s
mind also set him stirring in a different direction. Never
at a loss for ambition, he resolved to develop a general
theory of complex structures and dynamics in the natural
and computational worlds. He latched onto an obscure branch
of mathematic called cellular automata, and brought it to
the fore in the scientific world. Cellular automata were
first invented by the great mathematicians John von Neumann
and Stanislaw Ulam, as a way of capturing in a very simple
computational model the basic self-organizing and self-reproducing
processes of living systems. Von Neumann and Ulam used cellular
automata to create the first-ever example of a computational
system that could reproduce itself. They didn’t actually
write such a program, but they showed mathematically how
one could be created. Since that time, others have made
their vision more concrete, a pursuit related to the active
research field of artificial life (for instance, Tom Ray’s
Tierra system is a current computational framework that
includes self-reproducing computer code). Wolfram, throughout
the early 80’s, released one after another a series
of exciting new results showing that very simple cellular
automata systems could produce extremely complex behaviors.
This work has a major influence on the newly developing
“science of complex systems,” along with other
concurrent developments such as chaos theory and neural
networks. Wolfram developed several practical applications
of his ideas, including a new way of generating random numbers
for computer programs, and a new approach to computational
fluid dynamics, both of which proved highly successful.
His new book follows up the work on cellular automata, dealing
with a wider class of simple computational systems, and
showing how one after another phenomenon that “looks
like” physics, or life, or intelligence, can emerge
from these systems. These are fascinating and very suggestive
results. They don’t quite amount to anything yet in
terms of science, but Wolfram suggests that, if pursued
by teams of researchers over a period of years or decades,
they will. He is not suggesting that he’s found the
next Newton’s Laws of Motion, but perhaps that he’s
found the next equivalent of the calculus, which was the
mathematical foundation of Newton’s Laws. But this
calculus is not a precise set of mathematical definitions
and equations, it’s a broad class of interesting computer
systems that gives behaviors roughly analogous to interesting
natural phenomena.
Finally,
going even further out of the mainstream, Tony Smith, a
maverick physicist in Georgia (who pays his rent as a criminal
lawyer), has published a more complete computational physics
theory than anyone else. His theory (the “D4-D5-E6-E7-E8
model”) is large and complex, presenting the universe
as an 8-dimensional machine operating according to specific
mathematical equations that he has articulated. Mainstream
physicists like the superstringers have nothing but contempt
for this kind of work. On the other hand, the amount of
work that has gone into a theory like Smith’s is virtually
nothing compared to the amount of work that has gone into
superstrings, and from a bird’s-eye distance the results
are not all that much less impressive. Smith’s theory
runs into some subtle conceptual tangles in cases where
superstring theory is clear and simple (for example, pion
decay). On the other hand, Smith’s theory makes more
concrete empirical predictions than superstring theory,
a fact that has led Smith on an interesting excursion into
the sociology of experimental science.
Specifically, the D4-D5-E6-E7-E8 model predicts that a particular
kind of quark called the Top Quark should have a mass of
about 130 GeV. On the other hand, the standard model predicts
a mass around 170 GeV instead. Fermilab and other institutions
have done experiments leading to numerical predictions of
this mass, which should allow empirical selection between
different theories like these. But when you get right down
to it, what you find is that the interpretation of the empirical
data is a very subtle matter. The estimation of quark mass
is based on averaging values obtained over different “events”
of observing various kinds of particles. But it’s
not always clear when a particle has been observed –
often experimental noise looks just the same as a particle
observation. Whether quark mass comes out around 130 GeV
or around 170 GeV turns out to depend on whether a few borderline
cases are classified as particle observations or not. There’s
a surprising amount of subjectivity here, right at the forefront
of experimental physics, the hardest hard science on earth.
Whether Smith is right is not the point here – in
all probability his theory has some interesting insights,
but doesn’t capture the whole story. Perhaps it’s
total bunk (as my friend Matt Strassler, a physicist at
U. Penn and a brilliant but relatively conservative soul,
strenuously tells me); perhaps it’s 90% of the way
to the holy grail. Perhaps this theory, and Fredkin’s
theory, and Wolfram’s theory, and superstrings, all
capture different aspects of the underlying truth. The point
I want to make is that, even though this is hard empirical
science, things are pretty much wide open at this point.
The leading theory, superstrings, has virtually no empirical
support, and is valued primarily because of the elegance
of its mathematics. Indeed it was created almost entirely
based on advanced mathematics rather than conventional physics.
There are dozens of radically different theories, including
conceptually fascinating ones like “the universe as
a computer,” which exist at various stages of development.
We don’t have the experimental tools to test these
theories yet, and the tools that we have give results that
are difficult to interpret, leading to potentially significant
theoretical bias in the production of apparently hard experimental
data. For advanced theoretical physics, this is a time of
exciting exploration, not a time of solidity and stable
continuous progress.
Today
quantum gravity is an obscure research field, mired in conceptual
confusion and terrifyingly complex mathematics. One day,
however, it will likely be at the center of practical technology.
Quantum computing will give way to quantum gravity computing,
with properties we can’t yet imagine. And yet more
dramatic things such as time travel may be even be possible.
Indeed the question of whether time travel is possible is
deeply wrapped up in the quantum theory / general relativity
confusion. The great mathematician Kurt Godel, best known
for his contributions to mathematical logic, proved that
general relativity, considered on its own, does allow time
travel. If the universe as a whole is twisted in a certain
way, it’s possible to fly a spaceship in a certain
direction, and wind up back in one’s original location
earlier than one left. Now, there’s no evidence that
the universe is in fact twisted in such a way. However,
in the far future, there is the possibility of twisting
the universe to order – since, in general relativity,
the curvature of the cosmos is determined by the distribution
of matter within it. Maybe by moving matter around one could
somehow cause the universe to achieve the kind of shape
described by Godel, and thus make time travel possible.
The notion of time travel gives rise to all sorts of bizarre
conceptual paradoxes, which have been explored extensively
by science fiction authors. What if you go back in time
and kill your parents, do you then cease to exist? Furthermore,
if there are going to be time travelers in the future, then
where are they now? Why don’t we see any, say, popping
up sporadically in mid-air while we’re on the way
to the grocery store? Of course, these objections are not
definitive. Perhaps time travel will only be achieved in
the far future when intelligence has achieved a nonhuman
form, and perhaps these nonhuman time travelers simply have
no interest in coming back here to pester us primitive biological
life-forms. Perhaps the paradoxes of time travel lead to
a completely different order of experienced reality, which
we can barely fathom, living in the limited spacetime regime
we do.
Stephan Hawking, renowned physicist and author of the bestseller
A Brief History of Time, conjectured that quantum theory
might rule out traveling back in time. Some calculations
by William Hiscock and Deborah A. Konkowski seemed to support
this notion. But further work has led to the rejection of
this conclusion. Li and Gott, in a paper in the late 1990’s
showed that in fact there is no inconsistency between quantum
theory and the time-travel-friendly configurations of the
general-relativistic universe.
Li and Gott believe that the notion of time travel may be
essential to the origin of the cosmos itself. "The
universe wasn't made out of nothing," Gott says. "It
arose out of something, and that something was itself. To
do that, the trick you need is time travel." In other
words: How did the universe get here? Why, it traveled back
in time and created itself, of course. "The laws of
physics may allow the universe to be its own mother."
Very speculative, at this point, to be sure. But, still,
a preliminary indication of the wonders that may be ours
once the mysteries of quantum gravity are resolved. What
quantum gravity based technology will be like is pure speculation
at the moment, but, based on what we know today, it’s
far from impossible that time travel will be a part of it.
So, let’s suppose the string theorists, or the universal
computists, or some other group finally achieves their end
goal -- unifying quantum theory and gravitation, creating
one single equation, accounting in principle for all phenomena
in the universe. What happens then? Do the angels descend
from heaven, dancing and singing in the streets and on the
rooftops, serving out the wine in holy grails laminated
with spinning black holes, bringing peace on earth at long
long last? More seriously: does science immediately enter
a whole new era, where every phenomenon observed is analyzed
in terms of the one true equation?
Well,
parts of physics will surely be revolutionized. There will
be new technologies, maybe those we envision now like quantum
gravity computers or time travel machines, maybe other types
of things we can hardly imagine now. Perhaps, as some maverick
theorists believe, new light will be shed on the mysteries
of biological processes like cell development.
But no serious scientist really believes that such a “Theory
of Everything” (TOE) in principle will really be a
theory of everything in practice. There are a number of
technical and conceptual points that will stop this from
happening.
As superstring theorist John Schwarz says, “the TOE
phrase is very misleading on several counts….. [And]
it alienated many of our physics colleagues, some of whom
had serious doubts about the subject anyway. Quite understandably,
it gave them the impression that people who work in this
field are a very arrogant bunch. Actually, we are all very
charming and delightful.”
For one thing, if string theory or universal computation
or some other approach succeeds in making quantum theory
and gravitation play nice together, this still doesn’t
explain why our universe is the way it is, because it doesn’t
explain what physicists call the “initial conditions”
of the universe: it only explains how things evolved from
their starting-point. This may seem a small technical point
but it’s a big one in practice: there may be many,
many very different universes consistent with equations
as general as those of string theory.
Next, we may find that one thing the “universal equation”
binding quantum theory and gravitation teaches us is that
some very important things can’t be explained or understood.
Just as quantum theory has taught us that particles don’t
have definite states, the next wave of physics may open
up new kinds of indeterminacy and unknowability. We may
wind up learning, in ever more exquisite detail, why our
own finite, macroscopic minds are not up to the task of
understanding the real world underlying the shadow-world
they’ve evolved to see.
And this latter point ties in with the most serious problem
with these approaches to physics: the problem of intractable
calculations. Right now, the equations of string theory
are so hard to solve that we can only really understand
them in very special cases. There is an assumption that
the mathematics of the next century will allow us to make
more progress in this regard. But to what extent will this
really be true? Right now, we can’t do explicit quantum
theory calculations to understand proteins or neurons –
only very simple molecules. And we can’t do string
theory calculations to understand electrons and protons.
To bring string theory to the next level, we’ll have
to be able to use it to understand electrons and protons,
but that may be as far as it goes – the elucidation
of the implications of these micromicromicro-level equations
for macroscopic phenomena may remain too difficult for mathematicians
or computer-simulators or even AI’s to resolve for
hundreds or thousands of years, or even forever. It’s
no coincidence that Wolfram moved from physics to Mathematica
– but Mathematica isn’t enough. We need not
only calculational prowess, but superhuman intelligence
to guide the calculations. And even our AI superminds may
run into mathematical obstacles that we can’t yet
foresee or conceptualize.
In a way, it all comes down to Plato and his cave. From
studying these shadows on the wall, we can’t really
figure out what the birds and trees and squirrels are. But
we can learn more and more about them. We can make deeper
and deeper theories, some of them explaining the interaction
of very small bits of shadow, some of them explaining particular
kinds of large shadow. It’s vain to think we can fully
understand reality, or even fully understand what “reality”
means. The very notion of “everything,” when
taken seriously, intrinsically goes beyond our understanding.
The hubristic search to understand everything is valuable
insofar as it give us the passion to understand more and
more, but in more reflective moments, we have to acknowledge,
as Schwarz did in the quote given above, that we can’t
really capture the whole big universe in any little box
-- not even a box composed of the most sophisticated and
fantastic equations.
My own suspicion is that we’re going to need AI’s
to make sense of it all. AI’s for two reasons: one,
to do the increasingly intractable math; and two, to recognize
the relevant patterns in the masses of experimental-physics
data. For us the subatomic, high-energy world is counterintuive
and mysterious, and this is largely because our brains and
intuitions are evolved to deal with a particular, different
aspect of the physical universe. What if an AI were given
sensors and actuators attuned to the subatomic world? Then,
to it, our everyday world would be the strange one. What
if this AI were also put in communication with us, or other
AI’s more accustomed to the macro domain? What kinds
of patterns would such a system recognize?
Our physical theories about things like gravity, force and
power, electromagnetism and fluid flow, all come out of
our everyday intuition. The mathematics is grounded in real-life
experience, and real-life experience is often useful in
navigating and structuring the mathematics. On the other
hand, our physical theories about high-energy physics and
unified quantum gravity are driven by very abstract mathematics
and invented, ungrounded conceptual structures. No wonder
they are so much more complex, so much less natural and
workable. For an AI with appropriately rich subatomic sensors
and actuators, the conception of simple, not quite exact
but tremendously powerful “Newton’s Laws of
super-high-energy quantum gravity” will be a lot more
plausible than for us. We may never “understand”
the physics theories that such systems come up with, but
we will be able to follow the mathematics at least as a
high level, and appreciate and utilize the empirical results.
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