To me, some of what passes for the most advanced theory
in particle physics these days is not really science. When I found myself on a panel recently with
three distinguished theorists, I could not resist the opportunity to discuss what I see as major
problems in the philosophy behind theory, which seems to have gone off into a kind of metaphysical
wonderland. Simply put, much of what currently passes as the most advanced theory looks to be more
theological speculation, the development of models with no testable consequences, than it is
the development of practical knowledge, the development of models with testable and falsifiable
consequences (Karl Popper's definition of science). You don't need to be a practicing theorist
to discuss what physics means, what it has been doing, and what it should be doing.
When I began graduate school,
I tried both theory and experiment and found experiment to be more fun. I also concluded that first-rate
experimenters must understand theory, for if they do not they can only be technicians for the theorists.
Although that will probably get their proposals past funding agencies and program committees,
they won't be much help in advancing the understanding of how the universe works, which is the goal
of all of us.
I like to think that progress
in physics comes from changing "why" questions into "how" questions. Why is the sky blue? For thousands
of years, the answer was that it was an innate property of "sky" or that the gods made it so. Now we know
that the sky is blue because of the mechanism that preferentially scatters short-wavelength light.
In the 1950s we struggled
with an ever-increasing number of meson and baryon resonances—all apparently elementary
particles by the standards of the day. Then Murray Gell-Mann and George Zweig produced the quark
model, which swept away the plethora of particles and replaced them with a simple underlying structure.
That structure encompassed all that we had found, and it predicted things not yet seen. They were
seen, and the quark model became practical knowledge. Why there were so many states was replaced
with how they came to be.
A timelier example might
be inflation. It is only slightly older than string theory and, when created, was theological speculation,
as is often the case with new ideas until someone devises a test. Inflation was attractive because
if it were true it would, among other things, solve the problem of the smallness of the temperature
fluctuations of the cosmic microwave background radiation. Inflation was not testable at first,
but later a test was devised that predicted the size and position of the high angular harmonic peaks
in the cosmic microwave background radiation. When those were found, inflation moved from being
theological speculation to a kind of intermediate state in which all that is missing to make it practical
knowledge is a mathematically sound microscopic realization.
The general trend of the
path to understanding has been reductionist. We explain our world in terms of a generally decreasing
number of assumptions, equations, and constants, although sometimes things have gotten more
complicated before they became simpler. Aristotle would have recognized only what he called the
property of heaviness and we call gravity. As more was learned, new forces had to be absorbed—first
magnetic, then electric. Then we realized that the magnetic and electric forces were really the
electromagnetic force. The discovery of radioactivity and the nucleus required the addition
of the weak and strong interactions. Grand unified theories have pulled the number back down again.
Still, the general direction is always toward the reductionist—understanding complexity
in terms of an underlying simplicity.
The last big advance in
model building came a bit more than 30 years ago with the birth of the standard model. From the very
beginning it, like all its predecessors, was an approximation that was expected to be superseded
by a better one that would encompass new phenomena beyond the standard model's energy range of validity.
Experiment has found things that are not accounted for in it—neutrino masses and mixing and
dark matter, for example. However, the back-and-forth between experiment and theory that led
to the standard model ended around 1980. Although many new directions were hypothesized, none
turned out to have predicted consequences in the region accessible to experiments. That brings
us to where we are today, looking for something new and playing with what appear to me to be empty concepts
like naturalness, the anthropic principle, and the landscape.
Theory today
I have asked many theorists to define
naturalness and received many variations on a central theme that I would put as follows: A constant
that is smaller than it ought to be must be kept there by some sort of symmetry. If, for example, the
Higgs mass is quadratically divergent, invent supersymmetry to make it only logarithmically
divergent and to keep it small. The price of this invention is 124 new constants, which I always thought
was too high a price to pay. Progress in physics almost always is made by simplification. In this
case a conceptual nicety was accompanied by an explosion in arbitrary parameters. However, the
conceptual nicety, matching every fermion with a boson to cancel troublesome divergences in the
theory, was attractive to many. Experiment has forced the expected value of the mass of the lightest
supersymmetric particle ever higher. The Large Hadron Collider at CERN will start taking data
in 2008 and we will know in a couple of years if there is anything supersymmetric there. If nothing
is found, the "natural" theory of supersymmetry will be gone.
An even more interesting
example to an amateur theorist like me is the story of the cosmological constant. Standard theory
gives it a huge value, so large that the universe as we know it could not exist. It was assumed that
if the cosmological constant was not huge, it had to be zero. Unlike supersymmetry, there was no
specific symmetry that made it zero, but particle physicists expected one would be found eventually.
No one took seriously the possibility of a small cosmological constant until supernova observations
found that the Hubble expansion seemed to be speeding up. Naturalness seemed to prevent any serious
consideration of what turned out to be the correct direction.
At the time Sheldon Glashow,
John Iliopoulos, and Luciano Maiani developed the GIM mechanism, the naturalness concept was
not in the air.1 They realized that suppressing flavor-changing neutral currents
required restoring a certain kind of symmetry to the quark sector. They added the charmed quark
to create that symmetry, and the experiments of my group and Sam Ting's showed the charmed quark
was there.
The score card for naturalness
is one "no," the cosmological constant; one "yes," the charmed quark, though naturalness had nothing
to do with it at the time; and one "maybe," supersymmetry. Naturalness certainly doesn't seem to
be a natural and universal truth. It may be a reasonable starting point to solve a problem, but it
doesn't work all the time and one should not force excessive complications in its name. Some behaviors
are simply initial conditions.
For more than 1000 years,
the anthropic principle has been discussed, most often in philosophic arguments about the existence
of God. Moses Maimonides in the 12th century and Thomas Aquinas in the 13th used anthropic arguments
to trace things back to an uncaused first cause, and to them the only possible uncaused first cause
was God.
The cosmological anthropic
principle is of more recent vintage. A simplified version is that since we exist, the universe must
have evolved in a way that allows us to exist. It is true, for example, that the fine structure constant
α has to be close
to 1/137 for carbon atoms to exist, and carbon atoms are required for us to be here writing about cosmology.
However, these arguments have nothing to do with explaining what physical laws led to this particular
value of α. An interesting
relevant recent paper by Roni Harnik, Graham Kribs, and Gilad Perez demonstrates a universe with
our values of the electromagnetic and strong coupling constants, but with a zero weak coupling
constant.2 Their alternative universe has Big-Bang nucleosynthesis, carbon chemistry,
stars that shine for billions of years, and the potential for sentient observers that ours has.
Our universe is not the only one that can support life, and some constants are not anthropically
essential.
The anthropic principle
is an observation, not an explanation. To believe otherwise is to believe that our emergence at
a late date in the universe is what forced the constants to be set as they are at the beginning. If you
believe that, you are a creationist. We talk about the Big Bang, string theory, the number of dimensions
of spacetime, dark energy, and more. All the anthropic principle says about those ideas is that
as you make your theories you had better make sure that α
can come out to be 1/137; that constraint has to be obeyed to allow theory to agree with experiment.
I have a very hard time accepting the fact that some of our distinguished theorists do not understand
the difference between observation and explanation, but it seems to be so.
String theory was born
roughly 25 years ago, and the landscape concept is the latest twist in its evolution. Although string
theory needed 10 dimensions in order to work, the prospect of a unique solution to its equations,
one that allowed the unification of gravity and quantum mechanics, was enormously attractive.
Regrettably, it was not to be. Solutions expanded as it was realized that string theory had more
than one variant and expanded still further when it was also realized that as 3-dimensional space
can support membranes as well as lines, 10-dimensional space can support multidimensional objects
(branes) as well as strings. Today, there seems to be nearly an infinity of solutions, each with
different values of fundamental parameters, and no relations among them. The ensemble of all these
universes is known as the landscape.
No solution that looks
like our universe has been found. No correlations have been found such as, for example, if all solutions
in the landscape that had a weak coupling anywhere near ours also had a small cosmological constant.
What we have is a large number of very good people trying to make something more than philosophy out
of string theory. Some, perhaps most, of the attempts do not contribute even if they are formally
correct.
I still read theory papers
and I even understand some of them. One I found particularly relevant is by Stephen Hawking and Thomas
Hertog. Their recent paper "Populating the Landscape: A Top-down Approach" starts with what they
call a "no boundary" approach that ab initio allows all possible solutions.3 They
then want to impose boundary conditions at late times that allow our universe with our coupling
constants, number of noncompact dimensions, and so on. This approach can give solutions that allow
predictions at later times, they say. That sounds good, but it sounds to me a lot like the despised
fine-tuning. If I have to impose on the landscape our conditions of three large space dimensions,
a fine structure constant of 1/137, and so on, to make predictions about the future, there would
seem to be no difference between the landscape and effective field theory with a few initial conditions
imposed.
Although the Hawking and
Hertog paper sometimes is obscure to me, the authors seem to say that their approach is only useful
if the probability distribution of all possible alternatives in the landscape is strongly peaked
around our conditions. I'll buy that.
To the landscape gardeners
I say: Calculate the probabilities of alternative universes, and if ours does not come out with
a large probability while all others with content far from ours come out with negligible probability,
you have made no useful contribution to physics. It is not that the landscape model is necessarily
wrong, but rather that if a huge number of universes with different properties are possible and
equally probable, the landscape can make no real contribution other than a philosophic one. That
is metaphysics, not physics.
We will soon learn a lot.
Over the next decade, new facilities will come on line that will allow accelerator experiments
at much higher energies. New non-accelerator experiments will be done on the ground, under the
ground, and in space. One can hope for new clues that are less subtle than those we have so far that
do not fit the standard model. After all, the Hebrews after their escape from Egypt wandered in the
desert for 40 years before finding the promised land. It is only a bit more than 30 since the solidification
of the standard model.
Burton Richter
is former director of the Stanford Linear Accelerator Center and former Paul Pigott Professor
in the Physical Sciences at Stanford University.
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