Theory in particle physics: Theological speculation versus pract

To me, some of what passes for the mostadvanced theory in particle physics these days is not really science. When Ifound myself on a panel recently with three distinguished theorists, I couldnot resist the opportunity to discuss what I see as major problems in thephilosophy behind theory, which seems to have gone off into a kind ofmetaphysical wonderland. Simply put, much of what currently passes as the mostadvanced theory looks to be more theological speculation, the development ofmodels with no testable consequences, than it is the development of practicalknowledge, the development of models with testable and falsifiable consequences(Karl Popper's definition of science). You don't need to be a practicing theoristto discuss what physics means, what it has been doing, and what it should bedoing.

When I began graduate school, I tried both theory and experiment and foundexperiment to be more fun. I also concluded that first-rate experimenters mustunderstand theory, for if they do not they can only be technicians for thetheorists. Although that will probably get their proposals past fundingagencies and program committees, they won't be much help in advancing theunderstanding 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 ofyears, the answer was that it was an innate property of "sky" or thatthe gods made it so. Now we know that the sky is blue because of the mechanismthat preferentially scatters short-wavelength light.

In the 1950s we struggled with an ever-increasing number of meson and baryonresonances—all apparently elementary particles by the standards of the day.Then Murray Gell-Mann and George Zweig produced the quark model, which sweptaway the plethora of particles and replaced them with a simple underlyingstructure. That structure encompassed all that we had found, and it predictedthings not yet seen. They were seen, and the quark model became practicalknowledge. 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 stringtheory and, when created, was theological speculation, as is often the casewith new ideas until someone devises a test. Inflation was attractive becauseif it were true it would, among other things, solve the problem of thesmallness of the temperature fluctuations of the cosmic microwave backgroundradiation. Inflation was not testable at first, but later a test was devisedthat predicted the size and position of the high angular harmonic peaks in thecosmic microwave background radiation. When those were found, inflation movedfrom being theological speculation to a kind of intermediate state in which allthat is missing to make it practical knowledge is a mathematically soundmicroscopic realization.

The general trend of the path to understanding has been reductionist. Weexplain our world in terms of a generally decreasing number of assumptions,equations, and constants, although sometimes things have gotten morecomplicated before they became simpler. Aristotle would have recognized onlywhat he called the property of heaviness and we call gravity. As more waslearned, new forces had to be absorbed—first magnetic, then electric. Then werealized that the magnetic and electric forces were really the electromagneticforce. The discovery of radioactivity and the nucleus required the addition ofthe weak and strong interactions. Grand unified theories have pulled the numberback down again. Still, the general direction is always toward thereductionist—understanding complexity in terms of an underlying simplicity.

The last big advance in model building came a bit more than 30 years agowith the birth of the standard model. From the very beginning it, like all itspredecessors, was an approximation that was expected to be superseded by abetter one that would encompass new phenomena beyond the standard model'senergy range of validity. Experiment has found things that are not accountedfor in it—neutrino masses and mixing and dark matter, for example. However, theback-and-forth between experiment and theory that led to the standard modelended around 1980. Although many new directions were hypothesized, none turnedout to have predicted consequences in the region accessible to experiments.That brings us to where we are today, looking for something new and playingwith what appear to me to be empty concepts like naturalness, the anthropicprinciple, and the landscape.

Theory today

I have asked many theorists to define naturalness and received manyvariations on a central theme that I would put as follows: A constant that issmaller 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 tomake it only logarithmically divergent and to keep it small. The price of thisinvention is 124 new constants, which I always thought was too high a price topay. Progress in physics almost always is made by simplification. In this casea conceptual nicety was accompanied by an explosion in arbitrary parameters.However, the conceptual nicety, matching every fermion with a boson to canceltroublesome divergences in the theory, was attractive to many. Experiment hasforced the expected value of the mass of the lightest supersymmetric particleever higher. The Large Hadron Collider at CERN will start taking data in 2008and we will know in a couple of years if there is anything supersymmetricthere. If nothing is found, the "natural" theory of supersymmetrywill be gone.

An even more interesting example to an amateur theorist like me is the storyof the cosmological constant. Standard theory gives it a huge value, so largethat the universe as we know it could not exist. It was assumed that if thecosmological constant was not huge, it had to be zero. Unlike supersymmetry,there was no specific symmetry that made it zero, but particle physicistsexpected one would be found eventually. No one took seriously the possibilityof a small cosmological constant until supernova observations found that theHubble expansion seemed to be speeding up. Naturalness seemed to prevent anyserious consideration of what turned out to be the correct direction.

At the time Sheldon Glashow, John Iliopoulos, and Luciano Maiani developedthe GIM mechanism, the naturalness concept was not in the air.¹They realized that suppressing flavor-changing neutral currents requiredrestoring a certain kind of symmetry to the quark sector. They added thecharmed quark to create that symmetry, and the experiments of my group and SamTing's showed the charmed quark was there.

The score card for naturalness is one "no," the cosmologicalconstant; one "yes," the charmed quark, though naturalness hadnothing to do with it at the time; and one "maybe," supersymmetry.Naturalness certainly doesn't seem to be a natural and universal truth. It maybe a reasonable starting point to solve a problem, but it doesn't work all thetime and one should not force excessive complications in its name. Somebehaviors are simply initial conditions.

For more than 1000 years, the anthropic principle has been discussed, mostoften in philosophic arguments about the existence of God. Moses Maimonides inthe 12th century and Thomas Aquinas in the 13th used anthropic arguments totrace things back to an uncaused first cause, and to them the only possibleuncaused first cause was God.

The cosmological anthropic principle is of more recent vintage. A simplifiedversion is that since we exist, the universe must have evolved in a way thatallows us to exist. It is true, for example, that the fine structure constant αhas to be close to 1/137 for carbon atoms to exist, and carbon atoms arerequired for us to be here writing about cosmology. However, these argumentshave nothing to do with explaining what physical laws led to this particularvalue of α. An interesting relevant recent paper by Roni Harnik, Graham Kribs,and Gilad Perez demonstrates a universe with our values of the electromagneticand strong coupling constants, but with a zero weak coupling constant.²Their alternative universe has Big-Bang nucleosynthesis, carbon chemistry,stars that shine for billions of years, and the potential for sentientobservers that ours has. Our universe is not the only one that can supportlife, and some constants are not anthropically essential.

The anthropic principle is an observation, not an explanation. To believeotherwise is to believe that our emergence at a late date in the universe iswhat forced the constants to be set as they are at the beginning. If youbelieve 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 anthropicprinciple says about those ideas is that as you make your theories you hadbetter make sure that α can come out to be 1/137; that constraint has to beobeyed to allow theory to agree with experiment. I have a very hard timeaccepting the fact that some of our distinguished theorists do not understandthe difference between observation and explanation, but it seems to be so.

String theory was born roughly 25 years ago, and the landscape concept isthe latest twist in its evolution. Although string theory needed 10 dimensionsin order to work, the prospect of a unique solution to its equations, one thatallowed the unification of gravity and quantum mechanics, was enormouslyattractive. Regrettably, it was not to be. Solutions expanded as it wasrealized that string theory had more than one variant and expanded stillfurther when it was also realized that as 3-dimensional space can supportmembranes as well as lines, 10-dimensional space can support multidimensionalobjects (branes) as well as strings. Today, there seems to be nearly aninfinity of solutions, each with different values of fundamental parameters,and no relations among them. The ensemble of all these universes is known asthe landscape.

No solution that looks like our universe has been found. No correlationshave been found such as, for example, if all solutions in the landscape thathad 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 somethingmore than philosophy out of string theory. Some, perhaps most, of the attemptsdo not contribute even if they are formally correct.

I still read theory papers and I even understand some of them. One I foundparticularly relevant is by Stephen Hawking and Thomas Hertog. Their recentpaper "Populating the Landscape: A Top-down Approach" starts withwhat they call a "no boundary" approach that ab initio allows allpossible solutions.³They then want to impose boundary conditions at late times that allow ouruniverse with our coupling constants, number of noncompact dimensions, and soon. 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 spacedimensions, a fine structure constant of 1/137, and so on, to make predictionsabout the future, there would seem to be no difference between the landscapeand effective field theory with a few initial conditions imposed.

Although the Hawking and Hertog paper sometimes is obscure to me, theauthors seem to say that their approach is only useful if the probabilitydistribution of all possible alternatives in the landscape is strongly peakedaround our conditions. I'll buy that.

To the landscape gardeners I say: Calculate the probabilities of alternativeuniverses, and if ours does not come out with a large probability while allothers with content far from ours come out with negligible probability, youhave made no useful contribution to physics. It is not that the landscape modelis necessarily wrong, but rather that if a huge number of universes withdifferent properties are possible and equally probable, the landscape can makeno real contribution other than a philosophic one. That is metaphysics, notphysics.

We will soon learn a lot. Over the next decade, new facilities will come online that will allow accelerator experiments at much higher energies. Newnon-accelerator experiments will be done on the ground, under the ground, andin space. One can hope for new clues that are less subtle than those we have sofar that do not fit the standard model. After all, the Hebrews after theirescape from Egypt wandered in the desert for 40 years before finding thepromised land. It is only a bit more than 30 since the solidification of thestandard model.

Burton Richter is former director of the Stanford LinearAccelerator Center and former Paul Pigott Professor in the Physical Sciences atStanford University.

References

1. 1.S. Glashow, J. Iliopoulos, L. Maiani, Phys. Rev. D 2, 1285 (1970) [INSPEC].
2. 2.R. Harnik, G. D. Kribs, G. Perez, Phys. Rev. D 74, 035006 (2006) [SPIN].
3. 3.S. W. Hawking, T. Hertog, Phys. Rev. D 73, 123527 (2006) [SPIN].