Thursday, November 30, 2006

Baby and bathwater

Penetrating so many secrets, we cease to believe in the unknowable. But there it sits, nevertheless, calmly licking its chops. - Mencken

My multi-month musings on the failure of string theory might make it sound as if I just don't like string theory, period, but that's not my purpose. My real target is the warped sociology of high-energy/particle theory, because it is this, not string theory per se, which has brought physics to its present impasse.

String theory started in the late 1960s. The first string "revolution" occurred in 1984, when the absence of certain infinities in a small number of string theories was proved. Because these theories included supersymmetry, bore a striking resemblance to grand unified gauge theories, treated "forces" and "motion" on an equal footing, and required (for free) a force that looks like gravity, they immediately became the prime candidate for a "theory of everything."

There were good reasons in the 1980s to welcome string theory as a big step in the right direction. But by 1990 or so, fatal problems had arisen: the partially-defined nature of string theory and the riot of non-unique solutions that string theory apparently implies. (The complete "theory" is really a conjecture, not a theory - the dynamics is defined only through second order in a Taylor series - and the "solutions" are actually conjectures as well). The second string "revolution" of the 1990s solved none of these problems. Instead, it added conjecture upon conjecture ("M-theory") and eventually led to the dead-end of the anthropic principle as an escape hatch for undefined theories and their very large number of conjectured solutions. I've posted before about the anthropic principle and won't belabor the point.

A final, and fatal, objection to string theory is that it is not background-independent and therefore cannot be a true quantum theory of gravity (QG). That is, it assumes a spacetime in which the other fields move, rather than solving for that spacetime as a result of the gravitational dynamics. This feature of string theory violates one of the basic features of general relativity. At best, string theory is another semiclassical theory of gravity, one that assumes a classical spacetime background, then perturbs it with small quantum fluctuations. The fact that string theory gives certain results consistent with other semiclassical approaches to quantum gravity should therefore come as no surprise - it has to. Such results don't deserve the hype that surrounds them - they're special cases of the general result first proved by Beckenstein and Hawking.

And of course, there's no contact with experiment or observation. String theory says nothing definitive or new about the most important beyond-the-Standard-Model phenomena of the last 20 years: neutrino masses, dark matter, and dark energy. It also has nothing to say about impending high-precision asotrphysical tests of fundamental symmetries or cosmological inflation. By historical standards, this is a bizarre situation. String theory, the supposed "theory of everything," adds no explanatory power to already-existing unified field theories and, in fact, detracts from them.

It's time to back up and ask where string theory went wrong. Setting aside the question of observing any distinctively "stringy" physics, we should pause to consider the striking fact that the only real theoretical progress has been in good old quantum field theories (QFTs) related to strings through duality and higher dimensions (including branes). We should also go back to string theory's origins as a "rubber-band" picture of quarks bound inside hadrons by chromodynamic flux tubes. Unlike electrodynamics, the color force does not decrease with distance, but becomes constant. That looks like a string under tension, and hadronic strings form the basis of a good effective theory for strong interactions under restricted kinematic conditions.

String theorists should take the hint. Gauge theories can be formulated in terms of flux lines. Is string theory (or brane theory) an effective theory of extended blobs occurring in a more basic gauge-like theory, but one that is background-independent, i.e., does not assume a fixed spacetime background? Such irreducible flux blobs abound in nonstring approaches to quantum gravity, like loop QG and spin networks. They begin with elementary events, the atoms of spacetime, then add a causal structure and a topology. Space and time as we know them emerge from these more primitive elements. They're not assumed as they are in string theories. True QG theories fully respect the symmetries of general relativity and are background- independent by construction. They also keep the zero-wavelength divergences that typically plague QFTs from ever happening in the first place, because spacetime has an atomic structure.

This whole conclusion should not come as a surprise - string theory was invented by particle theorists, for whom gravity and general relativity are usually tacked on as an afterthought. Particle theorists look at the world through the prism of elementary particles and subnuclear gauge forces (electromagnetic, weak, strong). Strings apparently allow a common language of gauge and gravity forces (open and closed strings), but only to low order in the perturbation series. OTOH, QG theorists do know how to cope with gravity as a full theory in its own right, but tend to shove matter and non-gravitational forces into the stress-energy tensor as an afterthought. Maybe that should change.



More is needed for these approaches to constitute a theory of everything. General relativity is a theory of spacetime, while particle physics tells us about matter and energy. There's a third category, sometimes ignored in such discussions, that needs attention: information. We already have a theory of information and its evil twin, entropy, called thermodynamics. We already know general relativity is connected to thermodynamics, e.g., black hole and other horizon entropies. But the usual concept of information is too restricted. The information in a physical system requires initial and boundary conditions, which in a dynamical theory of spacetime, cannot be specified in the usual way. Perhaps horizons have some connection to this and also to the so-called "holographic principle" (everything inside a system is specified by its boundary).

There is also the information embedded in the phases and other properties of the non-gravitational dynamics. This information has to be protected from quantum decoherence at the Planck scale. There should be some effective symmetry that does so. Perhaps the symmetry is not exact and is broken at some tiny level that can be measured, either in the laboratory or through astrophysical measurements.

We also need to generalize standard quantum mechanical concepts in an appropriate way to cope with gravity. For example, the vacuum cannot be the ground state in the usual sense. It's not the state of lowest energy, since the spacetime is dynamical, and there are no energy states. Perhaps we can substitute with cosmologies of maximal symmetry. If such maximal symmetries are overwhelmingly probable ones, then that explains why we're in the universe we're in. Perhaps the universe starts in an ensemble of horizons, then evolves towards the universe of maximal symmetry. QG should also have a "ceiling state" as well, a state of minimal symmetry. These might also be defined in terms of curvature: smallest curvature, largest curvature.

Such issues are connected to the cosmological constant question and why it's nonzero but very small. The answer to that question will probably connect the quantum properties of spacetime (the smallest events) to the global properties of spacetime - that is, the whole universe (the largest event).



A short and not-too-technical review of nonstring approaches to quantum gravity can be found in Lee Smolin's article in the November 2006 Physics Today (requires subscription). In spite of the incompleteness of non-string theories of quantum gravity, they have led to real predictions that can be measured, without appeal to multiverses. These theories include semiclassical canonical QG (black hole thermodynamics, Hawking radiation, gravitational back-reaction in high-curvature situations) and the spin network and loop QG approaches (extensions of Poincaré invariance, small breakings of fundamental symmetries, a suppressed but nonzero cosmological constant). Here, for all its limitations, is real progress, unlike strings. Unfortunately, most of this progress is happening outside the US, because American fundamental theoretical physics is still in the deadly grip of string/M-theory.

The full story can be found in Smolin's friendly, readable, but sometimes-heavy-going The Trouble with Physics. Smolin cares about fundamental physics and about why it's floundering. His primary point is not to bash string theory, although he deftly dismantles the twenty-year-long hype surrounding it. He states clearly the critical contribution string theory has made to unifying subnuclear gauge forces with gravity (the open and closed strings), but also makes clear string theory's otherwise consistent failure to answer any fundamental questions about phyiscs.

Smolin uses the term "craftspeople" for the majority of physicists; the technical virtuosos of physics are the cream of the crop of these craftspeople. A much smaller, but critical, group is now missing in academia, and those are what Smolin calls the "seers." Einstein and Bohr were seers: they saw and outlined the peaks that the virtuosos then climb up and over, better and better. Filling in the gap between seers and craftspeople is a third group that Smolin misses, one that might be called "valleycrossers." They are the critical group that takes the visions of the seers and turns them into reliable machinery for "normal" science. They also serve as forerunners of scientific revolutions, because they identify connections that no one saw before. And they pursue interdisciplinary science that ignores academic boundaries, which nature knows nothing of. Science adds up to far more than its separate branches because of them. If the seers have visions of mountain peaks and virtuosos compete with one another's climbing skills, the valleycrossers are the first to make the actual journey from old peaks to new ones. They discover or make the paths that later virtuosos follow and hone to perfection. The present sociological structure of academic physics renders them an endangered species.

Smolin smartly uses these concepts in analyzing the pathology of institutionalized and academic science in the last part of his book. In it, he explains how and why the scientific situation degenerated into the desperate condition it's in now: the sociological forces driven by the structure and hierarchy of academia, the funding agencies, and the peer-review system. This system gives us, and can only give us, more technical virtuosos who can't grasp foundational questions or who wilfully ignore them. For physics to get out of this deadend, that situation has to change.

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