This will be my last Physics post, concluding the theme I’ve been running with over the last week or so. This one is about all the Physics that physicists don’t like to talk about; namely, the major problems in Physics. The post also goes over the successes and failures of string theory. I’ve gotten the suggestion to spice things up with pictures, so that’s what I’ll try to do. Enjoy!
Lee Smolin’s The Trouble with Physics is a sobering examination of the current state of
Physics. Smolin writes with a special clarity that is rare and satisfying. Of all the books assigned during the course, Smolin’s is the most insightful, informative, and enjoyable book with respect to the status of modern Physics. More than anything, the book gives an honest account of the triumphs and failures of string theory to date, which is a topic with which I had little familiarity beforehand. The book is so important, in fact, that I would suggest placing it earlier in the reading order. Smolin’s book might fit better in juxtaposition with Lemonick’s Echo of the Big Bang.
One important aspect of Smolin’s book is his account of the deepest problems facing fundamental Physics. The first problem in fundamental Physics is the lack of one consistent theory to describe the universe in its entirety. There are two wildly successful theories in modern Physics: the Standard Model of Quantum Theory, and Einstein’s Theory of General Relativity. The Standard Model describes the strange universe at the unimaginably small scale of subatomic particles and their interactions by way of electromagnetic, strong, and weak forces, whereas General Relativity describes the beautiful universe at the incomprehensibly large scales of stars and galaxies by way of gravity. Each of these theories is a triumph of predictive precision, yet quantum theory and relativity are mutually exclusive. Physics is in a strange situation when its two most successful theories are incompatible. Smolin’s first problem is apt because the large objects in the universe are ultimately made up of the small ones, and so nature only allows for one fundamental theory.
The second problem of Physics that Smolin mentions is the incomprehensible nature of quantum theory. Smolin argues that a new formulation of quantum theory or a deeper understanding of the universe will make quantum behavior amenable to realist philosophy, which argues that the underlying physics of any situation can be understood without making a special distinction between the observer and the observed. In other words, there is a real world out there that not subject to an observer. Smolin may be showing his philosophical bias here, as it is possible that a realist interpretation of quantum events is impossible. Nature doesn’t have to bend itself to the preferences of the physicists who study it. In fact, it might be that quantum phenomena are so sensitive that any possible observation of a quantum system disturbs it so greatly that one can never distinguish the features of the system from the effects of observation.
The third problem is a plea for a final, simple fulfillment of Democritus’ atomism. Smolin hopes that an ultimate theory will explain all of the particles and forces as manifestations of one simple, fundamental entity.
Problem four serves as a plea for simplicity. There are about twenty constants in the Standard Model that seem to be arbitrary. It seems unlikely that a tidy universe would allow for so many seemingly arbitrary constants that are so important to the properties and interactions of particles. Again, Smolin might be showing philosophical bias (admittedly, I share his bias). Perhaps there is no rhyme or reason to the constants. This would not be very satisfying or likely given the sensitivity of the constants, but it may be the case.
Finally, the most frustrating and mystifying problem in Physics is that of the existence of Dark Matter and Dark Energy. Dark matter is the cold, dark stuff that permeates the galaxies, and Dark Energy is the incessant fuel that drives the accelerating expansion of the universe at large. Dark Energy is more precisely thought of as a cosmological constant—a constant that was supposed to be an artifact of Einstein’s General Relativity, but seems to be an observed fact of the cosmos. Dark Matter might be easily explained by the discovery of a new particle, but Dark Energy is much more troubling. There is currently no understanding of what Dark Energy is, and as it will become clear, Dark Energy throws a wrench in many attempts to combine quantum mechanics and relativity.
Smolin has been persuasive in his argument that string theory does not answer these major problems in fundamental Physics as it stands. Smolin provides a catalog of evidence to suggest that any successful theory of quantum gravity must capture Einstein’s masterful insight that the geometry of space and time is dynamic and evolves with time. Such theories are known as “background independent” theories. Theories that have fixed background geometry are known as “background dependent.” Smolin catalogs the sordid history of string theory, and through all the breakthroughs and pitfalls one idea is clear: there is no single, coherent string theory. String theories have the basic feature that a fundamental rubber band like entity called a string vibrates differently in multiple dimensions to make up all of the particles and forces. Smolin classifies four major kinds of string theories. The first category consists of well-understood string theories with no cosmological constant operating in a fixed geometry. These theories are not good theories of quantum gravity since they are background dependent and predict behavior of particles that contradict observation. The second class of string theories contains theories that describe a universe with a negative cosmological constant. If the Maldacena conjecture is true, then these theories are connected and equivalent to gauge theory, which implies an exact description for string theory. This seems incredibly promising, but Dark Energy rears its ugly head with the fact that the cosmological constant is observed to be positive, not negative. The third class of string theories comes from the Stanford Group, who found reason to conjecture that there are more than 10^500 string theories that are stable in higher dimensions, many of which are background independent with positive cosmological constants. The evidence for conjecture consists of the satisfaction of certain necessary but insufficient conditions. The final class of string theories contains only one theory in twenty-six dimensions with tachyons, an unstable particle that leads to inconsistencies. In light of these four broad categories of string theories, it is hard to say what string theory does and does not answer, since there is not a particular string theory to speak of. The structure of the string lends itself to the unification of fermions and bosons, but until there is a single concrete, consistent formulation of string theory, there is no way to tell if string theory solves the problems of fundamental Physics.
Although Smolin beautifully argues his point that string theory does not currently fulfill the promises that it made at its inception and does not solve the major problems he outlined, it seems that the story of string theory is not over. There is reason to believe that all the different string theories point to a deeper unified theory codenamed M-Theory. All of the particular string theories may all be just particular solutions to M-Theory that is a fundamental theory of the universe. Another explanation, albeit less scientific, is that there are many universes, each of which a manifestation of a different string theory of the 10^500 string theories proposed by the Stanford Group. In any case, Physics is in trouble. No one can explain Dark Energy; there are too many adjustable constants in the standard model; there is no known fundamental entity that makes up all the particles and forces; and gravity is incompatible with quantum mechanics. Do not believe anyone who says the universe is well understood.