I remember when I went to school, I went to the library to borrow a book about specific and general relativity. Thereafter, I read a
book
about quantum physics, I remember it was at the time when just the
successor book
had been published, which I then also read. Subsequently, I thought it would make sense to read a book about string theory, but I didn’t read one. They were not that readily available at that time. At about the same time, I had also read, or better seen, an article by Lee Smolin in Scientific American that was, at least I thought so, about string theory. Therefore, reading this book, reading this book, I could now finally make up for the omission twenty years ago and read a book that treats, among others, string theory. Twelve years ago, I have read a history book and had beforehand the feeling that I might discover that didn’t understand it and that I wouldn’t care. But I really liked the book and have since more or less read noting but history books. Now with this book about string theory, I discovered that I kind of didn’t understand and didn’t care. I’m not sure how many more books about string theory I will read. The good thing then is that it turns out that Lee Smolin considers it unlikely that string theory accurately describes our universe. I might thus not have to read many more books about string theory.
In an introductory chapter, Lee Smolin states that between 1780s and the 1970s, there have been every year breakthroughs in theoretical physics that have been tested experimentally, or in experimental physics where the theory was then later on derived for. But since the 1970s, these breakthroughs have been lacking. Trying to figure out what is going wrong, he lists the five fundamental questions the current physical theories leave open. As such, what happens in a Black Hole with the singularity, is one of them. Moreover, Lee Smolin does not like the observer-based framework of quantum mechanics. It cannot be the world differs, depending on whether we look or not. Then, he looks at how in physics, progress is made by unifying theories. From unifying theories, new predictions will come out that can be tested. Some of the new predicts will be surprising and new to new approaches. His primary example is the unification of electricity and magnetism by James Clerk Maxwell. The electric and the magnetic forces were explained by forces and it was recognized that one can lead to the other, that effectively, they have to two faces of the same phenomenon. Moreover, when Maxwell determined the speed of the electromagnetic field, he arrived at the speed of light, thus hinting at the unification with light. However, Lee Smolin says, not all unifications are valid. Therefore, experimental confirmation is important. As at that time, materialism was prevailing it was thought that light as well has to be a wave in matter. It was thus postulated that light is a wave in the aether. When afterwards electrons and atoms were discovered, it was theorized that atoms are knots in magnetic field lines and different knots were different atoms. This led to the mathematical knot theory.
Once the aether had to be abolished, and the idea that fields, like everything else, was matter, the idea came up that matter might be made up from fields. Einstein unified motion with rest, and then later, when he was thinking about gravity, he unified gravity with acceleration. About the same time, Gunner Nordström, unified electromagnetism with gravity. He only added one additional dimension into the electromagnetism equations and gravity was covered. Lee Smolin also says that as we don’t see this extra dimension, it might be circular and, as we cannot see it, very small. Only one of these unifications could be correct, in Einstein’s theory, gravity would bend space, in Nordström’s not. Happily, this could be tested experimentally, and at a solar eclipse 1919, Einstein’s theory of general relativity could be proven.
Thereafter, when it was tried to unify general relativity with electromagnetism, and the way to go seemed to be again an additional dimension. Theodor Kaluza, like Nordström before, applied a fifth dimension to Einstein’s general theory of gravity, and found electromagnetism. However, the fifth dimension again has to be small circles too small to see, and whereas general relativity describes spacetime the whole time changing, the radius of these circles has to be frozen in time and space. But if the radius was allowed to change, the electrical charge would also change. Thus, for this unification to be a true unification, have to be treated the same and the fifth must also be allowed to change. To safe the theory, it was tried to hide this this limitation, but play around a little bit with the radius and either the circle collapse in a singularity, or become visible. The discovery of the strong and weak nuclear forces led ten to the further demise of this theory.
It was still tried to incorporate those two forces by adding more dimensions, but these dimensions again had to be very small. The more dimensions included, the higher the price paid to freeze their geometry. Again, only few solutions were stable, there were an infinite number of ways the higher dimensions could be curled up and, again, no new predictions followed out of these theories. Moreover, they ignored quantum theory. Einstein had hoped that if he would once have his unified field theory formulated, the quantum phenomena would be included therein. So, in the end, unification attempts invoking higher dimensions was abandoned.
Electromagnetism has been unified with the weak and the strong nuclear force by breaking the symmetry. As such, the force is transmitted by similar particles, just the electromagnetic over an unlimited distance, where the other two are only exerting influence over short distances. The particles were derived by breaking the symmetry: Just like with a pencil that stands on its tip, it’s symmetrical but unstable. When the symmetry was broken, the particles got the features and became stable. The standard model of elementary particles has been derived like this and has been confirmed many times in the last thirty years. In order to achieve the next unification, supersymmetry was proposed. Here, all the elementary particles have a symmetry with the force particle. The problem with supersymmetry is that supersymmetry has 125 parameters that have to be fine-tuned, and that the unknown particles again have to be hidden; they are thus suggested to be so heavily that they have not been observed yet. Lee Smolin says that the LHC might show supersymmetry. I know that the Higgs particle has been found by the LHC, but supersymmetry hasn’t.
And next the discussion is on quantum gravity. Here, approaches are normally not in a background-independent context, simply, because when they were first developed, no one knew how to apply quantum theory to general relativity. Field theories first had to be developed. Electromagnetism was soon unified with quantum theory in QED, in a background dependent way. But doing the same with gravity proofed much more elusive. In the 1970s, supergravity was calculated as an idea, but successes were limited again.
In the late 1960s, the idea came up to think about what would happen if particles were not the usually imagined points, but actually as vibrating rubber bands, or, with more dignity, strings. Firstly, applied only to the strong nuclear force, it was soon seen to be applicable to all particles, by using supersymmetry. Supersymmetry was thus discovered by route of string theory and string theory is only stable with supersymmetry, as otherwise there would be faster than light tachyons. But before 1984, string theory was ignored, until a paper was published that year that showed that the theory was finite and consistent. From then on, everybody wanted to work in string theories and conferences were established everywhere. In string theory, there are 10 dimensions. Once in time and three in space are observable, thus there have to be six hidden ones. However, it was realized that string theory had not only one solution, but many solutions. No new predictions about the world could be made. Some physicists started to doubt that string theory really explained the world. As string theory is background dependent, it was realized that string theory might not be a fundamental theory, so that all solutions are valid solutions depending on the background, and a deeper fundamental theory that is background independent would be needed. It was even appreciated that the background and therewith the constants and the universe might have evolved. In the 1990s, some people thus left the field disheartened, and the split between believers and skeptics deepened.
In the mid-1990s, it was realized that the five consistent string theories that had been established should be unified. In order to do this, dualities between two each were analyzed. In the end, the idea came up to unify them all by describing strings not in a 9-dimensional space as one dimensional, but as two-dimensional surfaces, like membranes, in a 10-dimensional space. As these surfaces can then spread in all the other hidden dimension, they are actually not two-dimensional, but more. Therefore, they are not membranes and are therefore called D-branes. Black holes can be interpreted as extremal brane systems in which the branes holding the maximum amount of electrical and magnetic charge are wrapped around an extra dimension. However, as with supersymmetry there remains the question if this similarity to black holes is purely coincidental or not.
But then, dark energy was discovered and string theory was in crisis. Even though many theories had been established that were all thought to govern a different region of a multiple universe, they all had Einstein’s cosmological constant, the energy density, at zero, or if not, then negative. Einstein himself has introduced the cosmological constant when it was recognized that the universe might be expanding; he wanted to save therewith the static universe. When Edwin Hubble could show that the universe was indeed expanding, Einstein quickly and embarrassed withdrew the constant. Soon, however, it was recognized that quantum theory said something about the cosmological constant as the vacuum energy of the universe, that, according to quantum theory because of the uncertainty principle, had to be huge. However, with such huge cosmological constant, no universe would ever have formed. Most theorists tended thus to ignore it, as the observed cosmological constant was zero. When in 1998 it had been established that the universe was expanding, it was clear that the cosmological constant has to be positive. First, the sentiment was that a positive cosmological constant could not be a solution for string theory. Then, a solution was discovered that could not only accommodate a positive cosmological constant, but that also solved the problem why the extra dimensions were stable and would not end in a singularity or become visible. The solution involved wrapping branes around the geometry in which the moduli are stable. Antibranes were then used wrapped around to get the cosmological constant small and positive. But the problem now was there were 10500 solutions. And in contrast to before, where it was taken that eventually, one unique and correct would be found, now, all the 10500 solutions were taken to represent genuine solutions. This led to split in the string theory society; whereas some were happy that string theory had been saved, other believed that it had been reduced ad absurdum.
Moreover, Lee Smolin says that some theorists are absolutely happy with this number of solutions. They say, eternal inflation gave rise to an infinite population of universes, with bubbles appearing where the expansion is slower. Our universe happens to be such a bubble where live happens to exists. All the solutions thus exist somewhere in the multiverse, predictions become difficult to obtain, as all the laws will happen somewhere and we just happen to live in a universe where live is possible, applying therewith an anthropic solution. Lee Smolin thinks it would be better to look for the features that are required by string theories; extra dimensions, supersymmetry, and the forces becoming unified. Finding them would not proof string theory, but if one of them would be confirmed to not exist, that would do the same with string theory. In an additional chapter, Lee Smolin analyses in how much string theory actually solves the five basic questions that he had listed in the beginning. Even though it neatly unifies particle and forces, it was set up for that purpose, it can only unify gravity and quantum theory in approximation, when certain conditions are met. As such, black holes are only understood under very special conditions, that do not occur in reality. The three remaining problems are neither solved by string theories. As such, Lee Smolin considers it better not to put all eggs in one basket and diversify research. He also points to experimental findings which actually seem to contradict our current physical understanding and where new theories made be derived from. As such, the cosmological constant defines the scale R over which it curves the universe. R is about 10 billion light years. The cosmic microwave radiation seems to show anomalies at this scale. Moreover, it turns out that at the acceleration c^2/R, the behavior of stars in galaxies changes. In the center of galaxies, the behavior of stars can be well calculated using Newtonian laws. Outside a certain radius, it can’t, and here dark matter seems to play a role. Now the border between these two regions is not defined by a specific radius. Rather, as a star reach the acceleration c^2/R, dark matter starts to have an effect. Moreover, as the two Pioneer spacecrafts have left the solar system, their trajectories are off from predicted values by about c^2/R. Thu, Lee Smolin says, even though these discrepancies are far from being understood, they might even turn out to be statistical flukes, they might nevertheless hint at things that we do not understand yet.
Moreover, there recently have been hints that at extreme energies at the velocity close to the speed of light, Einstein’s special relativity might be breaking down. By breaking down, Lee Smolin states that either the theory might turn out to be wrong, or we might not completely understand it yet and further research will deepen our understanding. As such, Lee Smolin was involved in work establishing a theory where not only the speed of light, but also the Planck length is constant. Therefore, not the speed of photons would be constant, but their energy, or depending on version, their momentum limited to a maximum. For low-energy photons, this would then mean effectively that their speeds are constant but this would only be a specialty below a certain energy threshold. In this theory inflation would be unnecessary, as shortly after the big bang, there was a lot of energy in the universe and light might thus have travelled faster in the early universe. The theory is called deformed or doubly special relativity, short DSR. Again, if DSR should be confirmed, all string theories that were build based on special relativity would face troubles. However, it would be possible again to construct string theories that are based on DSR.
Looking at physics beyond string theory, Lee Smolin looks at the developments of quantum gravity. Taking only discrete building blocks and causality as given, these theories have the idea that classical spacetime will emerge out of them. Spacetime is thus not a precondition, but will emerge. Moreover, many background-independent quantum theories of gravity have elementary particles in them as emergent states, so that string theories has nothing more to bring to the table than they do.
In the last part, Lee Smolin then comes to talk about his reason why he wrote the book. He says that string theorists believe too much in their theories without proofs, almost akin to religion. He says that whereas normally skepticism is normal in science, the field of string theory is dominated by sociologist ideas, where people want to adhere to the mainstream idea so they rather refer to a famous person in the field than to think independently for themselves. For example, the finiteness of string theory has always been assumed to be true from the 80s to the time the book was written, even if has actually never been proven.
Moreover, generally analyzing science, Lee Smolin thinks that the current way of doing physics is not useful anymore, in that big, aggressive research programs are supported, whereas smaller, more introspective ones are not. Looking at how normal science is done he says that there are the craftsmen and the seer. The craftsmen are good in calculating with the current theories, they contribute therewith to incremental progress. The seers on the other hand think deeply about a problem. They might not have an out for ten or twenty years but until then come up with revolutionary ideas. The seers always had had it difficult; Einstein himself did not get a job in academia but worked in a patent office. But nowadays, how the academic institution has developed, there is absolutely no room anymore for seers. There are some that support themselves, as Lee Smolin introduces them. He states, we should make more room for the seers, they might one day explain how the universe works. Presenting how science really works, he discloses that peer-review is not really done by peers, but by people older and more powerful. This has the implication that to get papers published, a first grant and then tenure, it is always easier and much less risky to do mainstream science. Seers always elicit mixed reviews, being either praised or criticized, so that they quite often fail in the peer-reviewed system. In a concluding section, Lee Smolin looks again at his five questions from the beginning and remarks that the developments in academia have organized it in such a way that revolutionaries are rare. He states that we should fights the symptoms of groupthink and open the doors to a wide range of independent thinkers. He thinks that projects others than string theory should be given priority so that new original research programs can gain some ground. All in all, this is a good critique in which Lee Smolin explains why he thinks it unlikely that string theory explains the universe. And for me, it has now been ten years since I regularly review (or summarize) the books I’ve read.
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Good and informative read - see review for details.
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