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by
Lee Smolin
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August 31 - October 1, 2019
Their ideas were lost to history. I have worked on the problem of quantum gravity all my life and I learned of them only while finishing this book.
Entropy is a measure of disorder, and there is a famous law, called the second law of thermodynamics, holding that the entropy of a closed system can never decrease.
The result is precisely Einstein’s general theory of relativity. This is not how Einstein found his theory, but had Einstein not lived, it is how general relativity might have been found.
Soon they were carrying around huge art pads, the biggest they could find. They covered each page in tiny, precise handwriting. Each pad represented months of work. The word “monastic” comes to mind. I was terrified. I stayed a week and fled.
I was not only a loser who had abandoned the true faith, I was a loser in danger of becoming a rival.
Some people need to think through everything very carefully, and this takes time, as they get easily confused. It’s not hard to feel superior to such people, until you remember that Einstein was one of them.
string theory, as a fundamental theory, could be consistent with special relativity and quantum theory only if several conditions were satisfied. First, the world had to have twenty-five dimensions of space. Second, there had to be a tachyon—a particle that goes faster than light. Third, there had to be particles that could not be brought to rest. We refer to these as massless particles, because mass is the measure of a particle’s energy when it is motionless.
According to Kuhn, a scientific revolution is preceded by the piling up of experimental anomalies. As a result, people begin to question the established theory. A few invent alternative theories. The revolution culminates in experimental results that favor one of the new alternatives over the old established theory.4
No more reliance on experiment to check our theories. That was the stuff of Galileo. Mathematics now sufficed to explore the laws of nature. We had entered the period of postmodern physics.
I knew many physicists who were sure that supersymmetry and the extra dimensions were there, waiting to be discovered. I knew as many who jumped ship at that point, because it meant accepting too much that had no foundation in experiment.
Why, you may ask, do the string theorists insist that space is nine-dimensional? Simply because string theory doesn’t make sense in any other kind of space. . . .
an example of the familiar but rarely understood phenomenon of emergence, a term that describes the arising of new properties in large and complex systems.
A bunch of protons, neutrons, and electrons may combine to produce a metal; others, of equal number, may combine to produce a living cell.
This is so pretty that it’s hard not to believe in the existence of the eleven-dimensional unifying theory. The only problem left open was to discover it.
Could our three-dimensional universe be such a surface in a higher-dimensional world? This is a big idea, and it makes a possible connection to a field of research called brane worlds, in which our universe is seen as a surface floating in a higher-dimensional universe.
I was told not to worry, it was just that the theory was smarter than we were. We cannot, I was told, ask the theory questions directly and expect answers. Any direct attempt to solve the big problems was bound to fail. Instead, we should trust the theory and follow it, content to explore the parts it was willing to reveal using our imperfect methods of calculation.
M-theory remains a tantalizing conjecture. It’s tempting to believe it. At the same time, in the absence of a real formulation, it is not really a theory—it is a conjecture about a theory we would love to believe in.
IN THE TWO STRING revolutions, observation played almost no role.
Einstein was the most creative and successful theoretical physicist of the preceding two centuries, but even he could not imagine the universe as anything but eternal and immutable.
a more productive lesson is just how hard it is for even the most adventurous thinkers to give up beliefs that have been held for millennia.
If an attempt to construct a unique theory of nature leads instead to 10500 theories, that approach has been reduced to absurdity.
Today string theorists are ready to accept the existence of a landscape containing a vast number of theories, based on much less evidence than we needed twenty years ago to convince ourselves that a single theory existed.
The scenario of many unobserved universes plays the same logical role as the scenario of an intelligent designer. Each provides an untestable hypothesis that, if true, makes something improbable seem quite probable.
I know of no successful predictions that have been made by reasoning from a multiverse with a random distribution of laws.
In spite of the fact that the anthropic principle has not led to any real predictions and is not likely to, Susskind, Weinberg, and other leading theorists have embraced it as signaling a revolution not just in physics but in our conception of what a physical theory is.
What is remarkable to me is the number of distinguished scientists who seem unable to accept the possibility either that string theory or the hypothesis of a random multiverse is wrong.
It is not an exaggeration to say that hundreds of careers and hundreds of millions of dollars have been spent in the last thirty years in the search for signs of grand unification, supersymmetry, and higher dimensions. Despite these efforts, no evidence for any of these hypotheses has turned up.
sooner or later science is supposed to accumulate evidence that allows us to reach a consensus about the truth of a theory.
the hope that string theory may describe our world rests wholly on a belief in string theories whose existence is only conjectured.
This captures Einstein’s essential insight that the geometry of spacetime is dynamical and evolving.
The approximation method, perturbation theory, gives answers to any physical question by a sum of an infinite number of terms. For the first several terms, each one is smaller than the one before, so you get an approximation just by calculating a few terms. This is what is usually done in string theory and quantum field theory.
If such gifted physicists have tried and failed, and if every attempt remains incomplete, it may be because the conjecture they are trying to prove is false.
The reason that mathematics invented the idea of proof and made it the criterion for belief is that human intuition has so often proved faulty.
The known string theories, as noted, all disagree with observed facts about our world:
If there are no other ideas, well, let’s invent some. Since there is no near-term hope that string theory will make falsifiable predictions, there is no particular hurry.
there are hints of new experimental discoveries, not anticipated by string theory, that, if confirmed, will point physics in new directions.
Consider R divided by the speed of light: R/c. This gives us a time, and the time it gives us is roughly the present age of our universe. The inverse, c/R, gives us a frequency—a very low note, one oscillation per lifetime of the universe.
as noted, it is the rate at which the universe’s expansion accelerates. There is no obvious reason for this scale to play any role at all in the dynamics of an individual galaxy. The realization that it does was forced on us by the data. I recall my amazement when I first learned about it. I was shocked and energized. I walked around for an hour in a daze, muttering incoherent obscenities.
Almost every principle once proclaimed has been superseded.
The principles that are hardest to give up are those that appeal to our need for symmetry and elevate an observed symmetry to a necessity.
The new theory is called deformed or doubly special relativity—DSR for short. It came from asking a simple question, which seems to lead to a paradox.
The theorists and experimentalists whose work I described in the last two chapters have already inaugurated the post-string era in fundamental physics.
Traditionally, one sees what happens as primary and the relationships between what happens as secondary. Thus the events are real and the causal relations between the events are simply properties of the events. Penrose found that this way of looking at things can be reversed. You can take the elementary causal processes as fundamental and then define events in terms of coincidences between causal processes.
It could only be the invention of someone who does not just exploit mathematics but thinks strategically and creatively about the structure of mathematical knowledge and its future.
one way to tell whether a particular unification is successful is that there is immediately a sense that the idea agrees with nature.
Basically, string theory is the development of this visionary idea in a context of a fixed background of space and time. Loop quantum gravity is the same idea but developed in a completely background-independent theory.
That is, without gravity, you have to carry out a special procedure to isolate the infinite expressions and render them unobservable; with gravity, there simply are no infinite expressions.
Several calculations give evidence that the singularities inside black holes are removed. Thus time can continue beyond the point at which classical general relativity predicted it must end. Where does it go? It seems to go into newly created regions of space-time. The singularity is replaced by what we call a spacetime bounce. Just before the bounce, the matter inside the black hole was contracting. Just after the bounce, it is expanding, but into a new region that did not exist before.
this obviates the need to revise the scientific method by invoking the anthropic principle, as Leonard Susskind and others have advocated
What has led to the present crisis in physics is not this core idea but a particular kind of realization of it, worked out in a background-dependent context—a context that ties it to risky proposals such as supersymmetry and higher dimensions.
