Today we are blessed with two extraordinarily successful theories of physics. The first is Albert Einstein's general theory of relativity, which describes the large-scale behavior of matter in a curved spacetime. The second is quantum mechanics. This theory describes the properties and behavior of matter and radiation at their smallest scales.
But, while they are both highly successful, these two structures leave a lot of important questions unanswered. They are also based on two different interpretations of space and time, and are, therefore, fundamentally incompatible. We have two descriptions, but as far as we know, we've only ever had one universe. What we need is a quantum theory of gravity.
Approaches to formulating such a theory have primarily followed two paths. One leads to String Theory, which has for long been fashionable, and about which much has been written. But String Theory has become mired in problems. Combining clear discussions of both quantum theory and general relativity, this book offers one of the first efforts to explain the new quantum theory of space and time.
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Interesting but not Convincing
This has an included PDF which is not really necessary, but does have a few clarifying images.
Baggott's previous books have been OK but not great. I liked this book more than his earlier books. This is mostly because much of this book are direct quotes from Smolin and Rovelli (both fascinating physicists and great writers). I have read and greatly enjoyed Smolin's and Rovelli's books on modern physics. They are my two favorite physics authors, so I really appreciated their influences on this book.
This book presents a simplified history of Loop Quantum Gravity (LQG) highlighting the key steps in the progress of the theory and is frank regarding the current incompleteness and limitations of the theory. I was not convinced LQG will end up being a productive theory for a number of reasons.
Smolin points out that LQG has fewer "big ideas" than string theory and instead has only one "big idea" blended with numerous existing, tried and true, methods. I found this argument weak, in that, it seems quite possible the "existing methods" are masking numerous misunderstandings about the universe. I am particularly weary of the technique used to take a classical field theory and convert it to a quantum theory. Such a process may yield equations that work, and (of course) yield the classical results as a limiting case, but may completely hide the underlying quantum reality. To make real progress may require either a single "really big uncomfortable idea" or several "big ideas".
One of the reasons for LQG was Penrose's feeling that space should be quantized. I feel this is a fundamental mistake. Space does not need to be quantized, momentum does not need to be quantized, but 'action' does need to be quantized. Position and Momentum are complementary which LQG seems to ignore. The way LQG is built up allows it to be used to do some calculations (which String Theory can not), yet that is not a good reason to trust it.
Focusing on just quantum gravity (as opposed to a wider more revolutionary theory) seems to me to be a weak strategy.
Baggott believes Tegmark's Mathematical Universe Hypothesis—“reality is timeless mathematics” exalts mathematics and puts mathematics up on a pedestal. It seems to me Baggott does not fully understand Tegmark's idea, not that mathematics is exalted, but that our sensory perceptions of reality should not be exalted, and if we deliberately set aside our perceptions, what is left is a timeless equation. Smolin claims any equation of everything would necessarily have to be established 'outside' of the universe then we're right back with the 'God's eye view' that Einstein sought to banish. This does not seem necessary to me, as in Tegmark's view there is no inside and no stuff - the equation IS the universe. The inside, outside, and stuff are just products of our senses and should not be believed, regardless of how difficult it is not to believe them.
There is a chapter that attempts to describe Smolin's belief in the reality (and fundamentalness) of time. It seems both Baggott and Rovelli disagree with Smolin on this (and so do I). Although I read Smolin's "Time Reborn" and other such explanations, I still do not understand how Smolin can hold this viewpoint. Most of that chapter is simply waving of hands, but there is one line on point..."the relativity of time is replaced by the relativity of physical shape". That could be conceivably true, and relativity of shape is surely part of relativity, but given existing experiments I don't see how the relativity of time can be denied.
There is also a chapter exploring how Smolin and Rovelli differ on their interpretations of QM. Rovelli is comfortable with QM as it is and does not see any incompleteness, while Smolin seems to side with Einstein that something is missing.
One nit, when listing things that are quantized, is to include 'energy'. Energy is transferred in packets and some subsystems have quantized energies, yet energy itself is continuous. 'Action' (energy x time) is quantized. Almost every popularizations of modern physics make a similar mistake.
The author presents the Zeno paradox (Achilles races the Tortoise) and explains that quantized space resolves the paradox. This would make sense if there was some evidence that space actually is quantized. I prefer using the pure relational view where only the interactions of Achilles and the Tortoise with the environment are considered. This also resolves the issue and is consistent with QM and is purely quantum.
The narration is very good.
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