In this book Carlo Rovelli, a theoretical physicist, explains Loop Quantum Gravity (LQG) - a concept that merges Einstein's theory of general relativity with quantum mechanics.
Carlo Rovelli
The theory of LQG was developed over the past few decades by theoretical physicists using sophisticated mathematics.
Theoretical physicists, however, were hardly the first people to speculate about the nature of the universe. Rovelli's discussion starts with ancient philosophers like Anaximander, Plato, Socrates, and Archimedes - men who had surprisingly insightful notions about the world.
Anaximander
Plato
Socrates
Archimedes
Humanity's understanding of the cosmos then advanced over the ages, with huge breakthroughs by Sir Isaac Newton and Albert Einstein, and important contributions by many others. Rovelli gives us a brief overview of the work of many great thinkers.
Sir Isaac Newton
Albert Einstein
[Note: We'd probably be much farther along if women had been allowed to contribute throughout history. LOL 😎]
*****
LQG - being largely mathematical - is difficult to explain in plain English, but Rovelli does a creditable job conveying the general idea. I'll try to provide a VERY abbreviated explanation in this review, starting with general relativity and quantum mechanics.
The Theory of General Relativity posits that gravity results from the curvature of spacetime. For example, a planet bends space around itself, and a moon circles around a planet because it follows the curve of space (like a marble that rolls around a funnel). This explains the 'force of gravity' that prevents the moon from flying off into the galaxy.
Quantum Mechanics contends that the energy of a field is distributed in 'quanta', or packets of energy, like electrons in an electrical field. However the exact location of the quanta is unknowable because they exist in a haze of probability. That is, at any instant a quantum might show up at point A, or point B, or point C, etc....but some locations are more likely than others.
LQG combines these two concepts by trying to quantize space itself. In other words, LQG treats space like it comes in quanta or small chunks. The current idea is that space is a fabric of loops (closed filaments) and nodes woven together into geometrical assemblages called spin networks.
Filaments and Nodes
Spin networks are combined with time to produce spin foams. Thus, the universe is comprised of spin foams. [Rovelli mentions that this model doesn't explain dark matter, which we still don't understand. Get to work physicists!!!]
The components of spin networks are unimaginably small, having what is called a 'Planck length' of 1.6×10−35 meters. The size of the Planck length can be visualized as follows: "If a particle or dot about 0.1 mm in size (which is about the smallest the unaided human eye can see) were magnified in size to be as large as the universe, then inside that universe-sized dot, the Planck length would be roughly the size of an actual 0.1 mm dot.
LQG is useful because it tells us more about black holes and adds to our understanding of the origin of the universe (which could have been a 'big bounce' rather than a 'big bang').
Moreover, LQG eliminates pesky singularities in math and science (points at which an object's behavior is very bizarre and hard to explain).
"In the center of a black hole is a gravitational singularity, a one-dimensional point which contains a huge mass in an infinitely small space, where density and gravity become infinite and space-time curves infinitely, and where the laws of physics as we know them cease to operate."
Currently LQG is competing with String Theory to explain the universe, but that's a whole other book. 😊
I'm interested in theoretical physics so I enjoyed Rovelli's narrative. I'll admit, though, that I'm still not clear about everything the author says, which probably requires knowledge of the underlying math. Still, I'd recommend the book - which explains LQG a billion times more thoroughly than this review - to folks interested in the subject.
Rating: 3.5 stars
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