Physics Book Face Off: Hyperspace Vs. The Elegant Universe

I've always been interested in physics. It's the subject that tries to answer the ultimate question of how the universe, and everything in it, works at its most fundamental level. I took a few physics courses in college, but I started to shy away from the subject after taking modern physics and having it go way over my head. I had a hard time grasping the concepts at the time, but recently, like with mathematics, I've been thinking about getting back into studying it more.

To kick off that activity, I started with two popular physics books that may be a little outdated, but should still have plenty of relevant, intriguing material on what has happened in the field post-Einstein. Both books are by prominent string theorists. The Elegant Universe was written in 1999 by Brian Greene, a professor of physics and mathematics at Columbia University. Hyperspace was written five years earlier in 1994 by Michio Kaku, a professor in theoretical physics at The City University of New York.

The Elegant Universe front coverVS.Hyperspace front cover

The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory

From what I've read, there's a huge debate going on in theoretical physics right now about whether or not string theory is the future of how we will understand the universe, or if it's a dead end that will never produce meaningful predictions about how the universe works. I'm certainly not qualified to make any judgements about this debate, but I still believe that the investigations of string theory have merit because the exploration of ideas has value in and of itself. String theory has also made significant contributions to both mathematics and physics by developing new mathematical constructs and bringing various far-flung ideas between the two subjects together under one framework.

That, however, is not the point of this book. The point is to give the reader a basic understanding of what string theory is about and how it affects our idea of how space-time works. Brian Greene is an excellent writer, and he does a great job of conveying his ideas in a way that non-theoretical physicists can understand. He starts out with a detailed description of Einstein's theories of Special and General Relativity and how they change our concept of the flow of time and the structure of space. Then he leaves the expanse of space to describe the main features of the very small particles of quantum mechanics. He wraps up this introductory material by explaining how these two sides of the universe—the very large and the very small—are incompatible when viewed within the confines of relativity and quantum mechanics. The two fields even come in direct conflict when trying to calculate what happens inside black holes or during the Big Bang.

This conflict is what string theory attempts to resolve. The rest of the book describes what sting theory is, how it can combine relativity and quantum mechanics into one overarching Theory of Everything, and goes into a number of issues that the theory must address before it can be considered valid. Towards the end of the book, Greene gets caught up in generalities and doesn’t do as good of a job relating the physics he's describing to everyday reality. He talks about strings wrapping around curled up dimensions and branes covering tears in the fabric of space. It's very hard to visualize what he's talking about and what implications it has for the behavior of space-time, but maybe the vagueness betrays the fact that no one really understands what's going on here, yet.

His other explanations are quite good, and reading the book generated tons of questions in my mind about how the concepts he was describing could be extended. For example, when he was describing how it's known that gravity travels at the speed of light, he goes through a thought experiment about what would happen to the planets if the sun suddenly exploded. The gist of the explanation is that the planets would not immediately leave their orbits because it would take time for the change in gravity to reach each planet.

As I was reading, I wondered what would happen if instead of exploding, the sun disappeared entirely, just winking out of existence (never mind how that might physically happen). Would it still take time for the change in gravity to reach the planets? At first I struggled with this idea because it seemed to me that in the first case of the sun exploding, the change in gravity would move along with the remnants of the sun, so the fact that gravity would be limited to the speed of light wasn't surprising. The matter that was traveling outward from the blast would be limited by the speed of light, and the force of gravity would change based on what happened to the matter as it sped outward. What was surprising was that, according to Einstein, even if the sun just disappeared, the planets would still take time to notice the absence of the sun's gravity because the sun was warping the space around it and the planets were following the curve of space in their orbits. The speed at which space would flatten out in the absence of the sun would happen at the speed of light.

Another great part of the book was the discussion of how to visualize higher dimensions. The concept of extra dimensions beyond three is especially hard to grasp because we experience the world in three dimensions, and we have no reference for what a fourth dimension (let alone a tenth dimension) would be like. One way to think about this—the way the book describes—is to imagine how a being in a one dimensional world would see a two dimensional object, and then work your way up to higher dimensions. Another way, that the book doesn't go into, is to think about how we already see our three dimensional world as a two dimensional projection. Our eyes actually see in 2D, and we build 3D models of objects in our minds. Similarly, we can project 4D objects into 3D spaces with computer simulations to try to get a better idea of what they are. One common example is the tesseract, or four dimensional cube. Here is a video showing what it looks like to rotate and unwrap a tesseract:

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