Physics Book Face Off: The Hidden Reality Vs. Parallel Worlds

It has been an awfully long time since my last Physics Book Face Off, and that's mostly because it seems that I read pop physics books very slowly. I haven't even gotten into the real physics books that I eventually plan to read on relativity, string theory, and the like, but that's okay because I'm still enjoying these gentle forays into the technical details of the universe. For this pair of physics books, I dug into the idea of multiple universes with The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos by Brian Greene and Parallel Worlds: A Journey Through Creation, Higher Dimensions, and the Future of the Cosmos by Michio Kaku. Physicists have a bunch of different ideas about the possibility of other universes lying outside our own, and both of these books, by authors I've read before, set out to explore those wide-ranging ideas.

VS.

The Hidden Reality

Having thoroughly enjoyed The Elegant Universe, I had high hopes for this book, and Brian Greene did not disappoint. His clear and thoughtful explanations of complex topics in physics again made for an enjoyable and thought-provoking read. However, it is important to be careful and pay attention when he's discussing string theory, because he's prone to exaggerating the conclusions that can be drawn from the theory as it stands, lacking in essential evidence.

Let's not let that small failing get in the way of learning about the possibilities that Greene covers in this book, where he goes through nine different possibilities for multiple universes, otherwise known as the multiverse. He starts out simply by describing how we know that the universe we live in is far larger than what we can see, even through our most powerful telescopes. Because there is more universe that lies beyond what we can observe, we can consider that part of the universe separate from ours, giving us a patchwork quilt of universes, each containing its own galaxies, stars, and solar systems.

Assuming the universe is infinite leads to the conclusion that all possible configurations of solar systems, planets, and molecules would exist throughout the expanse of the universe, so there would be multiple copies of you and me inhabiting far off planets just like Earth that we would never be able to see. That's pretty incredible to think about, but things get weirder.

The inflationary universe is pretty well-accepted in the physics community today. The universe starts out with an inflationary period before the Big Bang where space is expanding at an incredible exponential rate. For some reason that scientists haven't worked out, yet, our region of space slowed down, the Big Bang occurred, and our universe developed. It follows that there are regions of space outside of our universe that are still in the inflationary state, and there are other universes separate from ours that have also fallen out of that state, giving us an inflationary multiverse.

At this point in the book, Greene introduces string theory along with some justifications and arguments in favor of it. Some of these arguments are surprisingly weak, such as:

As of today, then, the most promising positive experimental results would most likely not be able to definitively prove string theory right, while negative results would most likely not be able to prove string theory wrong.

This argument is essentially saying string theory is unprovable, at least with foreseeable and feasible technology. It's not a very convincing or satisfying argument, and it ends up putting on airs of complete speculation for the next three types of multiverses.

The first such multiverse is the brane multiverse, which speculates that our universe is contained on a brane—a 3-dimensional construct that exists within a higher dimension—and there are other branes containing other universes all around ours. The second string theory multiverse is a cyclical multiverse that occurs when brane universes collide, setting off another Big Bang and the creation of another universe. The last string theory multiverse combines the inflationary multiverse with brane universes, resulting in the idea that different universes within the multiverse landscape could have different string properties, and thus different numbers of dimensions or physical constants.

After the string theory multiverses, Greene delves into Quantum Mechanics and explores the Many Worlds hypothesis that follows from the statistical quantum nature of reality. The (very brief) argument goes as follows: since all particles have wave functions and their properties are not definitively known, all possible values of those wave functions are indeed possible and are a reality in some universe in existence. The combinatorial explosion involved in this multiverse is incredibly mind-boggling, but there it is.

Next, we come to one of my favorite multiverses, the holographic multiverse. Greene describes this multiverse especially well, starting with part of the introduction:

Arguably the strangest parallel world entrant, the holographic principle envisions that all we experience may be fully and equivalently described as the comings and goings that take place at a thin and remote locus. It says that if we could understand the laws that govern physics on that distant surface, and the way phenomena there link to experience here, we would grasp all there is to know about reality. A version of Plato’s shadow world—a parallel but thoroughly unfamiliar encapsulation of everyday phenomena—would be reality.

Then he goes into the reasoning of how black holes' size is determined by the amount of information that can be contained on the surface area of the event horizon and is directly related to the mass within that surface. The arguments of other scientists are used to generalize this result:

Susskind and ’t Hooft stressed that the lesson should be general: since the information required to describe physical phenomena within any given region of space can be fully encoded by data on a surface that surrounds the region, then there’s reason to think that the surface is where the fundamental physical processes actually happen.

And Greene concludes:

If this line of reasoning is correct, then there are physical processes taking place on some distant surface that, much like a puppeteer pulls strings, are fully linked to the processes taking place in my fingers, arms, and brain as I type these words at my desk. Our experiences here, and that distant reality there, would form the most interlocked of parallel worlds.

I found the discussion of how information is encoded on the event horizon of black holes especially fascinating, and he also had plenty to say about how time and possibly even causality can be distorted around black holes. Every time I read something new about black holes, I realize more how little I understand these exceptionally strange objects. I also get the sense that nobody really understands them, and there are many more questions than answers about just what happens in and around black holes.

The final two multiverses were nearly as interesting. One being the simulated multiverse, with the idea being that our universe is actually a simulation, and there are innumerable other simulated universes going on as well. The arguments here are especially hand-wavy, of course. This one tries to rationalize the simulation by saying that simulations and artificial life within our universe would necessarily prove that we are also, in fact, part of a simulation:

One future day, a cosmic census that takes account of all sentient beings might find that the number of flesh-and-blood humans pales in comparison with those made of chips and bytes, or their future equivalents. And, Bostrom reasons, if the ratio of simulated humans to real humans were colossal, then brute statistics suggests that we are not in a real universe. The odds would overwhelmingly favor the conclusion that you and I and everyone else are living within a simulation, perhaps one created by future historians with a fascination for what life was like back on twenty-first-century earth.

I'm not really sure why that would follow, but it still gets the mind turning just reading about these ideas. The last multiverse is fittingly called the Ultimate Multiverse, and it encompasses all of the multiverses already discussed as well as any other that can ever be conceived. I thought the justification for these imagined universes being reality just because they could be thought up was a bit weak, but reading about the ideas for the different universes was mind-expanding, as was this entire book. Greene is a great scientific writer, with his descriptions and explanations being quite clear and understandable. I've found his works to be pleasantly approachable and always good for taking my mind out for a stroll through wondrous physics concepts. The Hidden Reality is no different, and I highly recommend it.

Parallel Worlds

As the subtitle suggests, this book is about a little more than just multiple universes, adding in entire sections on the early history of the universe as well as predictions of how the universe will develop in the future. Michio Kaku's writing is just as understandable and approachable as Greene's, and he covers a lot of ground in his discussions of the potential of the muliverse.

He starts out with a discussion of what we now know about how the universe came to be. The Big Bang wasn't actually the beginning, and was preceded by at least a small fraction of a second of an inflationary period where empty space expanded at a tremendous rate before matter was created in the Big Bang. Kaku sometimes gets a little loose with his language, for example describing the Big Bang as a "fiery explosion." It wasn't an explosion of any kind remotely like what we're used to, since it was the creation of matter and anti-matter at temperatures and energies so high that not even protons and neutrons could form, only fundamental particles.

Some of the theories are out of date as well, as we see in Kaku's assumption that the Big Bang happened at a singularity. As described in The Hidden Reality, it is now believed that empty space was rapidly expanding everywhere in an inflationary state, and a localized area that would become our universe dropped out of that inflationary state, triggering the Big Bang and the creation of the matter, radiation, and energy that makes up the universe today. It wasn't a singularity, but a much larger area of unknown extent that expanded at a much slower rate after the Big Bang, and likely continues well beyond the visible extent of the universe.

Kaku incorrectly puts the inflationary period after the Big Bang, but we can excuse the inaccuracy since it was discoveries occurring after this book was published that corrected that perspective. The discussion of inflation leads to the first type of parallel universe we encounter in the book: the inaccessible universe just outside the visible universe. Because the universe is still expanding, meaning every point in space is moving away from us, and points farther away are moving faster, there is a horizon where space and everything in it is moving away from us faster than the speed of light. We will never be able to reach galaxies that far away, so it is as if they exist in a different universe, albeit one with the same physics as our universe.

Another potential separate universe exists within black holes, since they are created from matter having collapsed into a singularity from extreme gravity. All that matter at the singularity could be a new universe, or because the gravity is so immense at the singularity that it tears spacetime, there could be another universe at the other end of the black hole. This alternate universe is also inaccessible, and the reasons why are fascinating:

Moreover, you would be crushed to death if you ever fell into one. And since one could never pass through the magic sphere (since time has stopped), no one could ever enter this parallel universe.

The crushed-to-death reason is obvious, but the fact that time stops as well was new to me. Because time slows down as gravity increases, from an outside observer's point of view, someone falling into a black hole would never actually reach it. However, from the victim's point of view, time for them would continue as normal and the time of the rest of the universe around them would speed up considerably. They would literally see the universe flash before their eyes as they were crushed by the black hole. Ouch.

Things get more wild and crazy from there. Kaku goes on to explore the Many Worlds hypothesis with every possible quantum mechanical variation resulting in another possible universe, invisible membrane universes that exist right alongside our that result from M-theory (Greene called them Brane universes), and discussions of how to escape the universe and travel faster than the speed of light. Some of the discussions are quite fantastical, but it's always easy to tell when he's deviated from accepted theory into speculation even if he doesn't say so directly.

Parallel Worlds ends up being a fun romp through some known physics and some wild science fiction. The stories, explanations, and discussions are always entertaining, even if they sometimes deviate from being entirely accurate. It's good to expand your horizons and let your thoughts run free every once in a while, and this was a great book for doing just that.


Both of these books covered a lot of the same ground, with most of the same multiverses represented, but described from different perspectives. This is a good thing! Getting multiple points of view on any concept allows us to compare and contrast, learn the nuances more deeply, and get a better understanding from differing explanations. The Hidden Reality included a few extra multiverses, like the Simulated Multiverse and the Ultimate Multiverse. Parallel Worlds took diversions into other areas like how we could theoretically escape our own universe and visit other ones. They were both quite entertaining reads, and definitely worth a look.