If you know anything about String Theory then you probably are already familiar with physicist Brian Greene. Through his various media appearances, his previous books, and his great Nova special, Greene has made a name for himself in popularizing the physics of Strings - extremely tiny (and theoretical) bands of energy which are thought to be the essential building blocks of matter and energy in the universe. However, in his latest book, The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos (amazon), Greene takes a bit of a side-step from String Theory to talk about the various ways in which we might be part of a multiverse.
“Might” is the key word here. As Greene notes early in the book, “The subject of parallel universes is highly speculative. No experiment or observation has established that any version of the idea is realized in nature. So my point in writing this book is not to convince you that we’re part of a multiverse. I’m not convinced – and, speaking generally, no one should be convinced – of anything not supported by hard data.” But, of course, if that were the whole story then we wouldn’t really need such a book. But Greene continues on to note that, “I find it both curious and compelling that numerous developments in physics, if followed sufficiently far, bump into some variation on the parallel-universe theme. It’s not that physicists are standing ready, multiverse nets in their hands, seeking to snare any passing theory that might be slotted, however awkwardly, into a parallel-universe paradigm. Rather, all of the parallel-universe proposals that we will take seriously emerge unbidden from the mathematics of theories developed to explain conventional data and observation.” (Pg. 8-9)
To that end, Greene does an amazing job at showing his readers why we should take these theories seriously. Through his typical use of visual metaphors, the book is easily accessible to those wholly unfamiliar with the subject. In fact, The Hidden Reality has one of the best explanations for the Holographic Universe that I’ve ever read (to find out what this is, I suggest reading the book).
At the same time, Greene seems aware that he has to write about all of these ideas using many qualifying terms. In fact, he even dedicates a chapter to the potential problems with invoking a multiverse. For example, Greene notes that “By invoking a multiverse, science could weaken the impetus to clarify particular mysteries, even though some of those mysteries might be ripe for standard, nonmultiverse explanations. When all that was really called for was harder work and deeper thinking, we might instead fail to resist the lure of multiverse temptation and prematurely abandon conventional approaches. … But to turn away from a multiverse because it could lead us down a blind alley is equally dangerous. In doing so, we might well be turning a blind eye to reality” (Pg. 188). In other words, multiverse theories are certainly quick answers to various fine-tuning problems, but they may not be the right answer.
By the end of the book, we’ve left physics entirely and jumped into the domain of philosophy, where Greene admits he is wary to accept conclusions yet continues to write excitedly about the possibilities. Does Greene want us to take the idea of simulated universes seriously? His enthusiasm combined with his noted hesitations comes across as a mixed message. It seems that overall, as Greene puts it, “The most reasonable assessment of the parallel universe theories we’ve so far encountered is that the jury is out” (Pg. 189).