Finished reading Brian Greene's "The Hidden Reality - Parallel Universes and the Deep Laws of the Cosmos" last weekend. This is in some ways the third book in the trilogy he has authored. The Elegant Universe spoke about Physics and Universe in general, discussed theory of relativity in a very accessible prose, talked about Calabi-Yaushapes/spaces and string theory in an engaging way. The next one called The Fabric of Cosmos spoke about how space forms the fabric from which the entire cosmos is formed. This third book is focused on the following versions of multiverses plausibly floating around.
Quilted Multiverse:
Inflationary Multiverse:
Brane Multiverse:
Cyclic Multiverse:
Landscape Multiverse:
Quantum Multiverse:
Holographic Multiverse:
Simulated Multiverse:
Ultimate Multiverse:
I am listing them here since each multiverse is worth a Google search to get lost into. :-)
Book starts with a discussion of how Einstein blinked when theory of relativity ended up showing a universe that is forever expanding or contracting. Feeling quite unsettled, he added the cosmological constant to his math to make it static. Subsequent investigation and actual observations through experiments indeed showed the universe to be continuously expanding and in fact accelerating its expansion. This then leads to the follow up questions on the limits of the universe and the possibility of a quilted multiverse. In subsequent chapters we explore more than half a dozen versions of the multiverses that have been proposed. While the ideas are highly speculative for now, author suggests that this is similar to the days Einstein came up with the theory of relativity that couldn't be tested via experiments immediately due to the limits of technology at that time. But as we know, subsequently ToR was indeed experimentally verified and has practical application today in things like GPS since satellites that provide the signal for our handheld units need to take into account the space-time curvature without which it won't work..! Thus Greene proposes that even the existence of multiverse could one day be experimentally investigated and confirmed even though we may not be able to visit the other universes in existence. Currently there seems to be ideas floating around based on how atomic particles/strings may behave differently if there are additional dimensions that support additional universes. Some of these sections are not as clearly written as the initial and end chapters. Still, he presents very interesting discussions on what can happen if universes bump into each other, can LHC (Preview)create a small black hole that will grow and swallow the whole universe (not likely :-), etc.
One of the concepts discussed related to string theory is very neat. In the 1980s physicists figured that there are five different types of string theories named (certainly not by marketing experts :-) Type I, Type IIA, Type IIB, Heterotic-O and Heterotic-E. To work out the details of each theory, perturbative approach was usually used, where you do take into account just the major factors in the first round to determine the (rough) result and then go back for a second pass to fine tune it further taking into consideration the second order factors that are lot less influential, followed by another pass taking into account third order factors and so forth to incrementally arrive at as precise a result as possible as the computations get more and more complicated. This is similar to the way we may do mental math to land in the right ballpark first and then narrow down the answer to a more precise value. While doing the math, string coupling constant that defines the probability of one string bumping off of another is used as a factor. Until the early 90s each one of this string theory is considered a silo and physicists worked out the details assigning small values for string coupling constants. But in 1995 there was a big revelation showing that if you keep increasing the value of string coupling constant (SCC) in one of these 5 theories, it slowly morphs into one of the other 4 models with the SCC value dialed much smaller..! Thus, for example, when Type I string theory model's SCC value is raised, it morphs into Hererotic-O string theory with SCC value that is really small..! While this idea itself is very elegant, he explains using this image that looks like a photograph of Einstein when your eyes are close to the image and morphs into Marilyn Monroe when you move back considerably..! Pretty cute analogy. :-)
One other satisfying proposal put forth is that the multiverse concept will answer the gnawing philosophical question "Why is there a universe instead of nothing?". I can paraphrase it briefly as follows. Since multiverse supports a large number of universes in existence, human beings end up existing in this tangible universe since this happens to be one variation that is capable of supporting our life form. This answer won't work easily if there is only one universe where we exist, which may force us to create a God to explain the creation of the universe. :-) If you find this topicintriguing, do read this paper by Peter Lynds.
Last couple of chapters touch upon the ideas of the whole world we perceive being a simulation run by someone where human beings are all entities within that simulation (similar to The Matrix movie plot line) and one day can we build computers powerful enough to do such simulations ourselves that will collide back with the simulation we are already on and so forth. Quite an interesting ride/read.
Something should be said about Greene's book titles themselves. Titles like "What They Don't Teach You at Harvard" are amusing gimmicks to catch attention and sell books. If that book had been named "Some Useful Principles for General Management" which is what it was, I doubt if it would have sold that much. On the other end of the spectrum, I find titles of all three of Greene's books almost poetic and at the same time representing the content exactly (truth in advertising). Admirable.
-sundar.
