The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos by Brian Greene

The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos Reviews

• 
  Alex

  So my buddy Ryan introduced me and Jo to his new girlfriend this past weekend and she's a mathematician (who is clearly not very good at it, because Ryan with a girlfriend doesn't add up - ZING!), so I was like "Do you think we're all avatars in a big futuristic game of The Sims?" and her face just lit up, like "I've been waiting for someone to ask me this all my life!" She is adorable, and we geeked out about parallel universes for like half an hour while Ryan and Jo made big exaggerated sighing noises. "Hey, just let us know when you want to talk about something that's not the nerdiest bullshit ever." "Okay, that will be never because we're best friends now." I said that. Ryan's girlfriend probably thinks I'm just okay.
  
  And that's why this is a great book: because it's wicked fun to talk about all this shit. Whee, multiverse! says my avatar. You ever hear that theory that once computers get to the same complexity as human brains, they'll sortof automatically develop consciousness? Scientists think that because otherwise there's something we totally don't get about consciousness, and who needs that? Lame. So here's the thinking: if that's true, then we'll probably have it by around 2020. And once we have artificial intelligence, what are we going to do with it? Put it in video games, obviously. We know this because we are people, and that's the kind of shit people do. So we're going to have these virtual worlds, just like The Sims except the people in it will actually be aware, and we'll all play these virtual world games on our Playstations. And that means there'll be like thousands of virtual worlds with conscious inventions in them interacting with each other. And that means that only one of all the worlds featuring conscious beings is the real one; the rest are video games. And that means that odds are the world you're reading this in isn't the real one. Just statistically, it's more likely that you're a collection of sprites that some pimply teenager created near-arbitrary rules for.
  
  That's nuts, right? Crazy nuts! Batty! Among other things, it means that there's a God after all, because whatever pimply teenager is playing this particular game that I'm in can delete this game whenever he wants, or make an asteroid hit Brazil, or make Kate Middleton show her boobs. For most intents and purposes, that is God.
  
  Also, it raises this question: is Pimply God doing a good job? Is this particular world a good one? Or is he an asshole? If all the sim worlds were ranked from most pleasant to least, where would this one fall? If you set a bunch of AIs up in a fairly nice place, will they probably fight? Is there like an Aggression Slider so you can make us more or less likely to fight? What would this world be like if Pimply God was deep in the throes of puberty? Or is this it?
  
  Ryan's girlfriend thinks we are most likely virtual - seriously,
  serious people think this whole thing makes perfect sense - but I think I lost her with the puberty bit.
  
  This is the craziest and last theory in Hidden Reality. There are eight others, and they're not mutually exclusive. The least crazy theory is that space is infinite, and infinite is a lot, so somewhere beyond what we can ever see, given the speed of light, there will be a world just like ours, and infinite worlds just like ours, because infinite means everything, including a world where everything happens and will happen exactly like it has and will in this world you're in now with the sole exception that, instead of reading this review, you personally decided to make a sandwich.
  
  Infinite is a lot.
  
  Everything in Hidden Reality is theoretical, so there's no pressing reason for you to read this book. (Or any of the rest of the books, while we're on the subject. Pimply God read Shakespeare and was like "Ha, that's cool - it rhymes and everything, sometimes! I mean, it's terrible compared to real-life literature, but not bad for artificial intelligence.") But whee, multiverse! Four stars, says my avatar, as Pimply God happens to narrow his focus on my apartment just to see what's going on near Boston and says "That sim there just called me pimply! Here's a random asteroid, how ya like me n
• 
  Manny

  I've now read three books about the multiverse in rapid succession: the first two were Rees's
  Before The Beginning (1996) and Davies's
  The Goldilocks Enigma (2007). This one came out just a few months ago, so I'm hopefully up to date for the moment.
  
  Well: I'm starting to feel quite familiar with the arguments, but each book has an interestingly different slant. Rees concentrates on presenting the experimental evidence for the existence of other universes - basically, the physical constants of our own universe appear to be tuned exactly right for life to be possible, it's unlikely that this is just chance, so we're probably one of many universes - and explicitly avoids philosophical speculation. Davies recapitulates Rees's arguments, but then goes overboard on philosophy; whatever he says, it's clear that he very much wants there to be a spiritual dimension to the story. Greene is similar to Davies, but his obsession is with string theory. He's spent his life working on it, and he desperately wants it to be part of the final explanation.
  
  Despite Greene's exaggerated love for all things stringy, I liked this book and found it well worth reading. The author presents nine different theories which in one way or another involve multiple versions of the universe. Most of these I had already seen in Davies, but I found Greene's exposition clearer and more detailed. In particular, I thought his chapter on "inflation" was the best account I had read of this mysterious process, which got our universe started by exponentially blowing it up, in a minute fraction of a second, from the size of a proton to a macroscopic scale. People now seem to understand inflation moderately well, and there are mathematical models which can be checked against the detailed maps of the cosmic background radiation which have been constructed over the last 10 years.
  
  I also thought the chapter on the Many Worlds interpretation of quantum mechanics was excellent; I hadn't understood that it's just a historical accident that the Copenhagen interpretation is the mainstream one. As Greene points out, it's actually more conservative to think of quantum processes as continually splitting off new universes, each of which is equally "real". The Copenhagen interpretation, with its notion of the collapse of the wavefunction, is the one which involves hand-waving and mystical invocations of the privileged role of the "observer". If you just look at the underlying mathematics, the Schrödinger equation, the Many Worlds interpretation is the straightforward way to translate the numbers into words.
  
  As already noted, Greene wants string theory to be in there, so there are chapters on "brane-worlds" (our universe is floating in a higher dimensional space), the "Cosmic Landscape" (there is a multitude of universes reflecting different flavors of string theory) and the Holographic Universe (we are the projection of a lower-dimensional process on a distant boundary surface). Like Davies, he also speculates about the possibility that our universe is a simulation running in some gigantic computer, or that we live in a "Platonic Universe", where mathematical theories exist simply by virtue of being consistent, and there is no other reality.
  
  All of these ideas were fun to read about, but I felt they were somewhat beside the point; I was surprised not to see him focussing on what, at least to me, seems like the obvious account. As he says, the Many Worlds interpretation of quantum mechanics is perfectly respectable, and inflation has accumulated a great deal of credibility as a theory of how the universe got started. Inflation starts at a scale where quantum processes are all-important. We can actually see the fossils of those quantum processes in the unevenness of the background radiation.
  
  Putting those two things together, we already have a multiverse theory that's mainstream to the point of being conservative. The quantum realities which split off during the very early history of the universe will be substantially different from each other. The question is whether they're going to be different enough, since we want them to differ with respect to things including settings of the physical constants. This part is still speculative. But you need it anyway in order to make most of the other theories work, and there seem to be moderately well-worked out accounts of how it could happen. I am sure people must be exploring in this direction.
  
  It sounds incredible when you write it down, but we appear to be on the edge of constructing a coherent scientific theory of Life, The Universe and Everything. I'm really curious to see what happens next.
  
• 
  Jason Furman

  Brian Greene really is one of the best popular science writers. His books give you a real sense of being guided by someone who genuinely knows what they're talking about, who uses metaphors effectively, and who effectively weaves the traditional material in with the new points he is making. He also approaches science with curiosity untainted by dogmatism. He is very much open to speculation, but equally open to the speculation not panning out.
  
  This book is about different concepts of the Multiverse. Greene devotes a chapter to each of what he defines as the major types and then has one or two additional chapters on questions like whether these theories are testable and broader implications.
  
  The multiverse's he consider include the quilted multiverse (which is just our universe extending out infinitely, leaving the possibility of endless accidental repetition -- which follows from some cosmological theories that follow the big bang), the inflationary multiverse (a product of repeated episodes of inflationary expansion, which follows from the addition of inflation to the previous theories), three multiverses that come from different versions of string theory (brane, cyclic and landscape), a quantum multiverse (which is Everett's Many Worlds interpretation, and is more conceptual), a holographic multiverse (which comes from the study of black holes and string theory), and simulated and ultimate multiverses (the last two coming from computer simulations and a deeper mathematical world).
  
  In every case, Greene does a good job of describing the physical theories that lead, usually by accident, to the implication that there is a particular type of multiverse, discusses the scientific status of those theories, and addresses issues around testing them. In the end, Greene has some sympathy with Steven Weinberg's adage that the problem with physics is that we do not take our theories/equations seriously enough as a real description of the world. The example he cites is the Positron, which was a byproduct of Dirac's solution of a math problem that turned out to be real. Greene clearly leans towards the view that the same is true of the multiverse, but he doesn't do much to tip his hand about which one.
• 
  Michael

  Outstanding update of the current status of modern physics and the projections of parallel universes from various advances. I would have loved to have Greene for a teacher in college. The tour is suitable for laymen with some understanding of physics, as he provides plenty of concrete examples to explain challenging concepts and gives an opportunity to skip more technical sections. For the more knowledgeable reader, a copious appendix is available, replete with the relevant math equations.
  
  Greene works his way through the history and logic behind eight different models of parallel universes and their status in terms of development and progress toward potential verifiability. The only one that I was really familiar with, from a college course on quantum physics 40 years ago, Everett’s 1956 Many-Worlds Hypothesis, is the only one that really bothers me, with its continual “splitting” off of parallel realities at every point where uncertain outcomes like the position of an object is resolved. Greene makes it clear how the interpretation is simpler and more elegant than Bohr’s interpretation. The latter calls for the probability wave of a particle to mysteriously collapses when it is detected or interacted with, effectively throwing out Schroedinger’s equations out whenever a “measurement” is made and restricting their applicability only to very small objects. Despite Occam’s Razor favoring Everett’s hypothesis (the quantum mechanics equations apply deterministically at all times), the interpretation of possible alternative states as each being somehow “real” has always been too absurd to be true to me. It’s a relief then that there is still scientific resistance to the notion and demonstrations of problems with the theory. Greene notes: “I don't expect theoretical or experimental consensus to come in my lifetime concerning which version of reality-a single universe, a multiverse, something else entirely—quantum mechanics embodies.”
  
  The other theories are fascinating and do not call for my alternative choices to have an existence in parallel universes. It was great to get a perspective on the excitement and challenges with string theory and its promise for helping unify the gravitational force with the other forces already in the fold of quantum field theory. The discovery that our universe is undergoing an acceleration in its expansion, and the need for some kind of anti-gravity force (“dark energy”) was a shock to me, so the recent successes of the theory of empty space having an “inflaton” field was satisfying to read about. Greene’s coverage of recent work on the meaning of entropy and information in physical systems was also a pleasure for me.
  
  For a sense of the content of this book, here is a thumbnail sketch of the seven other parallel universe theories covered, as laid out in Greene’s summary chapter:
  Quilted Multiverse: Conditions in an infinite universe necessarily repeat across space, yielding parallel worlds
  Inflationary Multiverse: Eternal cosmological inflation yields an enormous network of bubble universes, of which our universe would be one
  Brane Multiverse: In string/M-theory’s braneworld scenario, our universe exists on one three-dimensional “brane”, which floats in a higher-dimensional expanse potentially populated by other branes—other parallel universes
  Landscape Multiverse: By combining cosmology and string theory, the many different shapes for string theory’s extra dimensions give rise to many different bubble universes
  Holographic Multiverse: The holographic principle asserts that our universe is exactly mirrored by phenomena taking place on a distant boundary surface, a physically equivalent parallel universe
  Simulated Multiverse: Technological leaps suggest that simulated universes may one day be possible.
  Ultimate Multiverse: The principle of fecundity asserts that every possible universe is a real universe…These universes instantiate all possible mathematical equations.
  
  The summary chapter was also satisfying to me in attempting to address the following questions:
  Is the Copernican pattern fundamental? --Following the 500-year trend that ”the more we understand, the less central we appear”, we now approach the perspective that “our universe itself may not be central to any cosmic order”.
  Can scientific theories that invoke a multiverse be tested? --The worry is that ”by invoking a multiverse, we enter the domain of theories that can’t be tested”. He argues that well founded multiverse theories should be able to make predictions which are likely to be testable in our own universe, and hence falsifiable.
  How does the multiverse affect the nature of scientific explanation? Traditional approaches for physics aims to use mathematical expressions of physical laws and a set of constants and initial conditions to project what measurable things will be like at a subsequent time. Multiverse theories usefully explore variability in these conditions and laws which must be accepted as a given in a single-universe theory.
  Should we believe mathematics? Based on past progress, mathematical theories have had a huge impact on guiding science toward hidden truth. Despite problems in testing the multiverse theories, pushing on them is likely to lead to progress.
  
• 
  Muhammad

  নৃতত্ত্ববিদ্যা মতে মানব সভ্যতার জন্ম ২০,০০০-৪৪,০০০ বছর আগে। এই সময়টার ভেতর মানুষ খুব ধীরে ধীরে নতুন নতুন প্রযুক্তির উদ্ভাবন করেছে। মানুষ আগুন জ্বালাতে শিখেছে, হাড় দিয়ে হাতিয়ার তৈরী করেছে, পশুর সাথে হুটোপাটি না করেও স্রেফ বিষ দিয়েই যে অনেক কম ক্লেশে শিকার করে ফেলা যায় তা জেনেছে, পাথুরে গুহার গায়ে আঁচড় দিয়ে ছবি এঁকেছে...। এভাবে এক সময় মানুষ জায়গার পরিমাপের নিখুঁত হিসেব করতেও শিখে গেলো। ভূখণ্ডের কতখানি নিয়ে একটি সাম্রাজ্য বা দেশ হয় তার আন্দাজ পেলো। এত এত সব বিদ্যা একটু একটু করে অর্জন করে এই বড়জোর হাজার পাঁচেক বছর আগে মাত্র মানুষ জানতে পেলো পৃথিবী সসীম এবং এর আকার গোল (কারণ সমুদ্রে দাঁড়িয়ে অনেক দূরে বিপরীত দিক থেকে আগুয়ান জাহাজ এর দিকে তাকালে জাহাজের সবচেয়ে উঁচু অংশ মাস্তুলটিই প্রথমে দেখা যায়, তারপরে একটু একটু করে পুরো জাহাজটা চোখে পড়ে। পৃথিবী যদি সমান হতো তাহলে প্রথমবারেই জাহাজের পুরোটা দেখে ফেলা যেতো!)। পৃথিবীর আকার সম্পর্কে নিশ্চিত হয়ে যাবার অনেক বছর পর মানুষ জানতে পারলো পৃথিবীই শেষ নয়, সৌরজগৎ আছে, যেখানে কিনা পৃথিবীর মতো এমন আরো খান দশেক গ্রহ ঘুরপাক খাচ্ছে। এরপর আবিষ্কৃত হল সৌরজগৎ আসলে নিতান্তই শিশু। তারা বাস করে গ্যালাক্সির ভেতর। আমাদের গ্যালাক্সি মিল্কিওয়ে (আকাশ গঙ্গা) তার পেটের ভেতর প্রায় ১০০,০০০,০০০,০০০ নক্ষত্র নিয়ে বসে আছে (প্রায় সব নক্ষত্রেরই নিজের নিজের সৌরজগৎ আছে। অতএব, হিসেব মতে, মিল্কিওয়েতে মোটামুটি ১০০,০০০,০০০,০০০ বা এর কাছাকাছি সংখ্যক সৌরজগৎ আছে)। এমন গ্যালাক্সিও আছে প্রায় এই একই সংখ্যক-ই। এই নিয়েই ইউনিভার্স বা সৃষ্টিজগৎ। এ যেন অনেকটা রাশান মাত্রিওশকা পুতুল, কিংবা ৫ বাটির লাঞ্চ বক্স সেট, প্রত্যেকটির ভেতরই এক সাইজ ছোটটি গুঁজে দেয়া।
  
  
  
  কিন্তু সবচেয়ে বড় শেষ বাটিটা কোথায়? ইউনিভার্স কি নিজেই এক সাইজ ছোট কোন বাটির ভেতর আছে? ধর্মগ্রন্থে বর্ণিত স্বর্গ-নরক, দোজখ-বেহেশত কিংবা রূপকথার গল্পের ভালহাল্লা-অজল্যান্ড যদি থেকে থাকে, সেগুলো কি এই ইউনিভার্স এর ভেতরেই? নাকি অন্য কোন সৃষ্টিজগৎ সেটি? আধুনিক বিজ্ঞানের অসাধারণ প্রসারের কারণে আজ আমরা সৌরজগৎ, গ্যালাক্সি ইত্যাদির সমন্বয়ে গঠিত ইউনিভার্সের পুরো চিত্রটির অনেকখানি ‘দেখে’ ফেলতে পারি, অন্য কোন ইউনিভার্স এর সম্ভাবনা নিয়ে ভাবতে পারি। তবে প্যারালাল ইউনিভার্স বা অল্টারনেট রিয়ালিটি নিয়ে মানুষ হাজার হাজার বছর আগেই চিন্তা করে গেছে। প্লেটোর দর্শনে এই সম্ভাবনার কথা এসেছে বারবার। এসেছে আধুনিক সাইন্স ফিকশনের অন্যতম জনক এইচ জি ওয়েলস এর লেখায়ও। আমার সবচেয়ে প্রিয় চলচ্চিত্রগুলোর মাঝে অন্যতম প্রধান দুটি হলো ‘দ্যা উইজার্ড অফ অজ’ (১৯৩৯) ও ‘ইট’স আ ওয়ান্ডারফুল লাইফ’ (১৯৪৬)। দুটি ছবিরই থিম অল্টারনেট রিয়ালিটি। মার্ভেল-ডিসি কমিক্স এর সুপারহিরোরা অহরহ এক বাস্তবতা থেকে অন্য বাস্তবতায় ভ্রমণ করছে দুষ্টের দমনে। এই ইউনিভার্স এর ভালো ব্যাটম্যান অন্য এক ইউনিভার্স এর মন্দ ব্যাটম্যান এর ভয়ঙ্কর কোন প্ল্যান নস্যাৎ করে দিচ্ছে। অল্টারনেট রিয়ালিটির অভিজ্ঞতা হয়েছে ১৩ বছরের হ্যারি পটারেরও। ধর্মগ্রন্থ থেকে শুরু করে দর্শনের বই, সাইন্স ফিকশন থেকে কমিক্স জগত – সবখানেই প্যারালাল ইউনিভার্স এর রাজত্ব। তবে কমিক্স বা ফিকশন মানুষের কল্পনা প্রসূত। আসলেই কি ভিন্ন এমন কোন বাস্তবতা আছে যেখানে আরেকজন আমি ঠিক এই লেখাটিই কি-বোর্ডের একই অক্ষর গুলো একই সময়ে চেপে চেপে লিখছে আর “ওইদিকের আমি’র” কথা ভাবছে? গণিত ও থিওরেটিক্যাল ফিজিক্স বলছে এমন টা আসলেই হতে পারে! প্যারালাল ইউনিভার্সের অস্তিত্ব ও তাদের সম্ভাবনা নিয়ে পশ্চিমা বিশ্বের অসংখ্য পদার্থবিদ ও স্ট্রিং থিওরিস্টরা কাজ করে যাচ্ছেন। কলম্বিয়া বিশ্ববিদ্যালয়ের অধ্যাপক ব্রায়ান গ্রিন এঁদেরই একজন। যে সম্ভাবনাময় বাস্তবতাগুলো আমাদের জগতের সাথে সমান্তরাল ভাবে বয়ে যাচ্ছে এবং (অবশ্যই) যাদের আমরা চোখে দেখতে পারিনা, গ্রিন তাঁর বই ‘দ্যা হিডেন রিয়ালিটিঃ প্যারালাল ইউনিভার্সেস অ্যান্ড দ্যা ডিপ ল’জ অফ দ্যা কসমস’ এ সেই লুকিয়ে থাকা বাস্তবতাগুলোর খোঁজ করেছেন। ফিজিক্সে হাতেখড়ি যাঁদের নেই, ধরিয়ে দিতে দেয়েছেন তাঁদের হাতে চিন্তার কিছু সুতো।
  
  প্যারালাল ইউনিভার্সের অস্তিত্ব নিয়ে ব্রায়ান গ্রিন এর যুক্তি প্রদর্শনের আগে সম্ভাবনার অন্য একটা গল্প উপক্রমণিকা হিসেবে বলি। তারপর মূল পর্ব! ১০ বন্ধু ম্যাট্রিক পাশ দেবার পর উদযাপন করতে রেস্টুরেন্টে খেতে গেলো। কিন্তু রেস্টুরেন্টের টেবিলে কে কার পাশে বসে খাবে তা নিয়ে তাদের মাঝে ঝগড়া শুরু হয়ে গেলো। একজন প্রস্তাব করছে জন্মমাস ক্রম অনুযায়ী বসা যাক, আরেকজন প্রস্তাব করছে উচ্চতার ক্রম হিসেবে বসলে ভালো হয়। কেউই মনস্থির করতে পারছেনা, রেস্টুরেন্ট ম্যানেজারও টেবিলে খাওয়া সার্ভ করতে পারছেননা। শেষে ম্যানেজার তাদের কাছে গিয়ে প্রস্তাব করলেন, আজকে যে যেভাবে বসেছেন সেভাবেই বসে যান, আর একটা কাগজে বসার ক্রমটা টুকে নিয়ে যান। কাল এসে আবার অন্য ভাবে বসবেন। এভাবে প্রতিদিন নতুন নতুন অর্ডারে বসে ১০ জনে যত রকম ভাবে বসা সম্ভব তা শেষ করতে হবে। যেদিন শেষ হয়ে যাবে, ম্যানেজার সেদিন বিনা খরচায় তাদের ইচ্ছেমতো খাওয়াবেন। প্রবাবিলিটি অ্যান্ড স্ট্যাটিস্টিক্স কিংবা পার্মুটেশন-কম্বিনেশন এর সাথে যাঁরা পরিচিত তাঁরা ভেতরের ‘ক্যাচ’টা জানেন। গণিতে ফ্যাক্টোরিয়াল (একে ‘!’ দ্বারা প্রকাশ করা হয়) বলে একটি বিষয় আছে যা নির্দেশ করে একটি ঘটনাকে কতভাবে ঘটানো সম্ভব। ৩ টি সংখ্যাকে (ধরুন ৭, ৮, ৯) ৩! অর্থ্যাৎ ৩x২x১ = ৬ টি অর্ডারে সাজানো সম্ভব (৭.৮.৯, ৭.৯.৮, ৮.৭.৯, ৮.৯.৭, ৯.৭.৮, ৯.৮.৭) । এর পর আবার সাজাতে গেলে এই ৬টির কোন একটির পুনরাবৃত্তি ঘটবে। অতএব সেই ১০ জন ছাত্রের প্রতিদিন নতুন অর্ডারে বসে খেতে গেলে ১০! = ১০x৯x৮x৭x৬x৫x৪x৩x২x১ = ৩,৬২৮,৮০০ দিন = ৯,৯৪১ বছরের কিছু বেশী সময় লাগবে! তাদের প্রতিশ্রুত সেই ফ্রি খাওয়া আর পাওয়া হবেনা কখনো। প্যারালাল ইউনিভার্স সম্পর্কে ব্রায়ান গ্রিনের যুক্তিও ঠিক এটিই। আমাদের ইউনিভার্স হয় অসীম নয়তো সসীম। যদি সসীম হয়, অর্থাৎ বালুকণা, নক্ষত্র, নক্ষত্রের রং, তাদের রাসায়নিক গঠন ইত্যাদির প্রত্যেকটির সংখ্যা যদি কোথাও গিয়ে শেষ হয়, যা পরিমাপ করা যায়, তাহলে পার্মুটেশন কম্বিনেশনের যুক্তিমতে তাদের নিশ্চয়ই ভিন্ন ভিন্ন অর্ডারে সাজানো সম্ভব! একজন মানুষ অসংখ্য ইলেক্ট্রন-প্রোটন-কোয়ার্ক এর সমন্বয়ে গঠিত। তার চেহারা, বর্ন, চিন্তার প্রকৃতি, কন্ঠস্বর ইত্যাদি কেমন হবে তার সবই নির্ভর করে কিভাবে তার শরীরের সাব-অ্যাটমিক পার্টিকেলগুলো একে অন্যের সাথে ক্রিয়া করছে। পৃথিবীতে ৭০০ কোটি মানুষ আছে, আমাদের সবার প্রকৃতি হয়তো এক কিন্তু তবুও আমরা প্রত্যেকেই আলাদা। কতভাবে সাব অ্যাটমিক পার্টিকেল গুলোকে আলাদা আলাদা ভাবে সাজালে ৭০০ কোটি মানুষ পাওয়া যেতে পারে? যে মানুষটি দেখতে প্রায় হুবহু আমার মতো, কন্ঠস্বর আমার মতো, চিন্তা একই রকম কিন্তু তবু এক নয়, তার সাথে হয়তো আমার মাত্র কয়েক কোটি কণার বেশকম। পার্থক্য নির্ধারণকারী এই অল্প কয়েক কোটি কণা যদি আমার শরীরে ঠিক যে কম্বিনেশনে ‘বন্টিত’ হয়েছে, সেভাবে তার শরীরেও বন্টিত হত,অর্থাৎ তার আর আমার মাঝে ১টি কণারও কোন পার্থক্য না থাকতো, তাহলে আমার পুনরাবৃত্তি ঘটতো! সে মানুষটি আরেকজন আমি-ই হতাম। আমাদের ইউনিভার্স যদি সসীম সংখ্যক কণার সমন্বয়ে গঠিত হয়, তাহলে সম্ভাব্য সব ভাবে তাদের সাজানোর পর কোন এক সময় আবার তাদের পুনরাবৃত্তি ঘটবে। এর অর্থ হয়তো ঠিক এমনই আরেকটি ইউনিভার্স খুব কাছেই কোথাও অথবা ভয়ানক দূরে কোথাও ঘাপটি মেরে আছে, যেখানে এখানের ঘটনাগুলোই ঘটে চলেছে, আয়নার প্রতিফলনের মতো। টিভির শোরুমে যেমন বিভিন্ন মডেলের অনেকগুলো টিভিতে একই চ্যানেল চালিয়ে রাখে, একই ঘটনা বেশ কয়েকটা টিভিতে একই সাথে ঘটতে দেখা যায় শোরুমের কাঁচের দরজার বাইরে থেকে, অনেকগুলো প্যারালাল ইউনিভার্স যদি আমরা ‘বাইরে’ থেকে কোনভাবে দেখি, এমনটাই কি দেখবো? পুনরাবৃত্তি ব্যতিরেকে অন্য যে সম্ভাবনাময় ইউনিভার্স গুলো আছে, সেগুলোর প্রত্যেকটাতেই ঐ কয়েক কোটি কণার এদিক ওদিক হয়ে যাবার কারণে আমি একেক জায়গায় একেক রকম। কোথাও হয়তো আমি ছ্যাঁচড়া চোর, কোথাও হয়তো রাজনীতিবিদ (‘সম্ভাবনাময়’ আমার এ দুটো সত্ত্বাই বোধ করি কণাগুলোর একই কম্বিনেশন দিয়ে পাওয়া সম্ভব!)।
  
  মাল্টিভার্স বা অন্যান্য ইউনিভার্স গঠিত হবার ৯টি সম্ভাব্য উপায় বা মডেল আছে, অন্তত আজকের পদার্থবিজ্ঞান মতে। ব্রায়ান গ্রিন একেক অধ্যায়ে খুব গুছিয়ে ব্যখ্যা করেছেন সবগুলোই। কোথাও স্পেসটাইম একটি কম্বল বা বিছানার চাদরের মতো যেখানে একটু পর পর ফুলের নকশা প্রিন্ট করা আছে। স্পেস্টাইম বিছানার চাদরের মতো হলে (এবং অসীমভাবে বিস্তৃত হলে) ইউনিভার্স হবে নকশার সেই শোভাবর্ধনকারী ফুলগুলো। কোন মডেল বলে ইউনিভার্স হল পাঁউরুটির একটি স্লাইস। এমন অনেক গুলো স্লাইস (অর্থাৎ ইউনিভার্স) নিয়ে বড় একটি 'মেমব্রেন' বা সংক্ষেপে ‘ব্রেইন’। এই মডেল থেকেই ‘এম থিওরী’ উদ্ভাবিত হয়েছে। কোন মডেল বলে দুটি পাঁউরুটি’র স্লাইস বা দুটি ইউনিভার্স এর সংঘর্ষের ফলে নতুন ইউনিভার্স তৈরী হয়। কোন থিওরী বলে আমরা সবাই আসলে হলোগ্রাম, বাস্তব নই! আলোর সামনে আপনি হাত নাচালে দে���়ালে যে ছায়া পড়ে তা হলাম আমরা। অন্য কোথাও কেউ সুতো নাড়াচ্ছে তাই আমরা নড়ছি, আমাদের কাজগুলো আমাদের ইচ্ছেপ্রসূত নয়! প্লেটো এমনকি এই সম্ভাবনার কথাও ভেবে গেছেন। কোন মডেল বলে আমরা হলাম কম্পিউটার সিমুলেশন। খুব শক্তিশালী কোন কম্পিউটার কোথাও খুব শক্তিশালী কোন প্রোগ্রামিং ল্যাঙ্গুয়েজ দিয়ে আমাদের তৈরী করেছে। আমরা সেই প্রোগ্রামার কে কোন দিন দেখতে পারবো কিনা তা নির্ভর করছে সেই প্রোগ্রামার আমাদের সেই ক্ষমতা দিয়ে প্রোগ্রাম করেছেন কিনা! এ ধরণের চিন্তা আপনাকে সৃষ্টিকর্তার দিকে নাকি প্রকৃতির দিকে ধাবিত করাবে সেটি আপনার ব্যাপার তবে উভয় ক্ষেত্রেই চিন্তাগুলো আপনাকে বিনয়ী করবে নিশ্চিত। এত বড় সৃষ্টিজগতে আমাদের সম্ভবত সরাসরি কোন ভূমিকা নেই। পিঁপড়াদের দলের কোন বিশেষ একটি দলপতির ভূমিকা আমরা কখনো আমাদের ইতিহাসের বইতে লিখে রাখিনা, কিন্তু পিঁপড়ারা কি আমাদের জীবনে কোনভাবে ভূমিকা রাখছেনা? ব্যাক্টেরিয়াদের আমরা চোখেই দেখতে পাইনা অথচ ভালো-মন্দ দুদিকেই তাদের কত অবদান আমাদের জীবনে! মহাবিশ্বেরও শেষ আছে। একদিন সব নক্ষত্রের আলো ফুরিয়ে যাবে, ধ্বংস হয়ে যাবে এর প্রতিটি কণাও। আমাদের এত এত কার্যকলাপ, এত গবেষণা, আবেগ, শিল্প, সাহিত্য, ভালোবাসার ইতিহাস এগুলো সবই ফুস করে একদিন হারিয়ে যাবে? কোথাও কোন রেকর্ড থাকবেনা? পিঁপড়াদের সেই দলপতির মত অবস্থা হবে আমাদের যার অস্তিত্বকে আমরা পাত্তাও দেইনি? কেন যেন মনে হয় যত ক্ষুদ্রই হই না কেন আমরা, যে বিশাল মহাবিশ্ব আমাদের তার বুকে স্থান দিয়েছে, পালন করছে, আমরা তার চোখ এড়িয়ে যাবোনা। কোথাও আমাদের ছবি ঠিকই তোলা হয়ে রইলো। ইউনিভার্স তার ব্যাক্টেরিয়াদের ঠিকঠিক গুণে গুণে রাখে!
  
• 
  Annie

  Brian = bae. Definitely one of the more accessible cosmology writers of today. Any book discussing quantum field theory and string theory is going to require your undivided attention, of course, but it’s much more readable than many others, especially if you’re fond of analogies.
  
  Turns out, there’s lots of theories that give rise to the possibility of alternate realities:
  
  Theory 1, aka Quilted Multiverse: If the universe is infinite (it might not be, but if it is) then at some point in the distant, distant universe, there are worlds where there is a human named Annie just like me, typing on Goodreads, except she typed “GR” instead of “Goodreads.” And there’s another world with an Annie whose life is exactly like mine except she’s allergic to red wine (I would have to kill myself because there would be no point in living). In some, the worst things I’ve ever imagined happening have happened. In others, my wildest dreams have been fulfilled. (Somewhere, there is an Annie who can write like Kafka, DH Lawrence, and Kundera combined).
  
  Theory 2 aka Swiss Cheese: If the universe is expanding (hint, it almost definitely is) then there would be places which expansion would create which would have to have lowered rates of expansion, and those are like holes in Swiss cheese. The number of cheese-holes, as the cosmos continues to expand, continues to increase, and each one of those is adorably termed a bubble universe, or pocket universe. Including our own. This also brings in the Quilted Multiverse idea: from inside a bubble universe, the bubble’s space is infinite, but from outside the bubble universe, it’s finite. What appears as endless time to an outsider appears as endless space, at each moment of time, to an insider. And if it’s infinite to an insider, then there have to be infinite worlds with infinite repetitions and variations, as in Theory 1.
  
  Theory 3 aka String Theory aka Bread Loaf aka Cyclic Multiverse: [Strictly speaking there are five types of string theory, no one: Type I, Type IIA, Type IIB, Heterotic-O, and Heterotic-E, which all sound like sexually transmitted diseases to me, and anyway they all get subsumed into the grander M-theory]. String theory (particles are vibrating loops of filaments, and the way it vibrates determines what particle it is/how it behaves) and corresponding M-theory predict that there are ten dimensions of space and the eleventh dimension of time. Seven more than the ones we’re aware of: length, width, height, and time. But maybe that’s not an issue, because the four dimensions we’re used to extend over massive (maybe infinite) distances, but maybe the other seven are curled up at the other end of the cosmos and we can’t find them and they don’t extend far enough to reach into our reality. For analogy Greene gives the example of a straw, normal sized, except it’s as tall as the Empire State building. It’s three-dimensional up close, but from the other side of the river, it would just look like a vertical line (two-dimensional). Here comes the multiverse idea: pretend the world has just two dimensions (it’s a flat plane) called a two-braneworld. Line up a couple of these flat planes (slices of bread) and you can add on other parallel universes, other braneworlds, as many as you like (“just add slices to the cosmic loaf” as Greene memorably says). Of course, we have three spatial dimensions, but looking at 2 dimensions makes it easier to wrap our 3D heads around. It’s also possible that different braneworlds would bounce into each other and restart the cosmological clock, so that, rather than one big bang, things are a gigantic bumper-car game and universes are constantly (though very slowly, from our perspectives) getting remade and unmade and remade.
  
  Theory 4 aka Landscape Multiverse: pretty much the idea that the cosmological constant (density of energy in space, creating certain gravitational values) might be necessary for life, and so what are the odds it happened to happen? Well, you’d need an infinite number of universes to make it likely that [at least] one of them just happened to have the exact necessary cosmological constant to make life happen.
  
  Theory 5 aka the Quantum Multiverse: all the possibilities of [our] reality are realized in another reality, another multiverse. All possible histories and all possible futures are real. One of them is the reality we live in; the (basically infinite) rest exist in some way. True reality is more like a tree than a line.
  
  Theory 6 aka Everything is a Hologram: The event horizon of a black hole (point of no return) is the surface of a black hole in some ways, and fluctuations in the surface of the event horizon contains all the information the black hole sucks in. Similarly, our 3D world may be a rendering of 2D-encoded information (on some cosmological event horizon-type-thing) which creates reality as we know it.
  
  Theory 7 aka Virtual Reality: we can create other universes virtually and, someday, could probably create reactions in the (potentially sentient) actors in those universes which could make it feel like reality, every bit as much as it does to us. If everything’s infinite (see theory 1) with infinite numbers of beings creating infinite numbers of virtual universes whose inhabitants can also create infinite numbers of infinite universes and so forth, doesn’t it seem like odds are good that we’re just one of those games? Could we even tell if we were in a simulation? Depends on your Simulator and if they want to reveal themselves (are you there, God? It's me, Annie).
  
  Other interesting things:
  
  Dark energy might be the answer to Einstein’s unproven cosmological constant (that he retracted after he suggested it because he couldn’t account for it). There’s some… some shit out there, and we can’t see it, describe it, or know of its existence other than through a few methods of measurement.
  
  The mind-boggling idea that if the universe is infinite, and you shrunk infinity, everything would be closer together but the universe would still be infinite. Rationally I understand that you can’t reduce infinity (infinity divided by two is still infinity) but my brain doesn’t want this to be a thing.
  
  About 1 percent of the snow on a television that’s tuned to a non-working channel is due to reception of the big bang’s photons (aka background radiation).
• 
  Mary Overton

  It took me 8 months to read this book. My system is to read until my head is ready to explode, then stop for a month, then restart at the beginning. On the 4th attempt, I made it to the last page (or rather, Kindle location.) ...no guarantee of how much was actually understood. This is a book that pried open my mind. The brain is sore and ecstatic from the experience.
  
  From the last chapter,
  Table 11.1 Summary of Various Versions of Parallel Universes
  1. Quilted Multiverse: Conditions in an infinite universe necessarily repeat across space, yielding parallel worlds.
  2. Inflationary Multiverse: Eternal cosmological inflation yields an enormous network of bubble universes, of which our universe would be one.
  3. Brane Multiverse: In string/M-theory's braneworld scenario, our universe exists on one three-dimensional brane, which floats in a higher-dimensional expanse potentially populated by other branes - other parallel universes.
  4. Cyclic Multiverse: Collisions between braneworlds can manifest as big bang-like beginnings, yielding universes that are parallel in time.
  5. Landscape Multiverse: By combing inflationary cosmology and string theory, the many different shapes for string theory's extra dimensions give rise to many different bubble universes.
  6. Quantum Multiverse: Quantum mechanics suggests that every possibility embodied in its probability waves is realized in one of a vast ensemble of parallel universes.
  7. Holographic Multiverse: The holographic principle asserts that our universe is exactly mirrored by phenomena taking place on a distant bounding surface, a physically equivalent parallel universe.
  8. Simulated Multiverse: Technological leaps suggest that simulated universes may one day be possible.
  9. Ultimate Multiverse: The principle of fecundity asserts that every possible universe is a real universe, thereby obviating the question of why one possibility - ours - is special. These universes instantiate all possible mathematical equations.
  Kindle location: 6079-6106
  
  "....scientific discovery has shifted humankind's perspective on its place in the cosmic order. Over the course of nearly five centuries, the Copernican progression has been a dominant theme.... experience abounds with clues suggesting that we're a central hub around which the cosmos revolves. But the objective methods of scientific discovery have steadily corrected this perspective. At nearly every turn, we've found that were we not here, the cosmic order would hardly differ. We've had to give up our belief in earth's centrality among our cosmic neighbors, the sun's centrality in the galaxy, the Milky Way's centrality among the galaxies, and even the centrality of protons, neutrons, and electrons - the stuff of which we're made - in the cosmic recipe. There was a time when evidence contrary to long-held collective delusions of grandeur was viewed as a frontal assault on human worth. With practice, we've gotten better at valuing enlightenment.
  "The trek in this book has been toward what may be the capstone Copernican correction. Our universe itself may not be central to any cosmic order.... The idea that reality based on a multiverse extends the Copernican pattern and perhaps completes it is cause for curiosity. But what elevates the multiverse concept above idle speculation is a key fact that we've now repeatedly encountered. Scientists have not been on a hunt for ways to extend the Copernican revolution.... Instead, scientists have been doing what they always do: using data and observations as a guide, they've been formulating mathematical theories to describe the fundamental constituents of matter and the forces that govern how those constituents behave, interact, and evolve. Remarkably, when diligently following the trail these theories blaze, scientists have run smack into one potential multiverse after another."
  Kindle location: 6109-6128
• 
  Javier Santaolalla

  Me descubro ante el que yo considero que es el mejor divulgador científico de nuestro tiempo, al menos en lo que respecta a la física. Claro, audaz, brillante, sabe cómo exponer hasta el contenido más enrevesado, lo desmenuza y ordena para que el lector lo entienda pero también lo disfrute. Que un científico de primera línea tenga además estas habilidades narrativas es algo que un lector apasionado como yo agradece, una combinación que por desgracia no es fácil encontrar.
  En este libro Greene nos habla de los universos paralelos y cómo esta hipótesis surge como elemento fundamental de muchas teorías. En concreto nos habla de 9 tipos diferentes de multiversos y para ello tiene que navegar por la teoría de cuerdas, la inflación cósmica, la holografía... contenido muy avanzado pero al alcance de un lector sin conocimientos científicos pero motivado hacia la física.
  NOTA importante: antes de leer este libro recomiendo pasarse por otros del mismo autor, por ejemplo, el universo elegante.
  Gracias Greene por tu maravillosa obra. Por muchos más libros como este.
  
  Como es un libro con contenido muy avanzado pero está bien explicado le doy una calificación de 2 rombos (1 rombo, libro muy sencillo; 3 rombos, libro muy complicado).
  
  Sobre este libro puedes encontrar esta reseña en mi videoblog:
  
  https://www.youtube.com/watch?v=6h86O...
• 
  hayatem

  هل كوننا هو الكون الوحيد الذي له وجود؟
  
  "استكشاف المجهول يتطلب التسامح مع عدم اليقين"—برايان جرين.
  
  شخصياً من المؤمنين بوحود عوالم أو أكوان أخرى موازية لهذا العالم، وكذلك من المؤمنين بوجود حياة أو حيوات خارج هذه المجرة، ف الله الذي خلق أدق الكائنات المجهرية وصولاً للكائنات الأكثر تعقيداً " نحن البشر" لا شك أنه أبدع في تصميم وتخليق هذا الكون، بصورة لم يصل لها خيالنا الحسي أو المادي حتى الآن!
  بلغة الرياضيات أو الواقع الرياضي" بصفة خاصة" لا يدعم بشكل جازم وجود أكوان متعددة أو موازية لهذا العالم، أما نظرياً لازال هناك شك، وشبه يقين بوجود عوالم أو أكوان أخرى فسيحة موازية لهذا العالم.
  نظرية الأوتار، على الرغم من الشكوك الكثيرة التي تحوم حولها، وإثارتها المستمرة للجدل في الحقول الفيزيائية على اختلافها، إلا أنها برأيي من أجمل النظريات الفيزيائية الحديثة !
  
  هذا الكتاب مثير ومحفز على التفكير والتأمل بكوننا الفسيح .
  
  برايان جرين فيزيائي ممتع!
  
• 
  Mike (the Paladin)

  The first book I had by Brian Greene was
  The Fabric of the Cosmos. I got it not long after it was releases and a friend asked me if he could borrow it not long after I received it in the mail, before I'd read it.
  
  I said yes.
  
  I didn't get the book back for several years, Dr. Greene had written more by then. Oh well.
  
  This book (as is Dr. Greene's wont) is an attempt to take highly technical and advanced ideas and make them understandable to "us", "we", "the great unwashed", "the masses". I suppose in all truthfulness it only succeeds to a point.
  
  I mean, I know I don't have the math to grasp the heart of what he's talking about and even I can see the imprecision.
  
  Still, he does take the difficult, sugar coats it and lays it out there. (I kept seeing that scene from Big Bang Theory where they have a cameo from Dr. Greene and Sheldon's reaction to him).
  
  If you're a physicist (theoretical or experimental I suspect) this book will probably be hopelessly imprecise and simple, if your like me, it's fairly interesting. Even though I know that what I'm getting is at best a broad outlined picture (where the multiverse is compared to a large Swiss Cheese with the bubbles pictured as the universes). The book is laid out largely in the format of here it is, here's what we think/know and here's why. It assumes that those reading it won't really be in a position to challenge it and tries to be as simple as possible.
  
  In short not a bad book and manages to lay things out in a relatively (get it "relatively") simple way when dealing with a difficult set of subjects (or subject if it's "unified" :) ). Enjoy. If it interests you I recommend it.
• 
  Keith Akers

  This was a good book. For a popularization, this had some pretty heavy science in it. I'm a pretty smart guy and will have to re-read it to really have a better understanding. Greene really is a good writer, because even when you don't quite understand what he's talking about, he gives you enough of the broad overview so that you can go to the next section and feel that you haven't missed anything critical.
  
  The section on "quilted multiverses" was pretty straightforward and I can claim to have understood it. The next few parallel universes were a bit trickier, but I was with the program. When I got to multiple dimensions and the "landscape multiverse" with so many possible shapes to the extra dimensions that "kazillions" doesn't even begin to cover the number, this was something I wasn't quite so sure that I could explain to my wife.
  
  I'm not sure I want to challenge Brian Greene on any scientific question, but the one point I would have some questions about were the simulated multiverses (as in the movie "The Matrix"). I think he may be shortchanging Goedel and Turing. I'm not convinced that a simulated multiverse sufficient to model all the creatures on our planet would not consume so much energy that it would be in effect less probable than an actual universe. My experience as a programmer is that the complexity of programs and the resources needed to design them go up exponentially with the size of the program. If that is the case, creating a universe might actually take less energy than modeling one. We would need to automate the process of computer design, but this essentially can't be done, and that's the point of Turing's thesis. There may be a practical upper limit on the size of simulated universes that we (or future super-intelligent beings in our universe) could ever design. This may be my confusion, but I'd like just a little more explanation here.
  
  So, this book is recommended, and I hope to come back to it.
  
  One final thing: though the book never utters a word about it, Brian Greene is a vegan. That anybody this smart is a vegan says something, I think, about where we should be headed as a species.
  
• 
  R.L.

  This is a pretty dense book. Some of the footnotes have footnotes!
  
  I'm sure it must be very difficult for an author of a popular science book to walk the line between, on the one hand, providing enough technical information to make the arguments cogent, and on the other, miring the reader in a bog of difficult concepts and facts. My hat's off to Dr. Greene for giving it a try on some of the most out-there ideas that can be imagined.
  
  Some very interesting ideas, but ultimately, for me this was all a little difficult to chew and swallow.
  
  My own theory [ unencumbered by fact or mathematics :) ] is that the universe is
  infinite in all directions. Our atoms and subatomic particles are someone--or something!--else's galaxies and universes, and likewise, that our galaxies are atoms and molecules to some other beings. And, boy, are those beings *big*!
  
  For some reason I also think that the hierarchy that I just described doesn't recurse infinitely in a fractal manner. Instead, it's all circular, so that if you start here and go up to galaxies or down to atoms (or down to gehanna or up to the throne!), or, for that matter, out to the edge of our universe, and (bringing it back to the topic of this book) to other universes in the multiverse, you would eventually end up here again. Weird.
  
  Not light reading, but recommended if you're interested in some of the current thinking in the fields of cosmology and physics.
• 
  Kathleen

  I always enjoy looking at scientific concepts through the lens of the fantastic or peculiar, so this exploration of parallel universes by popular physicist Brian Greene is perfect for me. Written in his usual clear, funny style, Greene naturally discusses string theory, relativity, number theory with respect to infinities, quantum theory, and numerous other scientific concepts. Instead of simply discussing these theories, however, we get to look at various proposals of multiple universes. From the familiar quantum multiverse suggested by the probability wave math of Schrodinger's equations and the idea--Greene refers to it as a "quilted multiverse--that if the universe is infinite than it obviously repeats, to more obscure suggestions from string theory, the many kinds of parallel universes are distinct and interesting. Perhaps the most intriguing is the idea of the simulated multiverse; if one day computing power reaches the extent that we can play at building self aware, self contained universes the way I play at Spore and The Sims, then we will essentially create our own multiverses.
  
  The most enjoyable aspect of this study is Greene's particularity in explaining the scientific concepts--and eventual testability--of these universes. This book never feels like a pointless philosophical exercise, which keeps it fun.
• 
  Cassandra Kay Silva

  Finally! Brian Greene you are the master at delivering exactly what I have naturally been pondering and lack the scientific mind to undertake. Thank heavens it wasn't another science book that I had to sit through the whole of physics from Einstein onward. Way to know your audience. We are sick of the constant repeating of every major scientific breakthrough of modern times. We have heard it all a dozen times, we know the history and want to know what is going on now! Yet again you deliver in your classically simply elegant style the various avenues mathematics has led us to opening up different scenarios for parallel universes in every flavor under high investigation. Giving comparisons for each approach and tackling them all in turn. Its such absolute brain candy. You are so damn brilliantly perfect for presenting this type of information. Bravo! This is why you are one of my all time favorite authors.
• 
  David Rubenstein

  Much of this book describes speculative hypotheses about parallel universes. Some of the explanations--for example, the bubble universes--really made my head hurt. The explanation for why string theory requires ten space-time dimensions seems a bit glib to me. Some of the analogies--imagine a bunch of clones of Cartman from South Park standing atop mountain peaks--are just too silly to take seriously. The description of black holes and entropy is fascinating. Some parts of the book are rather philosophical--do mathematicians "invent" math or "discover" it? And, if you enjoy reading about extremely speculative hypotheses--like our universe is a computer simulation, much like "The Matrix"--then this is the book for you.
• 
  Sara

  I wanted to get a glimpse of the current state of physics' understanding and theories about our universe and possible multiverses, mostly after an interesting piece on Radiolab a few months ago. I felt like this book got bogged down in string theory, and I have to admit the mechanics of the holographic universe escaped me. Still, I feel like I understand more about how theories of multiverses originate, and where they might be going. Not the most fun read, and I was expecting less theoretical physics and more descriptions of possibilities, but it was OK.
• 
  Raghu

  Modern Physics’ description of reality has baffled non-scientists for over a hundred years. The General Theory of Relativity told us that space has curvature. It showed that Time slows down or speeds up, depending on how fast you move relative to something else. Quantum theory says we determine reality only when we measure it, the Universe exists in quanta, and that the world is probabilistic. Since the 1970s, String theory tells us that reality has ten dimensions (excluding time) and that there may be not one, but 10^500 universes. This book by Brian Greene, a theoretical physicist cum pop-science exponent, explores the ideas behind multiple universes. It explains how String theory gives rise to ‘multiverse’ and why it is science, despite String theory remaining unproven.
  
  The Multiverse story starts out with the idea of parallel universes. Astronomy says space extends to infinity. This suggests there must exist many parallel realms. In these realms, copies of you and me and everything else could exist. These copies could enjoy alternate versions of the reality we experience here. This is the Quilted Multiverse.
  The next possibility is the Inflationary Multiverse. The cosmology of Inflationary theory posits an enormous burst of super fast spatial expansion during the universe’s earliest moments. Inflationary expansion in distant realms may spawn bubbles of Universes in sequence and continue to do so for all eternity.
  
  Recent developments in String theory give rise to three more kinds of parallel universes.
  One of them is the Brane multiverse. It hypothesizes that our universe is one of many “slabs” floating in a higher-dimensional space, much like a slice of bread within a grander cosmic loaf.
  A second is the Cyclic Multiverse. It emerges from Brane universes colliding with one another, wiping away all they contain and starting a new, fiery big bang–like beginning in each. This could happen over and over. Branes might collide, bounce apart, attract each other through gravity, and then collide again. Such cyclic processes generate parallel universes in Time.
  The third is the Landscape multiverse. String theory’s extra spatial dimensions give rise to the possibility of an enormous number of shapes and sizes. It combines elements from string theory and cosmic inflation to argue that we live in a multiverse made up of an infinity of “bubble universes”. Our perfectly calibrated universe is just one of them.
  
  Next on the menu is the Quantum Multiverse. This idea was first proposed in 1957 by Hugh Everett III, a Princeton graduate student. We call the basic quantum equation a wave function. It describes a particle, say an electron, occupying many probable positions. Quantum Mechanics associates each position with a probability. Such a wavy nature of the electron collapses when we measure or observe where it is. When we do so, it reduces to a well-defined position. Everett theorized the electron is present in all the positions defined by the wave function. It just so happens that each position occurs in a different parallel universe. Hence, the sum of all our copies experiences the totality of all experiences. As a result, the number of universes in a Quantum Multiverse comes to 1024^112.
  
  Holographic Multiverse, based on the Holographic principle, is the next possibility. It says some distant two-dimensional surface could contain all the data needed to describe our universe in its entirety. Projecting this data to appear in three dimensions creates the experience of our universe, like in a hologram. Hence, we may live on a flat surface like a TV screen, which looks like it has 3D-depth. The impetus for this reasoning comes in part from the complexity of theoretical physics itself. Black holes and Quantum Gravity become simpler to solve when you reduce the universe from three to two dimensions!
  
  The next possibility is the Simulated Multiverse. The idea owes its origin to Edward Fredkin in the 1960s. Nick Bostrom, an Oxford philosopher, posits the idea that advanced civilizations could create many simulated realities. Our universe could be one such simulation, along with many other simulated universes. We see examples in the worlds created by video games and the movie ‘The Matrix’. In principle, there could be more simulated realities than non-simulated ones. This idea is more philosophical than one related to Physics.
  ‘The Ultimate Multiverse’ is the last of the Multiverse Prof. Greene talks about. In this scenario, all universes exist. The question of why one set of laws describes a real universe—ours—while all others are sterile abstractions evaporates. There are no sterile laws. All sets of laws describe real universes. Within this multiverse, there would be a universe that comprises absolutely nothing. This idea also owes more to philosophy than Physics. The cosmologist Max Tegmark proposed this theory.
  
  When I reached the end of the book, I felt I had read a science-fiction in a popular science book. The principal reason is that String theory is only a mathematical construct and its consequence of multiverse is also its mathematical offspring. However, this is not reason enough to discount them as unscientific results. It was the mathematics underlying General Relativity that led Karl Schwarzschild and Alexander Friedmann to predict the existence of black holes and an expanding universe. Even Einstein considered them bizarre. So, why not give the same latitude to String Theory because its mathematics also suggests ideas which look bizarre today? But General Relativity stands proven through other conventional experiments. This is not true of String Theory.
  
  Brian Greene is well aware of it and discusses this question at various points in the book. He says the most promising positive experimental results would most likely not be able to prove string theory definitively right. Negative results would most likely not be able to prove string theory wrong either. Failing to find supersymmetry particles might mean they don’t exist, but it also might mean they are too heavy for even the Large Hadron Collider to produce. Not finding evidence for extra dimensions might mean they don’t exist, but it also might mean they are too small for our technologies to access. Failing to find stringy signatures in observations of gravitational waves or the cosmic microwave background radiation might mean string theory is wrong. But it might also mean that the signatures are too meager for current equipment to measure. On the question of String theory not making definitive predictions, Greene says the lack of a unique specification of the extra spatial dimensions is the main stumbling block. Hence, they are non-testable and non-falsifiable proposals, invoking hidden realms beyond our capacity to access. They seem a far cry from what most of us would want to call science, as Karl Popper taught us.
  
  We could also critique these theories from a layperson’s point of view. If eminent scientists themselves propound ideas we cannot test or falsify but still insist they are science, then where does science stand? After consuming immense resources for fifty years, is it not fair to call for a change in direction from this approach? Should we spend another $20 billion on the Large Hadron Collider in blind faith? When the world is facing many pressing problems like poverty, disease, education, and inequality, is it acceptable to indulge resources in colonizing Mars? When fresh drinking water eludes much of the developing world, is it fair to spend a billion dollars looking for water on Mars? Will the Large Hadron Collider remain an expensive toy to find just the Higgs boson?
  
  The title of the book suggests that Strings and Multiverse are reality hidden from us now. I feel it is misleading. We can call them hidden mathematical conjectures. The book is in popular science style, but I found it a tough read. Prof. Greene does his best to explain the ideas using examples that rely on our worldly experience. The concepts are abstract and I found the visualization hard. Often, I could grasp only the result. The logic underlying it eluded me. Greene writes with a passion for his science. As an admiring reader, I would like his ideas proven true. But then, I think about simulated and Holographic universes. I feel it may only be a fantasy of Theoretical Physics.
  
• 
  Tomislav

  The first half of this book is an expansion on some of the various multiverse concepts mentioned only briefly in Greene's earlier The Elegant Universe and The Fabric of the Cosmos (which overlap each other a little). I was an enthusiastic reader of those, and enjoyed this deeper look at what the scientific basis of those might be, in somewhat the same style. But for the second half of the book, I want to mention specific chapters, as I feel the value of the writing swings wildly.
  
  7. Science and the Multiverse. This chapter for me raised some troubling issues as to what is actually science and what is interesting philosophical speculation. He goes a bit further out on the limb of the anthropic principle than I am usually comfortable with. Greene reasons about infinities of universes in the a number of different types of multiverse, based on a sample size of 1. It could be we are typical, and the exclusion of so many possibilities is valid, or it could be we are atypical. I mean, it's cool, you know, but not so reasonable. The cover article of the April 2011 issue of Scientific American concerns related issues in inflationary cosmology. See "Quantum Gaps in Big Bang Theory; Why our best explanation of how the universe evolved must be fixed or replaced" by Paul J. Steinhardt, Princeton.
  
  8. The Many Worlds of Quantum Measurement. This is possibly the best explanation of the uncertainty principle I have ever read. It gave me new insights into the Schrodinger equations, and addressed some concerns that have bothered me for years. This chapter alone justified reading the book.
  
  9. Black Holes and Holograms. I think I understand about information and the event horizon of black holes, but the holographic multiverse totally baffled me. Too many analogies and twists and turns in the explanation. I guess you have to do the math to get this one - or take it on faith, which I refuse to do.
  
  10. Universes, Computers, and Mathematical Reality. This chapter concerns epistomology and information theory. What's it doing in a physics book?
  
  Taken as a whole, the book was fascinating and worthwhile - but gets patchy in the second half.
• 
  Laurie

  I really am a huge fan of Brian Greene. I've read both "The Elegant Universe" and "The Fabric of the Cosmos" and find his writing simply astonishing. With complete sympathy for readers without mathematical acumen, he explains concepts like quantum physics, the theory of relativity, the four major forces including gravity with such precision, and wit, that I found myself gasping for breath as I felt like I was reaching groundbreaking depths of understanding of how the universe works on almost every page. Even if you are not interested in physics and the workings of reality, he has a way of presenting these ideas in such an entertaining and interesting way, you can't help but get caught up in his enthusiasm. He's so tuned in to the general reader that he even suggests skipping large parts of his book if you don't want the nitty-gritty details and just want the big-picture show. NOT SO with this book. It almost felt like a different writer. Even he did not seem as enchanted with the subject. Most of the book was really a lot of twisty, topsy-turvy conversation about the probability of multiple universes, including exact replicas of our own.
  His exlanations were vague and his writing was even boring. Big thumbs down and I am so disappointed!
• 
  Nicholas

  I purchased this on the strength of Brian Greene's past works and find him to be consistent in his clarity of explanation.Although I had a few sticky moments with some elements of string theory and the explanation of the concept of infinity, on the whole it went well.I didn't get the same revelatory experience I got from "Fabric of the Cosmos",as most of the multiverse theories are extensions of areas he has covered previously and as the book is written with the assumption of little or no prior knowledge,it can be a little tedious having to go through same theories again.If you have any prior knowledge in this field then you will realise that all these ideas are at the present unverifiable,but at the end he posits the questions that must be answered to advance the possibility of validating any of them.
  On the plus side I'd say that the book is well structured and comprehensive without complexity (although the notes delve a little deeper).On the minus side there is little new if you are a reader of "New Scientist" or are an avid reader of popular science and I must admit I did find myself counting the pages a few times.
• 
  Bradley

  It was a good update for what I've missed in the last decade, and it even makes me rather curious about some of the math. I've got a real curiosity about holographic universes now that I've got a better image of them. I love the idea of a highly encoded piece of information undergoing a transform that then gets plastered upon the ceiling of the universe. Touching upon branes and seeing how they might possibly touch each other was also pretty cool. The writing in this book was quite good and nothing seemed out of place, not even the heavy speculation near the end. In fact, I really enjoyed that, too.
• 
  notgettingenough

  Not enough Simpsons in it.
  
  Written after Manny's here
  http://www.goodreads.com/review/show/..., because somebody has to stand tall and write the tough reviews.
  
  
• 
  Joseph

  This is one heady book. Brian Greene explores the various possibilities of infinite universes that come out of the current theoretical physics of string/M theory. I had a hard time following some of the physics, but I attribute that to my lack of familiarity with the subject, not to Greene’s ability to explain difficult concepts.
  
  My favorite sections included Greene expounding on several different universes - specifically the ones made entirely of math and of course the ones where we are currently living in a computer simulation. We can go to all sorts of interesting places if we follow the math/physics of what we’ve learned through string theory.
  
  My guess is that a few hundred years from now, our descendants will laugh at how little we understood of the universe, its fundamental laws, and “reality”. We may be onto some really deep understanding of the universe, but likely we will have to iterate on the current theories several times before we get to more concrete answers. That and we will have to figure out how to verify the theories through experimentation. Either that, or the machines will take over and we’ll live in the matrix.
• 
  Mohammad Zaid

  I am sold to Brian Greene after reading this one.I am fond of his simplicity for writing such a book.I was looking for such a book for quite sometime and when I learned about it I instantly decided to read it. It took me quite a time to read this book,to process it.It is a good book for the people who have little knowledge in this field.Want to broaden your knowledge on the topics like Multiverse,string theory? Read this book.
• 
  Umair Khan

  In The Hidden Reality, celebrated theoretical physicist Brian Greene explains the mind-boggling idea of a ‘Multiverse’ (plural of universe). A professor of physics at Columbia University, Greene is well-known for his two earlier works on popular science, The Elegant Universe and The Fabric of the Cosmos which sought to explain, among other things, the very nature of space-time. But The Hidden Reality, in explicating the idea of multiple realities, shows the layperson something that has never before been presented by a popular physicist.
  
  The Big Bang Theory, which suggests the origin of the universe some 14 billion years ago gives rise to several other theories which require our universe not to be the only one. Most readers of popular science are familiar with the many worlds interpretation of quantum physics and M-theory’s “braneworld”. Multiverses, Greene writes in this book, are “harder to avoid than they are to find” and he discusses nine theories in their support.
  
  From the plain old “Quilted Multiverse theory” that postulates that in this infinite universe, there is bound to be repetition of the order of things to the “Inflation theory” that suggests that our universe began as a rapidly expanding bubble of empty space, Greene discusses the all. “String theory” gained popularity because it successfully unifies General Relativity and Quantum Theory and verification is expected from the experiments currently being run at the Large Hadron Collider. Greene also ponders over the possibility of a “Simulated Multiverse”, theorising that we may all be part of a simulation, like that in The Matrix. Extending this supposition further, the creators of our simulation might themselves be part of another simulation and the final wonderland is the “Ultimate Multiverse” in which everything that can exist and is mathematically consistent actually exists ‘somewhere’.
  
  As a teenager I read scientists like Carl Sagan and Stephen Hawking on the queer nature of black holes, the philosophical dilemmas of time travel, and the creationist implications of the big bang theory. But compared to this, these problems seem mundanely simple. Some people call these theories scientific blasphemy, but in the words of George Bernard Shaw, “all great truths begin as blasphemies”. The Hidden Reality both popularizes scientific ideas and shines a light on the path that will be explored by future physicists.
  
  These theories are challenging simplistic notions about science by ‘enhancement’ of mathematical and scientific standards. In order to resolve the deepest mysteries and the greatest dilemmas of modern science, modern scientists are willing to accept these bizarre-looking properties of the natural world. We have reached the point where we need to re-examine what is and is not science and how theories can be verified and accepted as scientific.
  
  Greene has selected an apt title for this book because nothing can be more hidden than a reality which is neither accessible physically nor conceivable mentally. The insightful and engaging narrative makes this complex tour of state-of-the art theoretical physics much more exhilarating and informative. Greene’s
  greatest achievement is that even as you grapple with these allusive concepts, you start falling in love with these mysteries.
  
  Published in The Express Tribune, Sunday Magazine, May 1st, 2011
  
  
  http://tribune.com.pk/story/157862/th...
• 
  bup

  This book wouldn't be great without his first two. Or maybe it would, but it would seem like a fantasy ride in some bologna-artist's woo-science book. It's basically a survey of the different multiverse ideas out there - all the different ways people have dreamed up multiple universes.
  
  Given the scientific gravitas Brian Greene is able to bring, though, one has to take these ideas seriously. At least, as seriously as one can. I'm not a physicist, but I have taken a statistics course or two, and the thinking about how we can test some ideas of multiple universes by looking at our own universe make absolutely no sense to me. Our universe is one data point. It gives you zero degrees of freedom. There is no hypothesis one could reject by theorizing, as Greene seems to do, that multiverses where a universe like ours would be more likely, and then turning around and looking at the data from our universe.
  
  Whether or not he's right about testability, though, even constructing hypotheses is a portion of science. Science is the process of developing and testing hypotheses. There's nothing wrong with developing hypotheses you have no idea how to test, and calling it a scientific step. OK, so we have no idea how to test, or if we can. Don't let that limit the process of developing hypotheses.
  
  Two more nitpicks (I gave this 5 stars, too - why am I only complaining? I guess I found the book so intriguing and engaging that I want to talk back to it):
  
  1) He never talked about the fact that many of these multiverse theories are not mutually exclusive - bubbles in the expansion of space and simulated multiverses could obviously coexist, as could many of the others.
  
  2) In the ultimate universe model, the universe with nothing in it obviously exists - inasmuch as it needs to. We have to accept that one, just like we have to accept the empty set in every set of subsets ever.
  
  Anyway, good book. I recommend "The Elegant Universe" and "Fabric of the Cosmos" before this, but I guess it would work on its own, too.
• 
  Ericka Clou

  This was a great follow-up to
  The Fabric of the Cosmos: Space, Time, and the Texture of Reality. It didn't repeat a lot of material from the first one, it explained the progression in physics since then. It was pretty mind-blowing. I did get a little stuck in some sections, but I decided that a perfect understanding of the topic wasn't strictly necessary for my purposes.
  
  I do think Greene protests too much that every aspect of physics isn't a *miracle.* I've never heard of so many miracles I believe in, even if, as Greene claims it's statistically expected when dealing with such large numbers. It is a miracle to find "shoes in my size" as Greene's analogy goes.
• 
  Lee (Rocky)

  I think I may just be too dumb for books like this. There are parts I understand and that intrigue me, but then suddenly things get too complicated for me to wrap my head around. This is mostly, but not exclusively, true of the sections that delve deeply into math (though the author, thankfully, relegates a lot of the math to notes), but there are plenty of non-math sections that were equally incomprehensible to me. Some of the notes are useful for clarification, but others just muddy the waters more. The last two chapters, where he delves into simulation theories, and then concludes by more comprehensibly summarizes the entirety of the book were the most readable and enjoyable to me. I'm glad I read it but am not sure I learned as much as I should have.
• 
  Koray Sarıdoğan

  Kitaptan, havaalanında uçuş beklerken karşımda oturan mühendis tipli bir gencin elinde görerek haberdar oldum. İyi ki de olmuşum.
  
  Bugünün satış pazarlama modalarından biri haline gelen "kuantum, kozmos, paralel evrenler" gibi konularda az çok bilgisi olanların kolaylıkla anlayabileceği, dikkatli bir okumayla tüm bu konular için temel ve orta düzeyde altyapı sahibi olabileceğiniz, Brian Greene'nin rahat, anlaşılır ifadesi sayesinde modern fiziğin güncel bilgilerine erişebileceğiniz bir kaynak.