This is the book that initiated my current interest in 21st century physics concepts. I was initially introduced to it during an Elderhostel vacation. A very well written presentation of a very complex subject.
Greene explains relativity so I get it, quantum physics so I follow it, and string theory so that at least I get what it explains. For me, that's pretty good.
There are a few things in the book, layman though I am, that I know are already a little dated - he keeps referring to the age of the universe as 15 billion years, rather than 13.6 billion. Also, he talks about how some big expansion in the moments just after the big bang would explain how the universe is as big as it is, while gravity slows it down. The problem there is that a couple of years ago scientists realized the expansion of the universe is speeding up.
Then again, given that the book was written in 2000, that says a lot more about the rate of discoveries/new models in cosmological physics than it does about the book.
One thing that bugged me, that I suppose is my dilettante's misunderstanding of it all, is the models that seemed to be pulled out of somebody's butt, that because they could make numbers work in formulae, suddenly are 'proved.' Why strings? I understand (now) the problem in trying to reconcile Einsteinian physics with quantum havoc, but why strings? Why not teeny little clouds, or starfish-shaped thingies, or anything else?
Another thing - he consistently talked about a bunch of extra spatial dimensions beyond the 3 we're familiar with (that make the numbers work), but he talked about those dimensions being really small and curled up (small as in Planck length which is about (squeezes fingers together) thiiissss big). Isn't it more proper to say that our extent into that dimension is curled up and tiny? If I imagine a little picture of a bunny rabbit in a Euclidean plane, the two dimensions of the Euclidean plane aren't small and bunny-like, only my drawing is. If the drawing is on a very thin piece of paper, the dimension of height isn't tiny, only my piece of paper's extent into that dimension is.
But that made me think, well, if the universe is all there is, there's no 'ether' in which to sit and observe our universe's tiny forays into the 9th dimension, maybe the 9th dimension is tiny - it's meaningless to talk about anything else. Then in the last chapter he talks about multiverses, and maybe there are universes that expand out in the dimensions we don't but are tiny in ours, so then I thought I was right again. Plus I got the munchies real bad - I totally could have eaten a Dorito pizza.
Tangentially, this book made me spend some time trying to visualize a 4th spatial dimension. It, like many layman discussions of physics and universal models, uses the weasel-worded assertion that 'it's nearly impossible to visualize a 4th dimension at a right angle to each of the 3 we're familiar with.' Look, is it possible or not? If it is, I want to see it in my mind. Some guy on a message board says he can visualize a tesseract, so I have to believe him or not. In the meantime, I'm going to keep looking around different corners to see if any of them go off into the 4th dimension.
Einstein's special and general relativity, quantum physics with quarks, leptons, and the 4 forces are all here. The major focus however is on string theory itself and Calabi-Yau spaces.
Greene does an excellent job (and keeps out math for the most part) but the subject matter is inherently complex. Each reader will take away some new understandings, but to fully appreciate this book, one would need considerable science background.