Archive for the ‘Quantum Gravity’ Category

The sooner, the better

April 12, 2006

Rovelli’s “Quantum Gravity” arrived today (well, yesterday actually), sooner than i expected. And, albeit my plans were to finish Penrose (see reviews, specially this one) before starting this one, i’ve just read the first chapter and am hooked. This first chapter is an overview of Loop Quantum Gravity, a roadmap of what’s in store in the rest of the book. Basically, it’s an extended version of the essay i mentioned in my previous post, and is, i think, quite readable by non-specialists. The good news is that a draft version of the book is available online, so you don’t need to spend money to read it. I’ll try to keep (the two of) you posted on my readings.

Skimming the first chapter’s bibliography, i’ve remembered a couple of ‘popular-science’ books on QG i own: “Three Roads to Quantum Gravity” (Lee Smolin) and “Physics Meets Philosophy at the Planck Scale: Contemporary Theories in Quantum Gravity”. I read them 3 or 4 years ago, and they may be a good companion while commuting to work, and an alternative for those of you with a moderate interest in these matters.

If you have a bit of mathematical background, this introductory article is also a good way to get acquainted with the basics of LQG and one of its central results, the quantization of space. Have fun!

Rovelli on quantum gravity

April 11, 2006

Carlo Rovelli, one of the fathers of Loop Quantum Gravity, has just published a short, non-highly-technical overview of the conceptual issues to be faced by any theory merging Quantum Field Theory and General Relativity. I found specially enlightening his observations about time in General Relativity:

In general relativity, when we describe the dynamics of the gravitational field (not to be confused with the dynamics of matter in a given gravitational field), there is no external time variable that can play the role of observable independent evolution variable. The field equations are written in terms of an evolution parameter, which is the time coordinate x0, but this coordinate, does not correspond to anything directly observable. The proper time τ along spacetime trajectories cannot be used as an independent variable either, as τ is a complicated non-local function of the gravitational field itself. Therefore, properly speaking, GR does not admit a description as a system evolving in terms of an observable time variable. This does not mean that GR lacks predictivity. Simply put, what GR predicts are relations between (partial) observables, which in general cannot be represented as the evolution of dependent variables on a preferred independent time variable.

A deep point, easily overlooked even by experts in General Relativity.

Carlo’s article also contains an interesting bibliography for further study. I found there his A dialog on quantum gravity, a fun comparison of our two main contenders for a merger of quantum mechanics and GR. Although it is probably too jargon laden for casual readers (and, in my opinion, gives a totally biased view of the subject), the final remarks in this dialog prettily summarise a position which i tend to favour:

I think that string theory is a wonderful theory. I have a tremendous admiration for the people that have been able to build it. Still, a theory can be awesome, and physically wrong. The history of science is full of beautiful ideas that turned out to be wrong. The awe for the math should not blind us. In spite of the tremendous mental power of the people working in it, in spite of the string revolutions and the excitement and the hype, years go by and the theory isn’t delivering physics. All the key problems remain wide open. The connection with reality becomes more and more remote. All physical predictions derived from the theory have been contradicted by the experiments. I don’t think that the old claim that string theory is such a successful quantum theory of gravity holds anymore. Today, if too many theoreticians do strings, there is the very concrete risk that all this tremendous mental power, the intelligence of a generation, is wasted following a beautiful but empty fantasy. There are alternatives, and these must be taken seriously. Loop gravity is pursued by a far smaller crowd; has problems as well, as you pointed out, but is succeeding in places where strings couldn’t get, and is closer to reality. And if you think at the quantum excitations bulding up physical space, you truly see quantum mechanics and general relativity talking to one another. And is beautiful. I have an immense respect for string theorists, but I think it is time to explore something else. Don’t you think, to say the least, that both theories are worthwhile exploring?

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Quantum Gravity

April 6, 2006

My research has always trodden the classical realms of General Relativity. As a result, my knowledge of advanced quantum field theories is shaky to say the least. Not that i wasn’t interested in foundational questions or in the quest for unification, or that i didn’t read something about those issues. Now i’d like to learn a bit more.

I’ve always found the fixed-background approach taken by QFT and (to my limited knowledge) String theory at odds with General Relativity’s spirit. In my opinion, gravitation is not just another force to be quantized, but rather a description of the background were other forces live in geometrical terms. If anything is to be quantized to obtain quantum gravity, it’s geometry. And since we seem to live in a world where every relevant quantity is discrete, the idea of a granular structure of space-time itself is, to me, all but natural. Although i read (without really understanding) that String theory predicts a quantized, relational space-time, the repeated affirmations of people like Witten about our not yet (?) understanding M-theory, together with the dismal multiverse speculations i’ve read recently (Lee Smolin explains exactly how i feel about it), have turned my attention, again, to Loop Quantum Gravity as an appealing subject of study. These days, i’ve been skimming several introductory and review articles, including the delicious Quantum Gravity Seminar by John Baez, and Smolin’s PI Course. The latter will be specially useful when my copy of Rovelli’s Quantum Gravity arrives in a couple of weeks, which will be about time to start studying LQG seriously, most probably guided by Christine Dantas’ excellent reading list on the subject.

Roger PenroseIn the meantime, i plan to end Roger Penrose’s The Road to Reality, which i’m sure i don’t need to review for you. In my case, this amazing book is being an excellent way of reviewing and relearning a lot of stuff i was supposed to know from the old times, and then discovering a lot of new things. I respect Penrose opinions immensely, and after seeing these recent lectures (half-way the page), i’m specially curious about his critiques of both String theory and Loop Quantum Gravity.

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