Thanks John for the guidance. Time – fair cop – I realize what i meant was that the human concept of time is an illusion, or as you say naive.

]]>I’ll have a look. Young folks take note: the original diagrams were ASCII art.

]]>Philip wrote:

But there seems to be nothing on the topics which he works on between such popular works and really technical stuff.

Weeks 1–50 of *This Week’s Finds* has a lot about quantum gravity that fits into that gap. So do later weeks. Even though it’s old, progress in this subject is so slow that this material is still useful. The ‘problem of time’ discussed by Isham remains a problem, for example.

For starters I recommend Wald’s *General Relativity*, a classic textbook on that subject. It explains diffeomorphisms in Chapter 2. After that, for quantum gravity, I’d recommend Ashtekar’s *Lectures on Nonperturbative Canonical Gravity*, which explains the problems with quantizing gravity in a careful way. You don’t need to buy into his proposed solution—loop quantum gravity—to find this material useful!

Time is not a “human illusion”, nor is the fact that a watch on top of a tower will drift out of synch with one at the bottom of the tower. Time is just different than our naive common sense suggests.

]]>I’ve read many of Rovelli’s popular books. But there seems to be nothing on the topics which he works on between such popular works and really technical stuff. I like to try to understand things before getting into the mathematics, but that is difficult in this area.

]]>John, Thanks very much for guidance. I see Isham’s doco references the work of Rovelli, which is ironic as i have read his book “Reality is not what it seems” and am about to get “The order of time”. For quite a while now I have been thinking that time is a human illusion brought on by our unconscious observance of increasing entropy. Which goes a long way to explaining why such a measurement varies depending on your relative motion or on what sort of gravitational field you are in.

Lastly could you recommend any books that would start to get me up to speed mathematically with the approach that people like Rovelli takes please (diffeomorphisms etc.). I am familiar with the maths in G.R. if that gives you an idea of where to start me off.

Thanks,

]]>General relativity is a theory of spacetime, but there are many different ways to put coordinates on spacetime, and thus no god-given “time” coordinate.

This is a one-sentence summary of the “problem of time”. Chris Isham wrote a nice 125-page review article summarizing work on this problem, and you might enjoy the Preliminary Remarks. You can read a lot more about it in weeks 1–50 if you just search for “Isham” and (separately) “problem of time”.

]]>I want to ask a question of you re the following article i found in the ”50 weeks doco” please -see particular bit beow. (I am new to your site – So Apologies if i am posting this in the wrong place)

Week 6

February 20, 1993

1) Alexander Vilenkin, “Quantum cosmology”, talk given at Texas/Pascos 1992 at

Berkeley, available as gr-qc/9302016.

……edited out …….Let’s get warmed up. . . .

Quantizing gravity is mighty hard. For one thing, there’s the “problem of time”—the lack of a distinguished time parameter in classical general relativity means that the usual recipe for quantizing a dynamical system — “represent time evolution by the unitary operators exp(iHt) on the Hilbert space of states, where t is the time and H, the Hamiltonian, is a self-adjoint operator” — breaks down!

…..could you please explain the “lack of a distinguished time parameter” comment as I understood that GR is based on the 4 dimensions of space-time (although Kaluza extended it to 5 and out popped Maxwell’s Equations of electromagnetism as well)

Thank you,

D Ramsay

Yes, I do have This Week’s Finds on my website. Your point?

]]>January 19, 1993

This Week’s Finds in Mathematical Physics (Week 1) ]]>