No he does not say that and why bug him?

[marcus]

I think it is out of line to pester Carlo Rovelli with one's own specious arguments and put words in his mouth.

Carlo has not "conceded" or "admitted" anything remotely like "LQG is wrong" as one of our members claims.

The poster has an argument, to which I doubt Carlo would subscribe and which I believe is groundless, for why one should say "LQG is wrong". The argument depends on LQG being perceived as extremely rigid---dependent on one unique version of General Relativity. I know of no indication that LQG is, in fact, unable to accommodate higher-order terms in the Einstein-Hilbert action, or other slight modifications in General Relativity noticeable only at very high energies or small scale.

Indeed LQG is clearly not inflexible since Loop researchers already modify the dynamics. This is most evident in LQG cosmology where sufficient progress has been made recently (more than in the full theory) that there is well-behaved dynamics to play around with and try variations on.

Anyway the poster's argument seems to be that LQG is wrong because it can work only with one fixed version of Gen Rel, and that because one of the concepts of LQG (spin networks, a way of constructing a basis of a certain Hilbert space) is meaningless save when GR is JUST SO. This sounds quite far-fetched. In any case it is not what Rovelli says.
So my reaction is let's not put words in Rovelli's mouth

Theory wrong or theory right is not my concern here. If you want to make a big deal of it Einstein's GR was "wrong" in the sense that if you push it to far you run up against singularities where it won't compute. Maybe LQG will have its own different but analogous limitations. But since it is a flexible and adaptible theory still under development, I don't think anyone today is smart enough to say what the limits of applicability will be.

Instead, what I object to is two things:
1. Rovelli's time is valuable, why bug him with chaff?
2. Why misrepresent him as saying what he didnt say (not even "in effect"
as the latest version puts it)? That doesn't seem right.

 

[marcus]

So did Rovelli reply to your email, jeff?

I really think it is out of line to impute some position to a scholar which he has not taken. You have so far not presented any quote from Rovelli that supports your claim about what he says. Instead, you have quoted from Rovelli something which does not at all imply what you claim he says---and then given your own reasonings.

You wrote him email, as I understand it, to see what he actually says bearing on this. Did he get back to you? I don't approve of bugging him, but if he took the time to reply then it would be straightforward of you to share it with us.

Rovelli in effect concedes that lqg is wrong, but still worthwhile

---for reference, here is Jeff's post starting the second thread on this---
Let me begin by saying that I won't be the first to post inappropriate responses in this second attempt to treat the above subject.

I previously posted the following:

In the forward to rovelli's new book "Quantum Gravity", james bjorken states quite plainly that the effective field theory approach to quantum gravity correctly taught us that GR must be viewed as just an effective field theory, and in fact this is the universally shared view.


The problem for lqg is that the central construct in lqg, spin networks, only makes sense if GR is in fact exactly correct. I don't see why it would make sense for an author to allow a forward to be written by someone else, that contradicts the basic premise of the book.

In fact, rovelli defends lqg by stating...

"But the modification of the notions of space and time has to do with the diffeomorphism invariance and the background independence of the action, not with it's specific form."

In other words, it is inaccurate to view lqg as a genuine candidate quantum gravity theory, and thus as a rival of string theory. Rather, lqg is just a toy theory serving as a laboratory to explore a small number of fundamental issues in quantum gravity.

I'll respond on two levels. One is on the specific physics of lqg, and the other, on the plausibility of alternative interpretations of these statements.

For example, someone may have a physics reason for not believing that lqg requires that GR be treated as if the einstein-hilbert action remains uncorrected at arbitrarily high energies.

Another example would be that someone may believe that it's plausible that rovelli does in fact believe that GR is exactly correct. In this case the best thing to do is just to email him, which I've done and am waiting for his response. Of course, nobody is stopping anyone from doing the same thing.
---endquote---

 

[marcus]

Jeff: "I'll post his response the moment I receive it"

I've emailed rovelli the following:

Hi Carlo,

Congratulations on your book, which is better bound (and perhaps even better smelling, I think) than other books in that series.

Now, my understanding is that the construction of lqg requires that General Relativity must be treated as if it's exactly correct. But you write that even if it's not, it still makes sense to pursue lqg for the reasons you give. I appreciate this, but I'd still like to know whether you agree with Bjorken's statement in the forward to the effect that General Relativity must in fact necessarily be only a low energy effective theory, and if you don't agree, why?

Thanks,
jeff

I'll post his response the moment I receive it.

That was post #11 in Jeff's first thread about this, and was dated December 2, 7AM. So I'm hoping to see the response soon, or whenever you receive it.

If you don't mind, I will offer some criticism of the email---simply my opinion. I think the main premise is wrong.
I think that what you say is your understanding is simply not true:

"...my understanding is that the construction of lqg requires that General Relativity must be treated as if it's exactly correct."

That is just plain wrong. In order to construct LQG one does not have to assume that GR is exactly true to nature.
=====
You say at one place that spin networks are not meaningful unless one assumes that GR is exactly correct. That assertion appears to be groundless. One defines the networks----labeled graphs---without any reference to GR correctness. Their meaning is as well-defined functions on the space of configurations, which serve as a basis for the quantum states of the gravitational field.
=====
Another thing that concerns me is the lack of specificity with which your email paraphrases James Bjorken's Foreword to Rovelli's book.

"...with Bjorken's statement in the forward to the effect that General Relativity must in fact necessarily be only a low energy effective theory,..."

Bjorken was explicitly talking about one specific way in which GR can be an effective theory---to be approapriately corrected. What he actually said matters, I believe.

Bjorken: "...the Einstein-Hilbert action is no doubt only the first term in an infinite series constructed out of higher powers of the curvature tensor..."

The picture this evokes is one where GR is retained in its existing form and some higher-order correction terms are added. Bjorken does not suggest that the basic form of the theory (its background independence for instance) is invalid. So what this envisions preserving in classical GR, is precisely those basic features carried over into LQG.

The difference in nuance is important. Your paraphrase conveys the notion of replacing GR with something quite different (which I gather could appeal to the average Joe String-Theorist), while what Bjorken actually said confirms the notion that GR is on the right track and may in future be refined by adding some second or third order correction terms