Freidel talk on proposed new physical principle (relative locality)

[marcus]

Slides PDF available now at the ILQGS (International Lqg Seminar) http://relativity.phys.lsu.edu/ilqgs/
for Laurent Freidel's talk
on The principle of relative locality
http://relativity.phys.lsu.edu/ilqgs/freidel030111.pdf

Audio files are not available as yet. The talk was scheduled for Tuesday 1 March.
If things went as planned, audio will be posted online within a day or two.

 

[Fra]

I almost forgot about this thread since the last post.

Don't remember if I wrote this already but it seems to me that while there are irresistable ingredient in this, I have the some objection as I have to rovelli's RQM.

Is the the spacetime manifold directly measured by an inside observer? or is it just infered, in an observer dependent way? I think it's hard to disagree with Freidel here. I think they are right, but I think more right than they seem to think themselves.

But the very same argument applies to his phase-space, does it not? This is almost exactly analogous to when Rovelli simply assumes that observer-observer communication obey the known QM structure? Ie. Rovelli assumes that the observer relative inferences does fit together in a bigger invariant structure - this may be neat, but it seems unwarranted and unphysical, and in particular it does not seem to be an inference. How come?

This is what is incoherent about this.

Seriously, does anything this this objection is unfair or irrelevant, then can you explain why.

Note: What I am trying to say here is not that their argument is wrong, it's just that just like with rovelli, they aren't radical enough. If their general argument is correct, I simply don't see why it's applied only once, and in a particular way that looks like it's constructed? Again, no too unlike rovelli, who has constructed an interpretation to suit the LQG picture (or so is my impression).

Since some of the arguments are so good, it's annoying that it seems to not go all the way. Why? I don't get it. It's almost like they don't quite take the inference argument serious themselves??

/Fredrik

 

[marcus]

The audio for Freidel's talk is now available.
http://relativity.phys.lsu.edu/ilqgs/

It is a good talk. The ILQG audience was smaller than usual (many of the regulars were at the QG school) and only two people (Alex Corichi was the first one) asked questions at the end. For me the talk added a lot to the paper that we saw, making the ideas of Rel. Loc. easier to understand.

At this point, Fra, it would seem that the approach is phenomenological. They have a plausible approximate model of curved-momentum-space physics with hbar and NewtonG both zero.

and the squareroot of their ratio (ie. Planck mass) is non-zero which is what makes momentumspace curved. So then they have many predictions from the model, that one can test.

Indeed the predictions seem to me strange and outlandish---so that the Rel. Loc. model would seem not only falsifiable but quite possibly wrong! But I don't want to pre-judge. The problem now is not to make the theory better but to do the experiments to see if momentum space is actually curved or, as we used to assume, flat.

 

[Fra]

The problem now is not to make the theory better but to do the experiments to see if momentum space is actually curved or, as we used to assume, flat.

That is a reasonabler way of seeing it for those betting on the specific implementation of the theory. ie. it's plausible enough to motivate invest in experimental feedback before revising the idea more. Or put differently: It's investing in putting the theory to test; has higher odds of gain than investing in polishing it further before putting it to test.

But since I'm not personally going to do any experiments or investments, I only judge the plausability of the approach and reasoning.

They have a plausible approximate model of curved-momentum-space physics with hbar and NewtonG both zero.

I think this is what I tried to question.

I mean, how plausible is it really? Their original spirit is really plausible, but I don't find it plausible how they twist it in the end. So I suppose my personal opinon is that it's better to invest in developing the idea more (answering to some of the critiques I suggested) before trying to kill it. I think it won't live long enough. If we can smell this righ now, what is the rationale in ignoring that?

/Fredrik

 

[atyy]

http://pirsa.org/11020116

 

[John86]

This new Freidel paper (lLocal Relativity) is just posted yesterday

Gamma ray burst delay times probe the geometry of momentum space
Authors: Laurent Freidel, Lee Smolin
(Submitted on 29 Mar 2011)

Abstract: We study the application of the recently proposed framework of relative locality to the problem of energy dependent delays of arrival times of photons that are produced simultaneously in distant events such as gamma ray bursts. Within this framework, possible modifications of special relativity are coded in the geometry of momentum space. The metric of momentum space codes modifications in the energy momentum relation, while the connection on momentum space describes possible non-linear modifications in the laws of conservation of energy and momentum. In this paper, we study effects of first order in the inverse Planck scale, which are coded in the torsion and non-metricity of momentum space. We find that time delays of order Distance * Energies/m_p are coded in the non-metricity of momentum space. Current experimental bounds on such time delays hence bound the components of this tensor of order 1/m_p. We also find a new effect, whereby photons from distant sources can appear to arrive from angles slightly off the direction to the sources, which we call gravitational lensing. This is found to be coded into the torsion of momentum space.

 

[John86]

I really enjoyed this paper by Freidel and Smolin. But not being a physicist really makes you feel running into conceptual walls. Can anybody help tme to make any sense out of the concept of "Phase Space" that is often mentioned in this new paper ?...

Wiki really didn,t help me in a positive way !..

 

[marcus]

I really enjoyed this paper by Freidel and Smolin. But not being a physicist really makes you feel running into conceptual walls. Can anybody help tme to make any sense out of the concept of "Phase Space" that is often mentioned in this new paper ?...

Wiki really didn,t help me in a positive way !..

This really should be answered, and we have company coming (I'm already an hour late)
Someone else should reply.

Phase space is a classical idea, probably 19th century.

It's simple, for one particle you take the position and momentum, conceptually R³xR³. For N particles you take an N-fold cartesian product of that. So you are keeping track of all the particles position and momentum, in the observer's immedciate neighborhood.

The nontrivial angle to this is the possibility that MOMENTUMSPACE ACTS LIKE IT IS CURVED instead of flat. That gives a whole new interest to the idea of phase space. Momenta don;t add in a simple vectorspace way any more.

the tendency in Freidel's papers is to focus on just one copy of momentum space and just one particle, because that is where the interest is. And so it all boils down to that. You forget about N particles and N-fold cartesian products for the time being.

If somebody doesn't like this simplified answer, please give a better!

Did you try Wippy?
http://en.wikipedia.org/wiki/Phase_space

 

[atyy]

When the dynamics can be cast in Hamiltonian form, phase space is the space of the fundamental degrees of freedom. Ashtekar's variables were a change of variables in general relativity so that the theory had Hamiltonian form. The ADM formulation of general relativity is yet another Hamiltonian formulation of GR.

http://www.scholarpedia.org/article/Hamiltonian_systems
http://arxiv.org/abs/1007.0402
http://grwiki.physics.ncsu.edu/wiki/Hamiltonian_(ADM)

I guess the rough idea of relative locality is that the phase space variables are usually [position, momentum]. But here the variables are [x,momentum] where x is not position. Then position is reconstructed from [x,momentum]. The reason is that some LQG related formalisms give [x,momentum] with momentum curved. But Hossenfelder showed that such a theory is already likely falsified if x is position. So the question was, can we interpret x as something else, and still recover spacetime, at least locally.

 

[atyy]

The last reference in the relative locality paper http://arxiv.org/abs/1101.0931 is, rather amusingly, http://arxiv.org/abs/1007.2200 . LQG's destiny is condensed matter, just like string theory

 

[John86]

Thank you both

 

[John86]

A new paper by Smolin

http://arxiv.org/abs/1104.2822

A real ensemble interpretation of quantum mechanics
Lee Smolin
(Submitted on 14 Apr 2011)
A new ensemble interpretation of quantum mechanics is proposed according to which the ensemble associated to a quantum state really exists: it is the ensemble of all the systems in the same quantum state in the universe. Individual systems within the ensemble have microscopic states, described by beables. The probabilities of quantum theory turn out to be just ordinary relative frequencies probabilities in these ensembles. Laws for the evolution of the beables of individual systems are given such that their ensemble relative frequencies evolve in a way that reproduces the predictions of quantum mechanics. These laws are highly non-local and involve a new kind of interaction between the members of an ensemble that define a quantum state. These include a stochastic process by which individual systems copy the beables of other systems in the ensembles of which they are a member. The probabilities for these copy processes do not depend on where the systems are in space, but do depend on the distribution of beables in the ensemble. Macroscopic systems then are distinguished by being large and complex enough that they have no copies in the universe. They then cannot evolve by the copy law, and hence do not evolve stochastically according to quantum dynamics. This implies novel departures from quantum mechanics for systems in quantum states that can be expected to have few copies in the universe. At the same time, we are able to argue that the centre of masses of large macroscopic systems do satisfy Newton's laws.

 

[marcus]

Jeff Morton (Baez PhD 2007) has a blog. He recently commented on the Principle of Relative Locality, and a presentation of it by Laurent Freidel.
http://theoreticalatlas.wordpress.com/2011/03/

I found this helpful. Morton has acquired the knack of explaining things in a relaxed gradual way without unnecessary technicality. (Math jargon used selectively and intelligently---avoided when some other description will do as well.)
Maybe he is not as good at this as his thesis advisor Baez, but I didn't see that Baez covered Freidel's recent papers and talk about Rel Loc. So I'll take Morton's discussion of it.

Morton will be participating at the mid-June Zurich conference "Quantum theory and Gravitation"
http://www.conferences.itp.phys.ethz.ch/doku.php?id=qg11:participants
He may have some blog goodies on that, so I'll try to remember to look.

Here's a self-introduction page at Morton's site:
http://www.theoreticalatlas.net/

 

[atyy]

marcus just posted https://www.physicsforums.com/showpost.php?p=3320858&postcount=1486 in his bibliography.

Now, we know Freidel and company are interested in relative locality because of the non-commutative fields which seem to come out of some spin foams. But doesn't the philosophy behind a different (?) sort of non-commutativity seem very similar to to relative locality's? Compare http://arxiv.org/abs/1008.0985 and http://arxiv.org/abs/1101.0931

However, relative locality is not fundamental (assuming it exists), since Freidel et al explain that it is only a certain limit, and the underlying theory is presumably string theory or whatever you favourite one is. OTOH, I guess Chamseddine and Connes are thinking that the spectral action is fundamental (assuming it can be quantized).

 

[marcus]

...
However, relative locality is not fundamental...

?
This sounds strange.
I don't recall Freidel et al ever saying that the Rel Loc principle is "not fundamental".

You can implement the principle in a flat space limit in order to have something to test. There would be various possible theories satisfying/implementing it. But that doesn't prevent Rel Loc being proposed as a fundamental physical principle.

I suspect, in fact, that Rel Loc IS being proposed as a fundamental physical principle. That would make sense AFAICS. I can't say definitely--haven't read all that much about it.

 

[atyy]

?
This sounds strange.
I don't recall Freidel et al ever saying that the Rel Loc principle is "not fundamental".

You can implement the principle in a flat space limit in order to have something to test. There would be various possible theories satisfying/implementing it. But that doesn't prevent Rel Loc being proposed as a fundamental physical principle.

I suspect, in fact, that Rel Loc IS being proposed as a fundamental physical principle. That would make sense AFAICS. I can't say definitely--haven't read all that much about it.

Last paragraph of p1 and the first paragraph of p2. http://arxiv.org/abs/1101.0931

 

[marcus]

I may not be remembering Rel Loc correctly, it came out some weeks back and I've been concerned with other things in the meanwhile.
As I remember, it says there is no space time shared by all observers.

A given observer can construct for himself a model of space time and say what worldlines crossed and what events occurred. But a distant observer may construct a different model that will not coincide-- with different events.

The curvature of momentum space that this incompatibility depends on is testable. It can be falsified. It might be a good thing if it were. The Rel Loc idea is quite radical and disturbing. It essentially says that no collective space time exists, that we can all share.

I think Rel Loc would be bad news for AdS/CFT and everything else we are used to. No space time. No bulk. No boundary. Many physical theories would be disrupted.

In GR different observers have different time and see simultaneity differently, but at least they all share the same space time----they just slice it up differently into spatial slices. The Rel Loc principle goes beyond that. They no longer share the same model of space time. Their spacetimes are inconsistent. Rel Loc is worse than GR---in the sense of unpalatable and disruptive. IMHO.

Maybe there is some more general phase space that all observers can share, that we can say exists, and that can somehow be "sliced" differently by different observers in order to realize their separate spacetime models. Maybe you can find some mathematical construct of reality which is shared in common. I do not recall seeing such a thing when I examined the papers.

 

[marcus]

Last paragraph of p1 and the first paragraph of p2. http://arxiv.org/abs/1101.0931

I know that! That is my point. They implement the principle in an approximation, as they say. That gives a testable realization. I think I talked about that earlier.

Surely the Rel Loc principle is not confined to that approximate picture!

 

[atyy]

I know that! That is my point. They implement the principle in an approximation, as they say. That gives a testable realization. I think I talked about that earlier.

Surely the Rel Loc principle is not confined to that approximate picture!

It should be. The more fundamental picture is the spin foam picture, where we may not have locality, or smooth geometry even in momentum space.

Or have we given up LQG?

 

[John86]

"new scientist" talks about phase space proposal and principal of relative locality !..

http://www.newscientist.com/article/mg21128241.700-beyond-spacetime-welcome-to-phase-space.html