Is Information Theory the Key to Quantum Gravity? A Rant from an Angry Physicist

[marcus]

this new blog (that Peter Woit flagged) is potentially very interesting and
has an uninhibited style

http://angryphysicist.wordpress.com/about/
==quote==

Rantings of an Angry Physicist

An angry Physicist ranting about Quantum Gravity, foundational Quantum Mechanics, the gross incompotency of Economists, and other fun issues!
About

I’m a undergrad at UC Davis, I have been studying physics for two or three years. I started studying from the library at CalTech with a retired rocket scientist and have been fascinated by theoretical physics since. I have a few more years to go until they kick me out with a degree or two; next year I am going to be taking 13 to 15 math courses, so I don’t know how much I can keep up with this blog then. But for now, I read everything I get my hands on and try to read up on every technical paper I can (truth be told: Dr. Carlip is very intimidating with his demigod-like powers to mystically cite sources like one would recite one’s phone number! So that is something to aspire to!) and as a bad habit from my earlier years I tend to think critically (I’m a baaaad man!).

Consequently I reject String theory as it stands now (yes, I am a heathen heretic whatever). Hmmm…I think that if we had strings that are one Planck length long, all we could really know about these strings are their relative position perhaps desribable by a graph? The various graph-states would be superpositioned and we easily recover quantum mechanics via a sum over histories method, etc. But that’s not what bothers me about string theory the most. What bothers me the most is that they propose the existence of the graviton, which I see as cartoonish. I’m certain that String theorists will have a host of insults to hurl at me about this, but it doesn’t change the fact that it makes no sense to have a graviton. Saying “Well quantum field theory demands the existence of a mediating boson” is no better than saying “Well, the bible demands that evolution be false”; it’s a simple appeal to authority.[/color]

Anyways, I thought about being an economist a while back (since fifth grade!). I must confess that I have a soft spot in my heart for economics…well, criticisms of Neoclassical and marginalist economics now. I’m an “old school” Neo-Ricardian type of fellow. I might actually post some stuff criticizing Neoclassical economics later on, keep your eyes peeled for it!

Back to my history, uh well, that’s it I guess for now.
==endquote==

 

[Ratzinger]

This is great indeed!

He is really an undergraduate student?

 

[Angryphysicist]

This is great indeed!

He is really an undergraduate student?

Yep, I am a first year freshman at UC Davis; I am taking physics 250 (special topics taught by Steve Carlip - as you imagine by my blog and who the professor is, it is on quantum gravity), physics 9D ("modern physics" - basic quantum theory and relativity), Linear Algebra, and English.

 

[marcus]

High A.P,

Davis could be becoming a strong place, or even more of one.
Did you see Derek Wise's paper?
He should be part of Carlip's (and your) group soon:

http://arxiv.org/abs/gr-qc/0611154
MacDowell-Mansouri Gravity and Cartan Geometry.

 

[Angryphysicist]

Davis could be becoming a strong place, or even more of one.

Perhaps...Dr. Carlip's being at Davis was the reason I chose to go to UCD.

Did you see Derek Wise's paper?
He should be part of Carlip's (and your) group soon:

http://arxiv.org/abs/gr-qc/0611154
MacDowell-Mansouri Gravity and Cartan Geometry.

If I am not mistaken, he is a rather pleasant chap with a British accent.

If that's him, I haven't spoken to him very much. I was not aware of his paper before, it looks interesting. Thanks for the link :)

A number of physics grad students here (except for a handful that I've met) don't believe in a lot of the stuff they write though. Very disappointing to say the least; perhaps, hopefully, he'll be one of the handful exceptions.

 

[ensabah6]

this new blog (that Peter Woit flagged) is potentially very interesting and
has an uninhibited style

http://angryphysicist.wordpress.com/about/
==quote==

Rantings of an Angry Physicist

An angry Physicist ranting about Quantum Gravity, foundational Quantum Mechanics, the gross incompotency of Economists, and other fun issues!
About

I’m a undergrad at UC Davis, I have been studying physics for two or three years. I started studying from the library at CalTech with a retired rocket scientist and have been fascinated by theoretical physics since. I have a few more years to go until they kick me out with a degree or two; next year I am going to be taking 13 to 15 math courses, so I don’t know how much I can keep up with this blog then. But for now, I read everything I get my hands on and try to read up on every technical paper I can (truth be told: Dr. Carlip is very intimidating with his demigod-like powers to mystically cite sources like one would recite one’s phone number! So that is something to aspire to!) and as a bad habit from my earlier years I tend to think critically (I’m a baaaad man!).

Consequently I reject String theory as it stands now (yes, I am a heathen heretic whatever). Hmmm…I think that if we had strings that are one Planck length long, all we could really know about these strings are their relative position perhaps desribable by a graph? The various graph-states would be superpositioned and we easily recover quantum mechanics via a sum over histories method, etc. But that’s not what bothers me about string theory the most. What bothers me the most is that they propose the existence of the graviton, which I see as cartoonish. I’m certain that String theorists will have a host of insults to hurl at me about this, but it doesn’t change the fact that it makes no sense to have a graviton. Saying “Well quantum field theory demands the existence of a mediating boson” is no better than saying “Well, the bible demands that evolution be false”; it’s a simple appeal to authority.[/color]

Anyways, I thought about being an economist a while back (since fifth grade!). I must confess that I have a soft spot in my heart for economics…well, criticisms of Neoclassical and marginalist economics now. I’m an “old school” Neo-Ricardian type of fellow. I might actually post some stuff criticizing Neoclassical economics later on, keep your eyes peeled for it!

Back to my history, uh well, that’s it I guess for now.
==endquote==

So if you reject string theory, are you a LQG fanboy then? Or a fan of some other approach like CDT, noncommuntive geometry, twistor theory, supergravity, induced gravity etc.

Personally I'll suspend judgment on string theory until LHC goes live and hopefully finds Higgs bosons and SUSY-partners and possibly evidence of higher dimensions as a result of microblack hole production.

If LHC does not find Susy-partners, and no other evidence for strings, such as cosmic strings, is forthcoming, then I will be intensely skeptical of string theory, and in-the-closet LQG'er :)

BTW which approach to QG does Steve Carlip favor? I recall he, John Baez, and Lubos Motl debating it out over at Sci.physics.research

 

[Haelfix]

Carlip is known more as a GR guru. He wrote the book on 2+1 gravity, has done some LQG work and done some work on BHs. Hes open to new ideas, and tackled various problems of QG that are sort of pushed to the wayside in traditional mainstream research but important nonetheless.

 

[ensabah6]

Carlip is known more as a GR guru. He wrote the book on 2+1 gravity, has done some LQG work and done some work on BHs. Hes open to new ideas, and tackled various problems of QG that are sort of pushed to the wayside in traditional mainstream research but important nonetheless.

Interesting. It appears to me that String theorists feel that since their closed lowest vibration mode of a string is a graviton, and they believe it is renormalizable, that therefore, string theory is the only consistent and only candidate theory of quantum gravity.

 

[marcus]

If I am not mistaken, he is a rather pleasant chap with a British accent.

I have no idea---can't help guess. Derek will officially be in the math department at davis. Might not be around there regularly yet: his postdoc contract begins in the Fall. Here's a picture of John Baez students taken summer 2004
http://math.ucr.edu/home/baez/students.html

 

[Kea]

...since their closed lowest vibration mode of a string is a graviton...

You know, string theory isn't so bad once you get rid of the graviton and other silly things.

 

[ensabah6]

You know, string theory isn't so bad once you get rid of the graviton and other silly things.

If, as angry physicst rant says, you disbelieve in the graviton, then there's no basis for calling string theory a theory of quantum gravity.

 

[Angryphysicist]

So if you reject string theory, are you a LQG fanboy then? Or a fan of some other approach like CDT, noncommuntive geometry, twistor theory, supergravity, induced gravity etc.

I don't really favor any of the current approaches right now, to be honest. I was impressed with Loop Quantum Gravity until we got to the math behind it in Carlip's class, and then I got rather disappointed with some of the choices made (e.g. the SU(2) gauge group being used, etc.).

I'm intrigued by noncommutative geometry, but despite having read Connes book several times I must confess I still don't fully understand the math.

Supergravity I don't understand one bit.

Twistor theory actually has been appealing to me on occasions, but I do not know whether to look into it any further than I all ready have.

I've actually been entertaining the idea of general relativity and quantum theory both being approximations to some underlying theory, but I would have no clue how to approach that.

So all in all I don't really subscribe to any school of thought; as Goethe once said:
Somebody says: "Of no school I am part,
Never to living master lost my heart,
Nor any more can I be said
To have learned anything from the dead."
That statement - subject to appeal -
Means "I'm a self-made imbecile."

I have no idea---can't help guess. Derek will officially be in the math department at davis. Might not be around there regularly yet: his postdoc contract begins in the Fall. Here's a picture of John Baez students taken summer 2004
http://math.ucr.edu/home/baez/students.html

After looking at the picture, I can say that I haven't noticed him too much. He looks like a rather cool fellow though.

 

[Kea]

If ... you disbelieve in the graviton, then there's no basis for calling string theory a theory of quantum gravity.

Who did?

 

[ensabah6]

Who did?

well there's Lubos Motl :0

 

[f-h]

At least as perturbation theory gravitons should make sense.

That is: pick a semiclassical state and study thepropagation of gravitationaldisturbations with respect to it. (e.g. Rovellis work)

Angry Physicist, feel free todo it with a non compact gauge group, you are assured to become quite famous in the QG community if you succeed. In the mean time we'll have to make do with the maths we can handle, not the maths we wish we could handle, and try to do physics within this constraint. Otherwise we're not doing physics but just annoyingly whinning about.

 

[Thomas Larsson]

I was impressed with Loop Quantum Gravity until we got to the math behind it ...
having read Connes book several times ...
Supergravity ...
Twistor theory actually has been appealing to me on occasions...
general relativity and quantum theory...

I am a first year freshman.

Either you are an extreme child prodigy, or you are quite old for a first year undergraduate :!)

 

[marcus]

Either... child prodigy, or ... quite old for a first year undergraduate ...

if it were me I think I'd choose both, that is: to be a former brainy kid who went into economics first (self-educated) and made a bunch of money or anyway somehow had fun being a finance whiz, and then decided on a second career. I would be about 35 years old and have already gone thru around 5 years of sporadic physics self-education reading at the CalTech library

but it is really not worth speculating.
the main thing is it looks like a package of unusual past experience and decisions that led to a good preparation for addressing the problems in QG

they are interesting problems and call for interesting people (like yourself Thomas) to address them

which random combination of accidents prepares you does not matter as long as you can do original research that bears on the problem---ultimately what counts.

 

[josh1]

Either you are an extreme child prodigy, or you are quite old for a first year undergraduate :!)

It appears that he doesn't have a UC Davis email address either.

 

[Angryphysicist]

It appears that he doesn't have a UC Davis email address either.

My ucdavis email is the same as my gmail address, just change the "@gmail.com" to "@ucdavis.edu", I'm just overly paranoid about spam getting into my ucdavis email account as there is an extremely poor spam detector.

Go ahead and email me at my uc davis email account, I'd be delighted to talk with you.

[edit]: out of my sheer love for you, I have changed my email account here to be my ucdavis email account.

Either you are an extreme child prodigy, or you are quite old for a first year undergraduate

I'm told I look old...but not old enough to not get carded

 

[ensabah6]

My ucdavis email is the same as my gmail address, just change the "@gmail.com" to "@ucdavis.edu", I'm just overly paranoid about spam getting into my ucdavis email account as there is an extremely poor spam detector.

Go ahead and email me at my uc davis email account, I'd be delighted to talk with you.

[edit]: out of my sheer love for you, I have changed my email account here to be my ucdavis email account.

I'm told I look old...but not old enough to not get carded

Hey Anger,
Do you know that Josh1 is our angry pro-string theorists here? He aggressively promotes string theory. He's made remarks that offended Peter Woit

Not that there's anything wrong, but when LHC comes online and fails to find SUSY or higher dimensions, and maybe only a single higgs, I think that will set the tone for future string theory research, and give non-string approaches breathing room.

 

[Demystifier]

An angry Physicist ranting about ... foundational Quantum Mechanics ...

I could not found what exactly he said about foundational QM?

 

[josh1]

Hey Anger,
Do you know that Josh1 is our angry pro-string theorists here? He aggressively promotes string theory. He's made remarks that offended Peter Woit...Not that there's anything wrong...

If I’m annoyed, it’s not because of other approaches. It’s because some PF members misrepresent the amount of interest in other approaches shown by researchers in general as a way of preventing other members who don’t know any better from learning why string theory is still the theory of choice for researchers and that this is not a result of stringy people having a "bad attitude".

...but when LHC comes online and fails to find SUSY or higher dimensions, and maybe only a single higgs, I think that will set the tone for future string theory research, and give non-string approaches breathing room.

Actually, although there are strong arguments independent of string theory (in particular, the higgs naturalness argument) that if supersymmetry exists in nature, evidence for it is likely to be found at the electro-weak scale, this need not be the case for supersymmetry to exist. In fact, even string theory can be correct without supersymmetry existing. It’s just that we’ve stuck with supersymmetric solutions since the theory is more tractable mathematically there. So even if evidence for supersymmetry is not found, strings will continue to dominate until a better idea is found since all of the reasons we believe that the current alternatives are hopeless will still apply.

On the other hand, if evidence for supersymmetry is found, all other research programs will be greatly effected because it would become much harder to believe that the correct theory doesn't treat gravity along with the other forces in a way that is much more unified than in all the other approaches.

 

[ensabah6]

If I’m annoyed, it’s not because of other approaches. It’s because some PF members misrepresent the amount of interest in other approaches shown by researchers in general as a way of preventing other members who don’t know any better from learning why string theory is still the theory of choice for researchers and that this is not a result of stringy people having a "bad attitude".



Actually, although there are strong arguments independent of string theory (in particular, the higgs naturalness argument) that if supersymmetry exists in nature, evidence for it is likely to be found at the electro-weak scale, this need not be the case for supersymmetry to exist. In fact, even string theory can be correct without supersymmetry existing. It’s just that we’ve stuck with supersymmetric solutions since the theory is more tractable mathematically there. So even if evidence for supersymmetry is not found, strings will continue to dominate until a better idea is found since all of the reasons we believe that the current alternatives are hopeless will still apply.

On the other hand, if evidence for supersymmetry is found, all other research programs will be greatly effected because it would become much harder to believe that the correct theory doesn't treat gravity along with the other forces in a way that is much more unified than in all the other approaches.

I agree with you 100% if evidence of SUSY is found all other programs will dry up. In fact, I might even do graduate-level work in strings (though I'm more leaning to solid state physics) as I know the job prospects of string theorists are very good (though I might want to work for industry rather than academia).

But if LHC does not find SUSY, one argument for SUSY, that it stabilizes the electro-weak scale against radiative corrections, is undermined. If LHC does not find evidence of higher dimensions, for example, in an unexpected rate of microblackhole production, and no other observational evidence, such as cosmic strings in astronomy, is forthcoming, it's not clear to me that continuing to pursue string theory is rational. ONe argument against string theory is that it is monopolizing theoretical physics and taking away the best and brightest young students away from developing other approaches, such as non-SUSY 4D LQG, toward working on string theory problems. A same situation exist in the software industry where the vast majority of programmers are working on microsoft windows rather than say linux or mac os x. All researchers have worked on string theory, much like computer programers, so it is much better developed than competing approaches like LQG, and this fact then draws in more researchers in a feedback loop completely independant of experimental evidence.

I am well aware that SUSY could exist on any breaking scale, from energies accessible to LHC all the way to the Planck breaking scale. But usually the way science works is that would like to see positive evidence for a theory, not reasons for non-observance.

Tevatron hasn't found evidence of SUSY which puts tight bounds on it.
I'll suspend judgment until LHC goes online but my intuition tells me the world is 4D and non-supersymmetric.

 

[Angryphysicist]

I could not found what exactly he said about foundational QM?

Nothing...yet. I am a busy undergrad, mind you, I have stuff to do, people to see, women to try and date; all of these endeavors failures mind you!

Still, one has to quixotically tilt the windmills now and again

Besides, if I spent all my time writing about what I wanted to on my blog, and researching, and so forth, I would have no time for my studies, and end up not getting into a grad school. Then where would I be?

 

[LukeD]

A same situation exist in the software industry where the vast majority of programmers are working on microsoft windows rather than say linux or mac os x.

Horrible comparison by the way. Microsoft hires a very limited amount of software developers. The rest of people go to work at other companies where they are hired. Apple also hires a good deal of people to work on Mac OS X, but few of the programmers work on the kernel; the majority work on applications that go along with the OS. Few people work on Linux because few companies hire anyone to work on Linux, and the companies that do have poor control over the code because they need to open source all of it due to Linux's license. There's just not much money in developing Linux for the individual programmers because Linux itself is free, so there are a few people who get paid to work on it, but the majority just do it in their free time.

This is completely different than the state of theoretical physics where pretty much every solution is just like Linux. To get anywhere, you need to publish your work to the public domain, and you're not getting paid by the owners of the theory (whoever that would be), you're getting paid by interested parties. Since you're getting paid by interested parties, only people who work on things that people deem promising get paid. This is like the state of Linux where no one who works on "side projects" like Enlightenment gets anything at all, but a few people who are working on projects like KDE and Gnome are actually getting paid for it. Of course there are few people working on Linux who are getting paid at all, so that's not the only reason there are a lot more people working on projects like KDE and Gnome than Enlightenment. More people are currently using KDE and Gnome and view it as a stable solution even though Enlightenment may be extremely promising and "the solution" to get people to actually switch to Linux.

Anyway... didn't actually mean to rant about that for a while, sorry.

-------------

Josh: I just found it extremely interesting that you doubted the idea that he could be a first year undergrad simply because you didn't see his university email address. What benefit would someone actually have of presenting himself as a first year student if he were not? If someone were a grad student but presented himself as a first year, that would just completely destroy his credibility.
You just seem to be doubting quite literally everything that you hear from anyone here. Doubting people so entirely like that can set you back as a theoretical physicist.

 

[ensabah6]

Horrible comparison by the way. Microsoft hires a very limited amount of software developers. The rest of people go to work at other companies where they are hired. Apple also hires a good deal of people to work on Mac OS X, but few of the programmers work on the kernel; the majority work on applications that go along with the OS. Few people work on Linux because few companies hire anyone to work on Linux, and the companies that do have poor control over the code because they need to open source all of it due to Linux's license. There's just not much money in developing Linux for the individual programmers because Linux itself is free, so there are a few people who get paid to work on it, but the majority just do it in their free time.

This is completely different than the state of theoretical physics where pretty much every solution is just like Linux. To get anywhere, you need to publish your work to the public domain, and you're not getting paid by the owners of the theory (whoever that would be), you're getting paid by interested parties. Since you're getting paid by interested parties, only people who work on things that people deem promising get paid. This is like the state of Linux where no one who works on "side projects" like Enlightenment gets anything at all, but a few people who are working on projects like KDE and Gnome are actually getting paid for it. Of course there are few people working on Linux who are getting paid at all, so that's not the only reason there are a lot more people working on projects like KDE and Gnome than Enlightenment. More people are currently using KDE and Gnome and view it as a stable solution even though Enlightenment may be extremely promising and "the solution" to get people to actually switch to Linux.

Anyway... didn't actually mean to rant about that for a while, sorry.

-------------

Josh: I just found it extremely interesting that you doubted the idea that he could be a first year undergrad simply because you didn't see his university email address. What benefit would someone actually have of presenting himself as a first year student if he were not? If someone were a grad student but presented himself as a first year, that would just completely destroy his credibility.
You just seem to be doubting quite literally everything that you hear from anyone here. Doubting people so entirely like that can set you back as a theoretical physicist.

My point is that without any physical results, like SUSY-partners at Tevatron or LHC, researchers are like programmers, most programmers work on Microsoft Windows due to its dominate position and most QG researchers work on string theory due to its dominate position. Faculty hiring and H-index reinforce the status quo -- if in a gedenaken thought experiment we lived in a world where Witten, Susskind and everyone else worked on LQG rather than strings, then Witten would still have the highest H-index as his papers would be cited by other entrenched LQG'er theorists, and such a position would put pressure on young physicists to go into LQG rather than strings.

The TWP is that the string position is self-reinforcing due to economies of scale -- there's the slick marketing of string theory to the public, there are universities wanting to improve their reputation by hiring string theorists, there are string theorists who cite Witten's and fellow string theorists work, improving string theory's H-index, string theorists who decide who get hired and often that is fellow string theorists, undergrad physics majors only hearing of string theory and wanting to go into that, etc. In a gedenken thought experiment everyone was LQG in academia, then academics publishing LQG papers would be highly cited by other LQG'ers, improving those H-index, and of course, faculty hiring and positions, and even marketing to the public, would reinforce LQG dominance.

i.e Maldacena may be the most cited paper on AdS/CFT correspondence, but that does not mean nature actually respects AdS/CFT correspondence. Observational evidence is that nature is DS, not ADS. Witten's paper on supersymmetric Yang mills paper, the KKLT papers, Randall-Sundram models are all highly cited by other string theorists, and they enjoy a higher h-index as a result, but there's no emperical evidence any of this - KKLT constructions, supersymmetry, p-branes, etc., are physics.

 

[f-h]

I don't see how SUSY/no SUSY at CERN should influence the rationale or need for Background Independent approaches.
We're trying to generalize the class of QFTs we know how to build, whether or not TeV scale SUSY is a feature of them is not crucial.

 

[Chris Hillman]

High A.P,

Davis could be becoming a strong place, or even more of one.
Did you see Derek Wise's paper?
He should be part of Carlip's (and your) group soon:

http://arxiv.org/abs/gr-qc/0611154
MacDowell-Mansouri Gravity and Cartan Geometry.

This paper is remarkable for offering a clear explanation of rolling without sliding via a term new to physics, "hamsterball". Highly recommended for the rodent characters alone.

 

[Kea]

Chris

Haven't seen you around here much before. Attempts to block threads on CarlB's (quantum gravity) work might be a little more difficult in this forum.

 

[marcus]

This paper is remarkable for offering a clear explanation of rolling without sliding via a term new to physics, "hamsterball". Highly recommended for the rodent characters alone.

Chris, what happens to GR when you remodel it based on deSitter instead of Minkowski? I ask because you are the web's GuRu, so your hunch about it could be really interesting.

 

[Kea]

Chris, what happens to GR when you remodel it based on deSitter instead of Minkowski?

Yes, Chris, that's the same deSitter that can be interpreted in a varying speed of light framework, such as that arising in the flat space GA formalism.

 

[Haelfix]

Eh? Desitter is a solution to Einstein's field equations (much to Einstein's chagrin). What do you mean exactly by remodeling it?

 

[marcus]

Eh? Desitter is a solution to Einstein's field equations (much to Einstein's chagrin). What do you mean exactly by remodeling it?

See Hillman's post (#28 on this thread)
https://www.physicsforums.com/showthread.php?p=1331501#post1331501

Cartan geometry using deSitter space as the local model

in Cartan geometry the tangent space at a point does not have to be a vectorspace

Haelfix you are talking about GLOBAL deSitter as a possible solution to Einst. eqn. That is not what I am talking about. I mean have the manifold be locally deSitter instead of locally Minkowski.

Maybe it is impractical. Chris Hillman might have an idea

he referred to the paper "MacDowell-Mansouri Gravity and Cartan Geometry" so he knows about Cartan geometry and so he knows that the tangent space does not have to be flat (momentum space can be curved, as in DSR). So I wonder has he thought about rebuilding GR and the Einstein eqn. on a Cartan manifold.

 

[marcus]

I don't see how SUSY/no SUSY at CERN should influence the rationale or need for Background Independent approaches.
We're trying to generalize the class of QFTs we know how to build, whether or not TeV scale SUSY is a feature of them is not crucial.

This is clear. I suppose (Background Independent) non-string QG research will continue to grow and get interesting results regardless anything you can reasonably expect to see or not see at CERN

 

[Fra]

Angry physicist, keep up your attitude and think for yourself at all cost. I sometimes wonder what the probability is that someone that has invested 30 years will want change direction and walk back. I can feel the pain and resistance.

The triggering reason why I as a student didn't decide continue with my original plan to try to get a research position somewhere is that I was sincerely disgusted by the attitude I faced that put the off from the academic world with the political and granting issues it contains. Maybe I was just unfortunate to end up having a string theorist as as supervisor during my undergraduate studies but I realized that going that route would imply a lot of politics and compromising and I that is not what I wanted to spend my time on, and to get into string theory wasn't on the map, no matter what the supposed "experts" tried to induce in a innocent student. Maybe time is changing though. Anyway, at this point the only thing thay annoys me is that I think they feed on my tax money. But I'll let that pass, because politics is not my cup of tea :)

/Fredrik

 

[Haelfix]

Marcus, Cartan formulations of General relativity is formally equivalent to say the usual Palatini formulation (which is often used to study QG). Making your local patch Desitter instead of Minkowski is fine (in any formulation), assuming your vacuum is indeed Desitter (eg say a FRW asymptotic final state), but I don't see how a change of variables or language exactly buys you new physics, these things are rigorously isomorphic mathematically (see Spivak).

When we do field theory close to the QG scale, using the usual Poincare invariance, the cosmological constant term is basically explicit and manifest. If you change formulations, and the isometry group is now ds or Ads, you have to be a little more careful as the CC is somewhat hidden in the extra algebra, but be sure it has to be identical. At least semi classically.

Now quantization might be a different beast. For some reason (probably man made confusion) it seems half the time quantization doesn't commute with formalisms.

 

[marcus]

Making your local patch Desitter instead of Minkowski is fine...

Haelfix you seem to be talking about a local coordinate patch.
That is not what I was talking about.

If you are not familiar with the basics of Cartan geometry
(which does not use a tangent space, at least in the usual sense)
then why don't you have a look at Derek Wise's paper?

the link to it is back in my post, and also in Chris Hillman's post where he refers to the "hamsterball". Indeed it is the hamsterball that replaces the tangentspace, in Cartan geometry (so his comment is apposite here!)

 

[marcus]

Marcus, Cartan formulations of General relativity is formally equivalent to say the usual Palatini formulation (which is often used to study QG)...

This suggests to me that you don't realize what is meant by "Cartan geometry" in Derek Wise's title. Elie Cartan called it "generalized geometry".

(we aren't talking about moving frames or differential forms or other such things that Cartan is famous for having helped to invent. these were very popular in the 20th century. we are talking about something else.)

Cartan's generalized geometry was virtually ignored for most of the 20th century.

The only textbook on Cartan geometry I know of is by R.W.Sharpe
(Springer 1997). It is helping Cartan geometry become recognized after many years of neglect.
The textbook is called
"Differential Geometry: Cartan's Generalization of Klein's Erlangen Program."

I don't see how a Cartan manifold can be formally isomorphic to an ordinary differentiable manifold---they belong to two different categories. "isomorphic" AS WHAT?

Perhaps I am missing something remarkable here, but it seems obvious that a theory of gravity formulated on a Cartan manifold would HAVE to contain new physics, compared to GR (a theory of gravity formulated on a conventional differentiable manifold).

It might be wrong physics, but it would surely be different

 

[Haelfix]

Cartan connections is something one learns in a graduate class in differential geometry, and be sure its formally identical to the usual way of studying manifolds when restricted to say the known cases people are interested in (say GR manifolds). In fact his student Ehresman simplified this line of thought into the usual connections on principle bundles point of view so often used nowdays. This is straight out of Spivak.

I looked at Derek Wise's paper and indeed that's exactly what it is, he even makes it explicit, when he constructs MM gravity from the Palatini formulation. Lo and behold, we have the same field content between the two. They are formally isomorphic.

You are free of course to construct something else other than gravity, and the Cartan connection can be of use in dealing with some topological gauge theories like BF theory. Indeed its this latter that motivates the paper, since MM gravity looks a little bit like the former when you generalize the Cartan connection as Wise points out (what he calls generalized Cartan connections).

Anyway, the whole point of using different formalisms is that sometimes what appears hard in one context is easier to solve or intuitively 'see' in another, but so long as we are talking about general relativity the physics must be the same.

 

[marcus]

Cartan connections is something one learns in a graduate class in differential geometry, and be sure its formally identical to the usual way of studying manifolds when restricted to say the known cases people are interested in (say GR manifolds). In fact his student Ehresman simplified this line of thought into the usual connections on principle bundles point of view so often used nowdays. This is straight out of Spivak.

I looked at Derek Wise's paper and indeed that's exactly what it is, he even makes it explicit, when he constructs MM gravity from the Palatini formulation. Lo and behold, we have the same field content between the two. They are formally isomorphic...

Spivak "Comprehensive Introduction..." is an interesting book.
https://www.amazon.com/dp/0914098713/?tag=pfamazon01-20
great cover art.
dropped out of academia and started his own press called "Publish or Perish"

I guess you would be talking about volume two of the five-volume set.

still not sure we are communicating, however.

Elie Cartan invented a lot of stuff (some I referred to earlier) including (with others like his student Ehresmann) the usual idea of connection. But what I said was I'm not talking about that other stuff. Cartan geometry the phrase in Derek Wise title is different from just doing conventional diffy geom using apparatus invented by Cartan. Cartan called it "generalized geometry", Wise and others call it Cartan geometry.

If you want to learn about it, and see if it has "new physics" potential, then I don't think you can rely on volume 2 of Spivak. I think you may need Sharpe's textbook. Garrett Lisi, who sometimes posts here, has Sharpe. Also Derek Wise referred to Sharpe. Also by carefully reading Wise' paper you can probably find out a lot.

I haven't read Spivak volume 2, but from what I see about it, I wouldn't assume it would give an intro to Cartan ("generalized") geometry. Could be wrong of course, perhaps I'll have a look at the math department library

=========
PS, Haelfix here is an exerpt from a reviewer's summary of volume 2
"...Kozul's concept of the connection and this is introduced in Chapter 6. First, note that the connection here is one of the versions of the introduced by Kozul as a map of pairs of vector fields to a vector field. Another useful version, not studied in volume II, is to consider the connection as a Hessian which maps any smooth function to a bilinear form on the tangent space. Second, note that Chapter 6 is usually the starting point for most treatments of curvature in differential geometry (e.g Do Carmo's "Riemannian Geometry"). Without the motivating material from the previous chapters, it would be difficult to understand the need for(or the point of) Kozul's connection.

Cartan's theory of curvature via a study of moving frames is detailed in Chapter 7. The author is careful to intuitively motivate Cartan's deviation from Euclidean concept as represented in the structure equations. Cartan's curvature tensor is shown to agree with Riemann's tensor, the "Test Case" is revisited, and the well-known fact that the curvature determines the Riemannian metric is established.

Building on the orthonormal frames from the previous chapter, Spivak now considers Ehresmann's theory of connections in principal bundles in Chapter 8. The main results here introduce the Ehresmann connection on the frame bundle, and gives the Kozul connection as a Lie derivative, thought of as the Cartan connection obtained from the Ehresmann connection..."

 

[ccdantas]

Here is something about "Cartan geometry" written by John Baez over at n-Cat Cafe

http://golem.ph.utexas.edu/category/2006/09/klein_2geometry_v.html#c004832"

 

[john baez]

Derek Wise

If I am not mistaken, he is a rather pleasant chap with a British accent.

No - while he's quite pleasant, Derek Wise doesn't have a British accent. He's from Colorado; I've attached a photo of him.

He's my grad student in the U. C. Riverside math department. He just passed his thesis defense last Wednesday! He hasn't come up to U. C. Davis yet; he should show up in the late summer. He'll be in the math department, but he plans to talk a bunch with Steve Carlip. Look him up if you don't bump into him.

He needs to polish his thesis a bit more before submitting it and putting it on the arXiv. It'll be called Topological Gauge Theory, Cartan Geometry and Gravity. It will subsume our paper with Alissa Crans on http://arxiv.org/abs/gr-qc/0603085" .

The idea is, quite briefly, that Klein geometry is to topological gauge theories like 3d gravity as Cartan geometry is to the MacDowell-Mansouri formulation of 4d gravity and its relatives. I explained a few of the ideas at more length in http://math.ucr.edu/home/baez/week232.html" of This Week's Finds, but I'll try to give a more thorough explanation of what we're up to when his thesis hits the arXiv!

A number of physics grad students here (except for a handful that I've met) don't believe in a lot of the stuff they write though. Very disappointing to say the least; perhaps, hopefully, he'll be one of the handful exceptions.

Like me, he's a mathematician. We believe what we write, because we've proved it.

 

[Chronos]

As usual, john baez is right on track. The other universes are fortunate he does not reside there.

 

[Angryphysicist]

No - while he's quite pleasant, Derek Wise doesn't have a British accent. He's from Colorado; I've attached a photo of him.

He's my grad student in the U. C. Riverside math department. He just passed his thesis defense last Wednesday! He hasn't come up to U. C. Davis yet; he should show up in the late summer. He'll be in the math department, but he plans to talk a bunch with Steve Carlip. Look him up if you don't bump into him.

I'll definitely have to look him up when he gets here, I can say by the photo (and the fact that he's not in Davis yet ) that I have not met him yet.

From the title of his thesis, he sounds like a fascinating fellow. I'd be intrigued to read it.

 

[john baez]

From the title of his thesis, he sounds like a fascinating fellow. I'd be intrigued to read it.

Derek wants to polish his thesis a bit more before putting it on the arXiv. But, I'm giving a talk on it this Tuesday, and you (and everyone!) can see the transparencies now:

http://math.ucr.edu/home/baez/derek/" .

Looking back on this thread I see some discussion about whether the Cartan-geometric approach gives new physics or not. There are lots of things to do with Cartan geometry, but the most interesting one to Derek is MacDowell-Mansouri gravity. This is not primarily a new theory. It's just a reformulation of ordinary general relativity. In fact, it's completely equivalent when the coframe field is nondegenerate.

However, Cartan geometry explains how MacDowell-Mansouri gravity gets to have the DeSitter or anti-DeSitter group as gauge group! This seemed very mysterious before. Cartan geometry also explains how MacDowell-Mansouri gravity gets to be a perturbation of a topological quantum field theory. These things suggest some interesting new ways of studying gravity - see the talk and references for details.

(The business about the coframe field being nondegenerate sounds technical - but in fact Witten's new paper argues that this technicality is the door through which the Monster group sneaks into 3d quantum gravity!)

 

[marcus]

Derek wants to polish his thesis a bit more before putting it on the arXiv. But, I'm giving a talk on it this Tuesday, and you (and everyone!) can see the transparencies now:

http://math.ucr.edu/home/baez/derek/" .

YAY!
I can only admire your sense of timing. Cartanization of qg is ready to roll. If Derek spends too much time polishing he will miss the tide.

So you launch him now. Does both him and us a great favor. thanks

=========
Oh hell, you stop the story at slide 23, just when it is getting good!

 

[P=K2ρ4/3]

John Baez students

I have no idea---can't help guess. Derek will officially be in the math department at davis. Might not be around there regularly yet: his postdoc contract begins in the Fall. Here's a picture of John Baez students taken summer 2004
http://math.ucr.edu/home/baez/students.html

That's my new desktop background.

 

[duke_nemmerle]

has this blog shut or am I missing something? Either way, if it's down I'd love it if the angryphysicist could figure out a way to keep us up to date on his adventures.

EDIT: forgive me, I've been going to angryphysicist rather than angryphysics

EDIT AGAIN: Haha, I finally just went back to the very first post, it's http://angryphysicist.wordpress.com/about/

 

[Angryphysicist]

Latest adventures...

has this blog shut or am I missing something? Either way, if it's down I'd love it if the angryphysicist could figure out a way to keep us up to date on his adventures.

Sorry, I would be more active in the blog (and here too!) BUT I am out on the town with a few mates. You're only young once!

As for my latest adventures, it's writing a series on writing file systems and I have been thinking about trying to use the Einstein field equations to describe Higgs bosons as a sort of quantization of curved spacetime.

There is some intuitive feeling that the Field equations is intimately related to the Higgs field, but I need to think this through a little more thoroughly.

And then, I am attending the first ever D programming language conference in Seattle, Washington...and I would like to work on a demand to make D more low level and a way to kill the garbage collector for systems programming.

 

[Chronos]

Information theory is the right road to QG IMO, Angryphysicist. I am certain the universe will obey a QIT description once all the false gods are disrobed.