My head is still spinning a bit, trying to process all the information gathered here (well, it might also be the lack of oxygen in the air, it is rather polluted and the high altitude doesn't help either). It's been my first conference in this community, and I have to say from all the conferences where I've been this year, the Loops has been the nicest experience. The atmosphere has been very welcoming, openminded and constructive.

My talk yesterday on

*'Phenomenological Quantum Gravity'*(slides here) went well (that is to say, I stayed roughly in the time limit and didn't make any completely embarrasing jokes). Though I wasn't really aware that I would be the only one on this conference speaking about DSR. If I had known, I might have extended my summary of that topic (there was a DSR talk scheduled by Florian Girelli, see picture to the right, but he changed the topic shortly, which I didn't know).

However, after more than a month of traveling, I am sitting here in the inner yard of the hotel and try to remind myself why I go to conferences:

- Because it's just such a nice experience to arrive in a foreign city where nobody understands your language, without any baggage, after a 36 hour trip, with an 8 hour jetlag, having just figured out that the credit card doesn't work, and the hotel doesn't have a reservation for you - and then to find the conference site with familiar faces, the air filled with words like
*'propagator'*,*'manifold'*and*'background independence'*. - Because one meets old and new friends, because there's a conference dinner or reception, and plenty of free coffee and cookies.
- Because the registration fee includes a welcome package that usually features a more or less useful bag, a notebook and a pen, and a significant amount of tourist information. Occasionally there are some surprises to that, e.g. this time the bag was a woven shoulder bag, or one of the previous SUSY conferences featured a squeezable brain...
- Because some people present new and so far unpublished results.
- To see and be seen.
- To inspire and get inspired...
- And of course to blog about it ;-)

There has been a significant amount of braiding on this conference (for program and abstracts see here), talks by Yidun Wan, Jonathan Hacket, Lee Smolin and Sundance Bilson-Thompson, the latter shown in the picture below with John Swain

Olaf Dreyer, preparing his talk

Some people from the blogosphere that I've meet in person here. Garrett Lisi and Frank Hellmann:

Alejandro Satz (from *Reality Conditions*) and Yidun Wan (from *Road To Unification*)

And here is a photo of Carlo Rovelli and Abhay Ashthekar - pen, notebook and coffee included:

Admittedly, I found the phenomenlogical part on this conference somewhat underrepresented. Indeed, I found myself joking I

*am*the phenomenology of the conference! Likewise, Moshe Rozali (who you might also know from comments on this blog) has been

*the*String Theory of the conference. He gave a very interesting talk about the meaning of background independence.

This afternoon, I am chilling out (okay, actually I am writing referee reports that have been due about a month ago). Below a picture of the hotel's inner yard where I am sitting (taken yesterday, right now it is raining). Tomorrow I am flying back to Canada, and I am

*really*looking forward to sleeping in my own bed. A nice weekend to all of you!

[Try clicking on the photos to get a larger resolution.]

**Updates:**Chanda just sent me a photo she took yesterday evening. I am very pleased about the truly intellectual expression on my face, must be the glasses.

And Garrett took this nice photo.

## 69 comments:

So Bee, we can create artificial gravity - yet still can't produce anti-gravity?

I don't mean weighlessness or zero gravity, I mean anti-gravity or the technology that was once assumed by some to have been used Egypt's pyramids ...

you know the kind of rings used in Stargate-1 to beam matter, people and objects up and down from ships

Quasar, funny you should mention 'Stargate-1'. We called the 2000-year-old markings at the Norchia Tombs the same name: "Stargate", because the Etruscans believed there was a door to the afterlife. (Here are a few more photos from that day, playing in the tombs, some with descriptions of the Etruscan city.)

Dear Bee: The slides of your talk are great. I didn't understand it all, but I learned some new things, and I could (more-or-less) follow your talk. Thanks!

Hi Amara, thanks for the pics.

I wasn't meaning to go metaphysical re: the concept of souls, doors to the afterlife and/or other (living) dimensions.

I meant in 'this' physical plane, if there is matter and antimatter, there is gravity and antigravity

Producing one should give us insights into the composition of the other - whether gravitons are in another dimension or not - how gravity acts in our 3D+T.

After all even travelling at the speed of light we would not be able to travel far in the universe in a lifetime (human lifetime). Me I'd be happy just to travel the length of the Milky Way (for now!)But of course wormholes, folded space and extra dimensions, do presumably mean we could go to the other side of the observable universe in the blink of an eye - or after our last breath on this hearth!

Dear Bee,

thanks for the report and great photos!

The portrait of you is really nice :-)

Cheers, stefan

PS: Quasar9, I would say just because there is matter and antimatter, that's not a reason that there should be antigravitation. Maybe Bee will have some time sometimes to go into more details, since she has a lot to say about this topic.

Special Relativity does not obtain in loop gravitation? How is that a good thing?

It appears the Lorentz group remains an unbroken symmetry in loop gravitation. An Equivalence Principle parity violation - allowed by affine, teleparallel, and noncommutative gravitation theories - empirically falsifies loop gravitation. Somebody should look.

Glad to have this report. Nice shot of you---if I am not mistaken Thomas Thiemann is there in profile to the side of the picture.

Hi Bee, your photo actually gives me the impression that you have been physically drained? by the experience of a certain Timelord !

I do not know if what you have crammed into,in the last month, constitutes as the nearest experience to a human being,(being) gravitationally slingshot!, from conference to conference, :)

I wonder if the is such a thing as "gravitational_bends", like when deep sea divers have to re-acclimatate, or decompress?..then maybe you should take things easy for a while, best paul.

Quasar9, the facts of gravity work on the principle of matter, thus anti-matter is nothing more than the equation of zero matter. Building the pyramids( which as Im sure you know, contains matter) cannot be a possibility.

On th eother hand, there can be a devise that can counter gravitational effects?..the intense fields of specific gravitational electrons(GEONS), obtained by the reduced vacuum of certain conducting energies, can be used to "lever_tate" immense amounts of matter, ie 70 ton stones.

Superconduction is anti-gravity in the presence of matter.

Bee,

In the slides I particularly approved of your judicious choice of quotations from Max Planck, Freeman Dyson, and other deep thinkers. :-)

with, of course, the culmination being slide #26.

proud to be so honored,

Martin

Stefan - no anti-gravity???

Hi Paul - Superconductivity is a phenomenon occurring in certain materials at extremely low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field (the Meissner effect).

Superconductivity occurs in a wide variety of materials, including simple elements like tin and aluminium, various metallic alloys and some heavily-doped semiconductors. Superconductivity does not occur in noble metals like gold and silver, nor in most ferromagnetic metals.

But I get where you are coming from

Or should I say I was thinking along the same

linesCluster unveils a new 3D vision of magnetic reconnection

Hi Bee,

Thanks for the report and nice photos. Just say "Hi" from Mexico City. We just had a venture in a raining dark night in the City.

After our long lingering in the Anthropology Museum, we accidentally met Carlo Rovelli at the gift shop. This is apparently a "Small World" effect.

Have fun,

Frank & Yidun

Could you please say something about the science that was presented at the conference? Spending two days at Loops '05 I found that quite remarkable.

Dear All:

Unfortunately, my above expressed hope to sleep in my own bed turned out to be overly optimistic. The flight out of Morelia was delayed so much that I (and a couple of other people from the conference) got stranded in Houston. I had to spend the night in an airport hotel, where I am still. At least my-stupid-bag is here with me.

Dear Amara:

Glad you liked the slides :-) I put some effort in it once I realized I would be the only one speaking about phenomenology. Also, I had to notice repeatedly that most of the people from that community know little about the extra dimensional stuff (first part of the talk), which I still find very exciting even though I don't work on it any more.

I will probably post a brief write-up of my talk the next days.

Dear Marcus:

Nope, it's not Thomas Thiemann, he wasn't with us that evening. But he was at the conference, slightly intimidating as usual...

Dear Martin:

Well, your comment at ScienceForums fit very nicely in my line of argumentation, since I used it to explain why I am not working on the extra dimensions anymore (first part of the talk), but instead 'committed' myself to the minimal length stuff (second part of the talk).

I think I vaguely understand what you were trying to express, and you probably didn't mean it to be insulting, but let me point out that you are completely wrong. If I was unenthusiastic and uncommitted I wouldn't work on phenomenological quantum gravity to begin with. Neither am I pessimistic, I like to think about myself as being realistic - that being what a phenomenologist should be. I definitely don't approve of people running around proclaiming that there are quantum gravitational predictions that can be measured soon, when these predictions are in my opinion completely unreasonable and based on handwavy arguments. The word 'phenomenology' has become somewhat fashionable in the last years... not that I'd mind, but this kind of advertisement can only lead to disappointment on the long run.

I don't think there is any qg to be found in UHECRs nor in gamma ray burst. What I currently find more promising are CMB correlations, possible also neutrino/graviton background - should it be measured at some point.

Best,

B.

Dear Robert:

Well, the easiest way to get an impression of the topics covered at the conference is to look up the abstracts. Since all the plenary talks were recorded, and the slides are with the organizers, I think there should be more info available soon.

What I found most interesting on the conference was Fotini's talk about non-locality and possible effects on the CMB, possible solution to the horizon problem. I think it's mostly based on this paper. I have to say though that I found the argumentation why and how that would affect the CMB very weak - it seems to me like a remake of the argument with wormholes

Wormhole Cosmology and the Horizon Problem

Then, I really found Jonathan Hackett's talk about the braids very interesting, he has worked out some general mathematical structures that I think are a vast improvement to Sundance's and Lee's work, you find it in this paper

Locality and Translations in Braided Ribbon Networks

Another talk I really liked was Moshe Rozali's talk about background independence. I am still thinking about it, esp. exactly what we'd require of a theory to be background independent. E.g. I'd think that in a space-time with higher dimensions it would be sufficient to have background independence on the 4-d submanifold to recover GR?

Then, there was a discussion on the last day where everybody was asked what in the most optimistic case he/she would be talking about at the Loops 2017 and what the probability for that would be. This too was recorded and I hope the audio will be available on the websites soon.

All the best,

B.

Dear Quasar:

I meant in 'this' physical plane, if there is matter and antimatter, there is gravity and antigravityThe problem with antigravity is that if energy (matter) is not positive then the vacuum is not stable. The reason is roughly the following: a particle and anti-particle with opposite electric charges still both carry positive mass, so you need energy to produce them. If both also had opposite sign masses, then you could make them literally out of nothing. Due to always present quantum fluctuations this would happen all the time, for particles with arbitrarily high masses and energies - and is definitely in disagreement with observations. If you believe in anti-gravity, first thing you need to do is to explain why this problem doesn't occur. Best,

B.

Hi Bee,

Thanks for the nice report!

Christine

Lucky you, 1 year ago I was stranded at Houston airport & had to sleep overnight at the terminal.

B: you mention "recorded", this is video right? (I thought I read somewhere it was audio). That Strings '07 conference had a well-coordinated team which uploaded talk videos (100mb each) of talks: shortly after, not real-time). I was given permission to make iTunes versions available over here, which could also be displayed as embedded Flash videos. Over any other physics related website, or physics blog (like yours). This is a great way of enhancing the "Content/Distribution" model: more avenues of exposure for Strings '07. Lubos is already using it on his blog.

"Citizen journalism" means individaul conference attendees (like yourself) can file on-site reports. Me, I was actually video-blogging off-site (halfway around the world), re-distributing their 100mb videos. Downloading them, transcoding (using a quad G5), uploading to Blip.tv, providing the "hooks" for an iTunes video-podcast.

Having blogged from Loops '07 (you reported dicey WiFi, I think I heard similar at Strings '07), how would you improve it in terms of getting talks (videos) & other multimedia to the Physics community? Time immediacy is an obvious one.

I saw your exchange w/Lubos at his blog, & the above would have avoided the problem. How about submitting a video to Arxiv (in addition to abstract/paper)? This obviously would require a change in architecture, but video is hot stuff these days w/Broadband access (video-sharing sites like Blip.tv, Youtube, et al) If so, conferences could be held practically "virtually"..just sit back at home & watch videos. (iTunes videos are playable over AppleTV, a living-room set top box). What is the point of traveling thousands of miles (aggravation of airports, train stations, etc) to a conference to hear a talk?

I always found it ridiculous that the amount of time spent to get to a conference >> talk-time (30 min). The only value of going to a conference is making personal contacts. This is what a "virtual conference" cannot do.

Okay, a dumb question from an experimental physicist who just likes perusing loop quantum gravity papers for fun: If there are non-local identifications of points in LQG like those described that have one end located in the interior of a star, the other end will stick out like a sore thumb due to the radiation it will be emitting, right? Although the probability of having one end of a non-local link in a star is presumably tiny, the ease of seeing such a case seems to make it worth looking into.

Hi Bee,

I am suggesting that if we can create gravity, we CAN create anti-gravity. That means that therefore it is possible there is anti-gravity (or such-like effect).

Fixing Black Holes

While the researchers are still analyzing the huge quantity of data from this simulation, about ten trillion bytes, initial results are exciting. The major finding is the regulatory role black holes play in galaxies. It turns out that after gobbling gas for millennia, a black hole eventually begins pushing gas away, like a repentant overeater. “The black hole tries to swallow a lot of gas,” says Di Matteo, “but this growth is kind of suicidal. As it tries to swallow more and more it will radiate so much energy that it will affect its surroundings and stop more gas from flowing in.”

The first effect of this purging “feedback” process is that it shuts off the quasar associated with the black hole. Because gas is no longer falling in, gas at the lip of the black hole is no longer heated and eventually radiation stops. In effect, feedback blows out the candle of the quasar, which explains why — though virtually every galaxy had a quasar when the universe was young — now there are quasars in only about one in 10,000 galaxies.

Most detailed cosmological simulation to date

The largest black holes, called supermassive black holes, lie at the center of each galaxy.

They can arise initially when the first stars collapse under their own gravity.Surrounded by dense gas in their central locations, they consume surrounding material, both gas and stars, and rapidly grow to become monstrous in size, some with masses a billion times that of our sun. But evidence suggests that supermassive black holes are self-regulators — they don't feast forever and they never swallow a whole galaxy, Di Matteo said.In her cosmic simulation, as in reality, galaxies routinely collide. The supermassive black holes embedded at the center of these galaxies choreograph the dynamics of galaxy collision. The result is a tremendous belch of energy produced as the merging black holes form a luminous state called a quasar.

"Quasar formation really captures when the fun happens in a galaxy," Di Matteo said. "You can only use a simulation to follow a complex, nonlinear history like this to understand how quasars and other cosmic structures come about."

Dear Christine:

Yes... its kind of funny how much of a difference it made for me to be at a conference where I was actually interested in the talks. It's been a very inspiring meeting, and I also have to say that I appreciate very much that I had a plenary talk. It is very nice to see that at least some of my work is been taken seriously. Also, I got a lot of supportive feedback to my talk which - after all that travel in the last weeks - I found very encouraging.

Dear BYen:

Yes, I guess we were lucky. Continental Airlines booked us into a Hampton nearby the airport and we got some meal vouchers. It was much better than the Holiday Inn I had to stay at in Mexico City because Air France messed up the connection to Morelia (among other things the main problem was that they didn't speak English, and I have no idea what the Spanish word for 'toothbrush' is).

Regarding the conference: All the plenary talks were video- and audio recorded, at least I saw some camera in the auditorium. The parallel sessions were audio recorded, so was the discussion (though with the latter I saw that apparently the 2nd microphone was not recorded, so it might be hard to hear). I expect that these recordings will be available on the website soon, so should the slides to the talks.

I definitely don't want my research to be communicated via videos on the arxiv. I am a scientist and not a movie star. I don't even feel comfortable with giving talks in front of O(100) people, and I doubt I will ever get used to it. I just don't find that medium appropriate. Research and the discussion of scientific questions requires thought and takes time. Not every bright physicist is witty, or good looking, or has a pleasant voice - all things that score in videos. I don't even think comment sections in (average) blogs are a good way to do discuss science questions. Also, I know that what I say in my talks is hardly ever as precise and to the point as it should be. If you want to learn about my research, you'll have to do it the old fashioned way by reading my papers.

I have listed the points why I like to go to conferences in the post above. There's a plus point to face-to-face communication that videos and audios just can't replace. I think though that there is a large potential in video conferencing once there is a standard for the hard- and software. (One other question that comes with that are the time differences. I'd opt for a global physics time, preferably US west coast.).

Dear Quasar:

Very interesting, but you have just ignored what I've written above. Best,

B.

Dear Anonymous:

There are no dumb questions only dumb answers. So here is my dumb answer: yes, I have thought the same thing, and even talked to Lee and Fotini about it (so I think). I wasn't considering stars but more black holes and Hawking radiation (non-local links could provide a possibility for usually impossible information exchange through the horizon, think about the information loss problem.)

But anyway, I couldn't make any sense of that within their model. E.g. what I would expect for the non-local links is that their length isn't equally distributed, but the rarer the longer the link? Also, I would want to make sure only very high energetic (Planckian) particles can go through the link, etc. None of that (what I find necessary to have a decent model) is addressed in their work so far (indeed I asked a related question at the end of Fotinis talk, but I either didn't understand the answer, or it wasn't an answer).

I hope that I will be able to understand these issues better in the soon future, since I think there is potential to this investigation. The reason why I find this interesting is that the presence of non-local links might make a nice new example for a phenomenological model. I believe this is what Lee and Fotini have been working on with Chanda and I expect we will hear more about that.

Best,

B.

There is not spanish word for "toothbrush", you must use three, concretelly you must say "cepillo de dientes".

B.T.W. it is a pity that "you are not able to recognize patent stupidities from scientific arguments", I guess that I should stop reading your blog, or well, may be not ;-).

I don´t know if you choosed not to answer Lubos or if he deleted your posts but I got very surprised when I saw the phrase, I thought that Lubos respected your scientific knowledge.

Actually, I think that Lubos Motl's blog is an example of how pernicious blogging can be if it is not handled well. Experienced people know whether to take his comments seriously, but what about some young PhD student? Even if he/she doesn't understand the issues, he/she might well start to think that LQG is not respectable. After all, LM *was* a prof at Harvard. I myself found that I was being influenced by his rants, and it was only when he put forward his crackpot theory of the 2nd law of thermodynamics [according to which entropy has to increase because of the inner logic of the scientific method !] that I realised that he is not a reliable source of information on anything but string theory. I do hope that all bloggers realize that they have a serious responsibility in this regard. I think that this blog does a good job.

Hi Bee,

It is very nice to see that at least some of my work is been taken seriously.Yes! And congratulations for that! It sounds the conference was very stimulating and productive.

I look forward to attend a next Loops conference (next year or so)! I feel like I'm starting my career almost from scratch. Yes, of course I'll be making use of some experience from everything I did before, but I feel like I'm "renewed", finally working with physics again. After one month in my new job position, it's clear now that I have some freedom for research, even in quantum gravity, although my initial, main responsabilities will be to give support (from the point of view of physics) to the projects of the materials division. In any case, I look forward to work in quantum gravity when I get myself sufficiently organized...

Best,

Christine

Dear Javier:

B.T.W. it is a pity that "you are not able to recognize patent stupidities from scientific arguments", I guess that I should stop reading your blog, or well, may be not ;-).

I don´t know if you chose not to answer Lubos or if he deleted your posts but I got very surprised when I saw the phrase, I thought that Lubos respected your scientific knowledge.

Lubos isn't able to recognize scientific knowledge if it doesn't come with a sticker saying 'string theory', but as far as I can tell he has never deleted any of my comments. The primary reason why I didn't answer his offensive remark was that I've been stuck in airports the last days. But I am not particularly inclined to reply to his stupid offense anyhow. It seems he is just looking for a reason to insult people I known and like, which makes me sick. His dismissive comment about talks on a conference that he didn't attend is so obviously bullshit that its not worth a discussion.

I appreciate a good argument, but he has almost dropped any scientific reasoning and retreated to insults void of content which are neither original nor funny, and he repeats the same phrases like a broken record. He would probably go on saying that I 'parrott' other peoples 'nonsense'. It doesn't bother me much since everyone who has met me knows that this could hardly be further off the truth, but I don't want to provoke him going on about crackpots of various color.

Best,

B.

Dear Anonymous:

Actually, I think that Lubos Motl's blog is an example of how pernicious blogging can be if it is not handled well. Experienced people know whether to take his comments seriously, but what about some young PhD student? Even if he/she doesn't understand the issues, he/she might well start to think that LQG is not respectable. After all, LM *was* a prof at Harvard. I myself found that I was being influenced by his rants, [...] I do hope that all bloggers realize that they have a serious responsibility in this regard. I think that this blog does a good job.Admittedly I find your remark kind of scary! I would like you to question everything you read, no matter whether on my or Lubos blog. The sad thing is that I think most of Lubos' convictions are in principle based on a scientific argument, and if he would stick to that it might be worthwhile hearing his opinion. I suspect he has retreated to insults because it is a cheap and easy way to attract a lot of people to his blog. I am not sure why he is so keen on raising the number of visitors but maybe that public appearance is all he has left.

Do you really think that students are influenced by what he writes?

Best,

B.

So, Bee, is manifest background independence a useful principle or not? (encouraged by your - there are not dumb questions - but I'd disagree there, a repeated dumb question is a dumb question).

I'd say it is a useful principle but one has to clarify what it means. I would say it means that the physical laws we observe allow for a completely dynamical background geometry. This might not be completely obvious in the formulation of the theory. I.e. think about describing a 4 dimensional space-time with arbitrary curvature as embedded in a 10 dimensional flat space. Fixed background? Best,

B.

Bee,

Hope you don't mind - The 4-D spacetime embedded in 10 flat dimensions presumably comes from string theory.

But forget string theory for a moment. There is some theorem about being able to construct an embedding of a N dimensional Euclidean manifold in a sufficiently higher dimension R^M (perhaps M is 2N or 2*(N+1) or something like that? Aside, I've always wondered what the theorem would be for a N space, 1 time manifold?

So, whatever your theory, whether string-derived or not, I assume that it can be embedded in some (much) higher dimensional flat space.

The question is why would we want to do that?

The only good reason would be that the intrinsic properties of our 4-D manifold are not sufficient to describe our physics; extrinsic properties that arise from the fact of being embedded are relevant.

We must also answer whether the details of the embedding are important. i.e., any actual construction of an embedding would rely on charts of our 4D manifold and of the embedding manifold, are we sure we are coordinate-independent?

Given all this overhead, seems to me there needs to be a very compelling physical reason to try this.

Hi Arun, no I don't mind. My comment above had no specific reference to string theory, it was meant as an example (though one that deliberately reminds of ST).

The question is why would we want to do that?That is of course a question you

canask (and a good one), but not the question youdoask if you already have a framework and try to figure out if it does as desired. I am sympathetic to your argumentation, but I have to say I have no specific opinion about the matter. I find it possible that quantum gravity is indeed not background independent (with quantum gravity I do not necessarily mean also a TOE, though one can discuss whether or not we can have the one without the other).Best,

B.

PS: Regarding the embeddings in flat space, there is a theorem about it but I can neither recall the name nor the reference. Chances are, Peter Woit will know it - if you are really interested ask over at his blog. My guess would be N*(N+1)/2.

Embeding: At most 2N + 1 for a manifld of dimension N.

About the relevance of Lubns blog I guess that it has had a lot of relevance. I have found interstings his critics to LQG (of course not the insults to LQG people), and I have relearned string theory (the fact that i don´t like too much the spin foams also influenciated me, it is not only Lubos).

Actually I have seen that Distler and Jonshon have deleted, in their own blogs, the links to Lubos Blog. I gues that the reason is the insults, or at least disrespects, towards Clifford recently.

Dear Bee,

Thank you for your reply!

If I may rephrase my question in a more positive way - what useful mathematical gizmos do we gain by embedding our 4-D spacetime in another manifold?

(I have an indefinite plan to read Choquet-Bruhat/DeWitt-Morette some day so I can replace gizmos by something more meaningful.)

Apart from being able to do Kaluza-Klein-like stuff, do the extra dimensions give us some room to do analytic continuations around 4-D singularities? Just as going from the real line to the complex plane lets us work around singularities of a function on the real line, can we, e.g., work our way around the Big Bang singularity in a higher dimensional theory?

Dear Arun:

If I may rephrase my question in a more positive way - what useful mathematical gizmos do we gain by embedding our 4-D spacetime in another manifold?Can't think of any, except that sometimes it's just generally useful to have a second point of view. I.e. if I think about small perturbations, say on a 2-sphere, for me its easier to picture them via an embedding in a flat 3-dimensional space than as an intrinsic property of the 2-surface. Since both is mathematically equivalent anyhow (one has to be careful about defining curvature though) I don't see how one could be mathematically superior or inferior.

I like to think about the embeddings because one isn't a priory confined to 3+1-dimensions. I mean, consider you start with a higher dimensional theory with matter fields in a 9+1 dimensional flat space with a dynamics that results in matter being confined to a naturally arising 3+1-dimensional submanifold with general covariance (ST unfortunately doesn't provide such a dynamics). Such a theory would in my opinion fulfill the requirements of reproducing GR as we like and know it, background independent on the submanifold, plus explaining why we experience 3 spatial dimensions - on the expenses of background independence of the higher dimensional space time. I think that something like this would be possible, but it might also be completely wrong.

Best,

B.

Bee: From Lubos' post on Loops-

" But loop quantum theories have no Lorentz invariance at the Planck scale which is why it is infinitely unlikely that a form of matter should ever have the right equation of state."

If this is true, that LQG theories are not Lorentz invariant, is this not a huge problem?

Dear Gordon:

I am not actually sure what Lubos tries to say, maybe better ask him - does he mean if there's a lattice or some other discrete structure then there can be no smooth symmetry group? See, I am not working on LQG, have never worked on LQG, and have no plans on doing so in the soon future. So I can't say anything intelligent about it, except that I have no problem with anything that happens at the Planck scale as long as GR and the standard model is reproduced in the regime that we have observed it. Best,

B.

PS: That includes equations of state being modified at Planckian densities, a possibility that I personally find very plausible.

Bee: Sorry, I thought you were one of the LQG group, given the way Lubos talks to you---I have been away from active physics for so long that a

phenomenologist to me means someone who reads Edmund Husserl ;)

Dear Gordon:

No need to apologize. I've been told repeatedly that I actually don't write very much about my research on the blog. Maybe I should do so more often. I have no specific preferences for either Loops or Strings or other things. Or, as Garrett put it so nicely 'You don't believe in a lot of things'. This remark really made me think - it might be true. Not sure whether that's good or bad.

Just keep in mind that not everybody at PI is in LQG. Some people seem to think so, but we actually have other research fields here as well. Yes, also String Theory, and we all coexist very peacefully ;-)

Best,

B.

Admittedly I find your remark kind of scary!

That's because you are aware of your responsibilities. :-)

Do you really think that students are influenced by what he writes?

Not directly, but indirectly. For example, I have had it put to me that LM wouldn't say these things unless they reflected, in some perhaps distorted way, the opinions of the Harvard group. When you have this constant drumming of "LQG people are all nuts", well, there is a natural tendency to think that where there is smoke there is fire: maybe LM is just crazy enough to say in public what a lot of people are saying privately.

Bear in mind that while there are a lot of physics blogs out there, only a tiny minority have any kind of regular following. I still do read LM because occasionally he writes interesting stuff about string theory. But I only look regularly at 3 or 4 physics blogs, and I think that most people do the same. So the people running that hard core [and you are becoming one of them -- scared?

:-) ] have a responsibility to think carefully about the possible effects of their writings. Again: young people are uncertain about opinions in the wider world, and, yes, I do think that they [unconsciously] look to people like you as role models. But don't worry, you are doing fine. So far.

:-)

Dear Anonymous:

Hmm, will think about it. I generally make an effort to keep my posts as reasonable and balanced as possible (there are a lot of things I DON'T write, believe me). Nevertheless, I don't think the blogosphere is a good place to look for 'role-models' (Plus I'm not sure I make a good one. There's a reason why I drive my co-workers into sheer desperation and I'm not proud of it).

So the people running that hard core [and you are becoming one of them -- scared? :-) ] have a responsibility to think carefully about the possible effects of their writings. Again: young people are uncertain about opinions in the wider world, and, yes, I do think that they [unconsciously] look to people like you as role models. But don't worry, you are doing fine. So far.I am not scared by responsibility, I am just always scared by those who get their opinion by adapting other people's opinions - as you say, in most cases this is an unconscious process, which makes it even more scary. I think one of the reasons for it is the pressure to just HAVE an opinion - it's a bit like making a measurement and forcing the wave-function to collapse. In many cases an honest 'I don't know' would be better, but that seems to be a sentence one can't score with these days. Pressure to making up one's mind fast might be okay if buying a trolley at eBay, but it's not a good environment for careful scientific considerations.

Best,

B.

OK, I am looking forward to the recorded talks to make up my own mind. I will find out if there was something similar to the conference in Golm where one of the parallel session speakers became king of buzzwords when he claimed "the big bang is just a phase transition".

Re Lorentz invariance: I have no idea what Lubos is talking about. It might be related to deformed dispersion relations which at least naively seem to single out some rest frames.

OTOH, let me please reiterate that the fact that a theory has a dynamical length scale (call it minimal or not) does not necessarily introduce a granularity of space-time or break Lorentz invariance or transform dispersion relations: Pure SU(3) gauge theory (or QCD if you like) dynamically produces a mass scale (where the coupling is of order 1, called Lambda_QCD, roughly some 100 MeV setting the mass scale for protons and neutrons as those do not come from the quark masses). This scale roughly translates into 1fm and there is by no means a granularity and the theory is perfectly Lorentz invariant.

Hi Robert:

Re Lorentz invariance: I have no idea what Lubos is talking about. It might be related to deformed dispersion relations which at least naively seem to single out some rest frames.Yes. At least as naively as I have argued in my recent paper...

OTOH, let me please reiterate that the fact that a theory has a dynamical length scale (call it minimal or not) does not necessarily introduce a granularity of space-time or break Lorentz invariance or transform dispersion relations[...]Indeed. As I usually say in my talks (unless I forget) even though discrete approaches lead to a minimal length, a minimal length doesn't necessarily mean there is a discrete structure. So, I fully agree with you.

Best,

B.

It's not only no discrete structure is implied, it's also that you can have a Lorentz invariant characteristic length. Relativistic length contraction is not at work here.

If I measure the QCD scale I get the same value as somebody who is moving at 99%c. This is the case although I find the mass of the protons of that moving observer to be much larger than 1GeV (the value observed by him).

Bee:

"I would say it means that the physical laws we observe allow for a completely dynamical background geometry."

Dear Bee,

I think that what you said above descrives general covariance, not background independence.

After I heard Moshe's talk I understood that by background independence he meant that there is no spacetime manifold in a BI theory. Even though his talk was very clear, someone still asked him in the end about the "base manifold" of the quantum theory in the middle. He again said that geometrical manifolds, spacetime metric, etc. emerge only when you take a particular classical limit. According to AdS/CFT, the physical observable of the BI fully quantum theory of gravity is the partition function.

This approach to realizing BI is very different from the one taken by the LQG people. In this respect the whole LQG approach seems very naive and misguided.

K

Hi K,

I understood the point of the talk as: if you talk about BGI, make clear what you mean with it. I don't understand your remark above. General covariance is invariance under arbitrary coordinate transformations. You can have that on a fixed background. What I said is that the background itself should be subject to a general covariant evolution law, as e.g. it is in GR. Best,

B.

Bee:

"I don't understand your remark above. General covariance is invariance under arbitrary coordinate transformations. You can have that on a fixed background."

Sorry but once you fix the background you break the general covariance. GR has general covariance as a theory but the solutions of GR (i.e the metric) only preserve certain isometries. It's just like in the Standard model, the theory is SU(3)XSU(2)XU(1) gauge invariant but the vacuum(solution) is not.

yes, once you fix the background, your global solution only allows certain symmetries (turn around, world stays the same) under which the metric stays identical. you're still allowed to pick an arbitrary coordinate system, and the physical laws (and so the metric) will transform from one system to the other according to the transformation laws of general diff. invariance. e.g. you can transform a FRW background into any coordinate system that you like and compute within it. the point about FRW is that you've 'picked' a 'time' as the 'physical' one, its not BGI.

Hi Bee,

Bee:

"I understood the point of the talk as: if you talk about BGI, make clear what you mean with it."

Well, I think what Moshe meant by BI is that the full guantum theory of gravity has no space-time background manifold, metric, etc. Those only emerge in a particular classical limit.

Contrast this with the LQG approach where they start with what they refer to as a "bare manifold" and then recast GR theory in terms of new variables without metric and quantize it. So, they claim that because the metric is not specified the have a BI theory. Now, since they start with a spece-time manifold, this approach is a lot more background dependent than the AdS/CFT.

Yes, without a background manifold (metric) it's certainly background independent in a higher degree. I wonder though if we really need that. I.e. I would be fine with the type of background independence that we have in GR.

Bee:

"yes, once you fix the background, your global solution only allows certain symmetries (turn around, world stays the same) under which the metric stays identical."

That's exactly what I meant by saying that the fixed background is not diffeo invariant.

"you're still allowed to pick an arbitrary coordinate system, and the physical laws (and so the metric) will transform from one system to the other according to the transformation laws of general diff. invariance."

I agree of course but the word "transform" implies non-invariance under the general coord transformations. Only the subset of those will leave the metric invariant, as you said above. However, things like the scalar curvature will still remain invariant since they are invariant under general diffeos in the first place.

"I wonder though if we really need that. I.e. I would be fine with the type of background independence that we have in GR."

If you'd like to explain the origin of space-time you'd better have a theory of quantum gravity where the observables are defined without a space-time manifold(whether is has a metric is an extra requirement). So, what I find exciting is that the AdS/CFT appoach gives us a strong hint that the space-time manifold (geomery) is only emerging in a certain classical limit of the full quantum theory.

I think we have a disagreement on the word 'invariant'. You are referring to isometries. Under a coordinate trafo x -> x'=Gx the metric remains unchanged g'= g. I.e. the Minkowski line element remains the same under the Poincare group. I am talking about general covariance, under a coordinate trafo the metric transforms as a tensor g' = GgG^T, or equations of motions transform accordingly. The metric of flat space looks different in spherical coordinates but that doesn't change the physical laws if you transform them appropriately.

If you'd like to explain the origin of space-time you'd better have a theory of quantum gravity where the observables are defined without a space-time manifoldSince we don't know how that theory looks like, I think that's hard to say. I would be fine with a theory of quantum gravity that does not explain the origin of space time.

"I think we have a disagreement on the word 'invariant'. You are referring to isometries. Under a coordinate trafo x -> x'=Gx the metric remains unchanged g'= g. I.e. the Minkowski line element remains the same under the Poincare group. I am talking about general covariance, under a coordinate trafo the metric transforms as a tensor g' = GgG^T, or equations of motions transform accordingly. The metric of flat space looks different in spherical coordinates but that doesn't change the physical laws if you transform them appropriately."

Absolutely! But I think that there is no disagreement if we distinguish "invariant" and "covariant".

I claim that once you fix the metric, say Minkowski, the physical observables are those that are invariant under the Poincare group, not the full diffeo group (except for the things like R, etc.). For instance, each particle will correspond to some irrep of the Poincare group characterized by the the eigenvalues of the two quadratic Casimir invariants. (p_\mu)^2 is one of them and the other (Pauli-Lubanski)is related to spin. So the bottom line is that when you describe a particular state, the physics is described by the Poincare invariant compinations. When you do a general coordinate transformation, those observables do not stay invariant.

I can't make sense out of what you say. Observables are things one observes, and depend on the observer. You say

"the physical observables are those that are invariant under the Poincare group, not the full diffeo group". Minkowski is a special case because you can define your std. coordinates (x,y,z,t) (up to Lorentz trafos), it's a flat background. Exactly this is the problem in a general background. What is the 'right' t, and the corresponding energy (i \delta_t) or particle definition respectively? It doesn't make sense to say observables are defined only in a special coordinate system as they clearly aren't. If I look at the CMB from earth, there is a dipole moment from our motion relative to the background. The higher frequency of photons we're moving through is an observable. In the CMB restframe, this effect is not there. Of course you can make a transformation of that observable from one coordinate system to the other - by using general covariance. This observable clearly does not stay invariant under general coordinate trafos, and it shouldn't.To come back to your above comment, I hope I explained why general covariance does not imply BGI.

Bee:

" can't make sense out of what you say. Observables are things one observes, and depend on the observer."

OK, but if two different observers are to agree on something the observable has be some sort of invariant, right?

I mean, in my specific example, in a fixed background, the states are labeled by the eigenvalues of quadratic Casimirs of the corresponding isometry group. I mean, the electron has the same rest mass given by (p_\mu)^2 in all frames and that's an observable.

In particle collisions we measure things like the invariant mass etc.

Those are what I call "physical" observables.

"You say "the physical observables are those that are invariant under the Poincare group, not the full diffeo group".

I only gave that example because it is well known. If you choose a different background, say AdS, the

physical states will be classified by the eigenvalues of the quadratic Casimirs of SO(2,3)-the isometry group of AdS_4.

"To come back to your above comment, I hope I explained why general covariance does not imply BGI. "

I never said that you said this.

BI independence implies that no spacetime manifold is necessary to define the QG theory in the sence implied by Moshe.

Best,

K

Hi K,

you wrote above

"Sorry but once you fix the background you break the general covariance."I tried to explain why you can have general covariance on a fixed background. You instead referred to isometries that a fixed background might not have.You can have observable quantities that are not invariant under coordinate transformation, i.e. not scalars. For example, the redshift that I mentioned above. It's definitely observable, yet it depends on the observer/his coordinate system.

You write: OK, but if two different observers are to agree on something the observable has be some sort of invariant, right?They need to know how to relate the measurements from one observer to the other. A scalar is the simplest case, it remains invariant. Vectors and tensors don't, they depend on the observer. For example the density of a medium is not a scalar. Yet, if the both observers know their relative motion they can agree on the value of the density - but it will depend on the reference frame.

Best,

B.

Bee:

"They need to know how to relate the measurements from one observer to the other. A scalar is the simplest case, it remains invariant."

Under Poincare transformations, not under some general diffeos!

K

The last comment is of course for the well known case (Minkowski background) but generically the quantities labeling the quantum states are invariants of the isometry group of the background metric. We have been discussing quantum theories right?

In flat space, general covariance is replaced by Poincare/Lorentz covariance. The equation of motion of the graviton in flat space is "Box"h_\mu\nu=0 which tells you that the graviton is massless.

That equation is not generally covariant but only Lorentz covariant.

K

K - a scalar quantity is defined to be one that is invariant under general coordinate transformations. For example the line element is. And the particle mass. You can look that up in any textbook on differential geometry.

We have been discussing quantum theories right?I have mostly talked about GR, which I think so far is not a quantum theory.

You say: In flat space, general covariance is replaced by Poincare/Lorentz covariance. The equation of motion of the graviton in flat space is "Box"h_\mu\nu=0 which tells you that the graviton is massless.That equation is not generally covariant but only Lorentz covariant.

Look, you are again mixing up isometries with coordinate transformations. You can perfectly well describe physics in flat space using a non-euclidean coordinate system. The transformation into such a system is not an element of the Lorentz-group, yet it's an allowed transformation - of course the equations in Minkowski space are generally covariant.

The problem with the graviton is that the graviton (h) itself is the deviation from the background. Commonly one fixes a gauge for the graviton (look up the chapter in MTW) - it is the gauging that explicitly breaks general covariance (it forbids certain transformations). It's the same with the higgs. It is not the split in v+h that breaks the symmetry - if you introduce that decomposition the Lagrangian still has all symmetries. You break it if you constrain yourself to unitary gauge to get a 'physical' definition of the field h, and the correct mass terms. As we know, these mass terms break the full symmetry.

Best,

B.

Hi Bee,

Maybe one should distinguish "coordinate systems" and "physical frames of reference".

In flat space only inertial frames are "physical", i.e. physical laws stay the same under Lorentz transformations.

On the other hand, if you pick some arbitrary coordinate system corresponding to some rotating frame the physical law will not be the same in such a frame. Take one physical law - the Dirac equation, in flat space. It is said to be Lorentz covariant because its form is preserved under Lorentz transformations. On the other hand, if you perform a non-Lorentz transformation on it it will have a different form, i.e. it won't be covariant.

All of the above does not imply that you cannot use things like spherical coordinates etc, in flat space. However, those are not "physical" frames of reference but simply convenient choices of coordinates.

Sorry, but you are not making any sense. There is nothing 'unphysical' about the coordinate system of an accelerated observer. It's just a coordinate system. What you are trying to say is (I think) there is a difference between a local orthonormal base and a coordinate system. I agree on that. The Dirac eq. \slash D \Psi = 0 is a Lorentz scalar, and generally covariant (\slash D = gamma^nu D_nu with D_nu = \nabla_nu + Gauge fields_nu, and \nabla the Levi Civita Connection). Again, provided that you transform the quantities appropriately (e.g. the anti-comm. of the Clifford algebra is in general g_munu not \eta_munu).

if you perform a non-Lorentz transformation on it it will have a different form, i.e. it won't be covariant.All of the above does not imply that you cannot use things like spherical coordinates etc, in flat space.

It seems to me you still are confused about what general covariance means. The Dirac equation after a general coordinate transformation still has the same 'form' that is \slash D \psi = 0.

Hi Bee,

I thought that in Minkowski space the Dirac equation was (disregarding the gauge field connection and the mass):

\slash\partial\psi=0

which is only Lorentz covariant but not generally covariant. Where am I wrong?

Bee:

"The Dirac eq. \slash D \Psi = 0 is a Lorentz scalar, and generally covariant (\slash D = gamma^nu D_nu with D_nu = \nabla_nu + Gauge fields_nu, and \nabla the Levi Civita Connection). Again, provided that you transform the quantities appropriately (e.g. the anti-comm. of the Clifford algebra is in general g_munu not \eta_munu)."

And how will \psi (a Lorenztz bi-spinor) transform under some general diffeo?

K.

See e.g.

Geometry, Topology and Physics, by M. Nakahara Chapter 7.10.3

a scalar quantity is defined to be one that is invariant under general coordinate transformations.Strictly speaking it's not. An invariant under coordinate transformations is a constant scalar. You still have to transform the argument.

I think a lot of the invariant/covariant confusion comes from people mixing active and passive transformations but if you are careful what you mean you can get rid of all confusion.

I agree with the notion that Einstein's equations are invariant under diffeos while any solution (except for g_mn=0) breaks the diffeo group to its isometries. Still, the diffeos are around as symmetries but you have to pull back the tensors.

I discussed this at quite some length in my secon LQG string paper hep-th/0610193.

I think a lot of the confusion about observables comes from the fact we are used to think in terms of coordinates.

Of course

phi(t=17,x=23,y=0.9,z=-3E15)

is not observable. But what is observable is "the temperature in my office". When I use coordinates this is only short hand for definition of a space-time point in a relational setting: My office is not at specific coordinates but is constituted by a lot of concrete and a sign on the door showing my name (and the fact that I am currently in it). Thus giving the temperature for a specific date and at a certain latitude and longitude and height above ground is just an abbreviation about going to the place where the quark and electron fields conspire to make up a physics institute with a pattern on one of the doors that the absorption of visible light makes my name readable. In terms of these, the observable "temperature" is invariant under diffeos, it's just much more convenient to use a coordinate system tied to these reference positions.

Hi Robert,

yes, I agree, the argument still has to be transformed. Best,

B.

Bee:

"General covariance is invariance under arbitrary coordinate transformations. You can have that on a fixed background. What I said is that the background itself should be subject to a general covariant evolution law, as e.g. it is in GR."

I disagree!

There should be "no prior geometry" for a "geometric, coordinate independent formulation of physics".

By "prior geometry" one means any aspect of the geometry of spacetime that is fixed immutably, i.e., that cannot be changed by changing the distribution of gravitating sources.

The "no prior geometry" demand actually fathered general relativity, but by doing so anonymously, disguised as "general covariance", it also fathered half a century of confusion.

See MTW 17.6.

Hi Anonymous:

Neglecting the question of who fathered whom, I don't know what you disagree on, since you say exactly what I say too. Best,

B.

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