Unification and Why...

[ajb]

As an aside, I am going to a workshop and conference on Continuum and Lattice Approaches to Quantum Gravity at Sussex. Renate Loll and many others will be there also.

 

There is also a school on Non-Perturbative Methods in Quantum Field Theory.

Edited September 5, 2008 by ajb

[Martin]

As an aside, I am going to a workshop and conference on Continuum and Lattice Approaches to Quantum Gravity at Sussex. Renate Loll and many others will be there also.

 

There is also a school on Non-Perturbative Methods in Quantum Field Theory.

 

 

Great that you are making it to that workshop! I saw the announcements for the workshop (and the two-day preparation school preceding it) some weeks ago. It is exciting to get those people who are leaders in several different approaches together in the same workshop.

I have to say: I have the highest respect for both Renate Loll and for John Barrett (who organized the big annual quantum gravity conference this year at Nottingham). And the last time i looked at the program, some weeks back, it looked like BOTH Loll and Barrett were giving talks.

And even tho their approaches to QG are quite distinct------Barrett does spinfoam and also noncommutative geometry, while Loll does computer simulations and a Monte Carlo quantum geometry path integral based on simplex building blocks----there is some indication that their approaches are converging. Laurent Freidel, who I think will also be at the workshop, has just published a paper where he shows how spinfoam (Barrett's baby) can bridge over and connect to a triangulations path integral (Loll's baby). This issue of convergence is bound to come up. It will be interesting if you can see how Loll, Barrett, and Freidel interact (if they all do actually take part.)

There are other good things about that workshop. You are wise to attend. will you also attend the two-day school right before it? Let us know your impressions. It's one event I really wish I could be there.

Edited September 5, 2008 by Martin

[ajb]

I am going to be in the Brighton area at the time, so I thought I would attend. I will be finishing of my PhD in part in Brighton.

 

My primary interest was to get a better idea of non-perturbative methods. Hopefully, the workshop will not be to far away from my current expertise and I can get something out of it. Should be fun whatever.

[Martin]

... I will be finishing of my PhD in part in Brighton.

...

 

sound like some good news on the way. any estimate of when we can offer our congratulations?

[ajb]

I have be allowed another year to submit and have the viva. I have a few partial results, so I have an idea of what will develop for the thesis.

 

By 6 months time I really want to have some better results to present.

[Martin]

...

There are other good things about that workshop. You are wise to attend. will you also attend the two-day school right before it? ...

Let us know your impressions. It's one event I really wish I could be there.

 

I checked the list of participants and answered my own question. I feel sure that you'll get a lot out of it. the list of grad students and postdocs who are attending the two-day school is already very impressive.

 

Andy Randono is an Ashtekar Penn State postdoc who recently discovered a quantization of the size of the early universe in Loop Cosmology----stepwise expansion. He presented the result this spring at a conference in Greece.

His PhD research in LQG at U. Texas Austen was distinguished.

 

Pedro Machado and Dario Benedetti are current and former Loll Utrecht PhD students. Benedetti is now postdoc at Perimeter. Loll has a track record of raising strong PhDs. She and her co-author Ambjorn will be there too of course but I'm just looking at the young generation.

 

Frank Saueressig was a Utrecht postdoc who has worked with Reuter on asymptotic safety gravity---I think he's taken a research position at Saclay in France now. Reuter will be lecturing of course. The Reuter approach, asymptotic safety using the renormalization group fixed point, is very big at the school and workshop----Reuter, Percacci, Bonanno, Rahmede, Lauscher, Niedermaier, Saueressig, and possibly several others---all those people work on asymptotic safety. It's big because, using this novel non-perturbative approach, gravity (which was thought to be non-renormalizable) turns out to be renormalizable after all, in a certain sense. Among other things this undermines the rationale for more far-fetched stuff, extra dimensions and all, which might turn out in the end not to be needed. Some of the current excitement must be due to that, I think

 

To my ability to judge, the school participants look like a pretty select group---promising younger generation plus senior researchers whose research is making definite progress.

http://www.ippp.dur.ac.uk/Workshops/08/NPMQFT/Participants/

Edited September 6, 2008 by Martin

[ajb]

To my ability to judge, the school participants look like a pretty select group---promising younger generation plus senior researchers whose research is making definite progress.

http://www.ippp.dur.ac.uk/Workshops/08/NPMQFT/Participants/

 

Not quite, they let me in!

 

The school looks good, hopefully I will learn a bit more about nonperturbative methods, the renormalisation group and the asymptotic safety of gravity. All things I am awareof , but thats about it. Most of my studies so far comes from the geometric structures arising from perturbative field theory. To get a little more acquainted with other approaches is part of my "general education" as opposed to forming any part of my research at the moment. That said I am sure there will be lots of geometry here, even if it is a little hidden in the physics.

[vacuodynamic]

I think I understand your question. It has to do with what AJB said, namely "what is going on" and what math models you use to calculate what is going on---when two masses attract.

 

Don't be put off by fine verbal distinctions, if your question could just as well be phrased how do masses attract each other?

 

Is that what you mean to ask? Probably. Anyway it is a good question.

There are various mathematical models of what is going on, and the simplest is only approximate.

 

there are layers of explanation to dig down thru.

 

the Newtonian description of how two masses attract uses point masses and force vectors. it is only approximate. it doesn't give the right answer for Mercury's orbit, for example. but it is nearly right for most things

 

then you go deeper and ask how does it really work and what is really going on, and you get another better but still partial, answer. the dynamic metric (or distance function) of Gen Rel. there is a metric that descibes geometry, and the metric is influenced by mass, energy, momentum----in simple cases it boils down to the energy density in space. (and the masses you asked about have their energy equivalent and contribute to that density.)

 

so this is still pretty simple----the two masses affect the energy density in space, and that affects the shape, the geometry, by affecting the metric (the distance function, also called the gravitational field----in Gen Rel the gravitational field is simply the geometry of space and time, gravity is nothing but geometry)

 

the energy density affects the geometry and the geometry detrmines the orbits----and now we get the right answer for Mercury and all that good stuff out to the 6 or 7th decimal place. :smile:

 

but you can STILL ask, and you can still try to dig down to a deeper layer!

In science you can always do this. there is always a deeper law underlying the law you are told. you can ask how does the distribution of mass affect the geometry.

this gets into the field called quantum gravity. it asks some more basic questions that underlie Gen Rel.

What is the microscopic geometry of space and time?

At some level matter and geometry must be the same thing, described by the same mathematical descriptors (technically called degrees of freedom) so that since they are the same thing it is obvious how they interact. At a deeper microscopic level, matter must simply be a facet or aspect of geometry (according to some quantum gravity models at least) so it is no longer a problem of how one affects the other---they are the same.

OK that is speculative, but you see one way how it could go.

 

the basic question, then, at this level is What is the microscopic geometry of space and time? The degrees of freedom, the descriptors, that underlie Gen Rel---and probably behave according to quantum mechanics rules.

 

So if you want to see what the people are doing that dig down to that level, and it is work in progress, then one excellent window on it is the Loll article in Scientific American. I have the link in my sig. The Loll group does computer simulations of the microscopic structure of spacetime. they have little random universes pop into existence in the computer, and they study them. they run random walks that wander around inside them, they measure stuff about the little quantum universes. and they average them up to get a smooth largescale average somewhat like our observed universe.

 

there are many types of quantum gravity research, but this one has reached a very concrete stage, so it is an accessible example of where a lot of quantum gravity research is heading and will probably be going.

And it is an easy reading article with lots of pictures, so it gets across the main ideas.

 

so if you want to know what underlies Gen Rel and how matter affects geometry of spacetime, then the question you should have in mind is one that current research is working on (and hasn't answered conclusively yet) namely what is the quantum microscopic structure of spacetime that underlies General Relativity?

 

Does this respond to your question, or did you have something else in mind?

 

Dear Martin and friends,

 

Thank you for trying to explain the question. What I have in mind is that I am not understood why some physicists have so much trouble about “why” in science? For me it is quite straight forward. Of course it is not my own idea, what I asking is the same thing as Newton was asked about his theory of gravity and he had answered, as was told by Richard P. Feynman in his book “The Characteristic of Physical Law” page 37 as follow;

 

“… Newton was originally asked about his theory ….. He said, ‘….I have told you how it moves, not why.’ But people often are unsatisfied without a mechanism, ….”.

 

So what you have said that “what underlies GR and how matter affects geometry of space-time” is about the “mechanism” of gravity which will be the answer for the question “why masses attract each other” isn’t it?

 

Sincerely

[Martin]

...

So what you have said that “what underlies GR and how matter affects geometry of space-time” is about the “mechanism” of gravity which will be the answer for the question “why masses attract each other” isn’t it?

...

 

As long as we understand each other, let's not worry too much about which word we use---how or why.

 

In ordinary speech, I don't think there is a sharp consistent distinction.

 

If there is as useful distinction in the context of common everyday speech, then maybe it is this: how is recognized to have some degree of finality---why is more subject to infinite regress.

 

don't make too much of the difference, which is not all that hard and fast, but a child can keep asking why (because each successive reason you give can be the subject of the next inquiry)----but by contrast once you have given an adequate description of how something happens you have done what you set out to do and that's it: finito.

 

you can say that what Newton really did, by his response, was to shut the guy up. and this way of terminating the discussion is especially appropriate to mathematical sciences where one describes processes using a mathematical model.

 

once you give a mathematical model that accurately fits observation, that's it: finito. the only way to continue arguing is to discover data where the fit is not so good (like the orbit of Mercury) and demand an improvement of the mathematical model.

 

a mathematical model is very howish and not very whyish. because nobody knows why mathematical models work----why should Nature allow herself to fit into the corset and brassiere of mathematics? that's just how it seems to work.

 

if some joker asks why Nature obeys mathematical models, the only thing to do is kick him out of your office----nobody knows why and he should know better than to ask. tell him go over to the philosophy building. :biggrin:

 

etc etc I am oversimplifying and giving you a cartoon version.

 

from a common everyday language standpoint it's mostly just a quibble and probably best to pay no attention.

 

(if you hang around the physics building and listen to physicists talking among themselves, they use the word "why" all the time! it means "explain the mathematical model to me with some more fundamental mathematics, elucidate, prove, provide intuition....etc"------the rigid distinction is in part merely a defensive façade, a way of coping with nonmathematical outsiders and shutting off an infinite succession of verbal questions one is not prepared to answer)

Edited September 6, 2008 by Martin