Removal of BH singularity by LQG

[Martin]

this is a lively ongoing line of research in LQG

a new paper by Bojowald et al just came out this month

 

http://arxiv.org/gr-qc/0503041

A black hole mass threshold from non-singular quantum gravitational collapse

Martin Bojowald, Rituparno Goswami, Roy Maartens, Parampreet Singh

4 pages, 3 figures

Report-no: AEI-2005-020,IGPG-05/3-3

 

"Quantum gravity is expected to remove the classical singularity that arises as the end-state of gravitational collapse. To investigate this, we work with a simple toy model of a collapsing homogeneous scalar field. We show that non-perturbative semi-classical effects of Loop Quantum Gravity cause a bounce and remove the classical black hole singularity. Furthermore, we find a critical threshold scale, below which no horizon forms -- quantum gravity may exclude very small astrophysical black holes."

 

there is a link there to the full 4-page article, in PDF, if you want to read it

[Johnny5]

this is a lively ongoing line of research in LQG

a new paper by Bojowald et al just came out this month

 

http://arxiv.org/gr-qc/0503041

 

I keep hearing about Martin Bojowald' date=' what is his story?

 

Also, I began reading the link and two questions immediately sprang to mind, although I don't know if you can answer them.

 

1. On page two, he mentions the condition for quantization of gravity when he specifies a "fundamental length scale" then out of nowhere appears the following equation:

 

[math'] l_* = .28 \sqrt{j} _p [/math]

 

Where did that come from? <--- that's my first question

 

2. On page two, he mentions that while we intuitively think of gravity as always attractive, that in LQG there are conditions in which gravity is repulsive. He talks about a bounce. So my second question is, in LQG, can gravity be repulsive? Yes or no.

 

 

A followup question would be, "If gravity can be repulsive, does that invalidate the principle of equivalence?"

[Martin]

I keep hearing about Martin Bojowald' date=' what is his story?

 

[/quote']

 

Bojo is 31 this year (born in 1974)

he got his PhD at Aachen in 2000

his contributions to Quantum Gravity are among the most important

(his getting rid of the bigbang singularity in 2001 has led to a lot of other research by himself and other people----had major consequences for quantum cosmology----and he now seems to be nearing the goal of getting rid of the blackhole singularty, having done so in a model with simplified matter).

I expect Bojo will get a Nobel when he is older and the importance of current research in LQG is assessed.

 

Major physicists have a geneology that goes by who your thesis advisor was,who guided your first big research project. Martin is in this line:

 

Sommerfeld/Heisenberg/Kastrup/Bojowald

 

http://www.physcomments.org/wiki/index.php?title=Genealogy::nobel

 

Sommerfeld is the guy who first found the fine structure constant 1/137.036.. Sommerfeld had especially creative students and grand-students

in the geneology.

 

Sommerfeld begat Wolfgang Pauli and Hans Bethe and Werner Heisenberg

(if you look at the Wiki geneology link, scroll down to what the author calls the "founding fathers branch" and you will see where Bojo fits in.)

these guys pass a kind of intuition on to each other by working together on deep or leading edge questions. there is a transfer of creativity and the ability to cut thru the rubbish. also a self-selective process

 

after PhD Aachen, Bojowald went for a postdoc fellowship to Abhay Ashtekar at Penn State. that was where he removed the BB singularity.

then in Fall of 2003 he went to the Albert Einstein Institute at potsdam near berlin (Penn State, AEI, and perimeter are the 3 top places to be for quantum gravity and the postdocs circulate. Maybe also Marseille because Rovelli is there). Now if you are a bright grad student or postdoc in QG then one of the best places for you to go is AEI to work with Bojowald (and the other quantum gravitists there)

 

AEI is actually in a small place called Golm, right near Potsdam. these places are suburbs on the west side of Berlin. If someone says AEI-Golm he means the same thing as AEI-Potsdam. it can be confusing.

 

You say what is Bojowald's story? Do you have a more specific question. I am just giving a scatter of information.

 

there was a write up about him in the journal Nature not long ago

 

here is a link to a list of his papers:

http://arxiv.org/find/grp_physics/1/au:+bojowald/0/1/0/all/0/1

 

at present there are 50 papers in the list (he is a hard worker and the field of quantum gravity is very exciting, his subfield quantum cosmology is especially active, so it is not too surprising that he has done so much work in the past 5 or 6 years)

 

somewhere I have a link to a snapshot taken of Martin at a May 2004 conference organized by Rovelli at marseille.

yeah, here it is

 

http://perimeterinstitute.ca/images/marseille/marseille017.JPG

 

M.B. is the guy not looking at the camera. The guy with him is Alejandro Perez

 

If you want to listen to M.B. talk about QG cosmology go here:

http://phys.psu.edu/events/

select "Spring 2003" to get a list of the talks given at penn state that semester. somewhere on that list is

Bojowald:Quantum Cosmology: An Overview

and clicking on that gets you the slides and the audio.

download the audio first (because it takes a few minutes to download during which you can be doing other things) and then when you play the audio follow along with the slides.

In the same collection there is also another Bojowald talk, Quantum Cosmology: Formalism

[Johnny5]

Martin, I am looking at his lecture on Quantum Cosmology, and he is referring to l_p, is that the same l_p, that was in the paper I was just reading on LQG?

 

Also, I see him talking about the Wheeler Dewitt equation, he has ( l_p)² = n hbar, then he writes Planck length, then he writes phi: matter field.

 

Does any of that make sense to you?

 

Do you know what the Wheeler Dewitt equation is trying to say?

 

Thanks, because this is all new to me, LQG that is.

[Martin]

Martin' date=' I am looking at his lecture on Quantum Cosmology, and he is referring to l[sub']p[/sub], is that the same l_p, that was in the paper I was just reading on LQG?

 

Also, I see him talking about the Wheeler Dewitt equation, he has ( l_p)² = n hbar, then he writes Planck length, then he writes phi: matter field.

 

Does any of that make sense to you?

 

Do you know what the Wheeler Dewitt equation is trying to say?

 

Thanks, because this is all new to me, LQG that is.

 

I think when he says "ell-sub-p" he means the conventional planck length l_p

you get by setting |G|=|hbar|=|c| =1

 

it COULD also be the variant which you get by

|8piG|=|hbar|=|c| =1

 

but that only differs by a factor of about 5 from the conventional one (only 5 times longer) so it doesnt make a lot of difference. lets suppose he means the conventional planck length

 

the Wheeler de Witt ("WdW") equation is the famous old first equation of quantum gravity from around 1970----it was part of a failed attempt.

IIRC it is kind of like a Schroedinger equation where the scale factor of the universe is the variable (instead of the position of a particle) and so it describes an expanding universe of uncertain size.

 

IIRC the WdW equation was or gave rise to a kind of naive quantized version of the Friedman equation, which is the main equation of classic cosmology and the basis of the bigbang picture.

 

The TROUBLE with the WdW, or one of the troubles, was that it failed to remove the BB singularity. It failed to compute at the same place the classic theory did. It ran into infinities just like the unquantized thing!

 

I am not quite sure about these details so i would have to go refresh my memory. but basically it was the equation of quantum cosmology until circa 1990, the equation everybody knew wasnt right but couldnt think how to fix.

 

John Archibald Wheeler is a great figure in 20th Cent. theoretical physics. Feynmann was his postdoc at princeton. wheeler is old now.

Bryce DeWitt has died.

they figured the WdW eqn out at the Raleigh-Durham airport where Wheeler was waiting between planes

[Johnny5]

If you don't mind my saying so, you know too much about LQG to be just a physics watcher.

[Martin]

as you move forwards in time away from the big bang, away from the classical failure point,

 

then the WdW approximates the classic behavior of the Friedman equation, so it is good in that it has a nice familiar behavior as long as you stay away from its singularity

 

and the Bojowald equation for the evolution of the universe approximates the WdW very closely very quickly

 

after only 100 or so planck time units (an unimaginably short time) the Bojowald model has already homed in on the right semiclassical limit and is cranking out the same numbers as the semiclassical model.

 

So basically you only need to use the quantum LQG model for a tiny fraction of a second right around where the classical model breaks down!

the rest of the time you can compute everything with the familiar old Friedmann that has been used since 1922 or so! (a simplified version of Einstein 1915)

 

this is a really satisfying thing about the way bojowald quantum cosmology has worked out.

 

we should look for a kind of introductory textbook-style presentation. these journal articles are so concentrated and dont explain their notation. I will try to come up with an older more long-winded bojowald paper that is easier to get into

[Martin]

If you don't mind my saying so, you know too much about LQG to be just a physics watcher.

 

I am just a watcher and i would not kid you about this,

but thanks for the implied compliment!

actually (on another subject) the information terrain is very rough here.

I cant think of good introductory reading material

and that seminar talk that you can download the audio, that is also

very sketchy and rough. probably this is how new research paths tend to be.

I will keep an eye out for better than average exposition but cant be sure

of finding any. you can look on your own too and maybe you will be lucky

[Johnny5]

I am just a watcher and i would not kid you about this' date='

but thanks for the implied compliment!

actually (on another subject) the information terrain is very rough here.

I cant think of good introductory reading material

and that seminar talk that you can download the audio, that is also

very sketchy and rough. probably this is how new research paths tend to be.

I will keep an eye out for better than average exposition but cant be sure

of finding any. you can look on your own too and maybe you will be lucky[/quote']

 

Gotcha, yes I know it's going to be hard to find any average exposition. Don't try too hard, but if you happen to find one, then let me know. I have already learned a great deal. I am still intrigued by this notion that matter couples to space in GR, and I am intrigued by any suggestion that LQG goes against GR.

 

I am still just trying to absorb some of what I've seen you talk about.

 

Friedman equation

Wheeler Dewitt equation

Bojowald, Roselli

 

You also mentioned that we are trying to understand the structure of space. There is a lot for me to think about here. I've seen talk about matter coupling to space, and I have no clue what that means, or if it's even meaningful. I am a cautious thinker. I have a reason for understanding physics, which doesn't even have anything to do with LQG, yet it might for all i know.