Jump to content


Photo
- - - - -

Quantum Gravity


  • Please log in to reply
153 replies to this topic

#1 Martin

Martin

    Physics Expert

  • Resident Experts
  • 4,596 posts
  • LocationSF Bay Area

Posted 2 September 2004 - 09:39 PM

the aim of this thread is to gather essential information from all quantum gravity-related threads into one, to get some focus
Links to other SFN threads will be provided, and links to outside sources

Smolin: How far are we from the quantum theory of gravity?
http://arxiv.org/abs/hep-th/0303185

Rovelli: Quantum Gravity Cambridge Press 2004
http://www.cpt.univ-...li/rovelli.html

Smolin: Invitation to Loop Quantum Gravity
http://arxiv.org/hep-th/0408048

Ashtekar Lewandowski: Background Independent Quantum Gravity; a Status Report
http://arxiv.org/abs/gr-qc/0404018

Smolin: Scientific Alternatives to the Anthropic Principle
http://arxiv.org/hep-th/0407213

Rovelli:Strings, loops and others: a critical survey of the present approaches to quantum gravity
http://arxiv.org/gr-qc/9803024
(Plenary lecture on quantum gravity at the GR15 conference)

Vaas:The Duel: Loops versus Strings
http://arxiv.org/abs/physics/0403112

to be continued
  • 0
Loll quantum gravity SciAm
http://www.signallak...uantumJul08.pdf
cosmology SciAm
www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
http://www.einstein-...logy/index.html

#2 Martin

Martin

    Physics Expert

  • Resident Experts
  • 4,596 posts
  • LocationSF Bay Area

Posted 2 September 2004 - 10:01 PM

Quantum Gravity Phenomenology conference Feb 4-14, 2004
http://ws2004.ift.un...oc.pl/html.html
(some talks are downloadable in PDF at this site, others are posted
at arxiv.org)

Vaas: Beyond Space And Time
http://arxiv.org/physics/0401128

Vaas: The Inverted Big-Bang
http://arxiv.org/physics/0407071

Smolin: Atoms of Space and Time
Scientific American, January 2004

NIST fundamental constants website:
http://physics.nist.gov/cuu/Constants/

Click "Universal" and you will get
http://physics.nist....4&Universal.y=9

which has values for planck units, among other things
Planck Length
Planck Mass
Planck Temperature
Planck Time

We discussed the fine structure constant (one of those
which Smolin has suggested may have evolved by
Cosmological Natural Selection or CNS) in this SFN thread:
http://www.sciencefo...89&page=1&pp=20

CNS is strengthened by Quantum Gravity's elimination of the black hole
singularity so that inflation of a new universe can begin at the bottom
of any black hole

Modesto: Disappearance of the Black Hole Singularity in Quantum Gravity
http://arxiv.org/gr-qc/0407097
  • 0
Loll quantum gravity SciAm
http://www.signallak...uantumJul08.pdf
cosmology SciAm
www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
http://www.einstein-...logy/index.html

#3 Martin

Martin

    Physics Expert

  • Resident Experts
  • 4,596 posts
  • LocationSF Bay Area

Posted 2 September 2004 - 10:16 PM

The planck scale is so basic to LQG that the flat limit in that approach to quantizing gravity will be apt to be a modified version of Special Rel in which more than just the speed of light appears the same to all observers.

this will involve a modification of the energy-momentum relation in SR.

predictions based on this are due to be tested by GLAST starting around 2007 and are discussed in several papers such as Smolin's "Invitation" and "How far are we..." links given earlier.

some papers about modified Special Rel and related matters:

Girelli Livine Oriti:Deformed Special Relativity as an effective flat limit of quantum gravity
http://arxiv.org/gr-qc/0406100

Smolin,Jerzy K-G: Triply special relativity
http://arxiv.org/hep-th/0406276

Moffat: Modified Gravitational Theory as an Alternative to Dark Energy and Dark Matter
http://arxiv.org/astro-ph/0403266

Notale: The Pioneer anomalous acceleration: a measurement of the cosmological constant at the scale of the solar system
http://arxiv.org/gr-qc/0307042

a seemingly obscure point: doubly and triply special rel are mathematically possible---one can have not just one scale (speed of light) invariant but one can have 2 and even 3 invariant scales (a special length a special energy...) but one cannot have 4 or more. triply special, or 3-scale relativity is the max. could that be what we have?
Chryssomalakos and Okon
Linear Form of 3-scale Relativity Algebra and the Relevance of Stability
http://arxiv.org/hep-th/0407080
  • 0
Loll quantum gravity SciAm
http://www.signallak...uantumJul08.pdf
cosmology SciAm
www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
http://www.einstein-...logy/index.html

#4 Martin

Martin

    Physics Expert

  • Resident Experts
  • 4,596 posts
  • LocationSF Bay Area

Posted 2 September 2004 - 11:52 PM

Usenet Physics FAQ
http://math.ucr.edu/home/baez/physics/

some John Baez links:

http://math.ucr.edu/home/baez/FUN.html

How many fundamental constants are there?
http://math.ucr.edu/.../constants.html

The meaning of Einstein's equation.
http://math.ucr.edu/...n/einstein.html

General Relativity tutorial:
http://math.ucr.edu/...baez/gr/gr.html
(includes "Short Course Outline")

From Baez Quantum Gravity writings,
an article on Planck scale physics:
http://math.ucr.edu/...nck/planck.html
and a continuation of that "Quantum Quandaries"
http://math.ucr.edu/...um/quantum.html

Baez Quantum Gravity seminar notes:
http://math.ucr.edu/home/baez/QG.html
  • 0
Loll quantum gravity SciAm
http://www.signallak...uantumJul08.pdf
cosmology SciAm
www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
http://www.einstein-...logy/index.html

#5 Thales

Thales

    Atom

  • Senior Members
  • 359 posts
  • LocationQueenstown, NZ

Posted 3 September 2004 - 01:37 AM

Wow, now thats alot of information to trudge through. Good work Martin. i'll get started on it after I finish the latest paper I am working on. Still a while off yet, trying to gain a qualative understanding of the universe is almost as difficult as a quantative one.
  • 0
I am the exception that makes the rules...

#6 Martin

Martin

    Physics Expert

  • Resident Experts
  • 4,596 posts
  • LocationSF Bay Area

Posted 3 September 2004 - 01:39 AM

Ambjorn Jurkiewicz Loll: Emergence of a 4D World from Causal Quantum Gravity
http://www.arXiv.org.../hep-th/0404156


Gambini Porto Pullin: Realistic clocks, universal decoherence and the black hole information paradox
http://arxiv.org/abs/hep-th/0406260

Gambini Porto Pullin: No black hole information puzzle in a relational universe
http://arxiv.org/hep-th/0405183.

Fleischhack: Representations of the Weyl Algebra in Quantum Geometry
http://arxiv.org/abs/math-ph/0407006
(goes beyond earlier results of Sahlmann, Thiemann, Lewandowski, Okolow.)

three recent ones of Date Hossain:
Genericity of Big Bounce in isotropic loop quantum cosmology
http://arxiv.org/gr-qc/0407074

Effective Hamiltonian for Isotropic Loop Quantum Cosmology
http://arxiv.org/abs/gr-qc/0407073

Genericity of inflation in isotropic loop quantum cosmology
http://arxiv.org/abs/gr-qc/0407069

an odd one by some people i havent heard of before:
Corrections to the Planck's radiation law from loop quantum gravity
http://arxiv.org/abs/hep-th/0407072
  • 0
Loll quantum gravity SciAm
http://www.signallak...uantumJul08.pdf
cosmology SciAm
www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
http://www.einstein-...logy/index.html

#7 Martin

Martin

    Physics Expert

  • Resident Experts
  • 4,596 posts
  • LocationSF Bay Area

Posted 3 September 2004 - 01:43 AM

Wow, now thats alot of information to trudge through. Good work Martin. i'll get started on it after I finish the latest paper I am working on. Still a while off yet, trying to gain a qualative understanding of the universe is almost as difficult as a quantative one.


be selective Thales, no need to trudge thru it all.
I gathered the links so they wouldnt get mislaid and I could hopefully find something about future topics as they come up. Sorry if it gave the impression that one person should wade thru everything!

while I'm here I will stash some links to Stacy McGaugh's MOND site
(modified newtonian dynamics---to explain galaxy rotation curves w/o dark matter)
http://www.astro.umd...d/mondvsDM.html
http://www.astro.umd...ond/litsub.html
http://www.astro.umd.edu/~ssm/mond/
http://www.astro.umd.edu/~ssm/

and here is an collection of QG links assembled by a string theory enthusiast who has acquired a taste for LQG as well. the Egyptian artwork is a bonus:
http://wc0.worldcros...5Y.43@.1de0f3cf
I havent examined these links in detail so can't vouch for their overall quality or usefulness but I can see there are some goodies in the batch.
  • 0
Loll quantum gravity SciAm
http://www.signallak...uantumJul08.pdf
cosmology SciAm
www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
http://www.einstein-...logy/index.html

#8 Martin

Martin

    Physics Expert

  • Resident Experts
  • 4,596 posts
  • LocationSF Bay Area

Posted 3 September 2004 - 01:48 AM

Thales, actually I am wondering now what paper(s) you are reading and what particular qualitative questions about cosmos you have in mind now.
did you start a thread about it? delighted if you'd start a thread about your own focus of interest

whoa! I answered my own question, sort of, by checking out the threads you'v been posting----double slit, entanglement, lightspeed, basic concepts.
So I have a rough idea.

if you ever have time and want discussion about quantum gravity basics, or an introduction to it, let me know.
  • 0
Loll quantum gravity SciAm
http://www.signallak...uantumJul08.pdf
cosmology SciAm
www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
http://www.einstein-...logy/index.html

#9 Martin

Martin

    Physics Expert

  • Resident Experts
  • 4,596 posts
  • LocationSF Bay Area

Posted 3 September 2004 - 03:01 AM

in LQG the operators of measuring area and volume are quantized and have discrete spectra. measuring such geometrical quantities can only give as a result certain multiples of the planck area and the planck unit of volume.
the planck units play an essential and pervasive role in LQG, to an extent that the flat limit of the theory may have to be a modified Minkowski space in which one or more other quantities besides the speed of light appear the same to all observers. familiarity with planck units is helpful in understanding Quantum Gravity.

NIST fundamental constants website:
http://physics.nist.gov/cuu/Constants/

Click "Universal" and you will get
http://physics.nist....4&Universal.y=9

which has values for planck units, among other things
Planck Length
Planck Mass
Planck Temperature
Planck Time


in a certain sense the Planck units are all one unit. the unit energy is that of a photon with unit wavelength and unit frequency, the unit temperature is the one associated by E=kT to unit energy, the unit mass is the one corresponding by E=mc2 to unit energy, the unit of speed (one unit of distance per unit of time) is the speed of light.

there is a thematic quote from Italo Calvino from the first paragraph of chapter 3 of "Six Memos" where he recalls a lecture on the Egyptian worldview given by the philosopher George Santayana:

"For the ancient Egyptians, exactitude was symbolized by a feather that served as a weight on scales used for the weighing of souls. this light feather was called Maat, goddess of the scales. The hieroglyph for Maat also stood for the unit of length---the 33 centimeters of the standard brick---and for the fundamental note of the flute."
  • 0
Loll quantum gravity SciAm
http://www.signallak...uantumJul08.pdf
cosmology SciAm
www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
http://www.einstein-...logy/index.html

#10 5614

5614

    Genius

  • Senior Members
  • 6,428 posts
  • LocationLondon, UK

Posted 3 September 2004 - 09:01 AM

now theres a lot and a lot of links, does anyone read ALL of them? if i saw one, two maybe three links, i'd read it, but that would takes hours to go through, which are the best?
  • 0
[acr=Hi, hope you have a nice day :)]Jonathan aka 5614[/acr][acr=Hi, hope you have a nice day :)]
---
So, is the universe indeterministic? Probably![/acr]

#11 Martin

Martin

    Physics Expert

  • Resident Experts
  • 4,596 posts
  • LocationSF Bay Area

Posted 3 September 2004 - 03:20 PM

now theres a lot and a lot of links, does anyone read ALL of them? if i saw one, two maybe three links, i'd read it, but that would takes hours to go through, which are the best?


the short list consists of just one paper

Smolin: An Invitation to Loop Quantum Gravity
http://arxiv.org/hep-th/0408048
  • 0
Loll quantum gravity SciAm
http://www.signallak...uantumJul08.pdf
cosmology SciAm
www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
http://www.einstein-...logy/index.html

#12 Severian

Severian

    Scientist

  • Senior Members
  • 4,077 posts

Posted 3 September 2004 - 03:53 PM

I personally prefer String Theory to LQG. I am not terribly sure why to be honest, perhaps it is just more of a gut feeling (no pun intended).

Perhaps the biggest attraction of string theory is supersymmetry. Supersymmetry has lots of nice features and motivations for both low and high energy. For me, only one of these is sufficient: it is the only (non-trivial) way to extend the Poincare algebra. So far every particle physics theory which has been successfully confirmed by experiment is based on symmetry groups. The question automatically arises 'why these symmetry groups?'. I feel that this will probably be answered by finding that the symmetry of the universe is the largest symmetry we can have without being inconsistent, and the low energy symmetry groups we see now are the remaining fragments left over from some symmetry breaking mechanism. If that is true, then shouldn't space-time be maximally symmetric too, which in turn would naturally lead to supersymmetry.

Now while I admit that LQG can be consistent with supersymmetry, it is rather auxilliary. By contrast in string theory, gravity is local supersymmetry - supersymmetry is completely fundamantal.
  • 0

#13 Martin

Martin

    Physics Expert

  • Resident Experts
  • 4,596 posts
  • LocationSF Bay Area

Posted 3 September 2004 - 04:18 PM

I personally prefer String Theory to LQG. I am not terribly sure why to be honest, perhaps it is just more of a gut feeling (no pun intended).

Perhaps the biggest attraction of string theory is supersymmetry...


hi Severian, nice to hear from you! As you correctly point out, there does seem to be a debate around the String vs. Loop issue, with some partisanship.
(I consider string and loop to be inevitable collaborators in the search for a quantum theory of gravity---the rivalry is secondary and something of a distraction)

We should gather some links about the debate between rival viewpoints.
Here is the Susskind-Smolin debate at Edge:
http://www.edge.org/...ve/edge145.html

Here is a discussion involving Lee Smolin, Urs Schreiber, Thomas Larsson, and others at the Not Even Wrong site:
http://www.math.colu...ves/000072.html

Here is a recent perspective on string theory from Warren Siegel, author of a textbook on Quantum Field Theory, and a substantial corpus of string theory research:
http://insti.physics.../research.shtml

Yesterday a continuation of the Not Even Wrong discussion:
http://www.math.colu...ves/000076.html
---sample---
Serenus Zeitblom: "I suspect that this blog has a large audience of believers in string theory [like me]"

Thomas Larsson: "Serenus, while I don't doubt your sincerity, I seriously don't understand how you (and others) can believe in string theory.

We all know that string theory makes no hard predictions (hence the name of this forum), but it makes quite a few soft predictions, e.g. supersymmetry, extra-dimensions, new gauge bosons, a negative cosmological constant, new long-range forces (massless scalar particles associated with moduli), etc. The most striking thing with these soft-predictions is that none of them has been confirmed by observation, and some (the negative cosmological constant and perhaps also supersymmetry) may even be in direct conflict with experiments..."

Posted by Thomas Larsson at September 2, 2004 06:55 AM
---end quote---
  • 0
Loll quantum gravity SciAm
http://www.signallak...uantumJul08.pdf
cosmology SciAm
www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
http://www.einstein-...logy/index.html

#14 Severian

Severian

    Scientist

  • Senior Members
  • 4,077 posts

Posted 3 September 2004 - 09:07 PM

Well, we only have to wait until 2008 to see if supersymmetry is true. If it is, the next big question will be 'How is it broken?'
  • 0

#15 Martin

Martin

    Physics Expert

  • Resident Experts
  • 4,596 posts
  • LocationSF Bay Area

Posted 4 September 2004 - 12:48 AM

Well, we only have to wait until 2008 to see if supersymmetry is true. If it is, the next big question will be 'How is it broken?'


Meanwhile, let's build up some online LQG sources as a reference or bibliography. (If you want to start a String or a Supersymmetry bibliography in another thread that would be great!)

Thales gave me this: Seth Major's page of Quantum Gravity links.
http://academics.ham.../resources.html

this is the intense green page which has a lot of the links we have already plus others we dont.
  • 0
Loll quantum gravity SciAm
http://www.signallak...uantumJul08.pdf
cosmology SciAm
www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
http://www.einstein-...logy/index.html

#16 Martin

Martin

    Physics Expert

  • Resident Experts
  • 4,596 posts
  • LocationSF Bay Area

Posted 5 September 2004 - 01:10 AM

Severian reminded us of the Particle Data Group annual
(the most cited pages in all high energy physics)
http://pdg.lbl.gov/
it has tons of data on particles, as the name suggests
here's a more detailed table of contents
http://pdg.lbl.gov/2...nts_sports.html

earlier I put a link to the NIST fundamental constants site
which gives just one (recommended) value, with error bounds, for
each physical constant

the PDG report is far more inclusive and
lists all the measurements made of something
and says who measured it. it is a monument of information
but you may have to download many PDF pages
whereas with NIST it is all immediately available in HTML
and somebody has boiled it down some.
  • 0
Loll quantum gravity SciAm
http://www.signallak...uantumJul08.pdf
cosmology SciAm
www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
http://www.einstein-...logy/index.html

#17 Dave

Dave

    boing

  • Administrators
  • 5,086 posts

Posted 5 September 2004 - 11:35 AM

Like to say that this thread has some pretty much invaluable information in it, so I'd like to thank Martin for it. I'm also making the thread sticky.
  • 0
Dave
SFN Administrator

Blog and photoblog | Get on IRC! | #sfn statistics

#18 Thales

Thales

    Atom

  • Senior Members
  • 359 posts
  • LocationQueenstown, NZ

Posted 5 September 2004 - 01:21 PM

I'll second that one Dave. Thanks Martin!
  • 0
I am the exception that makes the rules...

#19 Martin

Martin

    Physics Expert

  • Resident Experts
  • 4,596 posts
  • LocationSF Bay Area

Posted 5 September 2004 - 06:59 PM

thanks also to Dave!
I'm very glad the thread met with his approval.
We can keep it a bibliography (online reference shelf) thread
in line with being a sticky, and start separate threads for
discussion, questions about QG.
  • 0
Loll quantum gravity SciAm
http://www.signallak...uantumJul08.pdf
cosmology SciAm
www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
http://www.einstein-...logy/index.html

#20 Martin

Martin

    Physics Expert

  • Resident Experts
  • 4,596 posts
  • LocationSF Bay Area

Posted 6 September 2004 - 01:02 AM

Olaf Dreyer
Background Independent Quantum Field Theory and the Cosmological Constant Problem
http://arxiv.org/hep-th/0409048

this is a 4-page paper addressing the cosmological constant problem in a new way.

here is the author's summary:
"We introduce the notion of background independent quantum field theory. The distinguishing feature of this theory is that the dynamics can be formulated without recourse to a background metric structure. We show in a simple model how the metric properties of spacetime can be recovered from the dynamics. Background independence is not only conceptually desirable but allows for the resolution of a problem haunting ordinary quantum field theory: the cosmological constant problem."
  • 0
Loll quantum gravity SciAm
http://www.signallak...uantumJul08.pdf
cosmology SciAm
www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf
http://www.einstein-...logy/index.html




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users