View Full Version : How active is research on other quantum gravity theories than loops or strings
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>According to some physicists (for instance John Baez\nand Peter Woit), both string theory and loop quantum\ngravity have not made much progress recently.\n\nHow active are other approaches like noncommutative\ngeometry, euclidean quantum gravity, discrete\napproaches (Lorentzian, Regge calculus, ...), twistor\ntheory, topos theory, supergravity, Ads/CFT, emerging\nproperties (Robert Laughlin)...?\n\n\n\n\n___________________________ ___________________________\nClick here to donate to the Hurricane Katrina relief effort.\nhttp://store.yahoo.com/redcross-donate3/\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>According to some physicists (for instance John Baez
and Peter Woit), both string theory and loop quantum
gravity have not made much progress recently.
How active are other approaches like noncommutative
geometry, euclidean quantum gravity, discrete
approaches (Lorentzian, Regge calculus, ...), twistor
theory, topos theory, supergravity, Ads/CFT, emerging
properties (Robert Laughlin)...?
__{_______________________________________________ _____}
Click here to donate to the Hurricane Katrina relief effort.
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I.Vecchi
Sep7-05, 06:29 PM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>EvT ha scritto:\n\n> According to some physicists (for instance John Baez\n> and Peter Woit), both string theory and loop quantum\n> gravity have not made much progress recently.\n>\n> How active are other approaches like noncommutative\n> geometry, euclidean quantum gravity, discrete\n> approaches (Lorentzian, Regge calculus, ...), twistor\n> theory, topos theory, supergravity, Ads/CFT, emerging\n> properties (Robert Laughlin)...?\n>\n\nVery interesting question, to which I would like to append my own.\n\nIs there (or better, on the basis of the current knowledge can one make\nan educated guess about) a common issue underlying the diffuculties of\nthe different theories?\n\nAnd here is another volley.\nIn 1984 Wald wrote ([1]) "... consider a state of matter where , with\nprobability 1/2, all the matter is located in a certain region O1 of\nspacetime and , with probability 1/2, the matter is located in a region\nO2 disjoint from O1 ... Suppose now that we make a measurement of the\nlocation of the matter. We then find the matter to be entirely in O1 or\nin O2 ... after we have resolved the quantum state of the matter by\nthis measurement , then the gravitational field must change in a\ndiscontinuous , acausal manner ... These difficulties apparently can be\navoided only by treating the space time metric in a probabilistic\nfashion , i.e. by quantising the gravitational field".\n\n\nMy further questions are :\nIs Wald\'s formulation of the problem still considered appropriate?\nIf yes, which of the currently fashionable theories of gravity provide\na concrete answer to the issue above?\nAre there explicit (possibly simplified) models available?\nIs the dearth of relevant experimental results due only to the fact\nthat we don\'t know how to put planets or stars into the quantum state\ndescribed by Wald and that with smaller objects the effects are too\nsmall to measure?\n\nCheers,\n\nIV\n\n[1] R.M. Wald "General Relativity", 14.1\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>EvT ha scritto:
> According to some physicists (for instance John Baez
> and Peter Woit), both string theory and loop quantum
> gravity have not made much progress recently.
>
> How active are other approaches like noncommutative
> geometry, euclidean quantum gravity, discrete
> approaches (Lorentzian, Regge calculus, ...), twistor
> theory, topos theory, supergravity, Ads/CFT, emerging
> properties (Robert Laughlin)...?
>
Very interesting question, to which I would like to append my own.
Is there (or better, on the basis of the current knowledge can one make
an educated guess about) a common issue underlying the diffuculties of
the different theories?
And here is another volley.
In 1984 Wald wrote ([1]) "... consider a state of matter where , with
probability 1/2, all the matter is located in a certain region O1 of
spacetime and , with probability 1/2, the matter is located in a region
O2 disjoint from O1 ... Suppose now that we make a measurement of the
location of the matter. We then find the matter to be entirely in O1 or
in O2 ... after we have resolved the quantum state of the matter by
this measurement , then the gravitational field must change in a
discontinuous , acausal manner ... These difficulties apparently can be
avoided only by treating the space time metric in a probabilistic
fashion , i.e. by quantising the gravitational field".
My further questions are :
Is Wald's formulation of the problem still considered appropriate?
If yes, which of the currently fashionable theories of gravity provide
a concrete answer to the issue above?
Are there explicit (possibly simplified) models available?
Is the dearth of relevant experimental results due only to the fact
that we don't know how to put planets or stars into the quantum state
described by Wald and that with smaller objects the effects are too
small to measure?
Cheers,
IV
[1] R.M. Wald "General Relativity", 14.1
Igor Khavkine
Sep10-05, 10:10 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>EvT wrote:\n> According to some physicists (for instance John Baez\n> and Peter Woit), both string theory and loop quantum\n> gravity have not made much progress recently.\n>\n> How active are other approaches like noncommutative\n> geometry,\n\nThere are definitely a few people working on this. From what I\nunderstand, not many are trying to construct a quantum theory of\ngravity from some basic principles assuming noncommutative geometry\n(although there are some, John Madore being an example). Instead,\npeople seek to express some sector or limit of an underlying theory\n(like strings or loops) in terms of the language of noncommutative\ngeometry. This is how noncommutative field theory makes an appearance\nin string theory or even in condensed matter physics.\n\n> euclidean quantum gravity,\n\nDon\'t know a whole lot about this, except that it\'s Stephen Hawking\'s\nfavorite way of looking at the problem.\n\n> discrete approaches (Lorentzian, Regge calculus, ...),\n\nThe recent work on Lorentzian dynamical triangulations by Loll,\nAmbjorn, and Jurkiewicz seems to be showing some promise like emergence\nof a smooth large scale limit. I don\'t know that very many people\noutside their group are working on it.\n\n> twistor theory, topos theory, supergravity,\n\nSorry, don\'t know much about those.\n\n> Ads/CFT,\n\nJudging merely from the number of citations that Maldacena\'s paper has\ngathered since its appearance, it is definitely an active area of\nresearch. Unfortunately, I don\'t know enough about it to say whether\nit\'s leading anywhere or not.\n\n> emerging properties (Robert Laughlin)...?\n\nThese ideas are mostly favored by physicists with a condensed matter\nbackground. There are definitely examples where features like Lorentz\ninvariance make an appearance as a low energy emergent phenomenon.\nHowever, I have not seen any proposals of this sort of a significantly\ngreater sophistication. You can add a few more names to the list of\nsupporters of such ideas. These include Volovik, who\'s tried to say\nsomething about the cosmological constant problem, and Xiao-Gang Wen,\nwho is trying to construct the standard model as an emergent system.\n\nOne approach that you haven\'t mentioned is that of causal sets. Again,\nthere is a small group of people working on such models centered around\nRafael Sorkin. From what I\'ve seen, the main attraction of these models\nis there generality (all you need is discreteness with a built-in\nnotion of causality) and potential for richness (as a theory). However,\ndefinite results that can be thought of as progress toward quantum\ngravity are in short supply.\n\nHope this helps.\n\nIgor\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>EvT wrote:
> According to some physicists (for instance John Baez
> and Peter Woit), both string theory and loop quantum
> gravity have not made much progress recently.
>
> How active are other approaches like noncommutative
> geometry,
There are definitely a few people working on this. From what I
understand, not many are trying to construct a quantum theory of
gravity from some basic principles assuming noncommutative geometry
(although there are some, John Madore being an example). Instead,
people seek to express some sector or limit of an underlying theory
(like strings or loops) in terms of the language of noncommutative
geometry. This is how noncommutative field theory makes an appearance
in string theory or even in condensed matter physics.
> euclidean quantum gravity,
Don't know a whole lot about this, except that it's Stephen Hawking's
favorite way of looking at the problem.
> discrete approaches (Lorentzian, Regge calculus, ...),
The recent work on Lorentzian dynamical triangulations by Loll,
Ambjorn, and Jurkiewicz seems to be showing some promise like emergence
of a smooth large scale limit. I don't know that very many people
outside their group are working on it.
> twistor theory, topos theory, supergravity,
Sorry, don't know much about those.
> Ads/CFT,
Judging merely from the number of citations that Maldacena's paper has
gathered since its appearance, it is definitely an active area of
research. Unfortunately, I don't know enough about it to say whether
it's leading anywhere or not.
> emerging properties (Robert Laughlin)...?
These ideas are mostly favored by physicists with a condensed matter
background. There are definitely examples where features like Lorentz
invariance make an appearance as a low energy emergent phenomenon.
However, I have not seen any proposals of this sort of a significantly
greater sophistication. You can add a few more names to the list of
supporters of such ideas. These include Volovik, who's tried to say
something about the cosmological constant problem, and Xiao-Gang Wen,
who is trying to construct the standard model as an emergent system.
One approach that you haven't mentioned is that of causal sets. Again,
there is a small group of people working on such models centered around
Rafael Sorkin. From what I've seen, the main attraction of these models
is there generality (all you need is discreteness with a built-in
notion of causality) and potential for richness (as a theory). However,
definite results that can be thought of as progress toward quantum
gravity are in short supply.
Hope this helps.
Igor
Eugene Stefanovich
Sep10-05, 10:10 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>I.Vecchi wrote:\n\n> > Is there (or better, on the basis of the current knowledge can one make\n> > an educated guess about) a common issue underlying the diffuculties of\n> > the different theories?\n\nIn my opinion, the fundamental problem facing modern theoretical physics is\nthe deep contradiction between quantum mechanics and Einsteinian\nrelativity (both special and general relativity). In Einsteinian relativity\nspace and time are interchangeable. In quantum mechanics,\nposition is an observable that depends on the state of the system, and time\nis a numerical parameter. A big difference.\n\nA consistent theory cannot keep these two conflicting views at the same\ntime.\nSomething\'s got to give. I think that eventually the QM approach to\ntime and position will prevail.\n\nEugene.\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>I.Vecchi wrote:
> > Is there (or better, on the basis of the current knowledge can one make
> > an educated guess about) a common issue underlying the diffuculties of
> > the different theories?
In my opinion, the fundamental problem facing modern theoretical physics is
the deep contradiction between quantum mechanics and Einsteinian
relativity (both special and general relativity). In Einsteinian relativity
space and time are interchangeable. In quantum mechanics,
position is an observable that depends on the state of the system, and time
is a numerical parameter. A big difference.
A consistent theory cannot keep these two conflicting views at the same
time.
Something's got to give. I think that eventually the QM approach to
time and position will prevail.
Eugene.
Ilja Schmelzer
Sep13-05, 01:17 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>"I.Vecchi" <vecchi@weirdtech.com> schrieb\n> In 1984 Wald wrote ([1]) "... consider a state of matter where , with\n> probability 1/2, all the matter is located in a certain region O1 of\n> spacetime and , with probability 1/2, the matter is located in a region\n> O2 disjoint from O1 ... Suppose now that we make a measurement of the\n> location of the matter. We then find the matter to be entirely in O1 or\n> in O2 ... after we have resolved the quantum state of the matter by\n> this measurement , then the gravitational field must change in a\n> discontinuous , acausal manner ... These difficulties apparently can be\n> avoided only by treating the space time metric in a probabilistic\n> fashion , i.e. by quantising the gravitational field".\n>\n>\n> My further questions are :\n> Is Wald\'s formulation of the problem still considered appropriate?\n\nI think it is appropriate.\n\n> If yes, which of the currently fashionable theories of gravity provide\n> a concrete answer to the issue above?\n\nIn gr-qc/0001101 I consider superpositions of gravitational fields in a\nmore specific thought experiment. My conclusion is that quantum gravity\nneeds a common background.\n\nIlja\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>"I.Vecchi" <vecchi@weirdtech.com> schrieb
> In 1984 Wald wrote ([1]) "... consider a state of matter where , with
> probability 1/2, all the matter is located in a certain region O1 of
> spacetime and , with probability 1/2, the matter is located in a region
> O2 disjoint from O1 ... Suppose now that we make a measurement of the
> location of the matter. We then find the matter to be entirely in O1 or
> in O2 ... after we have resolved the quantum state of the matter by
> this measurement , then the gravitational field must change in a
> discontinuous , acausal manner ... These difficulties apparently can be
> avoided only by treating the space time metric in a probabilistic
> fashion , i.e. by quantising the gravitational field".
>
>
> My further questions are :
> Is Wald's formulation of the problem still considered appropriate?
I think it is appropriate.
> If yes, which of the currently fashionable theories of gravity provide
> a concrete answer to the issue above?
In http://www.arxiv.org/abs/gr-qc/0001101 I consider superpositions of gravitational fields in a
more specific thought experiment. My conclusion is that quantum gravity
needs a common background.
Ilja
Ilja Schmelzer
Sep13-05, 01:17 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>"Igor Khavkine" <igor.kh@gmail.com> schrieb\n> > If yes, which of the currently fashionable theories of gravity provide\n> > a concrete answer to the issue above?\n>\n> None as far as I know.\n\nWhat do you think about gr-qc/0205035?\n\nIlja\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>"Igor Khavkine" <igor.kh@gmail.com> schrieb
> > If yes, which of the currently fashionable theories of gravity provide
> > a concrete answer to the issue above?
>
> None as far as I know.
What do you think about http://www.arxiv.org/abs/gr-qc/0205035?
Ilja
Charles Francis
Sep15-05, 12:40 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>In message <20050905174410.33275.qmail@web32010.mail.mud.yaho o.com>, EvT\n<vantuyll@yahoo.com> writes\n>According to some physicists (for instance John Baez\n>and Peter Woit), both string theory and loop quantum\n>gravity have not made much progress recently.\n>\n>How active are other approaches like noncommutative\n>geometry, euclidean quantum gravity, discrete\n>approaches (Lorentzian, Regge calculus, ...), twistor\n>theory, topos theory, supergravity, Ads/CFT, emerging\n>properties (Robert Laughlin)...?\n>\n\nI have a paper currently in the hands of referees, so I suppose that is\nas active as I can make it, using quantum logic and a relational\ninterpretation. As far as I know, no one has seriously developed this\napproach since Von Neumann, though Rovelli has worked on it a bit.\nTwistors gave way to spin networks which lead on to LQG as I recall. I\nthink some of this may be useful, ultimately; mostly spin network bit. I\ndon\'t think topos theory got very far. Chris Isham was working on it\nwith Jeremy Butterfield, but I don\'t think he is now.\n\n\n\nRegards\n\n--\nCharles Francis\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>In message <20050905174410.33275.qmail@web32010.mail.mud.yahoo .com>, EvT
<vantuyll@yahoo.com> writes
>According to some physicists (for instance John Baez
>and Peter Woit), both string theory and loop quantum
>gravity have not made much progress recently.
>
>How active are other approaches like noncommutative
>geometry, euclidean quantum gravity, discrete
>approaches (Lorentzian, Regge calculus, ...), twistor
>theory, topos theory, supergravity, Ads/CFT, emerging
>properties (Robert Laughlin)...?
>
I have a paper currently in the hands of referees, so I suppose that is
as active as I can make it, using quantum logic and a relational
interpretation. As far as I know, no one has seriously developed this
approach since Von Neumann, though Rovelli has worked on it a bit.
Twistors gave way to spin networks which lead on to LQG as I recall. I
think some of this may be useful, ultimately; mostly spin network bit. I
don't think topos theory got very far. Chris Isham was working on it
with Jeremy Butterfield, but I don't think he is now.
Regards
--
Charles Francis
Juan R.
Sep23-05, 02:56 PM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>I.Vecchi wrote:\n> markw...@yahoo.com ha scritto:\n>\n> ..\n>\n> > Twistors emerge naturally from string theory -- a point made by Witten\n> > in a recent journal article this year.\n>\n> Maybe you are referring to [1], maybe not.\n> Anyways, such "natural emergence" reminds me of the marvellous\n> explanatory power of Aristotelian science.\n> I wonder whether there is ANYTHING that does not arise naturally from\n> string theory.\n>\n> IV\n>\n> [1] http://www.arxiv.org/abs/hep-th/0403199\n> -------------------------\n\nWell, the term \'arises naturally\' has, in string theory community, a\ndifferent connotation that in the rest of scientific commuities. In the\nrest of science, \'arises naturally\' means that can be derived from\nfirst principles on any underlying theory. In the string world, means\nother thing. In the own words of string theorists Seiberg:\n\n"string theorists are arrogant enough that whatever comes up in their\nresearch, they will call it string theory."\n\npage 6 of\n\nhttp://www.canonicalscience.com/stringcriticism.pdf\n\nIn\n\nhttp://www.math.columbia.edu/~woit/blog/archives/000161.html\n\nthis quote is incorrectly attributed to string theorist Maldacena. But\nPeter Woit corrects this below.\n\nIt is clear that Twistor theory has been introduced in recent versions\nof string theory, and thus, it now \'arises naturally\'...\n\n\nJuan R.\n\nCenter for CANONICAL |SCIENCE)\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>I.Vecchi wrote:
> markw...@yahoo.com ha scritto:
>
> ..
>
> > Twistors emerge naturally from string theory -- a point made by Witten
> > in a recent journal article this year.
>
> Maybe you are referring to [1], maybe not.
> Anyways, such "natural emergence" reminds me of the marvellous
> explanatory power of Aristotelian science.
> I wonder whether there is ANYTHING that does not arise naturally from
> string theory.
>
> IV
>
> [1] http://www.arxiv.org/abs/http://www.arxiv.org/abs/hep-th/0403199
> -------------------------
Well, the term 'arises naturally' has, in string theory community, a
different connotation that in the rest of scientific commuities. In the
rest of science, 'arises naturally' means that can be derived from
first principles on any underlying theory. In the string world, means
other thing. In the own words of string theorists Seiberg:
"string theorists are arrogant enough that whatever comes up in their
research, they will call it string theory."
page 6 of
http://www.canonicalscience.com/stringcriticism.pdf
In
http://www.math.columbia.edu/~woit/blog/archives/000161.html
this quote is incorrectly attributed to string theorist Maldacena. But
Peter Woit corrects this below.
It is clear that Twistor theory has been introduced in recent versions
of string theory, and thus, it now 'arises naturally'...
Juan R.
Center for CANONICAL |SCIENCE)
Hontas F. Farmer III
Oct2-05, 01:44 PM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>EvT wrote:\n\n> According to some physicists (for instance John Baez\n> and Peter Woit), both string theory and loop quantum\n> gravity have not made much progress recently.\n>\n> How active are other approaches like noncommutative\n> geometry, euclidean quantum gravity, discrete\n> approaches (Lorentzian, Regge calculus, ...), twistor\n> theory, topos theory, supergravity, Ads/CFT, emerging\n> properties (Robert Laughlin)...?\n>\n\nOn a scale of 1-10. Where 1 is almost no activity, and ten\nwould have everyone researching the subject. I would say\nalternative theories of gravity are at 3 but rising rapidly.\n\nI say this based on:\nThe number of proposals for new theories\nof gravity seen on this group.\nThe number of proposed alternative theories that have been published\nin recognized journals.\nThe growing dissatisfaction with Strings and Loop inability to\ndeliver results after all this time.\n\nIn short pursue an alternative theory of quantum gravity as a future\ncareer. The future belongs to something new.\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>EvT wrote:
> According to some physicists (for instance John Baez
> and Peter Woit), both string theory and loop quantum
> gravity have not made much progress recently.
>
> How active are other approaches like noncommutative
> geometry, euclidean quantum gravity, discrete
> approaches (Lorentzian, Regge calculus, ...), twistor
> theory, topos theory, supergravity, Ads/CFT, emerging
> properties (Robert Laughlin)...?
>
On a scale of 1-10. Where 1 is almost no activity, and ten
would have everyone researching the subject. I would say
alternative theories of gravity are at 3 but rising rapidly.
I say this based on:
The number of proposals for new theories
of gravity seen on this group.
The number of proposed alternative theories that have been published
in recognized journals.
The growing dissatisfaction with Strings and Loop inability to
deliver results after all this time.
In short pursue an alternative theory of quantum gravity as a future
career. The future belongs to something new.
thomas_larsson_01@hotmail.com
Oct4-05, 03:39 PM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>> As far as I\'m concerned, the one approach that\'s making\n> the most progress now is Causal Dynamical Triangulations,\n> which is a variant of the Regge calculus.\n\n> Not many people are working on this yet, in part because\n> it requires computer simulations, and most researchers\n> in quantum gravity still prefer pencil-and-paper work.\n> But, the results so far are impressive. They\'ve numerically\n> simulated quantum gravity, and found something surprising:\n> their spacetimes act 4-dimensional at large scales but\n> 2-dimensional at small scales!\n\nI tend to think about CDT as an experimental rather than\ntheoretical approach. In the absense of real experiments in quantum\ngravity, we have to settle for numerical ones. This is a situation\nquite familiar in statistical physics, where real experiments are\npossible, but computer experiments are often of higher quality.\n\nAlready there seems to be at least one qualitative question which\nCDT has answered, more or less conclusively: whether causality\nholds strictly or not. There are two ways to quantize the metric:\nsumming over metrics which are compatible with spacetime being a\ncylinder, and summing over all metrics irrespective of topology.\nAJL makes the distinction between Lorentzian vs Euclidean\nquantization; one could also talk about canonical vs path-integral\nquantization. The two agree in flat space, but it seems that they\ndiffer in QG, basically because the path-integral includes\ntrajectories that move backwards in time. If they differ, then both\ncannot be right (both could be wrong, of course). CDT seems to say\nthat canonical quantization is the right choice.\n\nSome time ago I looked at another of AJL\'s papers,\nhttp://www.arxiv.org/abs/hep-lat/9909129 , in which they dealt\nwith the 2D Ising model coupled to 2D gravity. In Euclidean\nquantization, the critical exponents change (to those of the spherical\nmodel, I think), but in Lorentzian quantization they stay at the\nOnsager values that they have in flat space. This is highly desirable,\nIMO. The Ising model is used for various phenomena in condensed matter\nphysics, i.e. for systems completely governed by electromagnetism. If\nthere is one thing we know for sure about QG, it is that it is\nunimportant compared to EM, the whole idea that we can ignore gravity\nin condensed matter is based on that. If QG would modify critical\nexponents, then it would not be possible to ignore gravity, which\nwould invalidate most of 20th century physics.\n\nI\'m less optimistic about CDT\'s prospects at producing quantitative\nresults in our lifetime. The reason for this pessimism is that\nlattice gauge theory has only started to produce high-quality data\nover the last five years or so, and I am unsure whether LGT\ncalculations are really used by experimentalists, more than as a\ntest of the feasibility of the method itself. There is no reason\nto expect that simulations in the much less understood QG will\nproceed faster.\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>> As far as I'm concerned, the one approach that's making
> the most progress now is Causal Dynamical Triangulations,
> which is a variant of the Regge calculus.
> Not many people are working on this yet, in part because
> it requires computer simulations, and most researchers
> in quantum gravity still prefer pencil-and-paper work.
> But, the results so far are impressive. They've numerically
> simulated quantum gravity, and found something surprising:
> their spacetimes act 4-dimensional at large scales but
> 2-dimensional at small scales!
I tend to think about CDT as an experimental rather than
theoretical approach. In the absense of real experiments in quantum
gravity, we have to settle for numerical ones. This is a situation
quite familiar in statistical physics, where real experiments are
possible, but computer experiments are often of higher quality.
Already there seems to be at least one qualitative question which
CDT has answered, more or less conclusively: whether causality
holds strictly or not. There are two ways to quantize the metric:
summing over metrics which are compatible with spacetime being a
cylinder, and summing over all metrics irrespective of topology.
AJL makes the distinction between Lorentzian vs Euclidean
quantization; one could also talk about canonical vs path-integral
quantization. The two agree in flat space, but it seems that they
differ in QG, basically because the path-integral includes
trajectories that move backwards in time. If they differ, then both
cannot be right (both could be wrong, of course). CDT seems to say
that canonical quantization is the right choice.
Some time ago I looked at another of AJL's papers,
http://www.arxiv.org/abs/http://www.arxiv.org/abs/hep-lat/9909129 , in which they dealt
with the 2D Ising model coupled to 2D gravity. In Euclidean
quantization, the critical exponents change (to those of the spherical
model, I think), but in Lorentzian quantization they stay at the
Onsager values that they have in flat space. This is highly desirable,
IMO. The Ising model is used for various phenomena in condensed matter
physics, i.e. for systems completely governed by electromagnetism. If
there is one thing we know for sure about QG, it is that it is
unimportant compared to EM, the whole idea that we can ignore gravity
in condensed matter is based on that. If QG would modify critical
exponents, then it would not be possible to ignore gravity, which
would invalidate most of 20th century physics.
I'm less optimistic about CDT's prospects at producing quantitative
results in our lifetime. The reason for this pessimism is that
lattice gauge theory has only started to produce high-quality data
over the last five years or so, and I am unsure whether LGT
calculations are really used by experimentalists, more than as a
test of the feasibility of the method itself. There is no reason
to expect that simulations in the much less understood QG will
proceed faster.
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