<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>\n\n\nRecently I browsed through the preprint gr-qc/0405091\nwhere Moffat talks about nonsymmetric gravitational theory. Anyway I\ndon\'t know much about the details of this theory and do not recall\nthis discussed in much detail on spr or sar.\nAnyhow since a couple of weeks back we discussed MOND maybe someone\ncan say a few things about this theory. The interesting thing\nwhich he mentioned in this paper is that his theory can explain both\nflat rotation curves of spiral galaxies as well as\nacceleration of the universe (thus avoiding the need for dark matter\nand dark energy). Plus I remember Steve mentioning that\nthe discrepancy observed in GR for DI hercules can be accomodated in\nNGT.\n\nanyhow can the experts on this forum give their latest critique of\nthis theory?\nDoes this theory agree with precision tests of GR at solar system\nscale?\nDoes it obey strong principle of equivalence?\nThanks\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>Recently I browsed through the preprint http://www.arxiv.org/abs/gr-qc/0405091
where Moffat talks about nonsymmetric gravitational theory. Anyway I
don't know much about the details of this theory and do not recall
this discussed in much detail on spr or sar.
Anyhow since a couple of weeks back we discussed MOND maybe someone
can say a few things about this theory. The interesting thing
which he mentioned in this paper is that his theory can explain both
flat rotation curves of spiral galaxies as well as
acceleration of the universe (thus avoiding the need for dark matter
and dark energy). Plus I remember Steve mentioning that
the discrepancy observed in GR for DI hercules can be accomodated in
NGT.
anyhow can the experts on this forum give their latest critique of
this theory?
Does this theory agree with precision tests of GR at solar system
scale?
Does it obey strong principle of equivalence?
Thanks
Jonathan Thornburg
Jun7-04, 08:59 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 sci.astro.research Melroy <melroysoares@hotmail.com> wrote:\n>\n> Recently I browsed through the preprint gr-qc/0405091\n> where Moffat talks about nonsymmetric gravitational theory. Anyway I\n> don\'t know much about the details of this theory and do not recall\n> this discussed in much detail on spr or sar.\n> Anyhow since a couple of weeks back we discussed MOND maybe someone\n> can say a few things about this theory. [[...]]\n\ngr-qc/0405068 (to appear in PRD) is an interesting experimental test\nof Moffat\'s theory:\n\nauthors: Oliver Preuss, Mark P. Haugan, Sami K. Solanki, Stefan Jordan\ntitle: An astronomical search for evidence of new physics:\nLimits on gravity-induced birefringence from the magnetic\nwhite dwarf RE J0317-853\nabstract:\nThe coupling of the electromagnetic field directly with gravitational\ngauge fields leads to new physical effects that can be tested using\nastronomical data. Here we consider a particular case for closer scrutiny,\na specific nonminimal coupling of torsion to electromagnetism, which\nenters into a metric-affine geometry of space-time. We show that under\nthe assumption of this nonminimal coupling, spacetime is birefringent\nin the presence of such a gravitational field. This leads to the\ndepolarization of light emitted from extended astrophysical sources. We\nuse polarimetric data of the magnetic white dwarf \\${RE J0317-853}\\$ to set\nstrong constraints on the essential coupling constant for this effect,\ngiving \\$k^2 \\lsim (19 {m})^2 \\$.\n\nMoffat\'s theory is parameterised by a parameter \\$k\\$, which has dimensions\nof length. \\$k=0\\$ gives general relativity (GR). This paper is basically\nsaying that \\$k\\$ is constrained to be very small, i.e. the theory can\nonly deviate a _very_ tiny amount from GR. (This is similar to the\nsituation for like Brans-Dicke theory with \\$\\omega\\$ very large.)\n\nSince Moffat\'s theory\'s experimental/observational predictions can be\nmade arbitrarily close to those of GR by making \\$k\\$ sufficently small,\nMoffat\'s theory can never be disproved by observations consistent with\ngeneral relativity. However, if \\$k\\$ is small enough, then Moffat\'s\ntheory\'s predictions don\'t differ significantly from GR\'s for interesting\nastrophysical systems, and Occam\'s razor suggests retaining GR as our\nworking-hypothesis theory of relativistic gravity.\n\n--\n-- "Jonathan Thornburg (remove -animal to reply)" <jthorn@aei.mpg-zebra.de>\nMax-Planck-Institut fuer Gravitationsphysik (Albert-Einstein-Institut),\nGolm, Germany, "Old Europe" http://www.aei.mpg.de/~jthorn/home.html\n"Washing one\'s hands of the conflict between the powerful and the\npowerless means to side with the powerful, not to be neutral."\n-- quote by Freire / poster by Oxfam\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 sci.astro.research Melroy <melroysoares@hotmail.com> wrote:
>
> Recently I browsed through the preprint http://www.arxiv.org/abs/gr-qc/0405091
> where Moffat talks about nonsymmetric gravitational theory. Anyway I
> don't know much about the details of this theory and do not recall
> this discussed in much detail on spr or sar.
> Anyhow since a couple of weeks back we discussed MOND maybe someone
> can say a few things about this theory. [[...]]
http://www.arxiv.org/abs/gr-qc/0405068 (to appear in PRD) is an interesting experimental test
of Moffat's theory:
authors: Oliver Preuss, Mark P. Haugan, Sami K. Solanki, Stefan Jordan
title: An astronomical search for evidence of new physics:
Limits on gravity-induced birefringence from the magnetic
white dwarf RE J0317-853
abstract:
The coupling of the electromagnetic field directly with gravitational
gauge fields leads to new physical effects that can be tested using
astronomical data. Here we consider a particular case for closer scrutiny,
a specific nonminimal coupling of torsion to electromagnetism, which
enters into a metric-affine geometry of space-time. We show that under
the assumption of this nonminimal coupling, spacetime is birefringent
in the presence of such a gravitational field. This leads to the
depolarization of light emitted from extended astrophysical sources. We
use polarimetric data of the magnetic white dwarf ${RE J0317-853}$ to set
strong constraints on the essential coupling constant for this effect,
giving $k^2 \lsim (19 {m})^2 $.
Moffat's theory is parameterised by a parameter $k$, which has dimensions
of length. $k=0$ gives general relativity (GR). This paper is basically
saying that $k$ is constrained to be very small, i.e. the theory can
only deviate a _very_ tiny amount from GR. (This is similar to the
situation for like Brans-Dicke theory with $\omega$ very large.)
Since Moffat's theory's experimental/observational predictions can be
made arbitrarily close to those of GR by making $k$ sufficently small,
Moffat's theory can never be disproved by observations consistent with
general relativity. However, if $k$ is small enough, then Moffat's
theory's predictions don't differ significantly from GR's for interesting
astrophysical systems, and Occam's razor suggests retaining GR as our
working-hypothesis theory of relativistic gravity.
--
-- "Jonathan Thornburg (remove -animal to reply)" <jthorn@aei.mpg-zebra.de>
Max-Planck-Institut fuer Gravitationsphysik (Albert-Einstein-Institut),
Golm, Germany, "Old Europe" http://www.aei.mpg.de/~jthorn/home.html
"Washing one's hands of the conflict between the powerful and the
powerless means to side with the powerful, not to be neutral."
-- quote by Freire / poster by Oxfam
puppet_sock@hotmail.com
Jun12-04, 07:10 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>melroysoares@hotmail.com (Melroy) wrote in message news:<1b7c3dda.0406051844.79990375@posting.google. com>...\n[snippers]\n> Plus I remember Steve mentioning that\n> the discrepancy observed in GR for DI hercules can be accomodated in\n> NGT.\n>\n> anyhow can the experts on this forum give their latest critique of\n> this theory?\n> Does this theory agree with precision tests of GR at solar system\n> scale?\n> Does it obey strong principle of equivalence?\n\nWell, I can\'t give a critique, exactly. I did my PhD "down the hall"\nfrom Moffat when he was at UofT. And I had many conversations with\nhim and his various grad students. And my PhD supervisor was one of\nMoffat\'s students. They usually called it NGT for non-symmetric gravity\ntheory.\n\nMoffat is a pretty cool guy to work with. He\'s very smart, has a lot\nof creative ideas, and knows a lot of physics. He\'s also prepared to\naccept criticism. If you show him something is wrong, he accpets it\nand moves on. His procedure for a working group is to surround himself\nwith the smartest people he can get, then pay attention to them.\nI have a lot of respect for him. He also has met lots of the big names\nin physics, and has plenty of great stories. He\'s a lot of fun to\nhave a beer with. He recently took a position at the Perimeter Inst.\n\nAnyway, it goes like so:\n\nThe starting point is, what if the metric is not really symmetric?\nHaving opened the can of worms, you now have to decide how you are\ngoing to introduce this. The theory has had a few different forms.\nBut basically, its got an issue that is both a source of annoyance\nand a source of resilliance. The parameter that says how big the\nanti-symmetric part of the metric is can be different for each mass.\nThis means that it is fairly hard to predict stuff, but also fairly\nhard to kill off the theory.\n\nAnyway, there are several papers on the consequences. Basically you\nsit down with Weinberg\'s gravity book and start doing the chapters\nwith a non-symmetric metric. So there\'s NGT solar system, NGT star\nformation and gravitational collapse, NGT cosmology, NGT gravity\nwaves, and so on. Moffat and his students and post-docs had quite\nan industry on this for about a decade. You should be able to find\nthe papers if you are interested.\n\nThe equivalent to Schwarzschild is that you get one extra parameter\nto define the anti-symmetric part. And this tends to decrease the\nsize of the perihelion advance of Mercury, but by a fairly small\namount. The smaller the parameter, the closer to general relativity\n(GR). When this parameter was zero, you got GR.\n\nOne thing Moffat worked very hard on was trying to relate the anti-sym\npart to some physical property. That way, there would be a strong\nprediction. Say, for example, it was Baryon number. Or it was some\nother thing like protons minus neutrons. Or some other such thing.\nThen he would be able to plug that in and get the anti-sym part, then\nwork out orbits or star formation or whatever. (Or actually, look up\nthe result from the papers already published giving these results as\na function of the anti-sym part.)\n\nThe problem was, all the experiments were consistent with GR, or were\nnot really trusted that well. You mentioned Di hercules. There was\na lot of discussion of that. And, of course, Moffat could fit it\nbecause he could tune the extra parameter to bring the periastron\nadvance to zero. People kept looking at the data, and doing new\nreadings. That\'s where things stood, last I looked, which was several\nyears ago.\n\nThere was a flurry of excitement for a while when somebody claimed\nto have measured one of the multipoles of the sun and it was too\nlarge for GR to accomodate. I\'m not sure what happened to that, but\nit does not seem to have survived. At least, I\'ve not seen any big\nstories about GR being disproved, so I\'m expecting the story is dead.\n\nAs far as the equivalence principle: Well, it gets modified but\nonly slightly. Two objects that had different values of the\nanti-sym part would follow different paths. In essence, this\nmeans there are potentially two kinds of "gravitational charge."\n\nHey, Uncle Al, are you reading this? Could the anti-sym part be\nrelated to chirality?\n\nSo, without some extra experimental evidence showing some departure\nfrom GR, it\'s hard to keep the excitement level for NGT. It\'s one\nof those theoretical alternatives that has to be checked. But until\nand unless some data points strongly in that direction, it\'s going\nto stay as just one more by-the-way gravity theory. But if some\ndata ever did contradict GR, Moffat would be on it like white on rice.\nSocks\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>melroysoares@hotmail.com (Melroy) wrote in message news:<1b7c3dda.0406051844.79990375@posting.google.com>...
[snippers]
> Plus I remember Steve mentioning that
> the discrepancy observed in GR for DI hercules can be accomodated in
> NGT.
>
> anyhow can the experts on this forum give their latest critique of
> this theory?
> Does this theory agree with precision tests of GR at solar system
> scale?
> Does it obey strong principle of equivalence?
Well, I can't give a critique, exactly. I did my PhD "down the hall"
from Moffat when he was at UofT. And I had many conversations with
him and his various grad students. And my PhD supervisor was one of
Moffat's students. They usually called it NGT for non-symmetric gravity
theory.
Moffat is a pretty cool guy to work with. He's very smart, has a lot
of creative ideas, and knows a lot of physics. He's also prepared to
accept criticism. If you show him something is wrong, he accpets it
and moves on. His procedure for a working group is to surround himself
with the smartest people he can get, then pay attention to them.
I have a lot of respect for him. He also has met lots of the big names
in physics, and has plenty of great stories. He's a lot of fun to
have a beer with. He recently took a position at the Perimeter Inst.
Anyway, it goes like so:
The starting point is, what if the metric is not really symmetric?
Having opened the can of worms, you now have to decide how you are
going to introduce this. The theory has had a few different forms.
But basically, its got an issue that is both a source of annoyance
and a source of resilliance. The parameter that says how big the
anti-symmetric part of the metric is can be different for each mass.
This means that it is fairly hard to predict stuff, but also fairly
hard to kill off the theory.
Anyway, there are several papers on the consequences. Basically you
sit down with Weinberg's gravity book and start doing the chapters
with a non-symmetric metric. So there's NGT solar system, NGT star
formation and gravitational collapse, NGT cosmology, NGT gravity
waves, and so on. Moffat and his students and post-docs had quite
an industry on this for about a decade. You should be able to find
the papers if you are interested.
The equivalent to Schwarzschild is that you get one extra parameter
to define the anti-symmetric part. And this tends to decrease the
size of the perihelion advance of Mercury, but by a fairly small
amount. The smaller the parameter, the closer to general relativity
(GR). When this parameter was zero, you got GR.
One thing Moffat worked very hard on was trying to relate the anti-sym
part to some physical property. That way, there would be a strong
prediction. Say, for example, it was Baryon number. Or it was some
other thing like protons minus neutrons. Or some other such thing.
Then he would be able to plug that in and get the anti-sym part, then
work out orbits or star formation or whatever. (Or actually, look up
the result from the papers already published giving these results as
a function of the anti-sym part.)
The problem was, all the experiments were consistent with GR, or were
not really trusted that well. You mentioned Di hercules. There was
a lot of discussion of that. And, of course, Moffat could fit it
because he could tune the extra parameter to bring the periastron
advance to zero. People kept looking at the data, and doing new
readings. That's where things stood, last I looked, which was several
years ago.
There was a flurry of excitement for a while when somebody claimed
to have measured one of the multipoles of the sun and it was too
large for GR to accomodate. I'm not sure what happened to that, but
it does not seem to have survived. At least, I've not seen any big
stories about GR being disproved, so I'm expecting the story is dead.
As far as the equivalence principle: Well, it gets modified but
only slightly. Two objects that had different values of the
anti-sym part would follow different paths. In essence, this
means there are potentially two kinds of "gravitational charge."
Hey, Uncle Al, are you reading this? Could the anti-sym part be
related to chirality?
So, without some extra experimental evidence showing some departure
from GR, it's hard to keep the excitement level for NGT. It's one
of those theoretical alternatives that has to be checked. But until
and unless some data points strongly in that direction, it's going
to stay as just one more by-the-way gravity theory. But if some
data ever did contradict GR, Moffat would be on it like white on rice.
Socks
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