[SOLVED] Re: open questions in physics [Archive]

John Baez

Apr7-04, 08:41 AM

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resize=yes,status=no,wi dth=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 article <53ca460a.0403241425.3ceb0632@posting.google.com>,\nUlmo <ulmo@cheerful.com> wrote:\n\n>When you ask why are there three generations of fermions, you could\n>say that if you take superstring theory, and compactify the extra\n>dimensions on the correct Calabi-Yau manifold, it predicts three\n>generations.\n\nWhere "correct" means "predicts three generations", right?\n\nI don\'t consider this much of an answer to the "why" question.\n\nIf I\'d had more energy I\'d have pointed out that there\'s an\nupper bound on the number of generations due to the requirement\nthat the gauge fields be asymptotically free. That\'s a nice fact.\n\n>You could also mention recent developments in string\n>theory such as Matrix Theory, AdS/CFT correspondance, brane worlds,\n>etc.\n\nI could, yes - but all these open questions have a huge theoretical\nliterature devoted to answering them, which I haven\'t had the energy\nto cite, in general.\n\n\n\n\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 article <53ca460a.0403241425.3ceb0632@posting.google.com>,
Ulmo <ulmo@cheerful.com> wrote:

>When you ask why are there three generations of fermions, you could
>say that if you take superstring theory, and compactify the extra
>dimensions on the correct Calabi-Yau manifold, it predicts three
>generations.

Where "correct" means "predicts three generations", right?

I don't consider this much of an answer to the "why" question.

If I'd had more energy I'd have pointed out that there's an
upper bound on the number of generations due to the requirement
that the gauge fields be asymptotically free. That's a nice fact.

>You could also mention recent developments in string
>theory such as Matrix Theory, AdS/CFT correspondance, brane worlds,
>etc.

I could, yes - but all these open questions have a huge theoretical
literature devoted to answering them, which I haven't had the energy
to cite, in general.

alistair

Apr7-04, 09:10 AM

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resize=yes,status=no,wi dth=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>Why does each generation of particles have precisely this structure:\ntwo leptons and two quarks?\nPerhaps its because each generation can in principle form its own\nversion of\na neutron.\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>Why does each generation of particles have precisely this structure:
two leptons and two quarks?
Perhaps its because each generation can in principle form its own
version of
a neutron.

Aaron Bergman

Apr7-04, 09:15 AM

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resize=yes,status=no,wi dth=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 article <c4kg6u\\$i1q\\$1@glue.ucr.edu>, baez@galaxy.ucr.edu (John Baez)\nwrote:\n\n> In article <53ca460a.0403241425.3ceb0632@posting.google.com>, \n> Ulmo <ulmo@cheerful.com> wrote:\n>\n> >When you ask why are there three generations of fermions, you could\n> >say that if you take superstring theory, and compactify the extra\n> >dimensions on the correct Calabi-Yau manifold, it predicts three\n> >generations.\n>\n> Where "correct" means "predicts three generations", right?\n>\n> I don\'t consider this much of an answer to the "why" question.\n\nIt\'s hardly a prediction, really. Different CYs give different numbers\nof generations.\n\nAaron\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 article <c4kg6u$i1q$1@glue.ucr.edu>, baez@galaxy.ucr.edu (John Baez)
wrote:

> In article <53ca460a.0403241425.3ceb0632@posting.google.com>,
> Ulmo <ulmo@cheerful.com> wrote:
>
> >When you ask why are there three generations of fermions, you could
> >say that if you take superstring theory, and compactify the extra
> >dimensions on the correct Calabi-Yau manifold, it predicts three
> >generations.
>
> Where "correct" means "predicts three generations", right?
>
> I don't consider this much of an answer to the "why" question.

It's hardly a prediction, really. Different CYs give different numbers
of generations.

Aaron

Arnold Neumaier

Jun16-04, 04:56 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>\nDear Prof. Baez,\n\n\nIn http://math.ucr.edu/home/baez/physics/General/open_questions.html,\nyou write,\n\n\'\'At the turn of the millennium, the Clay Mathematics Institute offered a\n\\$1,000,000 prize for providing a mathematically rigorous foundation for\nthe quantum version of SU(2) Yang-Mills theory in four spacetime dimensions,\nand proving that there\'s a "mass gap" - meaning that the lightest particle\nin this theory has nonzero mass. \'\'\n\nBut the prize is for answering this for all compact Lie groups,\nnot just SU(2). (Though the argument for SU(2) would probably extend to all\ncompact Lie groups...)\n\n\nAlso, I think that your comment on the collapse,\n\n\'\'Many physicists think these issues are settled, at least for most\npractical purposes. For example, many feel that "decoherence" is\nsufficient to produce the effects that previously were attributed to\ncollapse of the wavefunction, without introducing nonunitary time\nevolution.\'\'\n\nis inappropriate. The champions of the decoherence approach do not make\nsuch a claim; they are quite careful to delineate what decoherence can do\nand what it leaves open. If the big crowd has the cruder point of view\nreflected by your statement, it means nothing but lack of familiarity\nwith the details.\n\nWigner\'s analysis of the incompatibility of unitarity, superposition\nprinciple and collapse, Chapter II.2 in:\nJ.A. Wheeler and W. H. Zurek (eds.),\nQuantum theory and measurement.\nPrinceton Univ. Press, Princeton 1983,\nin particular pp. 285-288, is still unassailable, even in view of all\ndecoherence results.\nDecoherence applies only to ensembles, while the collapse is needed to\naccount for the interpretation of individual events. See also my\ntheoretical physics FAQ at\nhttp://www.mat.univie.ac.at/~neum/physics-faq.txt\n\n\nBest wishes,\n\nArnold Neumaier\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>Dear Prof. Baez,

In http://math.ucr.edu/home/baez/physics/General/open_questions.html,
you write,

''At the turn of the millennium, the Clay Mathematics Institute offered a
$1,000,000 prize for providing a mathematically rigorous foundation for
the quantum version of SU(2) Yang-Mills theory in four spacetime dimensions,
and proving that there's a "mass gap" - meaning that the lightest particle
in this theory has nonzero mass. ''

But the prize is for answering this for all compact Lie groups,
not just SU(2). (Though the argument for SU(2) would probably extend to all
compact Lie groups...)

Also, I think that your comment on the collapse,

''Many physicists think these issues are settled, at least for most
practical purposes. For example, many feel that "decoherence" is
sufficient to produce the effects that previously were attributed to
collapse of the wavefunction, without introducing nonunitary time
evolution.''

is inappropriate. The champions of the decoherence approach do not make
such a claim; they are quite careful to delineate what decoherence can do
and what it leaves open. If the big crowd has the cruder point of view
reflected by your statement, it means nothing but lack of familiarity
with the details.

Wigner's analysis of the incompatibility of unitarity, superposition
principle and collapse, Chapter II.2 in:
J.A. Wheeler and W. H. Zurek (eds.),
Quantum theory and measurement.
Princeton Univ. Press, Princeton 1983,
in particular pp. 285-288, is still unassailable, even in view of all
decoherence results.
Decoherence applies only to ensembles, while the collapse is needed to
account for the interpretation of individual events. See also my
theoretical physics FAQ at
http://www.mat.univie.ac.at/~neum/physics-faq.txt

Best wishes,

Arnold Neumaier