View Full Version : Lay person question: How string theory achieves closure for the
<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>This is another lay person question, this time aimed at understanding\nhow string theory reproduces general relativity.\n\nString theory begins by assuming that a string is evolving in\na spacetime having a prescribed metric. The metric, however, depends on\nthe energy and mass of the string in question\n(and all other strings as well) via a law that one knows only\nat the large scale, namely, general relativity.\nHow does string theory deal with this interdependence?\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>This is another lay person question, this time aimed at understanding
how string theory reproduces general relativity.
String theory begins by assuming that a string is evolving in
a spacetime having a prescribed metric. The metric, however, depends on
the energy and mass of the string in question
(and all other strings as well) via a law that one knows only
at the large scale, namely, general relativity.
How does string theory deal with this interdependence?
<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>Karim <gawains_chela@sbcglobal.net> wrote in message news:<e7ecf966.0404231615.42fab997-100000@posting.google.com>...\n\n> This is another lay person question, this time aimed at understanding\n> how string theory reproduces general relativity.\n>\n> String theory begins by assuming that a string is evolving in\n> a spacetime having a prescribed metric. The metric, however, depends on\n> the energy and mass of the string in question\n> (and all other strings as well) via a law that one knows only\n> at the large scale, namely, general relativity.\n> How does string theory deal with this interdependence?\n\nHere\'s my layperson response:\n\nFollowing the publication of General Relativity, Kaluza found he could\nunify both electromagnetism and relativity simply by extending the GR\nequations by 1 dimension. What\'s more, he showed that the photon was\na byproduct of empty 5D spacetime. That is, matter/energy was the\nresult of geometry.\n\nThe problem was that no one had observed the 5th dimension and he\nbasically restricted physics to 4D. I.e. our 4D universe was embedded\nin a 5D universe, but nothing happens on the 5th D. Klein came along\nand suggested that the 5th dimension was "compactified"...like the ant\non the garden hose analogy. The 5th dimension was rolled up so small\nthat we naturally couldn\'t observe it.\n\nThen came the strong and weak nuclear forces. Some thought that these\nshould easily be brought into this unification, so they naturally\nadded more dimensions and compactified them as before. The issue\nbecame how many D\'s you had to add. Also, other complexities were\nintroduced in later versions by adding mass here and there.\n\nSo, that\'s where GR comes in to play as far as superstring theory is\nconcerned.\n\n-stmx3\n\n[Moderator\'s note: Although one can obtain non-Abelian gauge symmetries by\nthe Kaluza-Klein mechanism with many dimensions - the gauge group is the\nisometry of the internal manifold - it is much more usual in string\ntheory to have some gauge symmetry already in 10 (or 11) dimensions to\nstart with, and the compactified dimensions play a role in the breaking\nof the original gauge group. LM]\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>Karim <gawains_chela@sbcglobal.net> wrote in message news:<e7ecf966.0404231615.42fab997-100000@posting.google.com>...
> This is another lay person question, this time aimed at understanding
> how string theory reproduces general relativity.
>
> String theory begins by assuming that a string is evolving in
> a spacetime having a prescribed metric. The metric, however, depends on
> the energy and mass of the string in question
> (and all other strings as well) via a law that one knows only
> at the large scale, namely, general relativity.
> How does string theory deal with this interdependence?
Here's my layperson response:
Following the publication of General Relativity, Kaluza found he could
unify both electromagnetism and relativity simply by extending the GR
equations by 1 dimension. What's more, he showed that the photon was
a byproduct of empty 5D spacetime. That is, matter/energy was the
result of geometry.
The problem was that no one had observed the 5th dimension and he
basically restricted physics to 4D. I.e. our 4D universe was embedded
in a 5D universe, but nothing happens on the 5th D. Klein came along
and suggested that the 5th dimension was "compactified"...like the ant
on the garden hose analogy. The 5th dimension was rolled up so small
that we naturally couldn't observe it.
Then came the strong and weak nuclear forces. Some thought that these
should easily be brought into this unification, so they naturally
added more dimensions and compactified them as before. The issue
became how many D's you had to add. Also, other complexities were
introduced in later versions by adding mass here and there.
So, that's where GR comes in to play as far as superstring theory is
concerned.
-stmx3
[Moderator's note: Although one can obtain non-Abelian gauge symmetries by
the Kaluza-Klein mechanism with many dimensions - the gauge group is the
isometry of the internal manifold - it is much more usual in string
theory to have some gauge symmetry already in 10 (or 11) dimensions to
start with, and the compactified dimensions play a role in the breaking
of the original gauge group. LM]
<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> So, that\'s where GR comes in to play as far as superstring theory is\n> concerned.\n>\n> -stmx3\n>\n> [Moderator\'s note: Although one can obtain non-Abelian gauge symmetries by\n> the Kaluza-Klein mechanism with many dimensions - the gauge group is the\n> isometry of the internal manifold - it is much more usual in string\n> theory to have some gauge symmetry already in 10 (or 11) dimensions to\n> start with, and the compactified dimensions play a role in the breaking\n> of the original gauge group. LM]\n\nIs it correct to say, therefore, that a string is just a feature upon\nspacetime?\n\n[Moderator\'s note: I am not sure whether I fully understand the point\nof this question, but: strings are today treated as objects that\npropagate on a pre-existing spacetime background. On the other hand,\none can show that a condensate of strings in a certain vibrational\nstate is equivalent to a change of the background - in this sense, the\nbackground itself is made of strings. All these statements\n"everything is made of strings" are only well-behaved at the weak\ncoupling, i.e. in the regions of the moduli space where the\ncoupling constant is small - in other regions, other objects become\nequally (or more) fundamental and important than the strings. LM]\n\nThe intent of my original post was to get a reference to the derivation\nof the Einstein equations as a limit of string theory.\n\n[Moderator\'s note: I recommend chapters 3 of the textbooks by\nGreen+Schwarz+Witten or Polchinski, although someone might point out\na better reference for this particular point. LM]\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>> ...
>
> So, that's where GR comes in to play as far as superstring theory is
> concerned.
>
> -stmx3
>
> [Moderator's note: Although one can obtain non-Abelian gauge symmetries by
> the Kaluza-Klein mechanism with many dimensions - the gauge group is the
> isometry of the internal manifold - it is much more usual in string
> theory to have some gauge symmetry already in 10 (or 11) dimensions to
> start with, and the compactified dimensions play a role in the breaking
> of the original gauge group. LM]
Is it correct to say, therefore, that a string is just a feature upon
spacetime?
[Moderator's note: I am not sure whether I fully understand the point
of this question, but: strings are today treated as objects that
propagate on a pre-existing spacetime background. On the other hand,
one can show that a condensate of strings in a certain vibrational
state is equivalent to a change of the background - in this sense, the
background itself is made of strings. All these statements
"everything is made of strings" are only well-behaved at the weak
coupling, i.e. in the regions of the moduli space where the
coupling constant is small - in other regions, other objects become
equally (or more) fundamental and important than the strings. LM]
The intent of my original post was to get a reference to the derivation
of the Einstein equations as a limit of string theory.
[Moderator's note: I recommend chapters 3 of the textbooks by
Green+Schwarz+Witten or Polchinski, although someone might point out
a better reference for this particular point. LM]
<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>> ...[Moderator\'s note: I recommend chapters 3 of the textbooks by\n> Green+Schwarz+Witten or Polchinski, although someone might point out\n> a better reference for this particular point. LM]\n\nI thank the moderator for his trying to help me out. I\'ll rephrase the\nquestion so it comes through better.\n\nStmx3 left me with the impression that according to string theory\n"everything is geometry of spacetime". Quoting him "matter/energy is the\nresult of geometry"(*). That is, I was left with the impression that\nwhile expositions of string theory begin by introducing strings as\nseparate entities existing within spacetime, ultimately, according to the\ntheory, all strings are just "excitations" of spacetime itself.\nThat is, in some sense, strings are to spacetime as waves are to the\nsurface of the ocean. Is this impression right?\n\n(*) Strictly speaking stmx3 was talking about Kaluza-Klein theory and I\nextraploated his statement to strings.\n\n-- Karim\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>> ...[Moderator's note: I recommend chapters 3 of the textbooks by
> Green+Schwarz+Witten or Polchinski, although someone might point out
> a better reference for this particular point. LM]
I thank the moderator for his trying to help me out. I'll rephrase the
question so it comes through better.
Stmx3 left me with the impression that according to string theory
"everything is geometry of spacetime". Quoting him "matter/energy is the
result of geometry"(*). That is, I was left with the impression that
while expositions of string theory begin by introducing strings as
separate entities existing within spacetime, ultimately, according to the
theory, all strings are just "excitations" of spacetime itself.
That is, in some sense, strings are to spacetime as waves are to the
surface of the ocean. Is this impression right?
(*) Strictly speaking stmx3 was talking about Kaluza-Klein theory and I
extraploated his statement to strings.
-- Karim
That is, I was left with the impression that
while expositions of string theory begin by introducing strings as
separate entities existing within spacetime, ultimately, according to the
theory, all strings are just "excitations" of spacetime itself.
That is, in some sense, strings are to spacetime as waves are to the
surface of the ocean. Is this impression right?
-- Karim
All particles/strings can transfer energy from one point to another. Since all particles have energy, wouldn't they have to distort the space-time around them. Fermions such as an electron have rest mass. But bosons such as a photon, for example, do not have rest mass. I take this to mean that there is no average net distortion at larger distances from the photon. Wouldn't this mean that photons curve space in both directions, if it shrinks space at some points, then it will stretch space in other so that there is not net space-time distortion? Does this sound right?
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