View Full Version : String theory and experimental practice
Alexander Blessing
May30-04, 09:06 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>Good evening.\nI\'ve come to read about string theory and it seems to be quite interesting,\nhard stuff. To unify the four known forces is a big deal, not even Einstein\ncould handle it ;-)\nAnyway, I would like to do some studies about this theory. Can anybody think\nof a way to show anything related to strings in a experiment or similar? I\nmean, I would like to do some research for school as a project, but pure\ntheory is probably not enough, I need something experimental to\nshow/prove/demonstrate/ask, whatever.\n\nWhat do you think, can this be realized? Do you have any ideas?\nThanks and regards,\nA. Blessing\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>Good evening.
I've come to read about string theory and it seems to be quite interesting,
hard stuff. To unify the four known forces is a big deal, not even Einstein
could handle it ;-)
Anyway, I would like to do some studies about this theory. Can anybody think
of a way to show anything related to strings in a experiment or similar? I
mean, I would like to do some research for school as a project, but pure
theory is probably not enough, I need something experimental to
show/prove/demonstrate/ask, whatever.
What do you think, can this be realized? Do you have any ideas?
Thanks and regards,
A. Blessing
Urs Schreiber
May31-04, 10:18 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>"Alexander Blessing" <Whatever5k@web.de> schrieb im Newsbeitrag\nnews:c9agts\\$qgm\\$03\\$1-100000@news.t-online.com...\n\n> Anyway, I would like to do some studies about this theory. Can anybody\nthink\n> of a way to show anything related to strings in a experiment or similar? I\n> mean, I would like to do some research for school as a project, but pure\n> theory is probably not enough, I need something experimental to\n> show/prove/demonstrate/ask, whatever.\n\nThat\'s an interesting question! ;-)\n\nI don\'t know what you know about string theory (but be sure to have a look\nat the section "popular resources" at our group\'s home page at\nhttp://schwinger.harvard.edu/~motl/sps/ and read as much as you can to get\nsome kind of idea what string theory is all about) or about physics in\ngeneral and I don\'t know what kind of school project you are faced with -\nbut let me nevertheless say a couple of things that I would maybe say when\nforced to do something like this in class:\n\nFirst experiment: Drop an apple. It falls to the ground. Thinking about a\nlittle more (the way Newton did) we can conclude that there is a universal\nattraction between lumps of matter, which we call gravity.\n\nSecond experiment: Set up a laser interferometer and measure the speed of\nlight in various directions (the Michelson-Morely experiment, look it up if\nyou don\'t know about it yet). This shows that the speed of gravity is\nindependent of the motion of the observer. Thinking about this for a while\n(the way Einstein did) shows that gravity is described by the curvature of\nspacetime.\n\nThird experiment: Measure the spectrum of thermal black body radiation.\nThinking about this for a while (the way Planck did) shows that nature is\ndescribed by quantum mechanics.(look up "Planck spectrum" and "quantum\nmechanics" with Google).\n\nFourth experiment: Enter \'"quantum gravity" string\' into Google to get an\nimpression of how these three experiments, when taken together, have led to\nan enormous activity among theoretical physicsts. After the standard model\nof particle phyiscs has successfully and elegantly given a unified picture\nof the quantization of three of the four forces of nature, namely\nelectromagnetism as well as the weak and the strong nuclear force, the big\ntask has been to find a sensible quantum theory of gravity. When people\nnoticed that the quantum mechanical motion of small 1-dimensional objects\n("strings") _implies_ the existence of gravity as well as of gauge forces,\nstring theory as a candidate theory of quantum gravity and unification was\nborn.\n\nThe theoretical "prediction" of gravity by string theory had a dramatic\nverification in a kind of "theoretical experiment" when it could be shown\nthat string theory precisely predicts microscopic degrees of freedom of\nblack holes that are expected to exists due to so-called "semi-classical"\narguments using general relativity and quantum field theory. This was and is\nregarded as strong evidence for the correctness of string theory as a\nphysical theory.\n\nBut the big problem with any theory of quantum gravity is, that the effects\nit describes occur at energies so immensely high (look up "Planck length")\nthat they can not be detected with currently available accelerators. Still,\neverybody in high energy physics is eagerly expecting the next-generation\naccelerator, the so-called "Large Hadron Collider" at CERN in Genf, to\nproduce experimental results that might hint at "physics beyond the standard\nmodel".\n\nSo currently the motivation for theorists to work on string theory is not\nmotivated by new experiments, but by theoretical considerations. You should\nbe well aware that it is so far just a guess that string theory really\ndescribes nature - though what one calls an "educated guess" - and not\neverybody in the high energy physics community shares this believe, even\nthough string theory is currently clearly the most popular candidate for a\ntheory of quantum gravity. (Another approach goes under the name "Loop\nQuantum Gravity", which is very different from string theory).\n\nPesonally I would tend to discourage school projects on something like\nquantum gravity. I think it would be better to spend the time in school to\nget an understanding of the established basics of physics, such as Newton\'s\nmechanics, quantum mechanics and maybe the theory of relativity. But on the\nother hand I could imagine that one can do an interesting project by\nconcentrating on the three experiments that I indicated at the beginning,\nsay something about how this has influenced our modern view of the world and\nmaybe in the end give a brief outlook on some modern developments, such as\nquantum gravity, maybe. If you feel that you can do it, that is... :-)\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>"Alexander Blessing" <Whatever5k@web.de> schrieb im Newsbeitrag
news:c9agts$qgm$03$1-100000@news.t-online.com...
> Anyway, I would like to do some studies about this theory. Can anybody
think
> of a way to show anything related to strings in a experiment or similar? I
> mean, I would like to do some research for school as a project, but pure
> theory is probably not enough, I need something experimental to
> show/prove/demonstrate/ask, whatever.
That's an interesting question! ;-)
I don't know what you know about string theory (but be sure to have a look
at the section "popular resources" at our group's home page at
http://schwinger.harvard.edu/~motl/sps/ and read as much as you can to get
some kind of idea what string theory is all about) or about physics in
general and I don't know what kind of school project you are faced with -
but let me nevertheless say a couple of things that I would maybe say when
forced to do something like this in class:
First experiment: Drop an apple. It falls to the ground. Thinking about a
little more (the way Newton did) we can conclude that there is a universal
attraction between lumps of matter, which we call gravity.
Second experiment: Set up a laser interferometer and measure the speed of
light in various directions (the Michelson-Morely experiment, look it up if
you don't know about it yet). This shows that the speed of gravity is
independent of the motion of the observer. Thinking about this for a while
(the way Einstein did) shows that gravity is described by the curvature of
spacetime.
Third experiment: Measure the spectrum of thermal black body radiation.
Thinking about this for a while (the way Planck did) shows that nature is
described by quantum mechanics.(look up "Planck spectrum" and "quantum
mechanics" with Google).
Fourth experiment: Enter '"quantum gravity" string' into Google to get an
impression of how these three experiments, when taken together, have led to
an enormous activity among theoretical physicsts. After the standard model
of particle phyiscs has successfully and elegantly given a unified picture
of the quantization of three of the four forces of nature, namely
electromagnetism as well as the weak and the strong nuclear force, the big
task has been to find a sensible quantum theory of gravity. When people
noticed that the quantum mechanical motion of small 1-dimensional objects
("strings") _implies_ the existence of gravity as well as of gauge forces,
string theory as a candidate theory of quantum gravity and unification was
born.
The theoretical "prediction" of gravity by string theory had a dramatic
verification in a kind of "theoretical experiment" when it could be shown
that string theory precisely predicts microscopic degrees of freedom of
black holes that are expected to exists due to so-called "semi-classical"
arguments using general relativity and quantum field theory. This was and is
regarded as strong evidence for the correctness of string theory as a
physical theory.
But the big problem with any theory of quantum gravity is, that the effects
it describes occur at energies so immensely high (look up "Planck length")
that they can not be detected with currently available accelerators. Still,
everybody in high energy physics is eagerly expecting the next-generation
accelerator, the so-called "Large Hadron Collider" at CERN in Genf, to
produce experimental results that might hint at "physics beyond the standard
model".
So currently the motivation for theorists to work on string theory is not
motivated by new experiments, but by theoretical considerations. You should
be well aware that it is so far just a guess that string theory really
describes nature - though what one calls an "educated guess" - and not
everybody in the high energy physics community shares this believe, even
though string theory is currently clearly the most popular candidate for a
theory of quantum gravity. (Another approach goes under the name "Loop
Quantum Gravity", which is very different from string theory).
Pesonally I would tend to discourage school projects on something like
quantum gravity. I think it would be better to spend the time in school to
get an understanding of the established basics of physics, such as Newton's
mechanics, quantum mechanics and maybe the theory of relativity. But on the
other hand I could imagine that one can do an interesting project by
concentrating on the three experiments that I indicated at the beginning,
say something about how this has influenced our modern view of the world and
maybe in the end give a brief outlook on some modern developments, such as
quantum gravity, maybe. If you feel that you can do it, that is... :-)
Urs Schreiber
May31-04, 10:23 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>On Mon, 31 May 2004, Urs Schreiber wrote:\n\n> you don\'t know about it yet). This shows that the speed of gravity is\n\nI am sorry for this typo. Of course I meant to say:\n\nThis shows that the speed of _light_ is indepdent of the motion of the\nobserver.\n\nOn the other hand what I wrote is still right, since the speed of gravity\nequals the speed of light! :-)\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>On Mon, 31 May 2004, Urs Schreiber wrote:
> you don't know about it yet). This shows that the speed of gravity is
I am sorry for this typo. Of course I meant to say:
This shows that the speed of _light_ is indepdent of the motion of the
observer.
On the other hand what I wrote is still right, since the speed of gravity
equals the speed of light! :-)
Rene Meyer
Jun1-04, 04:53 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>On Mon, 31 May 2004 10:18:47 -0400, Urs Schreiber wrote:\n> Second experiment: Set up a laser interferometer and measure the speed of\n> light in various directions (the Michelson-Morely experiment, look it up if\n> you don\'t know about it yet). This shows that the speed of gravity is\n> independent of the motion of the observer. Thinking about this for a while\n> (the way Einstein did) shows that gravity is described by the curvature of\n> spacetime.\n\nYou mean the speed of light, right?\n\nRené.\n\n\n[Moderator\'s note: Yes. I noticed this typo right after having posted it.\n-usc]\n\n--\nRené Meyer\nStudent of Physics & Mathematics\nZhejiang University, Hangzhou, China\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>On Mon, 31 May 2004 10:18:47 -0400, Urs Schreiber wrote:
> Second experiment: Set up a laser interferometer and measure the speed of
> light in various directions (the Michelson-Morely experiment, look it up if
> you don't know about it yet). This shows that the speed of gravity is
> independent of the motion of the observer. Thinking about this for a while
> (the way Einstein did) shows that gravity is described by the curvature of
> spacetime.
You mean the speed of light, right?
René.
[Moderator's note: Yes. I noticed this typo right after having posted it.
-usc]
--
René Meyer
Student of Physics & Mathematics
Zhejiang University, Hangzhou, China
Alexander Blessing
Jun1-04, 01:04 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>Hi Urs,\nthank you very much for your answer, it was very helpful for me.\nThe experiment you listed on your posting are quite interesting, but they\nare indeed (as you say yourself) rather concerning relativity theory or\nquantum mechanics.\nDon\'t you think that there is anything related to _string_ theory? Maybe\nanything from space?\n\nAnyway, thanks!\nDanke :-)\nAlexander\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>Hi Urs,
thank you very much for your answer, it was very helpful for me.
The experiment you listed on your posting are quite interesting, but they
are indeed (as you say yourself) rather concerning relativity theory or
quantum mechanics.
Don't you think that there is anything related to _string_ theory? Maybe
anything from space?
Anyway, thanks!
Danke :-)
Alexander
Urs Schreiber
Jun1-04, 01:50 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>"Alexander Blessing" <Alexander.Blessing@epost.de> schrieb im Newsbeitrag\nnews:pan.2004.06.01.20.59.12.46354.51 5-100000@epost.de...\n\n> Don\'t you think that there is anything related to _string_ theory? Maybe\n> anything from space?\n\nSome people are discussing/have proposed experiments that _might_ show\nsignatures of effects as one would maybe expect from some theories of\nquantum gravity. This includes looking for unusual effects in radiation that\nhits the earth which is either highly energetic or originates from very far\naway, or both. But so far these experiments have either not been done yet or\nhave yielded no new results.\n\nOn the other hand, there is one recent experiment that has had a great\ninfluence of string theory:\n\nThat\'s the observation that our universe is currently undergoing an\n_accelerated_ expansion (instead of a decelerated one or one with no\nacceleration or deceleration). Various pieces of data, such as measuremenets\nof far away supernova as well as of the cosmological microwave background\ndemonstrate that about 70% of the energy density of the universe is due to\nwhat is called "dark energy" nowadays, which is a term supposed to be\nslightly more general than the "cosmological constant", but it\'s essentially\nthe same idea.\n\nThe presence of the "dark energy" poses several problems to string\ntheorists: For one, it would have been somehow more natural from the string\ntheory perspective to have a negative cosmological constant than a positve\none, as it now turns out to be. Second, the value of the "dark energy",\nwhile with 70% of all there is seemingly large, is on the other hand for\ntheoretical reasons surprisingly small.\n\nThe problem is, one still does not know enough of string theory to be able\nto say if this experiment is maybe in contradiction to the theory or, if\nnot, how the observed value of the "dark energy" can be accounted for in the\ntheory.\n\nThis has now led some people to start a new intensive study of the "space of\nsolutions" of string theory, roughly. This abstract space, in which each\nclassical "background" spacetime in which strings propagate is one point is\ncalled the "string theory landscape" and their have been first indications\nthat indeed there are certain points in the landscape which describe\nuniverses with a small positive constant.\n\nYou can read about the accelerating universe for instance here:\n\nhttp://snap.lbl.gov/brochure/\n\nhttp://pancake.uchicago.edu/~carroll/preposterous.html\n\n\nFor more information on the string theory landscape see for instance the\nfollowing links:\n\nhttp://physicsweb.org/article/world/16/11/3/1\n\nhttp://www.phys.lsu.edu/mog/mog23/node8.html\n\nhttp://www.edge.org/3rd_culture/susskind03/susskind_index.html\n\n\n> Danke :-)\n\nKeine Ursache.\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>"Alexander Blessing" <Alexander.Blessing@epost.de> schrieb im Newsbeitrag
news:pan.2004.06.01.20.59.12.46354.515-100000@epost.de...
> Don't you think that there is anything related to _string_ theory? Maybe
> anything from space?
Some people are discussing/have proposed experiments that _might_ show
signatures of effects as one would maybe expect from some theories of
quantum gravity. This includes looking for unusual effects in radiation that
hits the earth which is either highly energetic or originates from very far
away, or both. But so far these experiments have either not been done yet or
have yielded no new results.
On the other hand, there is one recent experiment that has had a great
influence of string theory:
That's the observation that our universe is currently undergoing an
_accelerated_ expansion (instead of a decelerated one or one with no
acceleration or deceleration). Various pieces of data, such as measuremenets
of far away supernova as well as of the cosmological microwave background
demonstrate that about 70% of the energy density of the universe is due to
what is called "dark energy" nowadays, which is a term supposed to be
slightly more general than the "cosmological constant", but it's essentially
the same idea.
The presence of the "dark energy" poses several problems to string
theorists: For one, it would have been somehow more natural from the string
theory perspective to have a negative cosmological constant than a positve
one, as it now turns out to be. Second, the value of the "dark energy",
while with 70% of all there is seemingly large, is on the other hand for
theoretical reasons surprisingly small.
The problem is, one still does not know enough of string theory to be able
to say if this experiment is maybe in contradiction to the theory or, if
not, how the observed value of the "dark energy" can be accounted for in the
theory.
This has now led some people to start a new intensive study of the "space of
solutions" of string theory, roughly. This abstract space, in which each
classical "background" spacetime in which strings propagate is one point is
called the "string theory landscape" and their have been first indications
that indeed there are certain points in the landscape which describe
universes with a small positive constant.
You can read about the accelerating universe for instance here:
http://snap.lbl.gov/brochure/
http://pancake.uchicago.edu/~carroll/preposterous.html
For more information on the string theory landscape see for instance the
following links:
http://physicsweb.org/article/world/16/11/3/1
http://www.phys.lsu.edu/mog/mog23/node8.html
http://www.edge.org/3rd_culture/susskind03/susskind_index.html
> Danke :-)
Keine Ursache.
Peter Woit
Jun1-04, 04:53 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>Alexander Blessing wrote:\n\n>Hi Urs,thank you very much for your answer, it was very helpful for me.\n>The experiment you listed on your posting are quite interesting, but they\n>are indeed (as you say yourself) rather concerning relativity theory or\n>quantum mechanics.\n>Don\'t you think that there is anything related to _string_ theory? Maybe\n>anything from space?\n\nI don\'t want to stir up trouble on this newsgroup, but you deserve a\nmore straight-forward answer than you have gotten so far.\n\nAt the moment there is no known practical experiment that can be\ndone that would give any information about string theory and\nwhether the idea is right or wrong. This isn\'t controversial. If there\nwere such an experiment, even if it were very difficult to do, many\nphysicists would be hard at work on it.\n\n[Moderator\'s note: if you construct a Planckian accelerator for us,\nwe will be able to tell you many things what you will see, although\n- I admit - most of these predictions won\'t be unique, i.e. they will\nbe model-dependent, and many of them will be derived from effective\nfield theory and various other approximations. Yes, it is not possible\nto design an experiment that would safely tell us whether string theory\nis right or wrong tomorrow, but we can certainly get some hints. LM]\n\nThe question of why string theory is incapable of being\nexperimentally tested is a more controversial one.\n\n[Moderator\'s note: string theory is able to predict detailed things\n*in principle*, and, unlike any other theory we ever had, it would predict\nphysics uniquely, without any parameters, if we determined the correct\n"vacuum". Yes, these things are only "in principle". LM]\n\nOne point of view (mine) is that the theory is so ill-defined it can\'t be\nused to predict anything.\n\n[Moderator\'s note: string theory description of some of the most\nsymmetric backgrounds is as well-defined as you can get. Yes, we don\'t\nyet have a universal definition of (all backgrounds of) string theory. LM]\n\nOthers will tell you that the theory is just not well-enough understood to\nmake low-energy predictions and we don\'t have a practical way of probing\nwhat happens at the very high energies where the distinctive effects of\nstring theory are supposed to show up.\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>Alexander Blessing wrote:
>Hi Urs,thank you very much for your answer, it was very helpful for me.
>The experiment you listed on your posting are quite interesting, but they
>are indeed (as you say yourself) rather concerning relativity theory or
>quantum mechanics.
>Don't you think that there is anything related to _string_ theory? Maybe
>anything from space?
I don't want to stir up trouble on this newsgroup, but you deserve a
more straight-forward answer than you have gotten so far.
At the moment there is no known practical experiment that can be
done that would give any information about string theory and
whether the idea is right or wrong. This isn't controversial. If there
were such an experiment, even if it were very difficult to do, many
physicists would be hard at work on it.
[Moderator's note: if you construct a Planckian accelerator for us,
we will be able to tell you many things what you will see, although
- I admit - most of these predictions won't be unique, i.e. they will
be model-dependent, and many of them will be derived from effective
field theory and various other approximations. Yes, it is not possible
to design an experiment that would safely tell us whether string theory
is right or wrong tomorrow, but we can certainly get some hints. LM]
The question of why string theory is incapable of being
experimentally tested is a more controversial one.
[Moderator's note: string theory is able to predict detailed things
*in principle*, and, unlike any other theory we ever had, it would predict
physics uniquely, without any parameters, if we determined the correct
"vacuum". Yes, these things are only "in principle". LM]
One point of view (mine) is that the theory is so ill-defined it can't be
used to predict anything.
[Moderator's note: string theory description of some of the most
symmetric backgrounds is as well-defined as you can get. Yes, we don't
yet have a universal definition of (all backgrounds of) string theory. LM]
Others will tell you that the theory is just not well-enough understood to
make low-energy predictions and we don't have a practical way of probing
what happens at the very high energies where the distinctive effects of
string theory are supposed to show up.
Peter Woit
Jun3-04, 12:10 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>Alexander Blessing wrote:\n\n>Hi Urs,thank you very much for your answer, it was very helpful for me.\n>The experiment you listed on your posting are quite interesting, but they\n>are indeed (as you say yourself) rather concerning relativity theory or\n>quantum mechanics.\n>Don\'t you think that there is anything related to _string_ theory? Maybe\n>anything from space?\n\nI don\'t want to stir up trouble on this newsgroup, but you deserve a\nmore straight-forward answer than you have gotten so far.\n\nAt the moment there is no known practical experiment that can be\ndone that would give any information about string theory and\nwhether the idea is right or wrong. This isn\'t controversial. If there\nwere such an experiment, even if it were very difficult to do, many\nphysicists would be hard at work on it.\n\n[Moderator\'s note: if you construct a Planckian accelerator for us,\nwe will be able to tell you many things what you will see, although\n- I admit - most of these predictions won\'t be unique, i.e. they will\nbe model-dependent, and many of them will be derived from effective\nfield theory and various other approximations. Yes, it is not possible\nto design an experiment that would safely tell us whether string theory\nis right or wrong tomorrow, but we can certainly get some hints. LM]\n\nThe question of why string theory is incapable of being\nexperimentally tested is a more controversial one.\n\n[Moderator\'s note: string theory is able to predict detailed things\n*in principle*, and, unlike any other theory we ever had, it would predict\nphysics uniquely, without any parameters, if we determined the correct\n"vacuum". Yes, these things are only "in principle". LM]\n\nOne point of view (mine) is that the theory is so ill-defined it can\'t be\nused to predict anything.\n\n[Moderator\'s note: string theory description of some of the most\nsymmetric backgrounds is as well-defined as you can get. Yes, we don\'t\nyet have a universal definition of (all backgrounds of) string theory. LM]\n\nOthers will tell you that the theory is just not well-enough understood to\nmake low-energy predictions and we don\'t have a practical way of probing\nwhat happens at the very high energies where the distinctive effects of\nstring theory are supposed to show up.\n\n[This posting was re-posted again after the Harvard FAS newsserver\nproblems on June 2nd. 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>Alexander Blessing wrote:
>Hi Urs,thank you very much for your answer, it was very helpful for me.
>The experiment you listed on your posting are quite interesting, but they
>are indeed (as you say yourself) rather concerning relativity theory or
>quantum mechanics.
>Don't you think that there is anything related to _string_ theory? Maybe
>anything from space?
I don't want to stir up trouble on this newsgroup, but you deserve a
more straight-forward answer than you have gotten so far.
At the moment there is no known practical experiment that can be
done that would give any information about string theory and
whether the idea is right or wrong. This isn't controversial. If there
were such an experiment, even if it were very difficult to do, many
physicists would be hard at work on it.
[Moderator's note: if you construct a Planckian accelerator for us,
we will be able to tell you many things what you will see, although
- I admit - most of these predictions won't be unique, i.e. they will
be model-dependent, and many of them will be derived from effective
field theory and various other approximations. Yes, it is not possible
to design an experiment that would safely tell us whether string theory
is right or wrong tomorrow, but we can certainly get some hints. LM]
The question of why string theory is incapable of being
experimentally tested is a more controversial one.
[Moderator's note: string theory is able to predict detailed things
*in principle*, and, unlike any other theory we ever had, it would predict
physics uniquely, without any parameters, if we determined the correct
"vacuum". Yes, these things are only "in principle". LM]
One point of view (mine) is that the theory is so ill-defined it can't be
used to predict anything.
[Moderator's note: string theory description of some of the most
symmetric backgrounds is as well-defined as you can get. Yes, we don't
yet have a universal definition of (all backgrounds of) string theory. LM]
Others will tell you that the theory is just not well-enough understood to
make low-energy predictions and we don't have a practical way of probing
what happens at the very high energies where the distinctive effects of
string theory are supposed to show up.
[This posting was re-posted again after the Harvard FAS newsserver
problems on June 2nd. LM]
Lubos Motl
Jun3-04, 12:11 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>On Tue, 1 Jun 2004, Urs Schreiber wrote:\n\n> Some people are discussing/have proposed experiments that _might_ show\n> signatures of effects as one would maybe expect from some theories of\n> quantum gravity. This includes looking for unusual effects in radiation that\n> hits the earth which is either highly energetic or originates from very far\n> away, or both. But so far these experiments have either not been done yet or\n> have yielded no new results.\n\nWhat effects do you exactly expect to be measurable? What you say is\npotentially an interesting statement.\n\n> On the other hand, there is one recent experiment that has had a great\n> influence of string theory: (accelerating Universe)\n\nWell, it might however be a negative influence, what do you think? ;-)\n\n> > Danke :-)\n> Keine Ursache.\n\nMeinen Damen und Herren, es ist eine Englische Neuesgruppe! :-) Sorry for\nthis short and vacuous post; I am testing a new configuration of the\nFAS newsserver...\n___________________________________ ___________________________________________\nE-mail: lumo@matfyz.cz fax: +1-617/496-0110 Web: http://lumo.matfyz.cz/\neFax: +1-801/454-1858 work: +1-617/496-8199 home: +1-617/868-4487 (call)\n^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n\n[This posting was re-posted again after the Harvard FAS newsserver\nproblems on June 2nd. LM]\n\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>On Tue, 1 Jun 2004, Urs Schreiber wrote:
> Some people are discussing/have proposed experiments that _might_ show
> signatures of effects as one would maybe expect from some theories of
> quantum gravity. This includes looking for unusual effects in radiation that
> hits the earth which is either highly energetic or originates from very far
> away, or both. But so far these experiments have either not been done yet or
> have yielded no new results.
What effects do you exactly expect to be measurable? What you say is
potentially an interesting statement.
> On the other hand, there is one recent experiment that has had a great
> influence of string theory: (accelerating Universe)
Well, it might however be a negative influence, what do you think? ;-)
> > Danke :-)
> Keine Ursache.
Meinen Damen und Herren, es ist eine Englische Neuesgruppe! :-) Sorry for
this short and vacuous post; I am testing a new configuration of the
FAS newsserver...
__{_______________________________________________ _____________________________}
E-mail: lumo@matfyz.cz fax: +1-617/496-0110 Web: http://lumo.matfyz.cz/
eFax: +1-801/454-1858 work: +1-617/496-8199 home: +1-617/868-4487 (call)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[This posting was re-posted again after the Harvard FAS newsserver
problems on June 2nd. LM]
Urs Schreiber
Jun3-04, 12:13 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>On Wed, 2 Jun 2004, Lubos Motl wrote:\n\n\n> I meant to ask: What do you think that string theory predicts about these\nphenomena?\n\nAs far as I can see of the questions mentioned there are two where maybe\n(I\'d guess) a generic string prediction could be obtained in principle,\nthat is that concerning spacetime foaminess effects and that concerning\nextra dimensions.\n\nRe GZK violation:\n\n> Could you give us the reference to the relevant paper(s)?\n\nSee references [3,4,5] in\n\nGlennys R. Farrar, Tsvi Piran\nGZK Violation - a Tempest in a (Magnetic) Teapot?\nPhys.Rev.Lett. 84 (2000) 3527\nhttp://arxiv.org/abs/astro-ph/9906431\n\nI have heard (http://www.wissenschaft.de/wissen/news/149230.html) that\nSmolin and some other people believe that they can explain the GZK violation\nwith "doubly special relativity", that this is furthermore somehow a\nprediction of LQG and that they are hoping that the GLAST experiment in 2006\n(http://glast.gsfc.nasa.gov/) will say something about this (but I am having\ntrouble finding more details on this speculation, in hep-th/0401087 Smolin\nvaguely talks about some such experiments).\n\n\n> It will probably be more reliable to produce a LSP\n\nSure. But I guess it would already be interesting to directly (instead of\nindirectly by means of cosmological data and models) establish the mere\nexistence of some dark matter by such laboratory experiments. That would for\ninstance stop the growing MOND discussion.\n\nConcerning torsion:\n\nThere is an indian group working on (axion-)torsion in string theory. One\npaper is\n\nB. Mukhopadhyaya & S. Sen & S. SenGunpta & S. Sur\nParity violation in four and higher dimensional spacetime with torsion\nhep-th/0207165\n\nbut I haven\'t found much of interest in there.\n\n[This posting was re-posted again after the Harvard FAS newsserver\nproblems on June 2nd. 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>On Wed, 2 Jun 2004, Lubos Motl wrote:
> I meant to ask: What do you think that string theory predicts about these
phenomena?
As far as I can see of the questions mentioned there are two where maybe
(I'd guess) a generic string prediction could be obtained in principle,
that is that concerning spacetime foaminess effects and that concerning
extra dimensions.
Re GZK violation:
> Could you give us the reference to the relevant paper(s)?
See references [3,4,5] in
Glennys R. Farrar, Tsvi Piran
GZK Violation - a Tempest in a (Magnetic) Teapot?
Phys.Rev.Lett. 84 (2000) 3527
http://arxiv.org/abs/http://www.arxiv.org/abs/astro-ph/9906431
I have heard (http://www.wissenschaft.de/wissen/news/149230.html) that
Smolin and some other people believe that they can explain the GZK violation
with "doubly special relativity", that this is furthermore somehow a
prediction of LQG and that they are hoping that the GLAST experiment in 2006
(http://glast.gsfc.nasa.gov/) will say something about this (but I am having
trouble finding more details on this speculation, in http://www.arxiv.org/abs/hep-th/0401087 Smolin
vaguely talks about some such experiments).
> It will probably be more reliable to produce a LSP
Sure. But I guess it would already be interesting to directly (instead of
indirectly by means of cosmological data and models) establish the mere
existence of some dark matter by such laboratory experiments. That would for
instance stop the growing MOND discussion.
Concerning torsion:
There is an indian group working on (axion-)torsion in string theory. One
paper is
B. Mukhopadhyaya & S. Sen & S. SenGunpta & S. Sur
Parity violation in four and higher dimensional spacetime with torsion
http://www.arxiv.org/abs/hep-th/0207165
but I haven't found much of interest in there.
[This posting was re-posted again after the Harvard FAS newsserver
problems on June 2nd. LM]
Charlie Stromeyer Jr.
Jun4-04, 05:33 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>Alexander Blessing <Alexander.Blessing@epost.de> wrote in message news:\n\n> Hi Urs,\n> thank you very much for your answer, it was very helpful for me.\n> The experiment you listed on your posting are quite interesting, but they\n> are indeed (as you say yourself) rather concerning relativity theory or\n> quantum mechanics.\n> Don\'t you think that there is anything related to _string_ theory? Maybe\n> anything from space?\n\nI\'m not Urs, but I can answer your question even without the\nassistance of anything from outer space. There are already various\nproposals for how to test *possible* consequences of string theory\nright here at everyday low energy. As one example, here is a proposal\nfor a simple classical (i.e. non-quantum) experiment which may be able\nto distinguish between non-commutative (NC) space vs. the usual\ncommutative view of space:\n\nhttp://arxiv.org/abs/hep-th/0311134\n\nHowever, it is important to realize that some of these various\nexperiments would not be (yet, anyways) clearly indicative of string\ntheory.\n\nFor instance, if experiments like the above experiment were to suggest\nthat space is NC then there might be other explanations for why space\nshould be\nNC such as the NC QFTs of Madore, Majid or others as opposed to string\ntheory versions of NC QFTs such as the Witten-Seiberg NC QFTs (or\nperhaps they are called the Seiberg-Witten NC QFTs because I cannot\nremember right now who was the first author).\n\nHopefully, if there should be any positive experimental results they\ncould then be refined further to rule out some types of NC space vs.\nother types of NC space.\n\nI could also help you find other proposals for low energy experiments\nwhich may test for possible consequences of quantum gravity, but these\nexperiments will be too difficult for you to do by yourself unless\nperhaps you have or can get whatever money and guidance may be needed,\ne.g. you might need help from someone who could figure out how to\ncancel out possible quantum corrections for an experiment done at the\nclassical scale such as the experiment suggested in the paper above.\n\nThe advantage of such an experiment for your consideration is that it\nis classical which means that you would not have to learn and\nunderstand quantum theory first in order to think about the\nexperiment.\n\nIf you do want to start learning about QT then I have heard that some\nreviewers think that the new book by Brian Greene, "The Fabric of the\nCosmos", describes GTR and QM as well as any book ever written for a\npopular audience. You might also want to read the little book by\nRichard Feynman called "QED" because it is also a great book for\nlaymen, and AFAIK, QED and GTR are the most overall accurate\nscientific theories ever.\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>Alexander Blessing <Alexander.Blessing@epost.de> wrote in message news:
> Hi Urs,
> thank you very much for your answer, it was very helpful for me.
> The experiment you listed on your posting are quite interesting, but they
> are indeed (as you say yourself) rather concerning relativity theory or
> quantum mechanics.
> Don't you think that there is anything related to _string_ theory? Maybe
> anything from space?
I'm not Urs, but I can answer your question even without the
assistance of anything from outer space. There are already various
proposals for how to test *possible* consequences of string theory
right here at everyday low energy. As one example, here is a proposal
for a simple classical (i.e. non-quantum) experiment which may be able
to distinguish between non-commutative (NC) space vs. the usual
commutative view of space:
http://arxiv.org/abs/http://www.arxiv.org/abs/hep-th/0311134
However, it is important to realize that some of these various
experiments would not be (yet, anyways) clearly indicative of string
theory.
For instance, if experiments like the above experiment were to suggest
that space is NC then there might be other explanations for why space
should be
NC such as the NC QFTs of Madore, Majid or others as opposed to string
theory versions of NC QFTs such as the Witten-Seiberg NC QFTs (or
perhaps they are called the Seiberg-Witten NC QFTs because I cannot
remember right now who was the first author).
Hopefully, if there should be any positive experimental results they
could then be refined further to rule out some types of NC space vs.
other types of NC space.
I could also help you find other proposals for low energy experiments
which may test for possible consequences of quantum gravity, but these
experiments will be too difficult for you to do by yourself unless
perhaps you have or can get whatever money and guidance may be needed,
e.g. you might need help from someone who could figure out how to
cancel out possible quantum corrections for an experiment done at the
classical scale such as the experiment suggested in the paper above.
The advantage of such an experiment for your consideration is that it
is classical which means that you would not have to learn and
understand quantum theory first in order to think about the
experiment.
If you do want to start learning about QT then I have heard that some
reviewers think that the new book by Brian Greene, "The Fabric of the
Cosmos", describes GTR and QM as well as any book ever written for a
popular audience. You might also want to read the little book by
Richard Feynman called "QED" because it is also a great book for
laymen, and AFAIK, QED and GTR are the most overall accurate
scientific theories ever.
Rene Meyer
Jun5-04, 07:53 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>On Fri, 4 Jun 2004 17:33:17 -0400, Charlie Stromeyer Jr. wrote:\n\n> right here at everyday low energy. As one example, here is a proposal\n> for a simple classical (i.e. non-quantum) experiment which may be able\n> to distinguish between non-commutative (NC) space vs. the usual\n> commutative view of space:\n> http://arxiv.org/abs/hep-th/0311134\n> However, it is important to realize that some of these various\n> experiments would not be (yet, anyways) clearly indicative of string\n> theory.\n\nWhat does string theory have to do with NC space? I think string\ntheory still works with the normal commutative space, the target space\nis just an R^d.\n\n[Moderator\'s note: Rene, noncommutative geometry is a very important\nlimit of dynamics of open strings, and because the following paper\nby SW has more than 1000 citations, you should know about it:\n\nhttp://arxiv.org/abs/hep-th/9908142\n\nMoreover, even a broader context of string theory has some\nresemblance to noncommutative/fuzzy geometry and a very large\nfraction of string theory papers talk about such things. LM]\n\nRene.\n\n--\nRené Meyer\nStudent of Physics & Mathematics\nZhejiang University, Hangzhou, China\n___________________________________________ ____________________________________\nWeb page of SPS: http://schwinger.harvard.edu/~sps/\nPosted via: http://groups.google.com/groups?group=sci.physics.strings\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>On Fri, 4 Jun 2004 17:33:17 -0400, Charlie Stromeyer Jr. wrote:
> right here at everyday low energy. As one example, here is a proposal
> for a simple classical (i.e. non-quantum) experiment which may be able
> to distinguish between non-commutative (NC) space vs. the usual
> commutative view of space:
> http://arxiv.org/abs/http://www.arxiv.org/abs/hep-th/0311134
> However, it is important to realize that some of these various
> experiments would not be (yet, anyways) clearly indicative of string
> theory.
What does string theory have to do with NC space? I think string
theory still works with the normal commutative space, the target space
is just an R^d.
[Moderator's note: Rene, noncommutative geometry is a very important
limit of dynamics of open strings, and because the following paper
by SW has more than 1000 citations, you should know about it:
http://arxiv.org/abs/http://www.arxiv.org/abs/hep-th/9908142
Moreover, even a broader context of string theory has some
resemblance to noncommutative/fuzzy geometry and a very large
fraction of string theory papers talk about such things. LM]
Rene.
--
René Meyer
Student of Physics & Mathematics
Zhejiang University, Hangzhou, China
__{_______________________________________________ ______________________________}
Web page of SPS: http://schwinger.harvard.edu/~sps/
Posted via: http://groups.google.com/groups?group=sci.physics.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>> [Moderator\'s note: if you construct a Planckian accelerator for us,\n> we will be able to tell you many things what you will see, although\n> - I admit - most of these predictions won\'t be unique, i.e. they will\n> be model-dependent, and many of them will be derived from effective\n> field theory and various other approximations. Yes, it is not possible\n> to design an experiment that would safely tell us whether string theory\n> is right or wrong tomorrow, but we can certainly get some hints. LM]\n\nAnother interesting point is related. If string theory predicts the\n(positive cosmological constant) universe we live in is only meta-stable\nand that it is separated by a potential barrier order-of-magnitude the\nPlanck energy from a stable or perhaps a lower cosmological constant\nmeta-stable vacuum (which appears to be the case so far), a Planckian\naccelerator might be unable to give you any observable predictions as\nwell. This is for the simple reason that using this accelerator would\ndestroy the universe (similiar concerns were expressed for RHIC, but\nthese turned out to be groundless: however, RHIC is after all a much\nless than Planckian accelerator). More precisely, if it doesn\'t destroy\nthe universe, this might be an indication string theory is _wrong_. So,\nstring theory might be the first physical model in history which is\nfalsifiable but not verifiable ;-) OTOH, when the experiment is done,\nsomeone might claim that according to the Anthropic principle, even if\nwe have a slim chance of survival in this scenario, we would survive\n(because among all universes/places in which this scenario occured\nthose where we did survive would be the only ones where we would know\nthis). In fact, a similar claim was made regarding the RHIC. However,\nthis reminds of an amusing article claiming the the multi-world\ninterpretation of quantum mechanics may be put to experimental test in\na simple fashion: if it is true, you would experience living forever.\nThis is supposedly so because there\'s always a (highly improbable,\npossibly) world where you\'re still alive after any given amount of\ntime, and this is the only kind of world you would be able to observe.\nIt appears to me the same argument goes for the Anthropic principle...\n(at least in the form in which it would be able to save us from vacuum\ndecay)\n\nBest regards,\nSquark\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: if you construct a Planckian accelerator for us,
> we will be able to tell you many things what you will see, although
> - I admit - most of these predictions won't be unique, i.e. they will
> be model-dependent, and many of them will be derived from effective
> field theory and various other approximations. Yes, it is not possible
> to design an experiment that would safely tell us whether string theory
> is right or wrong tomorrow, but we can certainly get some hints. LM]
Another interesting point is related. If string theory predicts the
(positive cosmological constant) universe we live in is only meta-stable
and that it is separated by a potential barrier order-of-magnitude the
Planck energy from a stable or perhaps a lower cosmological constant
meta-stable vacuum (which appears to be the case so far), a Planckian
accelerator might be unable to give you any observable predictions as
well. This is for the simple reason that using this accelerator would
destroy the universe (similiar concerns were expressed for RHIC, but
these turned out to be groundless: however, RHIC is after all a much
less than Planckian accelerator). More precisely, if it doesn't destroy
the universe, this might be an indication string theory is _wrong_. So,
string theory might be the first physical model in history which is
falsifiable but not verifiable ;-) OTOH, when the experiment is done,
someone might claim that according to the Anthropic principle, even if
we have a slim chance of survival in this scenario, we would survive
(because among all universes/places in which this scenario occured
those where we did survive would be the only ones where we would know
this). In fact, a similar claim was made regarding the RHIC. However,
this reminds of an amusing article claiming the the multi-world
interpretation of quantum mechanics may be put to experimental test in
a simple fashion: if it is true, you would experience living forever.
This is supposedly so because there's always a (highly improbable,
possibly) world where you're still alive after any given amount of
time, and this is the only kind of world you would be able to observe.
It appears to me the same argument goes for the Anthropic principle...
(at least in the form in which it would be able to save us from vacuum
decay)
Best regards,
Squark
Lubos Motl
Jun6-04, 10:35 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>On Sat, 5 Jun 2004, Squark wrote:\n\n> Another interesting point is related. If string theory predicts the\n> (positive cosmological constant) universe we live in is only meta-stable\n> and that it is separated by a potential barrier order-of-magnitude the\n> Planck energy from a stable or perhaps a lower cosmological constant\n> meta-stable vacuum (which appears to be the case so far), a Planckian\n> accelerator might be unable to give you any observable predictions as\n> well. This is for the simple reason that using this accelerator would\n> destroy the universe (similiar concerns were expressed for RHIC, but\n> these turned out to be groundless: however, RHIC is after all a much\n> less than Planckian accelerator).\n\nI think that your conclusions are "derived" from a confusion of two\ndifferent meanings of the phrase "potential barrier comparable to the\nPlanck energy". If we talk about such a barrier in the context of\nspacetime physics, we mean something like a potential term in the energy\ndensity that is of order m_{Planck}^4 (well, the width of the barrier also\nmatters). It is simply not true that collecting a trans-Planckian energy\nis enough to tunnel through such a barrier. For example, the total energy\nof your body is roughly 10 orders of magnitude larger than the Planck\nenergy, and obviously it is not enough for the Universe to be destroyed.\n\nIt seems to me that you are not thinking about spacetime physics and\nquantum field theory, but rather about quantum mechanics for one particle\npropagating on the line. In that case, you could use the phrase "Planckian\nbarrier" in the same sloppy way, and your conclusion might be correct. But\nin spacetime\'s 4-dimensional (or higher-dimensional) physics, you must be\nable to concentrate the energy into a very small volume - your energy\n*density* must be large enough to convince the spacetime to jump. It\'s not\neasy because if you try to concentrate too much energy, you create a black\nhole whose radius grows with the energy, and the horizon protects the\nexterior (the rest of the Universe) from whatever disaster you might be\nimagining to happen inside. Very large black holes lead to small energy\ndensity again, and they are not helpful to initiate the tunneling. The\nPlanckian mass black holes are, in a sense, the densest objects in the\nworld (the Planckian density; let me now assume the conventional superhigh\nenergy Planck scale). Assuming that there has existed at least one black\nhole light enough so that it has already evaporated, it is safe to say\nthat having Planckian density is not enough for the Universe to decay.\n\nUsually we don\'t consider the effect of "catalysts", external objects that\nmight speed up the vacuum decay (perhaps because of their high energy). We\nusually consider the standard vacuum decay instanton, as Coleman has\ntaught us, which describes a process that spotaneously occurs in vacuum\nand does not require any external objects for it to be started. I think\nthat the reason why we don\'t consider the catalysts is that they don\'t\nreally matter. If the vacuum is stable enough, the density of the decay\nrate (probability per 4D worldvolume) is very small anyway, and it is\nprobably believed that it is very unlikely that a single process with some\ncatalysts could change the probability of decay. Well, maybe, there is a\nway to make a large probability of decay from a single collission of a\nparticular type, but I think we would agree that the fact that the\nUniverse has survived these 13.7 billion years of chaos is strong evidence\nthat it should not be *that* easy to destroy the Universe.\n\n> More precisely, if it doesn\'t destroy the universe, this might be an\n> indication string theory is _wrong_.\n\nIn other words, the only way to prove that string theory is correct is to\ndestroy the Universe? ;-) A proof of string theory would be cool, but this\nwould seem as a too large price to pay. :-)\n\nYour argument is amusing and the Universe is stable enough. But these two\nfacts don\'t imply that string theory is wrong; they imply that your\ncalculation of the probability of the vacuum decay is wrong.\n\n> In fact, a similar claim was made regarding the RHIC. However,\n> this reminds of an amusing article claiming the the multi-world\n> interpretation of quantum mechanics may be put to experimental test in\n> a simple fashion: if it is true, you would experience living forever.\n> This is supposedly so because there\'s always a (highly improbable,\n> possibly) world where you\'re still alive after any given amount of\n> time, and this is the only kind of world you would be able to observe.\n\nI don\'t think that this is a description of the many-world interpretation\nthat Everett and his followers would subscribe. It is not quite clear how\nto make their picture work equally well as the decohering histories, for\nexample, but it is more clear that they want to make it as\nindistinguishable from usual quantum mechanics as possible. The point is\nthat you can\'t choose in which parallel Universe you want to live. You are\n"randomly" assigned one of the parallel worlds. If it happens to be the\nworld in which you see an airplane that is going to hit the skyscraper\nwhere you are working at the moment on the 78th floor - which is something\nthat you *are* able to observe (you may replace the airplane by any other\nobservation that you do exactly before your death) - then the elementary\nlaws of physics (and common sense) imply that it is the Universe where\nyou\'re gonna die. If you wanted to survive, you should have chosen a\nluckier parallel Universe *before* you saw the airplane. I know, you did\nnot know about it - now it\'s too late. ;-)\n\nNo metalaws of physics or metaphysics can prevent you from dying as long\nas elementary laws of physics and common sense are respected.\n\nYou could speculate that the people who described how they returned from\nthe heaven/hell have really returned from a different parallel Universe in\nthe many-worlds interpretation because they decided that it is a bad idea\nto die, but I am not sure whether this would be the best newsgroup for\nsuch ideas.\n\n> It appears to me the same argument goes for the Anthropic principle...\n> (at least in the form in which it would be able to save us from vacuum\n> decay)\n\nI don\'t agree with the argument in either case. If the configuration of\nyour physical system is such that it is "programmed" for you to die in 0.1\nsecond - which is something that can certainly happen - there is no way to\nsurvive by jumping into a different parallel Universe, regardless whether\nthe parallel Universe results from the many-worlds interpretation of QM or\nfrom the anthropic reasoning.\n______________________________________ ________________________________________\nE-mail: lumo@matfyz.cz fax: +1-617/496-0110 Web: http://lumo.matfyz.cz/\neFax: +1-801/454-1858 work: +1-617/496-8199 home: +1-617/868-4487 (call)\n^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n\n[Posted for the 2nd time due to continuing problems with the FAS\nnewsserver. LM]\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>On Sat, 5 Jun 2004, Squark wrote:
> Another interesting point is related. If string theory predicts the
> (positive cosmological constant) universe we live in is only meta-stable
> and that it is separated by a potential barrier order-of-magnitude the
> Planck energy from a stable or perhaps a lower cosmological constant
> meta-stable vacuum (which appears to be the case so far), a Planckian
> accelerator might be unable to give you any observable predictions as
> well. This is for the simple reason that using this accelerator would
> destroy the universe (similiar concerns were expressed for RHIC, but
> these turned out to be groundless: however, RHIC is after all a much
> less than Planckian accelerator).
I think that your conclusions are "derived" from a confusion of two
different meanings of the phrase "potential barrier comparable to the
Planck energy". If we talk about such a barrier in the context of
spacetime physics, we mean something like a potential term in the energy
density that is of order m_{Planck}^4 (well, the width of the barrier also
matters). It is simply not true that collecting a trans-Planckian energy
is enough to tunnel through such a barrier. For example, the total energy
of your body is roughly 10 orders of magnitude larger than the Planck
energy, and obviously it is not enough for the Universe to be destroyed.
It seems to me that you are not thinking about spacetime physics and
quantum field theory, but rather about quantum mechanics for one particle
propagating on the line. In that case, you could use the phrase "Planckian
barrier" in the same sloppy way, and your conclusion might be correct. But
in spacetime's 4-dimensional (or higher-dimensional) physics, you must be
able to concentrate the energy into a very small volume - your energy
*density* must be large enough to convince the spacetime to jump. It's not
easy because if you try to concentrate too much energy, you create a black
hole whose radius grows with the energy, and the horizon protects the
exterior (the rest of the Universe) from whatever disaster you might be
imagining to happen inside. Very large black holes lead to small energy
density again, and they are not helpful to initiate the tunneling. The
Planckian mass black holes are, in a sense, the densest objects in the
world (the Planckian density; let me now assume the conventional superhigh
energy Planck scale). Assuming that there has existed at least one black
hole light enough so that it has already evaporated, it is safe to say
that having Planckian density is not enough for the Universe to decay.
Usually we don't consider the effect of "catalysts", external objects that
might speed up the vacuum decay (perhaps because of their high energy). We
usually consider the standard vacuum decay instanton, as Coleman has
taught us, which describes a process that spotaneously occurs in vacuum
and does not require any external objects for it to be started. I think
that the reason why we don't consider the catalysts is that they don't
really matter. If the vacuum is stable enough, the density of the decay
rate (probability per 4D worldvolume) is very small anyway, and it is
probably believed that it is very unlikely that a single process with some
catalysts could change the probability of decay. Well, maybe, there is a
way to make a large probability of decay from a single collission of a
particular type, but I think we would agree that the fact that the
Universe has survived these 13.7 billion years of chaos is strong evidence
that it should not be *that* easy to destroy the Universe.
> More precisely, if it doesn't destroy the universe, this might be an
> indication string theory is _wrong_.
In other words, the only way to prove that string theory is correct is to
destroy the Universe? ;-) A proof of string theory would be cool, but this
would seem as a too large price to pay. :-)
Your argument is amusing and the Universe is stable enough. But these two
facts don't imply that string theory is wrong; they imply that your
calculation of the probability of the vacuum decay is wrong.
> In fact, a similar claim was made regarding the RHIC. However,
> this reminds of an amusing article claiming the the multi-world
> interpretation of quantum mechanics may be put to experimental test in
> a simple fashion: if it is true, you would experience living forever.
> This is supposedly so because there's always a (highly improbable,
> possibly) world where you're still alive after any given amount of
> time, and this is the only kind of world you would be able to observe.
I don't think that this is a description of the many-world interpretation
that Everett and his followers would subscribe. It is not quite clear how
to make their picture work equally well as the decohering histories, for
example, but it is more clear that they want to make it as
indistinguishable from usual quantum mechanics as possible. The point is
that you can't choose in which parallel Universe you want to live. You are
"randomly" assigned one of the parallel worlds. If it happens to be the
world in which you see an airplane that is going to hit the skyscraper
where you are working at the moment on the 78th floor - which is something
that you *are* able to observe (you may replace the airplane by any other
observation that you do exactly before your death) - then the elementary
laws of physics (and common sense) imply that it is the Universe where
you're gonna die. If you wanted to survive, you should have chosen a
luckier parallel Universe *before* you saw the airplane. I know, you did
not know about it - now it's too late. ;-)
No metalaws of physics or metaphysics can prevent you from dying as long
as elementary laws of physics and common sense are respected.
You could speculate that the people who described how they returned from
the heaven/hell have really returned from a different parallel Universe in
the many-worlds interpretation because they decided that it is a bad idea
to die, but I am not sure whether this would be the best newsgroup for
such ideas.
> It appears to me the same argument goes for the Anthropic principle...
> (at least in the form in which it would be able to save us from vacuum
> decay)
I don't agree with the argument in either case. If the configuration of
your physical system is such that it is "programmed" for you to die in .1
second - which is something that can certainly happen - there is no way to
survive by jumping into a different parallel Universe, regardless whether
the parallel Universe results from the many-worlds interpretation of QM or
from the anthropic reasoning.
__{_______________________________________________ _____________________________}
E-mail: lumo@matfyz.cz fax: +1-617/496-0110 Web: http://lumo.matfyz.cz/
eFax: +1-801/454-1858 work: +1-617/496-8199 home: +1-617/868-4487 (call)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[Posted for the 2nd time due to continuing problems with the FAS
newsserver. LM]
Our esteemed moderator seems to be getting more conservative as the years go by =)
Actually, im a little confused. I thought that with a sufficiently powerful planckian accelerator (in principle), and enough events that we would certainly be able to distinguish between the toy models, or at least throw out the majority of them.
So lets say that we know everything in the world there is to know about the observable S Matrix, wouldn't that be enough to fix the compactification schemes?
Good evening. I've come to read about string theory and it seems to be quite interesting, hard stuff. To unify the four known forces is a big deal, not even Einstein could handle it ;-) Anyway, I would like to do some studies about this theory. Can anybody think of a way to show anything related to strings in a experiment or similar? I mean, I would like to do some research for school as a project, but pure theory is probably not enough, I need something experimental to show/prove/demonstrate/ask, whatever. What do you think, can this be realized? Do you have any ideas?
Thanks and regards, A. Blessing
(...) Anyway, I would like to do some studies about this theory. Can anybody think
of a way to show anything related to strings in a experiment or similar? I
mean, I would like to do some research for school as a project, but pure
theory is probably not enough, I need something experimental to
show/prove/demonstrate/ask, whatever.
What do you think, can this be realized? Do you have any ideas?
Thanks and regards,
A. Blessing
If your school doesn't have access to laser interferometers and infrared spectrometers... you could go back to Pythagoras and consider strings, actual bits of string (wire, etc.), vibrating at constant tension. With a few wires and bits of metal to stretch them between you could even make a model of strings between D-branes! Superstrings are more or less the same, except much smaller and with much greater tension.
However, I would echo Urs' warning that it's rather pointless to do this in school when you don't really have a hope of understanding the theory. If you really want to try, good luck!
Urs Schreiber
Jun10-04, 05:32 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>"tdent" <tdent@physicsforums.com> schrieb im Newsbeitrag\nnews:tdent.17k9lv-100000@physicsforums.com...\n\n> However, I would echo Urs\' warning that it\'s rather pointless to do\n> this in school when you don\'t really have a hope of understanding the\n> theory. If you really want to try, good luck!\n\nDepending on how this project is really supposed to look like I could\nimagine that it would be possible to _talk_ about some interesting\nexperiments that are being made, maybe showing some animations/web-sites. I\nhad compiled a list with links to such resources:\n\nhttp://golem.ph.utexas.edu/string/archives/000378.html\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>"tdent" <tdent@physicsforums.com> schrieb im Newsbeitrag
news:tdent.17k9lv-100000@physicsforums.com...
> However, I would echo Urs' warning that it's rather pointless to do
> this in school when you don't really have a hope of understanding the
> theory. If you really want to try, good luck!
Depending on how this project is really supposed to look like I could
imagine that it would be possible to _talk_ about some interesting
experiments that are being made, maybe showing some animations/web-sites. I
had compiled a list with links to such resources:
http://golem.ph.utexas.edu/string/archives/000378.html
<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>Lubos Motl <motl@feynman.harvard.edu> wrote in message\nnews:<20040607043522.E2574@mail.kolej.mff .cuni.cz>...\n\n> I think that your conclusions are "derived" from a confusion of two\n> different meanings of the phrase "potential barrier comparable to the\n> Planck energy". If we talk about such a barrier in the context of\n> spacetime physics, we mean something like a potential term in the energy\n> density that is of order m_{Planck}^4 (well, the width of the barrier also\n> matters). It is simply not true that collecting a trans-Planckian energy\n> is enough to tunnel through such a barrier. For example, the total energy\n> of your body is roughly 10 orders of magnitude larger than the Planck\n> energy, and obviously it is not enough for the Universe to be destroyed.\n\nHowever, this fact about my body is (fortunatelly, I guess)\nalso not enough to make it a testbed for quantum gravity!\nThe meaning of achieving Planckian energy in an accelerator\nis precisely packing up Planckian energy into very small\nspace (that of a few particles).\n\n> But in spacetime\'s 4-dimensional (or higher-dimensional) physics, you must be\n> able to concentrate the energy into a very small volume - your energy\n> *density* must be large enough to convince the spacetime to jump. It\'s not\n> easy because if you try to concentrate too much energy, you create a black\n> hole whose radius grows with the energy, and the horizon protects the\n> exterior (the rest of the Universe) from whatever disaster you might be\n> imagining to happen inside.\n\nYes, but if you believe vacuum decay can happen at all, there must\nbe cases in which this mechanism would not save us. If a quantum\nfluctuation can get over the barrier into the true vacuum without\ncreating a black-hole, why wouldn\'t an accelerator experiment\n(this time a comparatively deterministic event, _not_ a fluctation)\ndo the same?\n\n> Very large black holes lead to small energy\n> density again, and they are not helpful to initiate the tunneling.\n\nYes. Possibly a particle collision which is way over Planckian\nenergy would not destroy the universe since it would simply create\na (comparatively) macroscopic black-hole. However, such an\nexperiment would also be unable test non-trivial predictions of\nstring theory (probably). It appears to me, though, near-Planckian\nscale processes should be able to do both (test non-trivial\npredictions and destroy the universe; the second being a special\ncase of the first!)\n\n> The\n> Planckian mass black holes are, in a sense, the densest objects in the\n> world (the Planckian density; let me now assume the conventional superhigh\n> energy Planck scale). Assuming that there has existed at least one black\n> hole light enough so that it has already evaporated, it is safe to say\n> that having Planckian density is not enough for the Universe to decay.\n\nInteresting point. However:\n1) What are you basing your assumption on?\n2) A _one_ black hole might be insufficient: we might have been\nlucky. That is, it is possible that accidently no Planckian energy\n"modulons" of the appropriate type have been formed in the final\n"flash" of the black hole.\n3) It is possible that there is a protection mechanism against\nthis particular process, some symmetry cancellation say.\n\n> I think\n> that the reason why we don\'t consider the catalysts is that they don\'t\n> really matter. If the vacuum is stable enough, the density of the decay\n> rate (probability per 4D worldvolume) is very small anyway, and it is\n> probably believed that it is very unlikely that a single process with some\n> catalysts could change the probability of decay.\n\nWhy? Rememer this single process leads to extremely high energy densities.\nAnd it\'s sufficient for a small "vacuum bubble" to form: nothing can stop\nit then.\n\n> Well, maybe, there is a\n> way to make a large probability of decay from a single collission of a\n> particular type, but I think we would agree that the fact that the\n> Universe has survived these 13.7 billion years of chaos is strong evidence\n> that it should not be *that* easy to destroy the Universe.\n\nMaybe. Or maybe we\'ve just been lucky ;-)\n\n> In other words, the only way to prove that string theory is correct is to\n> destroy the Universe? ;-) A proof of string theory would be cool, but this\n> would seem as a too large price to pay. :-)\n>\n> Your argument is amusing and the Universe is stable enough. But these two\n> facts don\'t imply that string theory is wrong; they imply that your\n> calculation of the probability of the vacuum decay is wrong.\n\nFirstly, I am not trying to prove string theory wrong.\nSecondly, I think my argument is not yet falsified.\n\n> I don\'t think that this is a description of the many-world\n> interpretation that Everett and his followers would subscribe.\n\nDon\'t take me wrong, I don\'t think this idea to be\nanything more than an amusing curiosity.\n\n> > It appears to me the same argument goes for the Anthropic principle...\n> > (at least in the form in which it would be able to save us from vacuum\n> > decay)\n>\n> I don\'t agree with the argument in either case. If the configuration of\n> your physical system is such that it is "programmed" for you to die in 0.1\n> second - which is something that can certainly happen - there is no way to\n> survive by jumping into a different parallel Universe, regardless whether\n> the parallel Universe results from the many-worlds interpretation of QM\n> or from the anthropic reasoning.\n\nYou are right that the Anthropic principle can only serve to explain\nmiraculous survival posteriori rather than predicting such a priori\n(as opposed to the claim in the abovementioned article). There is,\nhowever, no difference from an observational standpoint, given that\nyou cannot observe you own death.\n\nBest regards,\nSquark.\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>Lubos Motl <motl@feynman.harvard.edu> wrote in message
news:<20040607043522.E2574@mail.kolej.mff.cuni.cz>...
> I think that your conclusions are "derived" from a confusion of two
> different meanings of the phrase "potential barrier comparable to the
> Planck energy". If we talk about such a barrier in the context of
> spacetime physics, we mean something like a potential term in the energy
> density that is of order m_{Planck}^4 (well, the width of the barrier also
> matters). It is simply not true that collecting a trans-Planckian energy
> is enough to tunnel through such a barrier. For example, the total energy
> of your body is roughly 10 orders of magnitude larger than the Planck
> energy, and obviously it is not enough for the Universe to be destroyed.
However, this fact about my body is (fortunatelly, I guess)
also not enough to make it a testbed for quantum gravity!
The meaning of achieving Planckian energy in an accelerator
is precisely packing up Planckian energy into very small
space (that of a few particles).
> But in spacetime's 4-dimensional (or higher-dimensional) physics, you must be
> able to concentrate the energy into a very small volume - your energy
> *density* must be large enough to convince the spacetime to jump. It's not
> easy because if you try to concentrate too much energy, you create a black
> hole whose radius grows with the energy, and the horizon protects the
> exterior (the rest of the Universe) from whatever disaster you might be
> imagining to happen inside.
Yes, but if you believe vacuum decay can happen at all, there must
be cases in which this mechanism would not save us. If a quantum
fluctuation can get over the barrier into the true vacuum without
creating a black-hole, why wouldn't an accelerator experiment
(this time a comparatively deterministic event, _not_ a fluctation)
do the same?
> Very large black holes lead to small energy
> density again, and they are not helpful to initiate the tunneling.
Yes. Possibly a particle collision which is way over Planckian
energy would not destroy the universe since it would simply create
a (comparatively) macroscopic black-hole. However, such an
experiment would also be unable test non-trivial predictions of
string theory (probably). It appears to me, though, near-Planckian
scale processes should be able to do both (test non-trivial
predictions and destroy the universe; the second being a special
case of the first!)
> The
> Planckian mass black holes are, in a sense, the densest objects in the
> world (the Planckian density; let me now assume the conventional superhigh
> energy Planck scale). Assuming that there has existed at least one black
> hole light enough so that it has already evaporated, it is safe to say
> that having Planckian density is not enough for the Universe to decay.
Interesting point. However:
1) What are you basing your assumption on?
2) A _one_ black hole might be insufficient: we might have been
lucky. That is, it is possible that accidently no Planckian energy
"modulons" of the appropriate type have been formed in the final
"flash" of the black hole.
3) It is possible that there is a protection mechanism against
this particular process, some symmetry cancellation say.
> I think
> that the reason why we don't consider the catalysts is that they don't
> really matter. If the vacuum is stable enough, the density of the decay
> rate (probability per 4D worldvolume) is very small anyway, and it is
> probably believed that it is very unlikely that a single process with some
> catalysts could change the probability of decay.
Why? Rememer this single process leads to extremely high energy densities.
And it's sufficient for a small "vacuum bubble" to form: nothing can stop
it then.
> Well, maybe, there is a
> way to make a large probability of decay from a single collission of a
> particular type, but I think we would agree that the fact that the
> Universe has survived these 13.7 billion years of chaos is strong evidence
> that it should not be *that* easy to destroy the Universe.
Maybe. Or maybe we've just been lucky ;-)
> In other words, the only way to prove that string theory is correct is to
> destroy the Universe? ;-) A proof of string theory would be cool, but this
> would seem as a too large price to pay. :-)
>
> Your argument is amusing and the Universe is stable enough. But these two
> facts don't imply that string theory is wrong; they imply that your
> calculation of the probability of the vacuum decay is wrong.
Firstly, I am not trying to prove string theory wrong.
Secondly, I think my argument is not yet falsified.
> I don't think that this is a description of the many-world
> interpretation that Everett and his followers would subscribe.
Don't take me wrong, I don't think this idea to be
anything more than an amusing curiosity.
> > It appears to me the same argument goes for the Anthropic principle...
> > (at least in the form in which it would be able to save us from vacuum
> > decay)
>
> I don't agree with the argument in either case. If the configuration of
> your physical system is such that it is "programmed" for you to die in .1
> second - which is something that can certainly happen - there is no way to
> survive by jumping into a different parallel Universe, regardless whether
> the parallel Universe results from the many-worlds interpretation of QM
> or from the anthropic reasoning.
You are right that the Anthropic principle can only serve to explain
miraculous survival posteriori rather than predicting such a priori
(as opposed to the claim in the abovementioned article). There is,
however, no difference from an observational standpoint, given that
you cannot observe you own death.
Best regards,
Squark.
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