Nima Arkani-Hamed
Apr30-04, 03:40 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>Here are the answers:\n\n> # Prof. Arkani-Hamed: imagine that you are a bookmaker, and you task is to\n> estimate the probability of the following events (of course, if you\n> write a more detailed answer, it will be appreciated):\n>\n> 1. The LHC will find convincing evidence of supersymmetry by 2010.\n> 2. A collider or another experiment will see evidence of large extra\n> dimensions or warped extra dimensions by 2015.\n> 3. An investigation of CMB nongaussianities will support the idea of the\n> ghost condensate by 2015.\n\nI think some sort of SUSY is still the most plausible possibility for\nphysics at the TeV scale, and this essentially only because of\ngauge coupling unification. It is possible that this success is preserved\nin the other scenarios, but it doesn\'t fall in your lap the way it does in\nSUSY. And at least the ``minimal" SUSY framework in getting more and more\nconstrained by the non-discovery of the Higgs and superparters. My odds\n(which are time-dependent!)\n\n40% :Standard SUSY\n30% :Some very non-standard version of SUSY\n15% :Low-scale quantum gravity\n15% : Something we haven\'t thought of at all\n\nAs for the ghost condensate triggering inflation: at this point the idea\nis too fresh for me to be able to give you an answer!\n\n> 4. String theorists will only be a small group in the math departments\n> in 2015.\n> 5. The cultural barrier between the phenomenologists and the string\n> theorists will significantly diminish by 2010.\n\nString theory is so deeply physical, that I can\'t see (4) ever happening:\nstring theory belongs in physics departments! But it may well be that\nthere will be fewer string theorists. The community is very large now, but\nthe large N hasn\'t particularly helped in moving the physics forward.\n\nThe cultural barrier between string theorists and phenomenologist has\nalready decreased a lot over the last 5 years or so; it would diminish to\n0 if quantum gravity is discovered at the LHC, and even if SUSY is\ndiscovered.\n\n> 6. It will be possible for string theory to give a successful and\n> completely new prediction about physics, even without knowing anything\n> about the "right vacuum" within the landscape.\n\nYou didn\'t put a time limit on this one, and given that the definition of\nstring theory keeps changing, I can safely say put the odds at 100%\n\n> 7. A nontrivial relation between the parameters of the Standard Model\n> will be calculated theoretically by 2030.\n\nBy 2030, we will very likely have a new Standard Model with an additional\nset of parameters. There will very likely be theoretically predicted\nbetween the values of these new couplings. I\'d put the odds at 80%. For\nthe SM of today parameters...I\'m less sure. The SM isn\'t all that special\nlooking, but there are already many plausible relationships between\ncouplings coming from GUT\'s and SUSY GUT\'s, not just the relatin between\nthe gauge couplings, but also the tau-bottom mass ratio, the relation\nbetween the Cabbibo mixing angle and the strange/down masses and some\nothers.By 2030, we may find conclusively out that some of these are\nactually right with 20%. And perhaps there is some totally new relation;\nsay 10%.\n\n> [Prof. Arkani-Hamed,]\n>\n> I think a number of your papers are quite interesting, but I would\n> like to probe into the proposed Cosmic?.. Bose-Einstein-Condensate?\n>\n> 1)Is there any chance of this being a Quark Condensate?..sort of like\n> a Promordial Fluid that all Matter evolves from?..and still be\n> ghost-condensate to our detectors.\n\nWe haven\'t thought yet about a microscopic theory for the condensate, only\ndescribed its low-energy excitations (which can largely be constrained by\nsymmetries). But it is conceivable that it arises from a quark condensate\nin strongly coupled chiral gauge theories.\n\n> 2)In the accelerated Expansion currently being detected, the expansion\n> would tear the fabric of space apart (under certain conditions), how\n> would a string worldline be continueous along the fabric of space, say\n> from our Galaxy to an area of immense expansion, and remain a bonafide\n> unbroken worldline?\n\nThis only happens in theories which are superaccelerating (where the\nparameter w = p/rho is less than -1). In this case the universe does\nindeed hit a ``big rip" in the future. I am extremely dubious of the\nconsistency of these theories; all known examples in the literature suffer\nfrom violent local instabilities, and are not physically consistent as far\nas I see. So your issue wouldn\'t come up; I wouldn\'t worry about it till\nsomeone shows that the big rip can happen in a physically consistent\ntheory.\n\n> Prof. Nima Arkani-Hamed,\n>\n> When one endeavors through life in the areas of interest I have found\n> you engaged in, there are certain fundamentals laws aside from the\n> known proofs, that become the basis of ones perceptions.\n>\n> In all fairness, I would show you this link:\n>\n> http://www.edge.org/q2004/index.html#randall\n>\n> ....to help you see what I am saying and then ask, what Law might\n> become Prof. Nima Arkani-Hamed\'s Law? sol\n\nI didn\'t quite get your question. I find this sort of thing (the edge\nlist) silly. Though I like Lisa\'s about there being no theorems in\nphysics. Thats a theorem (ah, Russell is rolling in his grave).\n\n> I\'d like to understand roughly what the concepts of "Ghost Inflation" are.\n>\n> What is the nature of the ghost field appearing here. Is ghost\n> inflation dealing with BRST ghosts (I guess not) or with \'phantom matter\'?\n\nThe ``ghost" is perhpas misleading; no, it has nothing to do with BRST\nghosts, and not really anything to do with phantom matter either. The\nbackground is one where a scalar field has non-zero (constant)\ntime-derivative. Around the origin of field space, the field would have a\nwrong-sign kinetic term (so would be a ``phantom"), but the point is we\nare far from the origin and the physics never explored the region close to\nthe origin either. Really our theory should be described as ``Gravity in\nthe Higgs phase".\n\n> Where would such a ghost field come from, physically? What is the\n> relation to string theory if any?\n\nI mentioned above one possibility for a UV completion of this physics:\nperhaps a lorentz-violating condensate in a chiral gauge theory.\nAs for string theory, it is clearly tied up with finding sensible\ntime-dependent solutions.\n\n> First, I would like to thank Nima Arkani-Hamed for sharing his opinions\n> and spending his valuable time explaining string theory to us.\n> Here are my questions. Feel free to edit them to make more sense or be\n> more generally applicable:\n>\n> String Theory and Axions At the American Physical Society meeting in\n> Philadelphia not too long ago, Ed Witten said "Supersymmetry is\n> compatible with the axion, but String Theory requires it"\n> Do you agree?\n\nYes, string theory requires ``axions". But no, it does not require nor\npredict OUR axion, needed to solve the strong CP problem in the Standard\nModel. The string theory axions could be made massive by physics in\nsectors other than the SM. Neither SUSY nor string theory will suffer\nif we never find OUR axion, and there other nice solutions to the strong\nCP problem.\n\n> Black holes and Horizons. Classical GR black holes have event horizons\n> and singularities. It seems to be generally accepted that string\n> theory black holes do not have singularities.\n> What about Horizons? Mathur at least seems to say that individual\n> string configurations do not have true event horizons. What is your\n> take on horizons and string theory based black holes?\n\nThese are all very interesting questions with no definite consensus on\nwhats going on. I find Mathurs papers interesting, but (among other\nthings) I don\'t understand how his conclusions can be extrapolated to the\ncase of Schwarzchild BH\'s.\n\n> Black holes and metrics. It seems to be accepted that string theory\n> leads either to a scalar tensor theory of gravitation or higher\n> derivative terms or perhaps both. Which (or what else) is your\n> preference. Are these extra terms likely to be big enough to be\n> measured by Ligo or Lisa.?\n\nThe higher derivative terms in the action are certainly there but totally\nirrelevant for low-energy/long distance physics, so they won\'t do anything\nto Ligo or Lisa. In supersymmetric compactifications, there are massless\nscalars corresponding to moving smoothly around in the continuosly\nconnected space of vacuaa. However SUSY is broken in our world, and the\nmoduli pick up masses, giving them a compton wavelength that is almost\nalways smaller than a millimeter or so. Thus, while some of them might be\nobserved in sub-millimeter tests of gravity, they wouldn\'t affect anything\nat distances larger than a mm or so. It is possible that some scalars stay\nmuch lighter, though you have to work to hard to ensure that they haven\'y\nbeen seen yet!\n\n> Black hole Information Loss: Well known issue, your comments?\n\nOnly that I think too many people are saying that this has been\n``understood", becasue we have manifestly unitary descriptions of quantum\ngravity in some cases (like AdS/CFT). We still don\'t exactly know how the\ninformation comes back out.\n\n> Finally, an issue that has already been beaten almost to death on\n> s.p.r: the Landscape and all that:\n> how predictive is String Theory? Or is it like Cartesian Geometry: A\n> very useful tool which predicts nothing?\n\nSince its been beaten to death I won\'t beat it further. I don\'t have a\nknee-jerk reaction against the idea that some things in physics can be\n``environmental", and I have always been very impressed with Weinber\'s\nargument about for the CC. Recently I\'ve been thinking with Savas\nDimopoulos about an extremely predictive picture but non-standard without\nlow-energy SUSY but with SUSY coupling unification,\nthat was inspired by the landscape. So it may be indeed be a useful tool\nto liberate us from our usual perspectives on naturalness and fine-tuning.\nBut I don\'t think we know nearly enough about the landscape yet, and in\nparticular the crucial question of cosmology, to get into long polemic\ndiatribes about it one way or another. If it turns out that predicting the\nvalue of the CC or the electron mass isn\'t a fundamental question but is\ninstead environmental, fine; we will only be convinced these are the\nwrong questions when we figure out what the right ones are.\n\n> Once again, TIA to N.A-H for sharing his wisdom with us. Jim Graber\n\n....\n\n> Even though I\'m eagerly trying to learn string theory, I should begin by\n> warning you that I\'m still a total novice.\n>\n> I\'ve noticed that there has been questions regarding *specific*\n> predictions in the string theory field. There has also been suggested\n> the very generic (but good) question of how predictive string theory is.\n>\n> I\'d like to know any kind of answers to a kind of in-between question:\n>\n> Which of currently planned experiments could give useful constraints for\n> string theory, guiding it in some direction, or perhaps pruning it?\n>\n> The trivial answer is of course that many, many, experiments have the\n> potential to have later implications for the theory, depending on\n> theoretical developments, but I\'d like to know if there are some\n> particularly interesting experiments on the horizon, and interesting\n> ways to use the theory to make constraints out of the results.\n>\n> Thanks, Mikael Djurfeldt\n\nThere are no definite experiments that can *exclude* string theory in the\nnear future. To my mnd, high energy collider experiments (the LHC in the\nnear future) by far have the best chance of revealing some evidence for\nstring theory, though the amount of evidence would vary depending on what\nis observed. In the most optimistic scenario, if the scale of quantum\ngravity is in the TeV range, we will make stringy excitations of SM fields\nat the LHC: a partner for every SM field with one greater spin, a spin 3/2\nelectron, spin 2 photon and gluon, and so on. It would be very hard to\nbelieve that it isn\'t string theory with this evidence! If we see some\nstandard sort of SUSY theory, this would at least show that some of the\ningredients needed in string theory exist in nature. In any case, at least\nuntil some major new experimental idea comes along, collider experiments\nare the way to go.\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>Here are the answers:
> # Prof. Arkani-Hamed: imagine that you are a bookmaker, and you task is to
> estimate the probability of the following events (of course, if you
> write a more detailed answer, it will be appreciated):
>
> 1. The LHC will find convincing evidence of supersymmetry by 2010.
> 2. A collider or another experiment will see evidence of large extra
> dimensions or warped extra dimensions by 2015.
> 3. An investigation of CMB nongaussianities will support the idea of the
> ghost condensate by 2015.
I think some sort of SUSY is still the most plausible possibility for
physics at the TeV scale, and this essentially only because of
gauge coupling unification. It is possible that this success is preserved
in the other scenarios, but it doesn't fall in your lap the way it does in
SUSY. And at least the ``minimal" SUSY framework in getting more and more
constrained by the non-discovery of the Higgs and superparters. My odds
(which are time-dependent!)
40% :Standard SUSY
30% :Some very non-standard version of SUSY
15% :Low-scale quantum gravity
15% : Something we haven't thought of at all
As for the ghost condensate triggering inflation: at this point the idea
is too fresh for me to be able to give you an answer!
> 4. String theorists will only be a small group in the math departments
> in 2015.
> 5. The cultural barrier between the phenomenologists and the string
> theorists will significantly diminish by 2010.
String theory is so deeply physical, that I can't see (4) ever happening:
string theory belongs in physics departments! But it may well be that
there will be fewer string theorists. The community is very large now, but
the large N hasn't particularly helped in moving the physics forward.
The cultural barrier between string theorists and phenomenologist has
already decreased a lot over the last 5 years or so; it would diminish to
if quantum gravity is discovered at the LHC, and even if SUSY is
discovered.
> 6. It will be possible for string theory to give a successful and
> completely new prediction about physics, even without knowing anything
> about the "right vacuum" within the landscape.
You didn't put a time limit on this one, and given that the definition of
string theory keeps changing, I can safely say put the odds at 100%
> 7. A nontrivial relation between the parameters of the Standard Model
> will be calculated theoretically by 2030.
By 2030, we will very likely have a new Standard Model with an additional
set of parameters. There will very likely be theoretically predicted
between the values of these new couplings. I'd put the odds at 80%. For
the SM of today parameters...I'm less sure. The SM isn't all that special
looking, but there are already many plausible relationships between
couplings coming from GUT's and SUSY GUT's, not just the relatin between
the gauge couplings, but also the \tau-bottom mass ratio, the relation
between the Cabbibo mixing angle and the strange/down masses and some
others.By 2030, we may find conclusively out that some of these are
actually right with 20%. And perhaps there is some totally new relation;
say 10%.
> [Prof. Arkani-Hamed,]
>
> I think a number of your papers are quite interesting, but I would
> like to probe into the proposed Cosmic?.. Bose-Einstein-Condensate?
>
> 1)Is there any chance of this being a Quark Condensate?..sort of like
> a Promordial Fluid that all Matter evolves from?..and still be
> ghost-condensate to our detectors.
We haven't thought yet about a microscopic theory for the condensate, only
described its low-energy excitations (which can largely be constrained by
symmetries). But it is conceivable that it arises from a quark condensate
in strongly coupled chiral gauge theories.
> 2)In the accelerated Expansion currently being detected, the expansion
> would tear the fabric of space apart (under certain conditions), how
> would a string worldline be continueous along the fabric of space, say
> from our Galaxy to an area of immense expansion, and remain a bonafide
> unbroken worldline?
This only happens in theories which are superaccelerating (where the
parameter w = p/\rho is less than -1). In this case the universe does
indeed hit a ``big rip" in the future. I am extremely dubious of the
consistency of these theories; all known examples in the literature suffer
from violent local instabilities, and are not physically consistent as far
as I see. So your issue wouldn't come up; I wouldn't worry about it till
someone shows that the big rip can happen in a physically consistent
theory.
> Prof. Nima Arkani-Hamed,
>
> When one endeavors through life in the areas of interest I have found
> you engaged in, there are certain fundamentals laws aside from the
> known proofs, that become the basis of ones perceptions.
>
> In all fairness, I would show you this link:
>
> http://www.edge.org/q2004/index.html#randall
>
> ....to help you see what I am saying and then ask, what Law might
> become Prof. Nima Arkani-Hamed's Law? sol
I didn't quite get your question. I find this sort of thing (the edge
list) silly. Though I like Lisa's about there being no theorems in
physics. Thats a theorem (ah, Russell is rolling in his grave).
> I'd like to understand roughly what the concepts of "Ghost Inflation" are.
>
> What is the nature of the ghost field appearing here. Is ghost
> inflation dealing with BRST ghosts (I guess not) or with 'phantom matter'?
The ``ghost" is perhpas misleading; no, it has nothing to do with BRST
ghosts, and not really anything to do with phantom matter either. The
background is one where a scalar field has non-zero (constant)
time-derivative. Around the origin of field space, the field would have a
wrong-sign kinetic term (so would be a ``phantom"), but the point is we
are far from the origin and the physics never explored the region close to
the origin either. Really our theory should be described as ``Gravity in
the Higgs phase".
> Where would such a ghost field come from, physically? What is the
> relation to string theory if any?
I mentioned above one possibility for a UV completion of this physics:
perhaps a lorentz-violating condensate in a chiral gauge theory.
As for string theory, it is clearly tied up with finding sensible
time-dependent solutions.
> First, I would like to thank Nima Arkani-Hamed for sharing his opinions
> and spending his valuable time explaining string theory to us.
> Here are my questions. Feel free to edit them to make more sense or be
> more generally applicable:
>
> String Theory and Axions At the American Physical Society meeting in
> Philadelphia not too long ago, Ed Witten said "Supersymmetry is
> compatible with the axion, but String Theory requires it"
> Do you agree?
Yes, string theory requires ``axions". But no, it does not require nor
predict OUR axion, needed to solve the strong CP problem in the Standard
Model. The string theory axions could be made massive by physics in
sectors other than the SM. Neither SUSY nor string theory will suffer
if we never find OUR axion, and there other nice solutions to the strong
CP problem.
> Black holes and Horizons. Classical GR black holes have event horizons
> and singularities. It seems to be generally accepted that string
> theory black holes do not have singularities.
> What about Horizons? Mathur at least seems to say that individual
> string configurations do not have true event horizons. What is your
> take on horizons and string theory based black holes?
These are all very interesting questions with no definite consensus on
whats going on. I find Mathurs papers interesting, but (among other
things) I don't understand how his conclusions can be extrapolated to the
case of Schwarzchild BH's.
> Black holes and metrics. It seems to be accepted that string theory
> leads either to a scalar tensor theory of gravitation or higher
> derivative terms or perhaps both. Which (or what else) is your
> preference. Are these extra terms likely to be big enough to be
> measured by Ligo or Lisa.?
The higher derivative terms in the action are certainly there but totally
irrelevant for low-energy/long distance physics, so they won't do anything
to Ligo or Lisa. In supersymmetric compactifications, there are massless
scalars corresponding to moving smoothly around in the continuosly
connected space of vacuaa. However SUSY is broken in our world, and the
moduli pick up masses, giving them a compton wavelength that is almost
always smaller than a millimeter or so. Thus, while some of them might be
observed in sub-millimeter tests of gravity, they wouldn't affect anything
at distances larger than a mm or so. It is possible that some scalars stay
much lighter, though you have to work to hard to ensure that they haven'y
been seen yet!
> Black hole Information Loss: Well known issue, your comments?
Only that I think too many people are saying that this has been
``understood", becasue we have manifestly unitary descriptions of quantum
gravity in some cases (like AdS/CFT). We still don't exactly know how the
information comes back out.
> Finally, an issue that has already been beaten almost to death on
> s.p.r: the Landscape and all that:
> how predictive is String Theory? Or is it like Cartesian Geometry: A
> very useful tool which predicts nothing?
Since its been beaten to death I won't beat it further. I don't have a
knee-jerk reaction against the idea that some things in physics can be
``environmental", and I have always been very impressed with Weinber's
argument about for the CC. Recently I've been thinking with Savas
Dimopoulos about an extremely predictive picture but non-standard without
low-energy SUSY but with SUSY coupling unification,
that was inspired by the landscape. So it may be indeed be a useful tool
to liberate us from our usual perspectives on naturalness and fine-tuning.
But I don't think we know nearly enough about the landscape yet, and in
particular the crucial question of cosmology, to get into long polemic
diatribes about it one way or another. If it turns out that predicting the
value of the CC or the electron mass isn't a fundamental question but is
instead environmental, fine; we will only be convinced these are the
wrong questions when we figure out what the right ones are.
> Once again, TIA to N.A-H for sharing his wisdom with us. Jim Graber
....
> Even though I'm eagerly trying to learn string theory, I should begin by
> warning you that I'm still a total novice.
>
> I've noticed that there has been questions regarding *specific*
> predictions in the string theory field. There has also been suggested
> the very generic (but good) question of how predictive string theory is.
>
> I'd like to know any kind of answers to a kind of in-between question:
>
> Which of currently planned experiments could give useful constraints for
> string theory, guiding it in some direction, or perhaps pruning it?
>
> The trivial answer is of course that many, many, experiments have the
> potential to have later implications for the theory, depending on
> theoretical developments, but I'd like to know if there are some
> particularly interesting experiments on the horizon, and interesting
> ways to use the theory to make constraints out of the results.
>
> Thanks, Mikael Djurfeldt
There are no definite experiments that can *exclude* string theory in the
near future. To my mnd, high energy collider experiments (the LHC in the
near future) by far have the best chance of revealing some evidence for
string theory, though the amount of evidence would vary depending on what
is observed. In the most optimistic scenario, if the scale of quantum
gravity is in the TeV range, we will make stringy excitations of SM fields
at the LHC: a partner for every SM field with one greater spin, a spin 3/2
electron, spin 2 photon and gluon, and so on. It would be very hard to
believe that it isn't string theory with this evidence! If we see some
standard sort of SUSY theory, this would at least show that some of the
ingredients needed in string theory exist in nature. In any case, at least
until some major new experimental idea comes along, collider experiments
are the way to go.
> # Prof. Arkani-Hamed: imagine that you are a bookmaker, and you task is to
> estimate the probability of the following events (of course, if you
> write a more detailed answer, it will be appreciated):
>
> 1. The LHC will find convincing evidence of supersymmetry by 2010.
> 2. A collider or another experiment will see evidence of large extra
> dimensions or warped extra dimensions by 2015.
> 3. An investigation of CMB nongaussianities will support the idea of the
> ghost condensate by 2015.
I think some sort of SUSY is still the most plausible possibility for
physics at the TeV scale, and this essentially only because of
gauge coupling unification. It is possible that this success is preserved
in the other scenarios, but it doesn't fall in your lap the way it does in
SUSY. And at least the ``minimal" SUSY framework in getting more and more
constrained by the non-discovery of the Higgs and superparters. My odds
(which are time-dependent!)
40% :Standard SUSY
30% :Some very non-standard version of SUSY
15% :Low-scale quantum gravity
15% : Something we haven't thought of at all
As for the ghost condensate triggering inflation: at this point the idea
is too fresh for me to be able to give you an answer!
> 4. String theorists will only be a small group in the math departments
> in 2015.
> 5. The cultural barrier between the phenomenologists and the string
> theorists will significantly diminish by 2010.
String theory is so deeply physical, that I can't see (4) ever happening:
string theory belongs in physics departments! But it may well be that
there will be fewer string theorists. The community is very large now, but
the large N hasn't particularly helped in moving the physics forward.
The cultural barrier between string theorists and phenomenologist has
already decreased a lot over the last 5 years or so; it would diminish to
if quantum gravity is discovered at the LHC, and even if SUSY is
discovered.
> 6. It will be possible for string theory to give a successful and
> completely new prediction about physics, even without knowing anything
> about the "right vacuum" within the landscape.
You didn't put a time limit on this one, and given that the definition of
string theory keeps changing, I can safely say put the odds at 100%
> 7. A nontrivial relation between the parameters of the Standard Model
> will be calculated theoretically by 2030.
By 2030, we will very likely have a new Standard Model with an additional
set of parameters. There will very likely be theoretically predicted
between the values of these new couplings. I'd put the odds at 80%. For
the SM of today parameters...I'm less sure. The SM isn't all that special
looking, but there are already many plausible relationships between
couplings coming from GUT's and SUSY GUT's, not just the relatin between
the gauge couplings, but also the \tau-bottom mass ratio, the relation
between the Cabbibo mixing angle and the strange/down masses and some
others.By 2030, we may find conclusively out that some of these are
actually right with 20%. And perhaps there is some totally new relation;
say 10%.
> [Prof. Arkani-Hamed,]
>
> I think a number of your papers are quite interesting, but I would
> like to probe into the proposed Cosmic?.. Bose-Einstein-Condensate?
>
> 1)Is there any chance of this being a Quark Condensate?..sort of like
> a Promordial Fluid that all Matter evolves from?..and still be
> ghost-condensate to our detectors.
We haven't thought yet about a microscopic theory for the condensate, only
described its low-energy excitations (which can largely be constrained by
symmetries). But it is conceivable that it arises from a quark condensate
in strongly coupled chiral gauge theories.
> 2)In the accelerated Expansion currently being detected, the expansion
> would tear the fabric of space apart (under certain conditions), how
> would a string worldline be continueous along the fabric of space, say
> from our Galaxy to an area of immense expansion, and remain a bonafide
> unbroken worldline?
This only happens in theories which are superaccelerating (where the
parameter w = p/\rho is less than -1). In this case the universe does
indeed hit a ``big rip" in the future. I am extremely dubious of the
consistency of these theories; all known examples in the literature suffer
from violent local instabilities, and are not physically consistent as far
as I see. So your issue wouldn't come up; I wouldn't worry about it till
someone shows that the big rip can happen in a physically consistent
theory.
> Prof. Nima Arkani-Hamed,
>
> When one endeavors through life in the areas of interest I have found
> you engaged in, there are certain fundamentals laws aside from the
> known proofs, that become the basis of ones perceptions.
>
> In all fairness, I would show you this link:
>
> http://www.edge.org/q2004/index.html#randall
>
> ....to help you see what I am saying and then ask, what Law might
> become Prof. Nima Arkani-Hamed's Law? sol
I didn't quite get your question. I find this sort of thing (the edge
list) silly. Though I like Lisa's about there being no theorems in
physics. Thats a theorem (ah, Russell is rolling in his grave).
> I'd like to understand roughly what the concepts of "Ghost Inflation" are.
>
> What is the nature of the ghost field appearing here. Is ghost
> inflation dealing with BRST ghosts (I guess not) or with 'phantom matter'?
The ``ghost" is perhpas misleading; no, it has nothing to do with BRST
ghosts, and not really anything to do with phantom matter either. The
background is one where a scalar field has non-zero (constant)
time-derivative. Around the origin of field space, the field would have a
wrong-sign kinetic term (so would be a ``phantom"), but the point is we
are far from the origin and the physics never explored the region close to
the origin either. Really our theory should be described as ``Gravity in
the Higgs phase".
> Where would such a ghost field come from, physically? What is the
> relation to string theory if any?
I mentioned above one possibility for a UV completion of this physics:
perhaps a lorentz-violating condensate in a chiral gauge theory.
As for string theory, it is clearly tied up with finding sensible
time-dependent solutions.
> First, I would like to thank Nima Arkani-Hamed for sharing his opinions
> and spending his valuable time explaining string theory to us.
> Here are my questions. Feel free to edit them to make more sense or be
> more generally applicable:
>
> String Theory and Axions At the American Physical Society meeting in
> Philadelphia not too long ago, Ed Witten said "Supersymmetry is
> compatible with the axion, but String Theory requires it"
> Do you agree?
Yes, string theory requires ``axions". But no, it does not require nor
predict OUR axion, needed to solve the strong CP problem in the Standard
Model. The string theory axions could be made massive by physics in
sectors other than the SM. Neither SUSY nor string theory will suffer
if we never find OUR axion, and there other nice solutions to the strong
CP problem.
> Black holes and Horizons. Classical GR black holes have event horizons
> and singularities. It seems to be generally accepted that string
> theory black holes do not have singularities.
> What about Horizons? Mathur at least seems to say that individual
> string configurations do not have true event horizons. What is your
> take on horizons and string theory based black holes?
These are all very interesting questions with no definite consensus on
whats going on. I find Mathurs papers interesting, but (among other
things) I don't understand how his conclusions can be extrapolated to the
case of Schwarzchild BH's.
> Black holes and metrics. It seems to be accepted that string theory
> leads either to a scalar tensor theory of gravitation or higher
> derivative terms or perhaps both. Which (or what else) is your
> preference. Are these extra terms likely to be big enough to be
> measured by Ligo or Lisa.?
The higher derivative terms in the action are certainly there but totally
irrelevant for low-energy/long distance physics, so they won't do anything
to Ligo or Lisa. In supersymmetric compactifications, there are massless
scalars corresponding to moving smoothly around in the continuosly
connected space of vacuaa. However SUSY is broken in our world, and the
moduli pick up masses, giving them a compton wavelength that is almost
always smaller than a millimeter or so. Thus, while some of them might be
observed in sub-millimeter tests of gravity, they wouldn't affect anything
at distances larger than a mm or so. It is possible that some scalars stay
much lighter, though you have to work to hard to ensure that they haven'y
been seen yet!
> Black hole Information Loss: Well known issue, your comments?
Only that I think too many people are saying that this has been
``understood", becasue we have manifestly unitary descriptions of quantum
gravity in some cases (like AdS/CFT). We still don't exactly know how the
information comes back out.
> Finally, an issue that has already been beaten almost to death on
> s.p.r: the Landscape and all that:
> how predictive is String Theory? Or is it like Cartesian Geometry: A
> very useful tool which predicts nothing?
Since its been beaten to death I won't beat it further. I don't have a
knee-jerk reaction against the idea that some things in physics can be
``environmental", and I have always been very impressed with Weinber's
argument about for the CC. Recently I've been thinking with Savas
Dimopoulos about an extremely predictive picture but non-standard without
low-energy SUSY but with SUSY coupling unification,
that was inspired by the landscape. So it may be indeed be a useful tool
to liberate us from our usual perspectives on naturalness and fine-tuning.
But I don't think we know nearly enough about the landscape yet, and in
particular the crucial question of cosmology, to get into long polemic
diatribes about it one way or another. If it turns out that predicting the
value of the CC or the electron mass isn't a fundamental question but is
instead environmental, fine; we will only be convinced these are the
wrong questions when we figure out what the right ones are.
> Once again, TIA to N.A-H for sharing his wisdom with us. Jim Graber
....
> Even though I'm eagerly trying to learn string theory, I should begin by
> warning you that I'm still a total novice.
>
> I've noticed that there has been questions regarding *specific*
> predictions in the string theory field. There has also been suggested
> the very generic (but good) question of how predictive string theory is.
>
> I'd like to know any kind of answers to a kind of in-between question:
>
> Which of currently planned experiments could give useful constraints for
> string theory, guiding it in some direction, or perhaps pruning it?
>
> The trivial answer is of course that many, many, experiments have the
> potential to have later implications for the theory, depending on
> theoretical developments, but I'd like to know if there are some
> particularly interesting experiments on the horizon, and interesting
> ways to use the theory to make constraints out of the results.
>
> Thanks, Mikael Djurfeldt
There are no definite experiments that can *exclude* string theory in the
near future. To my mnd, high energy collider experiments (the LHC in the
near future) by far have the best chance of revealing some evidence for
string theory, though the amount of evidence would vary depending on what
is observed. In the most optimistic scenario, if the scale of quantum
gravity is in the TeV range, we will make stringy excitations of SM fields
at the LHC: a partner for every SM field with one greater spin, a spin 3/2
electron, spin 2 photon and gluon, and so on. It would be very hard to
believe that it isn't string theory with this evidence! If we see some
standard sort of SUSY theory, this would at least show that some of the
ingredients needed in string theory exist in nature. In any case, at least
until some major new experimental idea comes along, collider experiments
are the way to go.