what do you think physics departments in universities should do with string theory and string theorists, including the tenured, post docs, and those graduate students pursuing phD's in string theory?

Aaron Wall got a phD in string theory and from univ of maryland and is now at stanford (!) doing string theory research.

he's also a christian, and believes string theory and christian theology go hand in hand. different take than someone like witten.

On that thread, "shantanu" said Witten should debate an LQG researcher: "Incidentally some of his own colleagues at Princeton such as Frans Pretorius are working on LQG." I replied:

"shantanu: in what sense is Princeton’s Frans Pretorius “working on LQG”? He specializes in numerical simulation of classical general relativity."

But Woit didn't allow this through... I used to have about a 50-50 success rate in getting my comments through at Woit's, but lately he doesn't allow anything.

same one as on PF? i recall u and urs think LQG is debunked, which made me wonder what you think 4-D GR should be quantized, if LQG is wrong or nonstandard.

would be interesting still to have a respected string theorist debate a respected LQG one on youtube

Quantum gravity is a difficult subject. I regard the foundation of it, as the low-energy effective field theory which comes from quantizing general relativity in the usual way. That framework becomes untenable at high energies, but at low energies it works, as has been demonstrated by John Donoghue and others.

String theory is attractive as an approach to quantum gravity for a number of reasons. It provides a UV completion, it predicts the same corrections to the black hole entropy as the low-energy effective theory does (at least, I think this is the significance e.g. of Ashoke Sen's work on logarithmic corrections to the entropy), it produced the 'weak gravity conjecture' which in turn seems to be confirmed by field-theory studies (I mean the work on naked singularities in AdS, and how they are prevented by the addition of a charged field satisfying the WGC).

All that came from string theory without anyone intending it or expecting it. So it's actually quite reasonable for someone to think that string theory is the right framework for quantum gravity, and that our universe must be some kind of string vacuum. However, as Witten said in his latest interview, it is quite possible that the known types of 'string vacua' are not all there is, and that another radical broadening of string theory could occur in the future (see his remarks on 'it from qubit').

The essence of string theory may be something very abstract and algebraic (like 'an S-matrix satisfying some kind of bootstrap property'), rather than 'quantum theory of extended objects'. It might encompass something as alien e.g. as what Marni Sheppeard tries to do. So that's one reason why one can take string theory very very seriously, but also take an interest in what appear to be other approaches. So I take an interest in asymptotic safety, conformal gravity, Salvio's adimensional gravity, twistors, Deur's QCD-like gravity, and so on.

what do you think of woit and sabine's criticism of strings plus the fact lhc hasn't found susy or kk?

since lhc hasn't found any evidence of susy and kk, and there are grave theoretical reasons to doubt susy and kk, how would you and urs recommend canonically quantizing GR, if lqg is "wrong", for those theorists who prefer QG w/o kk or susys, just 4D.

i know urs is not a fan of spinfoam, what do you think of it as an approach, or group field theory?

flavor changing neutral current, susy-breaking, fine tuning, swampland are well known reasons. in addition to non-observation

occam's razor would imply susy and kk are not realized in nature.

in the scenario that susy and kk are not realized in nature, and that quantum gravity implies quantized spacetime, what would be the way to arrive at such a theory?

I guess I was thinking of a "theoretical problem" as a barrier to a theory's existence, even as a mathematically well-defined possibility. But OK, if we are talking about something empirical which broadly presents a difficulty for a whole paradigm, perhaps we can call that a theoretical difficulty too.

In that sense, supersymmetry always had problems with FCNCs, also with proton decay, which were to be solved by R-parity. But I wouldn't call supersymmetry breaking a difficulty, just something that requires a mechanism, and here there are many theoretical possibilities...

The real problem I see for supersymmetry is empirical absence and empirical irrelevance. You can posit supersplit supersymmetry which is only restored at the Planck scale, but then supersymmetry isn't relevant to explaining how a light Higgs is possible.

" But I wouldn't call supersymmetry breaking a difficulty, just something that requires a mechanism, and here there are many theoretical possibilities."

all of which requires nature employ new unobserved physics, which may or may not be realized in nature.

what about occam's razor? isn't it simpler to posit the SM plus minimal extensions like SMASH, then to propose SUSY, the come up with speculation on how to deal with its many problems for which there's no evidence for

"what about occam's razor? isn't it simpler to posit the SM plus minimal extensions like SMASH, then to propose SUSY, the come up with speculation on how to deal with its many problems for which there's no evidence for"

If you have a problem to solve, a unified theory may very well be better under Occam's Razor than ad hoc extensions. Even if a minimal SUSY double then number of particles, it may only add one assumption in lieu if ad hoc minimal adjustments adding several assumptions.

But, when there is "no evidence" any theory beyond the minimal fails Occam's Razor. Also, worth recalling that Occam's Razor is not always right. It is a default against which we test other hypotheses, but sometimes evidence in some other part of physics can make an otherwise Byzantine solution the better one.

"If you have a problem to solve, a unified theory may very well be better under Occam's Razor than ad hoc extensions. Even if a minimal SUSY double then number of particles, it may only add one assumption in lieu if ad hoc minimal adjustments adding several assumptions."

some SUSY breaking theorists adds additional unseen particles, a hidden sector separate from the SM

## 13 comments:

witten, brian green eva silverstein et al are smart,

what do you think is really going on?

personally i wish he said something more substantive on LQG

Cognitive biases are strong.

what do you think physics departments in universities should do with string theory and string theorists, including the tenured, post docs, and those graduate students pursuing phD's in string theory?

Aaron Wall got a phD in string theory and from univ of maryland and is now at stanford (!) doing string theory research.

he's also a christian, and believes string theory and christian theology go hand in hand. different take than someone like witten.

On that thread, "shantanu" said Witten should debate an LQG researcher: "Incidentally some of his own colleagues at Princeton such as Frans Pretorius are working on LQG." I replied:

"shantanu: in what sense is Princeton’s Frans Pretorius “working on LQG”? He specializes in numerical simulation of classical general relativity."

But Woit didn't allow this through... I used to have about a 50-50 success rate in getting my comments through at Woit's, but lately he doesn't allow anything.

ru mitchell porter?

same one as on PF? i recall u and urs think LQG is debunked, which made me wonder what you think 4-D GR should be quantized, if LQG is wrong or nonstandard.

would be interesting still to have a respected string theorist debate a respected LQG one on youtube

That's me.

Quantum gravity is a difficult subject. I regard the foundation of it, as the low-energy effective field theory which comes from quantizing general relativity in the usual way. That framework becomes untenable at high energies, but at low energies it works, as has been demonstrated by John Donoghue and others.

String theory is attractive as an approach to quantum gravity for a number of reasons. It provides a UV completion, it predicts the same corrections to the black hole entropy as the low-energy effective theory does (at least, I think this is the significance e.g. of Ashoke Sen's work on logarithmic corrections to the entropy), it produced the 'weak gravity conjecture' which in turn seems to be confirmed by field-theory studies (I mean the work on naked singularities in AdS, and how they are prevented by the addition of a charged field satisfying the WGC).

All that came from string theory without anyone intending it or expecting it. So it's actually quite reasonable for someone to think that string theory is the right framework for quantum gravity, and that our universe must be some kind of string vacuum. However, as Witten said in his latest interview, it is quite possible that the known types of 'string vacua' are not all there is, and that another radical broadening of string theory could occur in the future (see his remarks on 'it from qubit').

The essence of string theory may be something very abstract and algebraic (like 'an S-matrix satisfying some kind of bootstrap property'), rather than 'quantum theory of extended objects'. It might encompass something as alien e.g. as what Marni Sheppeard tries to do. So that's one reason why one can take string theory very very seriously, but also take an interest in what appear to be other approaches. So I take an interest in asymptotic safety, conformal gravity, Salvio's adimensional gravity, twistors, Deur's QCD-like gravity, and so on.

hello mitchel,

small world. lol.

what do you think of woit and sabine's criticism of strings plus the fact lhc hasn't found susy or kk?

since lhc hasn't found any evidence of susy and kk, and there are grave theoretical reasons to doubt susy and kk, how would you and urs recommend canonically quantizing GR, if lqg is "wrong", for those theorists who prefer QG w/o kk or susys, just 4D.

i know urs is not a fan of spinfoam, what do you think of it as an approach, or group field theory?

What are the "grave theoretical reasons to doubt susy and kk"? All I see is the *empirical* fact, lack of evidence,

flavor changing neutral current, susy-breaking, fine tuning, swampland are well known reasons. in addition to non-observation

occam's razor would imply susy and kk are not realized in nature.

in the scenario that susy and kk are not realized in nature, and that quantum gravity implies quantized spacetime, what would be the way to arrive at such a theory?

I guess I was thinking of a "theoretical problem" as a barrier to a theory's existence, even as a mathematically well-defined possibility. But OK, if we are talking about something empirical which broadly presents a difficulty for a whole paradigm, perhaps we can call that a theoretical difficulty too.

In that sense, supersymmetry always had problems with FCNCs, also with proton decay, which were to be solved by R-parity. But I wouldn't call supersymmetry breaking a difficulty, just something that requires a mechanism, and here there are many theoretical possibilities...

The real problem I see for supersymmetry is empirical absence and empirical irrelevance. You can posit supersplit supersymmetry which is only restored at the Planck scale, but then supersymmetry isn't relevant to explaining how a light Higgs is possible.

" But I wouldn't call supersymmetry breaking a difficulty, just something that requires a mechanism, and here there are many theoretical possibilities."

all of which requires nature employ new unobserved physics, which may or may not be realized in nature.

what about occam's razor? isn't it simpler to posit the SM plus minimal extensions like SMASH, then to propose SUSY, the come up with speculation on how to deal with its many problems for which there's no evidence for

"what about occam's razor? isn't it simpler to posit the SM plus minimal extensions like SMASH, then to propose SUSY, the come up with speculation on how to deal with its many problems for which there's no evidence for"

If you have a problem to solve, a unified theory may very well be better under Occam's Razor than ad hoc extensions. Even if a minimal SUSY double then number of particles, it may only add one assumption in lieu if ad hoc minimal adjustments adding several assumptions.

But, when there is "no evidence" any theory beyond the minimal fails Occam's Razor. Also, worth recalling that Occam's Razor is not always right. It is a default against which we test other hypotheses, but sometimes evidence in some other part of physics can make an otherwise Byzantine solution the better one.

"If you have a problem to solve, a unified theory may very well be better under Occam's Razor than ad hoc extensions. Even if a minimal SUSY double then number of particles, it may only add one assumption in lieu if ad hoc minimal adjustments adding several assumptions."

some SUSY breaking theorists adds additional unseen particles, a hidden sector separate from the SM

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