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marcus

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The focus is on *professionally researched* foundational theory. You are right that Quantum Gravity is an effort in that direction. The aim is a quantum theory of the spacetime geometry in which matter lives. Once that goal is reached the door will be open to rebuilding the quantum field theory of matter on new geometrical foundations.I wonder if theories concerning the foundations of quantum mechanics, quantum field theories, and relativity are the same as Beyond the Standard Model concerns. Quantum Gravity is trying to find theories that encompass both QM and GR. So perhaps this is the same as trying to find an foundational theory for both...

So the ultimate goal of the professional research that gets reported and discussed here, including QG, has always been a fundamental theory of both Geometry AND Matter fields.

I sort of hope that QG will lead to a new understanding of what Geometry is, but also a new understanding of what Matter is----perhaps that at the quantum level they are both the same thing. So it would at last be understood how it is that they interact so closely and affect each other.

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marcus

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According to you, friend, what scale are the fluctuations in the CMB? It seems to me that they have *angular* scale---measured in fractions of a degree of angle. A wide range of angular scales....when they cannot directly measure things at the scale of quantum gravity. Are we even sure that QG is even measureable in principle?

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Yes, but that depends on your cosmological model, the CMB may be a red-herring wrt to QG if inflation is falseAccording to you, friend, what scale are the fluctuations in the CMB? It seems to me that they have *angular* scale---measured in fractions of a degree of angle. A wide range of angular scales.

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It might even depend on what type of inflation - slow role or otherwise.Yes, but that depends on your cosmological model, the CMB may be a red-herring wrt to QG if inflation is false

Correct me here if I'm wrong. But I think I remember LQG to be based on spin form networks that are a speculation about what spacetime itself is made of - I don't think it's a pure derivation from QG and QM, right? Also, CDT is also a guess on what spacetime is like, not a direct derivation, right? If so, then these seem to be based on principles they think may be common to QG and QM, but not purely derived from them. And so they are attempts at what is foundational to QG and QM, right? So is BSM really chasing after foundational issues?

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marcus

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Over the years, I've often quoted Rovelli to the contrary....I think I remember LQG to be based on spin form networks that are a speculation about what spacetime itself is made of...

It's not about what Nature is "made of", it's about how she responds to measurement. That's a paraphrase---he said it more emphatically with more examples.

That was at a seminar given by CR, Ashtekar, and Laurent Freidel, I think it was in 2006-2008 and would still online at the ILQGS.

I think Niels Bohr made essentially the same point about Quantum Theory in general. It is not about what it

Early universe phenomenologists seem to think they can get testable predictions out of Loop. It might even be possible to constrain or rule it out by CMB polarization observations. You have to make up your own mind. I'll get a link to some 60 papers relevant to that.

As you know, testable scientific theories are never verified, they simply survive testing until it is shown that they need to be replaced or improved.

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foundational questions about QM are usually very distinct from what serious theoretical physicists are trying to do with regards to quantum gravity. I mean, most of the prominent string theorists accept copenhagen + decoherence interpretation in their work, or maybe a few like to think MWI is the better idea.

The point is that foundational questions don't arise in their intricate calculations, you could probably even argue that bohmian pilot waves are consistent!

At no point in the foundations of string theory are the questions of randomness v determinstic qm considered - they are irrelevant to the mathematics.

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marcus

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Here's the link I promised. It is for papers related to early universe phenomenology, appearing in 2009 or later:...

Early universe phenomenologists seem to think they can get testable predictions out of Loop. It might even be possible to constrain or rule it out by CMB polarization observations. You have to make up your own mind. I'll get a link to some 60 papers relevant to that.

As you know, testable scientific theories are never verified, they simply survive testing until it is shown that they need to be replaced or improved.

http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+LOOP+SPACE+AND+%28QUANTUM+GRAVITY+OR+QUANTUM+COSMOLOGY%29+%29+AND+%28GRAVITATIONAL+RADIATION+OR+PRIMORDIAL+OR+inflation+or+POWER+SPECTRUM+OR+COSMIC+BACKGROUND+RADIATION%29+AND+DATE%3E2008&FORMAT=www&SEQUENCE=citecount%28d%29 [Broken]

http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+LOOP+SPACE+AND+%28QUANTUM+GRAVITY+OR+QUANTUM+COSMOLOGY%29+%29+AND+%28GRAVITATIONAL+RADIATION+OR+PRIMORDIAL+OR+inflation+or+POWER+SPECTRUM+OR+COSMIC+BACKGROUND+RADIATION%29+AND+DATE%3E2008&FORMAT=www&SEQUENCE=ds%28d%29 [Broken]

As of today this gets 63 papers and they are ranked by cites so you get some older ones first because they have been around longer---so have had more time to be cited.

Unusual, the message I get from your post is that it doesn't matter what your

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Yes, that makes sense. Of course our theories must be about what we can measure since we can never verify any theory about what nature is.Over the years, I've often quoted Rovelli to the contrary.

It's not about what Nature is "made of", it's about how she responds to measurement.

...

I think Niels Bohr made essentially the same point about Quantum Theory in general. It is not about what itisbut about how it responds to measurement---i.e. what we can say, what bets we can settle, what statements we can verify.

...

As you know, testable scientific theories are never verified, they simply survive testing until it is shown that they need to be replaced or improved.

But this only brings up the question about what we base our theories on when we want to make predictions about what we cannot directly measure, a.k.a. quantum gravity perhaps. It seems then we can only be guided by our intuition or reason. But that might lead us to a rather difficult dilemma. What happens if we were to derive physics from some sort of undeniable form of reasoning or logic? If that theory were falsified, then reality becomes illogical. But what recourse do we have if we can not measure what we predict? Indeed, what recourse do we have in our theorizing other than to rely on intuition and reason? How could we ever know that any theory we came up with was unique since if we can never know what the universe IS?

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marcus

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That's somewhat hypothetical, but it does not apply to Loop. The predictions derived from LQG discussed in the literature involve measurements which are possible to make. You can check some of the papers by phenomenologists (in particular those who not from Loop community and are just as interested in positive as negative results).... But what recourse do we have if we can not measure what we predict?

There should be some 63 papers there. Please let me know if you have trouble with the links and I will hunt up some different ones. Not all are phenomenology so you have to inspect to see which ones relate to observational tests.

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omg, calm down, you sound crayz! [sic] :OK, Descartes already solved this, there surely isn't a BAD GOD in charge of things.

Yes, that makes sense. Of course our theories must be about what we can measure since we can never verify any theory about what nature is.

But this only brings up the question about what we base our theories on when we want to make predictions about what we cannot directly measure, a.k.a. quantum gravity perhaps. It seems then we can only be guided by our intuition or reason. But that might lead us to a rather difficult dilemma. What happens if we were to derive physics from some sort of undeniable form of reasoning or logic? If that theory were falsified, then reality becomes illogical. But what recourse do we have if we can not measure what we predict? Indeed, what recourse do we have in our theorizing other than to rely on intuition and reason? How could we ever know that any theory we came up with was unique since if we can never know what the universe IS?

or...., chillax man, the next stage of particle physics is just 'round the corner

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i similar thoughts like yours.I have my doubts that they will unite gravity and theory without a better understanding of the first. I think they will have to start from scratch based on some general principle that encompasses everything (perhaps logic). And show from that how QM and GR are derived. It seems difficult to believe that they could just guess at the math and be sure it's correct when they cannot directly measure things at the scale of quantum gravity. Are we even sure that QG is even measureable in principle?

and derived from general relativity and special relativity.I derivation from QG and QM, right? Also, CDT is also a guess on what spacetime is like, not a direct derivation, right? If so, then these seem to be based on principles they think may be common to QG and QM, but not purely derived from them. And so they are attempts at what is foundational to QG and QM, right? So is BSM really chasing after foundational issues?

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Perhaps you've answered this already. But is LQG a pure derivation from QM and GR? Or, is it more as I suspect, an educated guess to postulate some more fundamental principle as the foundations of both? Hey, as long as they don't claim that it is the result of some undeniable logic, we should feel safe with their hypothesis, right?That's somewhat hypothetical, but it does not apply to Loop. The predictions derived from LQG discussed in the literature involve measurements which are possible to make. You can check some of the papers by phenomenologists (in particular those who not from Loop community and are just as interested in positive as negative results).

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Well also I think it's important whether colleagues think you're sane or not.Unusual, the message I get from your post is that it doesn't matter what yourideologyis. If that's the gist, I would agree. What matters is not your ideology but whether independent professional theory-testers think your theory istestable.

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marcus

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Right! Otherwise grants and jobs will not be forthcoming for a large percentage of the community's recent PhDs and postdocs.Well also I think it's important whether colleagues think you're sane or not.

I think we can take that for granted as an unstated assumption.

http://sites.google.com/site/grqcrumourmill/

As you can see things are looking pretty good in that regard.

These should all be familiar names to anyone who follows Loop research:

==quote==

2012

...

Cambridge U., DAMTP (philosophy of cosmology) - Offer to: David Sloan (Utrecht; accepted)

...

Louisana State U. (loop quantum gravity & cosmology) - Offer to: Edward Wilson-Ewing (Marseille; accepted)

Penn State U. (Fundamental Gravitational Theory, GR/QC) - Offer to: Yasha Neiman (Tel Aviv; accepted), Thomas Cailleteau (LPSC, Grenoble; accepted), Marc Geiller (APC, Paris; accepted), Norbert Bodendorfer (Erlangen - Nuremberg U.)

Perimeter Institute (quantum gravity, cosmology, ...) - Offer to: Flavio Mercati (Zaragoza), Philipp Höhn (Utrecht; accepted), Ryszard Kostecki (Warsaw; accepted)

...

Warsaw U. (loop quantum gravity) - Offer to: Emanuele Alesci (Erlangen; accepted)

2012

Francesca Vidotto Grenoble -> Utrecht (Rubicon Fellowship)

William Nelson Penn State U. -> Nijmegen (Marie Curie)

Muxin Han Marseille -> Marseille (Marie Curie)

2012

...

Hanno Sahlmann APCTP, Pohang -> Erlangen - Nuremberg U. (faculty)

James Ryan Potsdam, Max Planck Inst. -> UNAM, Mexico (tenure track) - declined

Razvan Gurau Perimeter Inst. -> CNRS, France (research position)

Leonardo Modesto Perimeter Inst. -> Fudan U., Shanghai (faculty)

==endquote==

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As I recall, there was a mathematician whose sanity they question but whose mathematical proof was flawless. I don't remember his name. He had a Beautiful Mind.Well also I think it's important whether colleagues think you're sane or not.

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Yeah, hollywood hype, it was an economics award remember, not a mathematical one, even Nash thought his embeddings ideas were his best work http://en.wikipedia.org/wiki/Nash_embedding_theoremAs I recall, there was a mathematician whose sanity they question but whose mathematical proof was flawless. I don't remember his name. He had a Beautiful Mind.

anyway, no one cares, you seem to want fundamental QG research to tackle fundamental QM foundations - but it doesn't, and some of the major figures have spoken about this, eg Witten at the end chapter of Brian Greene's 'The Elegant Universe', and Polchinski in a guest blog article http://blogs.discovermagazine.com/c...blogger-joe-polchinski-on-the-string-debates/

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Oh yes, of course. That's the whole point of physics. We certainly don't want anyone to succeed in deriving physics from undeniable logic. How ridiculous! That's crazy! That would be the end of all that funding money.Right! Otherwise grants and jobs will not be forthcoming for a large percentage of the community's recent PhDs and postdocs.

I think we can take that for granted as an unmentioned assumption.

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marcus

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Hi F.

I'm afraid you may have distorted the meaning of what I said by taking out of the original context and connecting it to your "undeniable logic" idea, which is entirely unrelated to what I was talking about with Unusualname.

I emphasized the importance of a theory being testable by observations/measurements that can really be performed (Loop QG is testable in this sense according to people whose job it is to invent and study tests of theories.)

Unusual responded (I imagine jokingly or at most half seriously) to the effect that it is also important to have the good opinion of colleagues.

Actually we know this can be a bad guide in many cases because colleagues with a rival approach can be envious, or resentful, or in denial about the value of a given line of investigation.

but as a practical matter you have to have enough respect and credibility in the wider scientific community to earn a living. So I replied to Unusual's post to reassure him that the Loop QG program is doing tolerably well in the respect/acceptance category.

==quote==

I think we can take that for granted as an unstated assumption.

http://sites.google.com/site/grqcrumourmill/

As you can see things are looking pretty good in that regard.

These should all be familiar names to anyone who follows Loop research:

==quote==

2012**Postdoc Positions**:

...

Cambridge U., DAMTP (philosophy of cosmology) - Offer to: David Sloan (Utrecht; accepted)

...

Louisana State U. (loop quantum gravity & cosmology) - Offer to: Edward Wilson-Ewing (Marseille; accepted)

Penn State U. (Fundamental Gravitational Theory, GR/QC) - Offer to: Yasha Neiman (Tel Aviv; accepted), Thomas Cailleteau (LPSC, Grenoble; accepted), Marc Geiller (APC, Paris; accepted), Norbert Bodendorfer (Erlangen - Nuremberg U.)

Perimeter Institute (quantum gravity, cosmology, ...) - Offer to: Flavio Mercati (Zaragoza), Philipp Höhn (Utrecht; accepted), Ryszard Kostecki (Warsaw; accepted)

...

Warsaw U. (loop quantum gravity) - Offer to: Emanuele Alesci (Erlangen; accepted)

2012**External Fellowships**:

Francesca Vidotto Grenoble -> Utrecht (Rubicon Fellowship)

William Nelson Penn State U. -> Nijmegen (Marie Curie)

Muxin Han Marseille -> Marseille (Marie Curie)

2012**Tenure Track/Faculty Positions**:

...

Hanno Sahlmann APCTP, Pohang -> Erlangen - Nuremberg U. (faculty)

James Ryan Potsdam, Max Planck Inst. -> UNAM, Mexico (tenure track) - declined

Razvan Gurau Perimeter Inst. -> CNRS, France (research position)

Leonardo Modesto Perimeter Inst. -> Fudan U., Shanghai (faculty)

==endquote==

This has nothing to do with methodology (whether you imagine people proceeding by "undeniable logic" or by educated guesses and analogy with what has worked in the past). It simply is the practical matter of respect and acceptance by peers.

These are just the Fall 2012 appointments. The past year or so has seen a run of them, including a significant number to faculty or comparable research positions. I can't list all---some that come to mind are Bianchi to Perimeter, Giesel to Erlangen, Engle to Florida Atlantic, Singh to LSU, Hellmann to MPI-Potsdam, Meusburger to Erlangen, You Ding to BNU-Beijing..as I say, this is not a complete list.

I'm afraid you may have distorted the meaning of what I said by taking out of the original context and connecting it to your "undeniable logic" idea, which is entirely unrelated to what I was talking about with Unusualname.

I emphasized the importance of a theory being testable by observations/measurements that can really be performed (Loop QG is testable in this sense according to people whose job it is to invent and study tests of theories.)

Unusual responded (I imagine jokingly or at most half seriously) to the effect that it is also important to have the good opinion of colleagues.

Actually we know this can be a bad guide in many cases because colleagues with a rival approach can be envious, or resentful, or in denial about the value of a given line of investigation.

but as a practical matter you have to have enough respect and credibility in the wider scientific community to earn a living. So I replied to Unusual's post to reassure him that the Loop QG program is doing tolerably well in the respect/acceptance category.

==quote==

Right! Otherwise grants and jobs will not be forthcoming for a large percentage of the community's recent PhDs and postdocs.Well also I think it's important whether colleagues think you're sane or not.

I think we can take that for granted as an unstated assumption.

http://sites.google.com/site/grqcrumourmill/

As you can see things are looking pretty good in that regard.

These should all be familiar names to anyone who follows Loop research:

==quote==

2012

...

Cambridge U., DAMTP (philosophy of cosmology) - Offer to: David Sloan (Utrecht; accepted)

...

Louisana State U. (loop quantum gravity & cosmology) - Offer to: Edward Wilson-Ewing (Marseille; accepted)

Penn State U. (Fundamental Gravitational Theory, GR/QC) - Offer to: Yasha Neiman (Tel Aviv; accepted), Thomas Cailleteau (LPSC, Grenoble; accepted), Marc Geiller (APC, Paris; accepted), Norbert Bodendorfer (Erlangen - Nuremberg U.)

Perimeter Institute (quantum gravity, cosmology, ...) - Offer to: Flavio Mercati (Zaragoza), Philipp Höhn (Utrecht; accepted), Ryszard Kostecki (Warsaw; accepted)

...

Warsaw U. (loop quantum gravity) - Offer to: Emanuele Alesci (Erlangen; accepted)

2012

Francesca Vidotto Grenoble -> Utrecht (Rubicon Fellowship)

William Nelson Penn State U. -> Nijmegen (Marie Curie)

Muxin Han Marseille -> Marseille (Marie Curie)

2012

...

Hanno Sahlmann APCTP, Pohang -> Erlangen - Nuremberg U. (faculty)

James Ryan Potsdam, Max Planck Inst. -> UNAM, Mexico (tenure track) - declined

Razvan Gurau Perimeter Inst. -> CNRS, France (research position)

Leonardo Modesto Perimeter Inst. -> Fudan U., Shanghai (faculty)

==endquote==

This has nothing to do with methodology (whether you imagine people proceeding by "undeniable logic" or by educated guesses and analogy with what has worked in the past). It simply is the practical matter of respect and acceptance by peers.

These are just the Fall 2012 appointments. The past year or so has seen a run of them, including a significant number to faculty or comparable research positions. I can't list all---some that come to mind are Bianchi to Perimeter, Giesel to Erlangen, Engle to Florida Atlantic, Singh to LSU, Hellmann to MPI-Potsdam, Meusburger to Erlangen, You Ding to BNU-Beijing..as I say, this is not a complete list.

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strangerep

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No, it just means that your "undeniable" form of reasoning or logic is wrong, or you have used incorrect inputs to the logic.[...] What happens if we were to derive physics from some sort of undeniable form of reasoning or logic? If that theory were falsified, then reality becomes illogical.

I think it was Feynman who said something like "it doesn't matter how smart you are -- if experiment contradicts your theory then you are wrong".

(Hmm, I think I hear ZapperZ approaching with his philosophy-murdering axe... :-)

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It may be a bit philosophical, but I think it is still a good question: How foundational are the issues in BSM? I've not seen a good place to pose questions about foundations in PF. But I thought this question would be of interest here. I'm beginning to think that QG won't be adequately addressed without answering the foundations of physics. Why is the universe both QM and GR? I think this is the same as asking what fundamental principle should give rise to both? That seems extremely foundational. And it would be nice to see a well motivated principle and some undeniable reasoning as the basis of all physics. Ultimately I think it has to be logical. For otherwise we're suggesting that at some basic level reality is somewhere not logical, and that's untenable.

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marcus

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Friend, as long is is not YOUR thinking, but is something that gets featured at major conferences and has professionals writing research papers about it it seems appropriate to BSM.

Your original question isn't philosophy. BSM discussion includes professionally-researched bids to subsume QM and GR in a general relativistic quantum field theory of geometry and matter.

Loop is a step in that direction: get an empirically testable quantum theory of geometry (and then add matter.)

Here is the Hossenfelder thread:

https://www.physicsforums.com/showthread.php?t=632220

I can't imagine how anything could be more "foundational". It denies that nature is fundamentally quantum. Quantum theories are only effective. A nonzero hbar is the result of symmetry breaking.

Hossenfelder is a pro, whose specialty is QG Phenomenology. She has already organized two international conferences on the Experimental Search for QG and her third one is coming up next month.

http://www.perimeterinstitute.ca/en/Events/Experimental_Search_for_QG/Experimental_Search_for_Quantum_Gravity%3A_the_hard_facts/ [Broken]

Friend, you asked a straightforward question which I think can be easily and clearly answered.

Your original question isn't philosophy. BSM discussion includes professionally-researched bids to subsume QM and GR in a general relativistic quantum field theory of geometry and matter.

Loop is a step in that direction: get an empirically testable quantum theory of geometry (and then add matter.)

It already is accepted to discuss efforts towards that goal as long as professionally researched....Quantum Gravity is trying to find theories that encompass both QM and GR. So perhaps this is the same as trying to find an foundational theory for both. Would such a foundational theory be acceptable here in Beyond the Standard Model forum?

Another example is the idea that Sabine Hossenfelder just came up with where planck's [STRIKE]h[/STRIKE] goes to zero at high energy. So nature is neither strictly quantum nor strictly classical, in her picture. She posted this on arxiv recently and we've discussed it here (mainly her and Demystifier). That's a good example of a fledgling theory that comes to grips with a "foundatiional" = fundamental issue. I'll get the link.The focus is on *professionally researched* foundational theory. You are right that Quantum Gravity is an effort in that direction. The aim is a quantum theory of the spacetime geometry in which matter lives. Once that goal is reached the door will be open to rebuilding the quantum field theory of matter on new geometrical foundations.

So the ultimate goal of the professional research that gets reported and discussed here, including QG, has always been a fundamental theory of both Geometry AND Matter fields.

...

Here is the Hossenfelder thread:

https://www.physicsforums.com/showthread.php?t=632220

I can't imagine how anything could be more "foundational". It denies that nature is fundamentally quantum. Quantum theories are only effective. A nonzero hbar is the result of symmetry breaking.

Hossenfelder is a pro, whose specialty is QG Phenomenology. She has already organized two international conferences on the Experimental Search for QG and her third one is coming up next month.

http://www.perimeterinstitute.ca/en/Events/Experimental_Search_for_QG/Experimental_Search_for_Quantum_Gravity%3A_the_hard_facts/ [Broken]

Friend, you asked a straightforward question which I think can be easily and clearly answered.

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strangerep

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Question: since that paper hasn't yet been published in a refereed journal, doesn't that mean it's outside the PF guidelines? I know that the BTSM forum tolerates more latitude, but I'm wondering where demarcation lies. (?)Another example is the idea that Sabine Hossenfelder just came up with where planck's [STRIKE]h[/STRIKE] goes to zero at high energy. So nature is neither strictly quantum nor strictly classical, in her picture. She posted this on arxiv recently and we've discussed it here (mainly her and Demystifier).

[Don't get me wrong -- I'm perfectly happy to discuss anything Bee comes up with. I'm just wondering about the strict application of PF rules to such cases.]

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I thought the policy was not to advocate speculative theories. So if someone always put his theory in the form of a question, would that be allowed on PF? Or if someone developed a relatively easy theory from well accepted principles, would that be acceptable on PF? Otherwise, I think you're limiting the scope to theories that have been worked on for 10, 20, 30 years without success from professionals who seem to have a financial interest in keeping their research programs alive. Is that even a recipe for success?Friend, as long is is not YOUR thinking, but is something that gets featured at major conferences and has professionals writing research papers about it it seems appropriate to BSM.

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