Peter Woit reports on a short series of posts on SPR by Lubos and Michael Douglas, and a question asked concurrently by David Kogan. Lubos referred to Woit's blog and was seemingly in agreement with part of what Peter said. I had a bit of difficulty identifying the PF instances because they occur in several threads. Thought it might be useful to gather the links together here so as to more easily follow the conversation:(adsbygoogle = window.adsbygoogle || []).push({});

Here is David Kogan's post

https://www.physicsforums.com/showthread.php?t=45246

---quote from Kogan---

Relating to string theory, how do physicists know when to abandon a theory?

What are some examples of "brick walls" that would cause string theory to

become nonviable and abandoned?

---end quote---

Here is a preliminary exchange between Lubos and Michael Douglas, about Landscaping: vacuum counts and averages. First Lubos, then Michael.

https://www.physicsforums.com/showthread.php?t=44344

Near the end of that exchange, Douglas says:

---quote from Douglas---

More importantly, as explained in 0303194, my recent 0409207, etc., I

think it is meaningless to average over different vacua, because we

only observe one vacuum. Rather, the goal of my own work, and what I

advocate doing, is to characterize the distribution of vacua well

enough to estimate the number N_SM of different vacua which satisfy

the many existing observational constraints (standard model,

cosmological, etc.) as well as possible future constraints (this might

lead to "predictions" as discussed in 0409207). Based on this

information, we can decide whether we should continue the search for

the right vacuum directly (appropriate if N_SM <= a few), look for

additional principles to cut down the number (if N_SM is large),or

give up and start making anthropic arguments or whatever (if N_SM is

ridiculously large).

---end quote---

The conversation continues, but not in the same thread. This next post is the one which caught Peter Woit's attention. In it, Lubos replies to Michael and also appears to agree with a comment made by Peter Woit

https://www.physicsforums.com/showthread.php?t=45506

---quote from Lubos, "Stringy Naturalness"---

Dear Michael,

thanks for these interesting comments here. I've read your constructively

provoking recent review

http://www.arxiv.org/abs/http://www.arxiv.org/abs/hep-th/0409207 [Broken]

Well, Peter Woit made some comments about the situation on his blog

http://www.math.columbia.edu/~woit/blog/

on September 20th - his summary is that you say that string theory

predicts that we can never see any physics related it. It would be too

difficult for me to pretend that I disagree with these Woit's remarks.

Do I understand well that all these predictions of yours about the

nonexistence of low energy SUSY and large dimensions critically rely on

your definition of stringy naturalness? You describe your notion of

stringy naturalness very explicitly:

[itex](**)[/itex]An effective field theory (or specific coupling or observable)

T1 is more natural in string theory than T2, if the number

of phenomenologically acceptable vacua leading to T1 is larger

than the number leading to T2.

I could not disagree more. This very definition of naturalness already

seems to contain - assume, in fact - Woit's result that the most typical

prediction of this approach to string theory will be that there are no

predictions. According to [itex](**),[/itex] the more ambiguous and unpredictive

something is, the better.

Also, I don't think that this counting "the more vacua, the more natural"

generalizes the notion of naturalness from physics "before" string theory

in any natural way. I would say that naturalness means - and always meant

- that the parameters that naturally appear in the description of physics

should be of order one. There are infinitely many more numbers (even among

integers!) :-) that are *not* of order one (for example 1235235236236236),

but this makes them *less* natural, not more, does not it?

If the notion of stringy naturalness were defined using the number of

vacua, I would probably choose a definition which seems to be nearly the

opposite of [itex](**),[/itex] namely

(##)An effective field theory or physical mechanism - or a value

of a coupling or another parameter - is natural from the

stringy viewpoint if it can be expected to be reproduced

in stringy backgrounds whose adjustable discrete parameters

are of order one, i.e. backgrounds that are "simple".

A more rigorous definition what is "simple" and what exactly should be of

order one requires some deeper knowledge of physics than what we have, but

the rough philosophy difference seems clear, I think.

Note that this definition more or less implies thatthe number of the, while your "natural" vacua are by definition members

discrete "natural" vacua with (approximately) the desired properties will

also be of order one

of huge families (unnatural families, in my language).

I think that it is (##), not (**), that naturally generalizes the previous

notions of naturalness. Naturalness means that the properly defined

parameters are of order one - not too small and not too large....

---end quote---

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# Motl and Douglas discuss naturalness

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