Motl and Douglas discuss naturalness

[marcus]

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:

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

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:

$(**)$ 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 $(**),$ 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 $(**),$ 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 that the number of the
discrete "natural" vacua with (approximately) the desired properties will
also be of order one, while your "natural" vacua are by definition members
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---

 

[marcus]

The comment of Peter Woit that Lubos referred to was this:

---quote Woit blog of September 20, 2004---

This Week's Predictions

A new preprint (http://www.arxiv.org/abs/hep-th/0409207 ) by Michael Douglas indicates that, at least this week, the latest "predictions" from string theory are for:

1. No large extra dimensions.

2. No low scale supersymmetry.

So it looks like the "prediction" of the string theory "Landscape" will be that no physics related to string theory beyond that of the standard model will ever be observable. Thus the only "prediction" of string theory will be that you can never see any physics related to it. This kind of "prediction" is great since it proves string theory must be true. Either you don't ever see any effects of string theory in which case you have confirmed its predictions so it must be true, or you do see effects of string theory, in which case string theory is even more true...
---end quote---
http://www.math.columbia.edu/~woit/blog/archives/000082.html

Lubos Motl then posted on SPR agreeing with Peter Woit's take on Douglas paper. Today Peter Woit rejoined the conversation and responded:

---quote Woit's blog of October 01, 2004---

Whatever

Over at sci.physics.strings there's the scary sight of Lubos Motl agreeing with me in a posting about "Stringy Naturalness". Well, maybe he isn't directly saying he agrees with me, but "It would be too difficult for me to pretend that I disagree with these Woit's remarks" is pretty close.

Lubos is criticizing the new sort of "naturalness" critierion advocated by Miichael Douglas in a preprint reviewing his recent work on the "Landscape". By this criterion a low energy effective QFT is more "natural" when there are more supposed string theory vacua that have this low energy limit. As Lubos points out, the danger with this criterion is that it tends to lead you to the conclusion that the most "natural" effective field theory is the one that is least likely to be able to predict anything new.

The posting immediately before Lubos's is from Michael Douglas himself, responding to an earlier thread. In it he explains the goal of his work as follows. He wants to estimate N_SM, the number of vacua consistent with the observed known Standard Model behavior, then

"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)."

The posting immediately before Douglas's asks for "what would cause string theory to become nonviable and abandoned", but hasn't gotten any responses. An obvious response would be that if it becomes clear that string theory has so many consistent vacua that it can't ever predict anything, the theory would have to be abandoned. Neither Douglas nor others working on the Landscape seem willing to mention this possibility in public, the closest he gets is the line about having to "give up and start making anthropic arguments or whatever".
---end quote---

So Mike Douglas paper is turning out to be an important one and we are watching a three-way conversation about it, with posts from Motl, Douglas, and Peter Woit. Peter started things rolling on Sept 20 by commenting on Douglas paper, after which Lubos posted something and Douglas replied including his ringing phrase about "anthropic arguments or whatever", then Lubos responded to Douglas and in doing so he agreed with Woit and gave two alternative defintions of naturalness, which he tagged (**) and (##). Now most recently (I hope not finally) Woit has contributed another post to the series.

I must say that Lubos Motl appears in an honorable and sympathetic role here. He makes a point of steadfastly rejecting Landscapery and the temptation to (in Douglas words) "give up" and resort to Anthropism. He explicitly says that things like the ratio of the Planck mass to the electron mass should be explained and that naturalness means having one vacuum not ten-to-the-hundredth different ones.

The name of the Douglas paper that started the discussion is
Basic Results in Vacuum Statistics
http://www.arxiv.org/abs/hep-th/0409207

 

[pat777]

Either you don't ever see any effects of string theory in which case you have confirmed its predictions so it must be true, or you do see effects of string theory, in which case string theory is even more true...

Of course, the absence of string theory also predicts that you do not see the effects of string theory because they don't exist. That doesn't really prove string theory is correct to me.