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Regarding Peter Woit, who keeps an anti-string theory blog called Not Even Wrong, Lubos Motl writes in his blog:

"One such a nutcase [Woit] has been writing this kind of garbage and nothing else for more than two years, if not twenty years, hoping that a lie repeated 1000 times becomes the truth. But what happened instead is that he became much greater a nutcase than he was at the beginning. String theory is undoubtedly falsifiable and those who don't understand why are just too severely limited to join any meaningful discussion about the subject."

And regarding physicist Lee Smolin (and in particular Smolin's ideas about black hole reproduction), Motl writes:
"Invent some crazy theory - for example that our universe is a f*cking universe that produces mutated children inside the black holes that evolve into new universes and follow Darwin's rules of natural selection, which is why our universe must be optimized for black hole production. If you think that this theory is far too crazy, believe me that it has been seriously proposed by a physicist [Smolin]. In fact, this physicist, a darling of the media, still proposes this nonsense even though at least a dozen of people have explained him why his theory is easily falsified."

Another interesting passage where he uses somewhat loaded language, to make a questionably valid point:
"In reality, it will probably be impossible to falsify string theory because string theory is probably correct and you can't ever falsify correct theories. ;-) But if string theory were wrong, there would be thousands of ways to falsify it, even in the very near future. Although string theory predicts many new phenomena whose details are not uniquely known, it also implies that many old principles are exactly valid. If string theory is correct, the superposition principle of quantum mechanics, Lorentz invariance, unitarity, crossing symmetry, equivalence principle etc. are valid to much higher accuracy than the accuracy with which they have been tested as of 2006."


Regardless of Motl's expertise, I think he often lets his emotions get out of control when he writes in his blog. It's embarassing to hear this kind of rhetoric coming from an academic... not the kind of wording you would normally expect to hear from a Harvard professor. I also think he sweeps various issues under the rug a bit, and presents a one-sided view on string theory (as opposed to Sean Carroll's views which are about as balanced and fair as one could hope to acheive in such a heated controversy). Nevertheless, I agree with Motl on his main point which is that string theory is undoubtedly falsifiable. I was somewhat shocked to see that Woit, who otherwise seems a reasonable person, chose to keep the title "Not Even Wrong" for his book when from reading his blog I get the sense that even he knows that this is at best a severe exaggeration. He has raised a fair number of concerns about string theory that might make sense, but surely this is not one of them. The only sense in which string theory might be called "neither right nor wrong" is if you're viewing it more as a framework, like quantum field theory, within which we construct new theories, rather than a single theory. Which is a perfectly reasonable way to view it. Other than this subtle qualification, it's pretty clear to me that string theory is well-defined and must be either right or wrong. Whether we will find any easy way of testing it for a while is another issue entirely, although there is a lot of ambiguity of what it means to "test" a theory. As Motl points out, there are some ways of looking at it, where one could argue it's passed several tests already. He states this as if it were a fact, but I'd like to emphasize that it depends on your definition of test which can mean different things in different contexts, and different things to different people.

What does it mean to test a theory? Naively, your first instinct might be to say "just see whether it has the right predictions or not!" However, that doesn't work here, because string theory has already predicted many of the features of the world, and so far gotten them all correct. While this is encouraging, most people would agree that you need more than just that in order to really test a theory. in order to really test a theory, you need a competing theory with different predictions to distinguish it from. However, in the case of string theory, it's the only one we've got so there is no competing theory. There are two objections to this statement I forsee: 1.) what about Quantum Field Theory + General Relativity (henceforth QFT+GR)?, and 2.) what about Loop Quantum Gravity, and other approaches to quantum graivty?

The second one is easier, so I'll handle that first: LQG and other approaches are not yet far enough along to be compared to string theory. They may predict the same things at the energy scales we can access right now, or even reduce to string theory entirely, or they may predict something completely different. We'll just have to wait and see. But the problem is not with string theory here, it's just that nobody is sure what the alternatives really say.

Regarding objection 1, "QFT + GR" isn't really a single theory, but rather two contradictory theories. String theory reduces to one in one limit, and the other in another limit. In either of these limits, the predictions of string theory agree with the predictions of QFT+GR. This is why the already confirmed predictions of string theory don't usually count--because any test you do in those regimes is just as much a test of QFT or GR, depending on which case you're in. When you go outside of those limits, QFT+GR becomes an invalid theory due to loss of consistency, whereas string theory holds up perfectly well. However, so far we've never been able to actually access the energies which would take us into the regime where the breakdown in GR+QM occurs. That doesn't mean it's impossible, or that we will never do that. It just means that, for now, our sole criteria for whether the theory is right is whether it's consistent in the currently inaccessible regimes. String theory is the only consistent theory of nature we know of, which is why it is funded so well. Of course, as with any statements like this, they are always a bit shaky... the proof that it is indeed consistent is to "physicists' standards" not to mathematicians' standards. The same is true for Quantum Field Theory, the most precisely tested and confirmed theory in all of history. We don't yet know how to axiomitize it, therefore a formal mathematical proof with all the steps filled in is out of the question. But usually when this happens in physics (and it does a lot) it's because we don't know exactly which assumptions we need to put in to help avoid the pathological "non-physical" cases that might make an otherwise true statement false under some conditions. Usually, these conditions are ones that nobody cares about, but you never know... occasionally they could turn out to be important. Which is why eventually we do need the mathematicians to finish the axiomitization projects. So for practical purposes we can say we "know" string theory is consistent, whereas for alternative theories it's more like we "guess" (or perhaps just "hope") that it's consistent. There are lots of fuzzy shades of grey in between these two, but my understanding is that string theory is a lot closer to the former category whereas other theories are much closer to the latter.

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spoonless
Oct. 8th, 2006 08:06 pm (UTC)

Of course it is consistent in some ways, if you decide certain parts of nature are ignorable.

The type of consistency I was referring to in that sentence is self-consistency, that it doesn't contradict itself anywhere. There is, of course, another kind of consistency... consistency with experimental data. But there is no known experiment that it's inconsistent with. The difference between Peter Woit and I is that Peter Woit, at least according to his mantra "Not Even Wrong" thinks that there is no possible way string theory could ever be inconsistent with the data, whereas I believe that it is possible... we haven't seen any such thing yet, but we may very well in the future.

But there's another issue here which Woit is correct on. Even though string theory is consistent with all known experiments (without ignoring any part of nature), that doesn't mean it predicts what the outcomes will be in all cases. And this is to many a disapointment, at least compared with the insanely high expectations some string theorists had at the beginning.

I think it should be noted, though, that none of the predecsessors of string theory has ever tried to predict all of reality from first principles before. This was a new kind of thing that people got excited about, because of the fact that there are no "free parameters" in pure string theory. However, it's become fairly clear by now, that in order to get anything useful out of it you have to choose certain things to match our local environmental conditions, such as the shape of spacetime. While some string theorists of the old guard may hold on to the hope that this will just pop out of the theory, I would say "sorry, ain't gonna happen".


nobody is sure what the alternatives really say.
having said what i said above, this comment could arguably be made about string theory as well.

Yes, I'll give you that. We do have a better idea in the case of string theory. But as you say, this could just be because it's had more time to develop.

is having an incredibly hard time accomodating and explaining the slightly larger than zero value of the cosmological constant.

I personally think that Steven Weinberg has already sufficiently explained the value of the cosmological constant with his anthropic calculation... and I've offered a public wager previously to anyone who thinks we will find a better explanation, but nobody has taken me up on it. String theory doesn't explain it at all, but it can definitely accomodate it as KKLT have shown in their famous paper which started all the buzz about the Landscape.

String theory also has this problem of having an infinite number of solutions and no way of telling us which one to pick out.

There are a some people in the field who think it is only a finite number, and there are some who think it may be infinite. Kindof depends on who you believe at this point. Regardless of which it turns out to be, I'll note again that every theory in the history of physics prior to string theory has had an infinite number of solutions. The one with the least amount of continuous adjustable parameters was the Standard Model, with only 20-something tunable parameters (of course, it doesn't include gravity, inflation, dark matter, etc. which would involve even more). This gives rise to an infinite number of solutions... and the only way to fit our universe is to just measure those parameters and set them to the measured values. At the offset, string theorists had the audacity to suggest that they could predict all of these parameters from first principles. But in retrospect, to me at least, that was a foolish and arrogant hope. Some of them are probably random and some may be what they are for anthropic reasons, but it doesn't seem likely to me that any theory will predict all of them from first principles. The only reason it appeared that way is because of the specific features of string theory--namely, that if you ignore the question of choosing a background, there are no free parameters in the theory.
spoonless
Oct. 8th, 2006 08:19 pm (UTC)

At the offset, string theorists had the audacity

meant to type "at the outset". However even that isn't really correct... I should say "at some point in the 80's, after people realized certain things about string theory and got a bit overexcited."
spoonless
Oct. 8th, 2006 08:26 pm (UTC)

i guess when i posted my entry on Cosmic Variance, you'll see my thoughts on how truly idiotic it is to homogenize what people are working on before you're even sure who is right and who is wrong.

I'm looking forward to read it.
onhava
Oct. 8th, 2006 09:41 pm (UTC)
I'm really getting exasperated with this debate. Personally, I think there are real sociological criticisms to make of high-energy physics, but they're certainly not the simple-minded nonsense people keep repeating all over the internets. And I find it kind of odd that, after arguing that string theory doesn't make concrete predictions about the real world, the recommended ways of dealing with this from Woit and Smolin are respectively "let's go think really hard about representation theory and the Dirac operator!" and "let's pursue an approach that isn't known to give flat space and doesn't appear to address the issue of renormalizability!" It's not that one might not get somewhere by pursuing those paths, but it's pretty obvious neither of them is talking about an idea that will clearly relate to the real world in the near future, if ever.

It's also clear that the dynamics of the Landscape are terribly understood, and I have no idea whether a better understanding of them could help with predictivity. The more I learn about eternal inflation, the more I'm fundamentally skeptical that it makes any sense as a mechanism for populating the Landscape. Hartle-Hawking sorts of ideas might turn out to be more relevant and predictive. Or something else. Who knows? No one, as far as I can tell, which is why we certainly shouldn't abandon the field now.

My exasperation has been driving me to waste too much time posting annoyed and/or snarky anonymous comments on blogs, so I should probably just stop reading physics blogs for a while.
spoonless
Oct. 8th, 2006 10:19 pm (UTC)

The more I learn about eternal inflation, the more I'm fundamentally skeptical that it makes any sense as a mechanism for populating the Landscape. Hartle-Hawking sorts of ideas might turn out to be more relevant and predictive. Or something else. Who knows?

I'm glad to hear you say this, because it makes me think "good, I'm not the only one!" I've been secretly hoping ever since people started talking about the Landscape that we will one day use Hartle-Hawking (or some other kind of Everett-style universal wavefunction) to populate it. I wonder why there has been so much attention on eternal inflation when the quantum multiverse in contrast has a perfectly well-defined measure on it which should make it much easier to get predictions out. I guess the problem is that most people are far less convinced than I that that sort of multiverse even exists... or maybe they just feel more comfortable talking about an eternal-inflation style multiverse.

I was pleased when this paper came out, but it didn't get much attention. Unfortunately, I still have a lot more to learn before I can really contribute to the technical discussion on these types of issues, or understand exactly what they're doing and how.

My exasperation has been driving me to waste too much time posting annoyed and/or snarky anonymous comments on blogs

yeah. I'm amazed at the level of confusion that gets involved here, even among physicists, but especially in the media... although I suppose that's nothing new. I wouldn't worry too much, though... this should blow over eventually. Even if a few important physicists have to take some time out from their career to explain to the right people what is going on here.
onhava
Oct. 9th, 2006 01:32 am (UTC)
Yeah, Hartle-Hawking is something I'm aware of because of stuff people here have done (e.g. this paper). I get the impression the concept is fairly popular at Harvard also, with all this OSV stuff I don't understand....

Recently we've had some weekly group meetings here to review lots of stuff about black holes, inflation, and so on, which is giving me a good incentive to dig into some of the older literature more. Last week I reviewed Coleman/deLuccia for the group, which I learned a lot from. I've also been reading some of these old Guth papers on bubbles of false vacuum, which have me pretty confused: evidently from the outside they look like black holes, but then what happens when those black holes evaporate? It looks like back then they thought the baby universe just pinches off and it's no problem, but given the current point of view on black hole information I'm not sure that makes sense?

That Hawking / Hertog paper is in the stack of stuff I'm wanting to read and understand soon, but probably I'll have to go back and learn more background first.

Anyway, it's all a lot of fun, and while I'm not going to completely give up working on QCD and collider physics anytime in the foreseeable future, I would like to be able to work on this sort of thing too, or at the very least keep up with the technical details of what's going on. Maybe I'm just being overly optimistic, but I feel like if we really understand quantum gravity at a deeper level it will probably become apparent that there are feasible experimental tests that no one has conceived of before.
spoonless
Oct. 9th, 2006 05:18 am (UTC)
Sarangi/Tye paper looks great, I'll put that on my list to read. I'm surprised all this stuff avoided my detection, I guess it must just be because eternal inflation is really big over here on the west coast. Everybody loves Linde.

There's a lot of work going on here about Coleman-deLuccia and bubbles of various things. I follow some of it, but I don't know the answer to your question about what happens when the black hole evaporates. Pinching off certainly doesn't sound right to me; I don't know.

while I'm not going to completely give up working on QCD and collider physics anytime in the foreseeable future

The stuff you're doing is pretty bad ass, I wouldn't feel too left out. I wish I knew more about AdS/QCD and holography, what an incredibly rare combination of fun and usefulness/testability!
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