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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.
"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.
