This week I’m quite busy so I’m taking a break from landscape-bashing. Instead I’ll just quote someone else; it’s up to you to guess who.
“Suddenly it’s not too important whether a theory teaches us something new about the real world – either predicts new unknown phenomena or previously unknown links between the known phenomena and objects. It’s more important that such an unpredictive scenario might be true and we should all work hard to show that the scenario is plausible because we should like this scenario, for some reasons that are not clear to me.”
“The anthropic strategy is to pick as complicated Calabi-Yau manifolds as possible, to guarantee that there will be a lot of mess, confusion, and possibilities, and that no predictions will ever be obtained as long as all the physicists and their computers fit the observed Universe… This means that you don’t want to start with Calabi-Yaus whose Betti numbers are of order 3. You want to start, if one follows the 2004 paper, with something like F_{18}, a toric Fano three-fold. That’s a 3-complex-dimensional manifold that is analogous to the two-complex-dimensional del Pezzo surfaces, in a sense. But you don’t want just this simple F_{18}. You take a quadric Z in a projective space constructed from this F_{18} and its canonical bundle. OK, finally the Euler character of the four-fold X is 13,248. Great number and one can probably estimate the probability that such a construction has something to do with the real world.”
“Do we really believe that by studying the orientifold of the weighted projective space CP^{4}_{[1,1,1,6,9]}, we will find something that will assure us (and others – and maybe even Shelly Glashow) that string theory is on the right track? … If we deliberately try to paint the string-theoretical image of the real world as the most ambiguous and uncalculable one, I kind of feel that it’s not quite honest.”
“Some people used to blame string theorists that they were only looking for the keys (to the correct full theory) under the lamppost. It’s unfortunately not the case anymore: most of the search for the keys is now being done somewhere in the middle of the ocean (on the surface). Maybe, someone will eventually show that the keys can’t stay on the surface of the ocean, and we will return to the search for the keys in less insane contexts.”
Of course, I can be wrong. It is obvious that all man is wronmg some time, but I follow the philosophy of sending copies of my papers to people that is directly critiqued on my work.
I sent a copy of Is this dynamics? to Weinberg and also to other people, specialist in different topics that I studied, and nobody find failures on the mathematical derivations. On specialist on molecular dynamics did a three page reply manuscript but I think that I show that his criticism was not satisfactory.
Weinberg found no failure.
My philosophy of scientific research is different and i am not proud of making mistakes.
Your philosophy is very strange for me but i respect it!!
Juan R.,
If I make the same mistakes as Weinberg, I’m proud of it.
According to the poster of 26April@1:44PM it was pointed out in 1957 by Wigner and Salecker that in QM there are no perfect clocks.
Ideal classical time evolution is unphysical (just a convenient approximation) and at a more fundamental level is replaced by correlations between observationscorrelations between readings on real quantum clocks (which have finite lives and do not necessarily agree) and other observables.
The poster cited 3 recent papers from Phys. Rev. Letters. I am wondering if anyone has looked at them? I found the arxiv numbers of the preprints and posted them also on 26
April in case anyone might wish to have a look.
I looked at the papers and I believe one of the main points is that if one describes time evolution by real-world clocks then time evolution can not, for theoretical reasons, be unitary. There is an interesting theoretical bound on how precise, and at the same time long-lived, a real clock can be.
Here is an exerpt of the earlier post:
“Unitarity is not needed to conserve probabilities. The existence of an ‘ideal classical’ time in which physics is unitary is simple ‘wrong’ within any QG framework. As emphasized originally by Salecker and Wigner (Rev.Mod.Phys. 29 (1957) 255) and more recently by Ng(Mod.Phys.Lett. A9 (1994) 335) and Camelia (Mod.Phys.Lett. A9 (1994) 3415-3422), there exist limits in nature to how ‘classical’ even the best possible clock can be. When one introduces realistic clocks, quantum mechanics ceases to be unitary and indeed, a fundamental mechanism of decoherence of quantum states arises…”
Thomas Larsson wrote:
“You cannot give up unitarity without making a drastic modification of quantum mechanics. I didn’t realize that this was controversial.”
It is not controversial that giving up unitarity will require modification of quantum mechanics. It is controversial that giving up unitarity means probabilities will not longer add up to 1. Recall –
“Unitarity encodes the idea that all probabilities must add up to unity. Although the technical requirement of unitarity might perhaps be relaxed in some way, I don’t understand how this underlying idea could be wrong. At some point Dirac had some strange ideas about probabilities taking values between -2 and +2 (according to Dyson), but that evidently didn’t work out either. ”
In the Copenhagen interpretation of QM, measurements do not preserve unitarity. If all time evolution is indeed unitary, i.e., measurement itself is described in QM, then QM is able to provide excellent approximations to non-unitary time evolution. Maybe, just like free quarks are unobservable, unitarity is not directly observable when gravity plays a role.
Canonical science shows that unitarity is an approximated concept.
Unitarity –> conservation of probability and conservation of norm for closed systems.
But the inverse is not true. One can maintain non-unatirity with conservation of probability and well defined quantum states.
non unitary evolution is responsible for collapse of wave function and also for the increase of entropy (second law).
Usual mathematical proofs on the adequacy of unitary are simply wrong. E.g. Chapter three of Weinberg QFT is full of mathematical mistakes. I demonstrated time ago (manuscript sent to Weinberg) that there at least three mathematical mistakes in Weinberg derivation of master equation.
I had pure states in my mind; I haven’t thought about mixed states for a long time and really don’t want to say anything about them. However, unitarity simply means that the norm and hence the total probability is preserved, so you cannot give up unitarity without making a drastic modification of quantum mechanics. I didn’t realize that this was controversial. That it is difficult to define an invariant inner product without a background Minkowski metric is another matter.
One way to ensure unitary time evolution is to have a unitary rep of some symmetry group containing time evolutions as a subgroup, e.g. the Poincare or diffeomorphism groups. However, the only unitary, proper rep of the diffeomorphism group is the trivial one, so there seems to be no time evolution at all in quantum gravity; this is one version of the problem of time. One can avoid this paradox, at least in 1D, by allowing for projective reps, since the Virasoro algebra has many non-trivial unitary reps with positive central charge. The only alternative is apparently to formulate quantum gravity holographically using some sort of AdS/CFT, but that idea has serious problems since AdS is ruled out experimentally.
Thomas – I think I would say unitarity amounts to the possibility of setting up an isolated system and assigning it a wave function.
-drl
Arun said:
I thought unitarity encodes such a rule against probability amplitudes. Any process where a quantum mechanical pure state evolves to a mixed state would violate unitarity.
Well that’s the whole point – a system that is fully described remains so. “Describe” means to give an exhaustive accounting of possibilities of measurements. Unitarity means new possibilities don’t just appear or disappear from the blue. Insofar as quantum theory has a variational principle, something *has* to have this property.
-drl
Thomas Larsson wrote :
“Unitarity encodes the idea that all probabilities must add up to unity.”
I thought unitarity encodes such a rule against probability amplitudes. Any process where a quantum mechanical pure state evolves to a mixed state would violate unitarity. Unitary time evolution cannot increase entropy, can it?
“…exist limits in nature to how ‘classical’ even the best possible clock can be. When one introduces realistic clocks, quantum mechanics ceases to be unitary and indeed, a fundamental mechanism of decoherence of quantum states arises.
A concrete model has been recently put forward in discrete quantum gravity
(Phys.Rev.Lett. 90 (2003) 021301) which also has the appealing consequence of ‘solving’ the BH information paradox. (Phys.Rev.Lett. 93 (2004) 240401)
These ideas, including a consistent discretization of gravity, can be also applied to loop geometry (Phys.Rev.Lett. 94 (2005) 101302).”
I looked up the preprints for these articles
(Phys.Rev.Lett. 90 (2003) 021301) preprint is: gr-qc/0206055
(Phys.Rev.Lett. 93 (2004) 240401) is: hep-th/0406260
(Phys.Rev.Lett. 94 (2005) 101302) is: gr-qc/0409057
“Unitarity encodes the idea that all probabilities must add up to unity. Although the technical requirement of unitarity might perhaps be relaxed in some way, I don’t understand how this underlying idea could be wrong.”
Unitarity is not needed to conserve probabilities. The existence of an ‘ideal classical’ time in which physics is unitary is simple ‘wrong’ within any QG framework. As emphasized originally by Salecker and Wigner (Rev.Mod.Phys. 29 (1957) 255) and more recently by Ng(Mod.Phys.Lett. A9 (1994) 335) and Camelia (Mod.Phys.Lett. A9 (1994) 3415-3422), there exist limits in nature to how ‘classical’ even the best possible clock can be. When one introduces realistic clocks, quantum mechanics ceases to be unitary and indeed, a fundamental mechanism of decoherence of quantum states arises.
A concrete model has been recently put forward in discrete quantum gravity
(Phys.Rev.Lett. 90 (2003) 021301) which also has the appealing consequence of ‘solving’ the BH information paradox. (Phys.Rev.Lett. 93 (2004) 240401)
These ideas, including a consistent discretization of gravity, can be also applied to loop geometry (Phys.Rev.Lett. 94 (2005) 101302).
all the best
String theorists are still looking under the lamp post, in that I have not found many willing to concede that the universe might not be unitary. Loop quantum gravity theorists are much more open to non-unitarity.
Unitarity encodes the idea that all probabilities must add up to unity. Although the technical requirement of unitarity might perhaps be relaxed in some way, I don’t understand how this underlying idea could be wrong. At some point Dirac had some strange ideas about probabilities taking values between -2 and +2 (according to Dyson), but that evidently didn’t work out either.
Anyway, the LQG people evidently have a unitary rep of their C* algebra, called the Ashtekar-Isham-Lewandowski representation, so I don’t think that they really give up unitarity. Unfortunately, this rep is not of type usually encountered in quantum theory, and apparently it leads to all kinds of trouble, like the energy not being bounded from below in the harmonic oscillator.
Being¨very skeptical about all kinds of new ideas, I would like quantum gravity to keep all the key concepts in quantum theory and general relativity, i.e. unitarity, locality, causality and general covariance. There is of course a no-go theorem forbidding the combination of locality and diff invariance (“there are no local observables in quantum gravity”), but this is evaded by allowing for projective reps of the diffeomorphism group (this was a major motivation for finding these). With a positive cosmological constant, I don’t really see what the alternative to locality could be, since there is no S-matrix and no good boundary on which a boundary theory could live.
For a while during the development of computer science, a ridiculously large fraction of the incoming students wanted to do the highly hyped field of Artificial Intelligence, while the number leaving with a thesis in AI was much smaller. They arrived, realized that AI was to a great extent a scientific failure, and changed fields at some point during their graduate career.
I spent a couple of years in my youth working for a startup company whose product was an implementation of the AI language Prolog. Like the rest of the AI industry, this company went bankrupt after the demise of the Japanese Fifth Generation project. We had a rather cool motto, though: Artificial intelligence is better than none.
“For those physicists forecasting doom and gloom, missteps of artificial intelligence don’t seem to have hurt the larger discipline of computer science much. ”
Well, then, what do the Japanese have to say about that (re: The Fifth Generation Project)…if the parallel is to hold such that an deliberately overly ambitious grandious program leads to unforseen benefits along the way.
Did the Japs benefit or not?
I hope nobody’s telling that narrative, because it certainly isn’t true. String theory is simply the best guess out there for something that unifies gravity and QM. It certainly does not come close to following from just naively attempting to unify QM and GR; it’s origin is much more convoluted.
I should also mention that the counternarrative by some LQG-types that string theorists are closeminded about other directions simply isn’t true. I came into grad school well-versed in these myths and was disabused of them. This is not to say that everyone knows everything about the flaws in the other approachs, but the idea that LQG (as the only other major example, really) is rejected simply out of prejudice is just plain wrong.
Perhaps Lubos’ terminology of the “haystack” (instead of the “landscape”) is more telling sign.
String theorists are still looking under the lamp post, in that I have not found many willing to concede that the universe might not be unitary. Loop quantum gravity theorists are much more open to non-unitarity.
I think this is because there’s a narrative (I can’t judge how true it is) that string theorists tell. This is that they start with the hypothesis that physics is unitary, is consistent with general relativity and quantum field theory, and they are led inexorably to a ten- or eleven-dimensional world with string theory. But if it is true, then at some point, if they start coming to more and more unbelievable conclusions, they might want to start questioning their hypotheses.
For graduate students, the relevant number is not the number of students going to grad school determined to do string theory, but the number of students leaving grad school having done string theory. For a while during the development of computer science, a ridiculously large fraction of the incoming students wanted to do the highly hyped field of Artificial Intelligence, while the number leaving with a thesis in AI was much smaller. They arrived, realized that AI was to a great extent a scientific failure, and changed fields at some point during their graduate career. In the defense of AI, let me say that since then it has reinvented itself, and now has much more modest goals which it is (to a large extent) truly succeeding in accomplishing.
I leave the parallels with string theory to the readers. For those physicists forecasting doom and gloom, let me note that the missteps of artificial intelligence don’t seem to have hurt the larger discipline of computer science much.
String theorists and Creationists give the anthropic principle a bad rap, which gets compounded by its only being a truism that’s necessarily as incomplete as Dirac’s Large Numbers hypothesis was flawed.
Tack on the entropic interpretation and you’ve got good reason for it if the universe is ‘closest to Einstein’s hand”
Based on the typical actions of humanity tho… it’s the only thing keeping us alive!… 😉
Arun – that was a well-turned phrase 🙂
-drl
It’s worth remembering that even a physicist as great as Wolfgang Pauli was moved to despair at certain points in the development of quantum mechanics. During one such episode he declared that he wished he had become a magician instead of a physicist.
JC: I don’t have any particular stats on the number of grad students entering ST… however, something that i have been noticing over the years is that more and more folks are coming into grad school with their minds set into ST; as if there was nothing else in physics other than ST. That is, apparently these folks have “bought” the “marketing” without knowing the “merchandise” well enough.
Anybody have any statistics on the number of graduate students entering string theory these days?
It would be interesting to compare it to the situation in the mid-late 1990’s and/or mid-late 1980’s.
Two more Lubos quotes:
If string theory is gonna require an infinite sequence of refinements and increases of the complexity (and decreases of expected predictivity) of its vacua to match reality, then I would definitely love to know this fact as soon as possible, because in such a case I would consider string theory to be a wrong theory of physics – much like other incorrect theories in the history of science that were first corrected 100 times before they were abandoned (recall Lorentz’s explanations of Morley-Michelson experiments) – and people should move on.
Of course I don’t believe that this is correct, but allowing something like that for a theory *does* mean to construct a scientifically unjustifiable theory.
More frustratingly, using the words of Steve Shenker, AdS/CFT now says that nearly every QFT may be assigned a “gravitational dual” – i.e. every quantum theory is, in some loose sense at least, a theory of quantum gravity (in a higher-dimensional space). We definitely don’t want to be *this* broad because then the term “quantum gravity” would become pretty vacuous.
Dan Friedan was quiet for decade before he declared that string theory was a complete scientific failure. Lubos hasn’t published for a year. Let’s see what happens in nine years.
science will not be on a firm footing until it rocognizes the primacy of consciousness. All we experience is the awareness that cognizes shapes and colors, and the stories we tell ourselves about those shapes and colors. The stories are always changing, the shapes and colors are always changing. Only the awareness abides.
Science survived Sir Isaac Newton. It will survive the anthropic principle and superstring theory.
Somebody asked:
Is Lubos starting to doubt? Is he experiencing a conversion moment?
Nah, but he hates the anthropic principle enough to put his foot right into his own mouth. He thinks he’s got a new and better idea:
http://motls.blogspot.com/2005/02/entropic-principle.html
… that he got from here:
http://www.arxiv.org/abs/hep-th/0502211
So now he’s decided to jump on a bandwagon of denial instead, because the anthropic coincidences don’t just go away because of this, and an “entropic” anthropic principle requires that the forces of the universe be constrained to produce intelligent life as a special and integral contributor to the thermodynamic process.
The probability that the cosmological evolution will end up as a Universe with a particular shape of the hidden dimensions (and particular values of the fluxes) is determined by the (exponentiated) entropy of a corresponding black hole whose geometry flows via the attractor mechanism to the given shape of the Universe near the horizon. Note that this contrasts sharply with the “anthropic principle” – which itself is not a principle, rather a lack of principles. In the anthropic principle, the corresponding probabilistic weight is determined by the ability of the Universe to support intelligent life.
Not quite, since Black holes and humans share a comparitively uniqueness when it comes to the entropy of the universe, since both can isolate the release of enough energy to make real particles from the negative energy of the vacuum, which serves to reverse the normally destructive consequences of the second law of themodynamics on a grand scale, while directly affecting the symmetry of the universe.
What they’ve actually managed to accomplish is to identify yet another of the vast and growing number of cosmic coincidences that define the many ecosystematic balances that are common to the anthropic principle, but in this context it’s become a “biocentric” principle, where intelligent life is now more probably required on most every banded sprial galaxy that has a black hole at its center.
Way to go, Lubos, et. al… you’ve turned it into an epidemic… 😉
I hadnt seen the most recent Lubos blog.
It’s all there. Sorry for being dense
http://motls.blogspot.com/2005/04/kennedys-landscape.html
everybody seems to know or assume that the quote is from Lubos Motl.
I didnt happen to see it on his blog or on the web anywhere.
can anyone give a link to where the quote can be found?
thx
Is Lubos starting to doubt? Is he experiencing a conversion moment?
On other hand, excessive landscape-bashing can derive on empiricism-bashing.
Lubos must be gratified: (sincere) quotation is the sincerest form of flattery
I guess Cumrun Vafa (basically because I admire him and this quote is admirably honest)
Lubos Motl. These are remarkable thoughts indeed.