Saturday, January 21, 2006

Who is an LQG expert

Peter Woit has made some strange comments about the recent review of LQG by Nicolai and Peeters. First of all, he dismisses Nicolai and Peeters for their knowledge of string theory; it should not be terribly surprising. Second of all, he expects them to repeat his own outsider misconceptions what string theory is and what string theory is not. Dear Peter, if Nicolai and Peeters were writing the same "material" about string theory as you do, then they would become the same high-energy physics ignorants as you are.

But there is one more comment that is pretty typical not only for Peter but also for others who have no factual arguments and who replace them by misleading ad hominem descriptions of the inconvenient thinkers:
  • ...I’m curious to hear from experts what they think of this article...
Whom does Peter exactly mean by the word "expert"? I suppose he means an LQG expert. Who is an LQG expert? According to my understanding, an LQG expert is someone who has learned the formalism and who is able to do research of these concepts and reach some conclusions, especially if other physicists find the research relevant.

As far as I can see, the previous paper about LQG by Nicolai, Peeters, and Zamaklar that we discussed here is by far the most cited LQG paper written in 2005. Is there some way to justify that these people are not experts? I don't think so. You may define experts as those who dogmatically insist on some Holy Scripture written by someone else in the past. But in this particular case, the Holy Scripture is known to be at least partially flawed and important analyses are missing in it altogether.




For example, LQG does not solve the UV problems with the infinite amount of undetermined higher-derivative terms; it just replaces them by an infinite number of unknown terms in the Hamiltonian constraint or parameters of the spin foam Feynman rules. Everyone who has a basic understanding of the physical concepts knows that the reason is quite clear and Peeters and Nicolai's arguments are robust. Peter tries to suggest that this particular serious flaw of LQG is analogous to string theory in some mysterious way. Of course it is not and only physics ignorants could buy such a statement. String theory does solve this problem and predicts a clear structure (and values) of all higher-derivative terms, including those in the gravitational sector. This is exactly what we mean by the well-established statement that string theory solves the UV problems of quantum gravity.

Similar observations hold for the ultralocality of LQG, a fatal flaw that makes the kinematical Hilbert space non-separable and prevents ordinary notions of continuity from appearing in the LQG framework. Of course that string theory does not suffer by any of these problems either. Its Hilbert space(s) is (are) always separable and continuity - in fact, the exact physics of GR and QFT - follows from string theory at low energies.

We could go on and on and on. String theory reproduces special relativity; LQG does not. String theory is consistent with the existence of other forces and fields (which moreover seem necessary not only according to the experiments but also because of internal consistency of quantum gravity) and it in fact predicts them; LQG does not. There is no "analogy" or "equality" and whoever thinks that for every argument against LQG, there must exist an argument against string theory (that's probably the ultimate approach of political correctness), is crazy.

Peter may criticize that string theorists investigate the "landscape" of solutions and he may dislike the large number of discrete solutions. But regardless whether we understand the landscape, its size and its interpretation correctly, string theory is the first theory we ever had (and the only theory we have as of 2006) that allows us to address the question about "possible Universes" at all. In all other "theories" (including LQG), the space of possibilities is only bounded by our imagination and the time we invest to write down new terms. The space of possibilities is an infinite-dimensional continuum. In string theory, the discrete set of possibilities is made out of solutions to rather well-defined rules and can be classified. This ability to discriminate what is possible and what is not is desirable; in fact it is necessary if we want to argue that we have mastered quantum gravity. It is because in quantum gravity, we can in principle get from any "phase" to any other "phase" (think about cosmology that has no superselection sectors) and a complete theory simply must know about all these "phases"; in the stringy jargon, I really mean the "backgrounds".

Should we really use the word "expert" exclusively for those who misunderstand these basic points? Do we really want to return to the Middle Ages when the quality of scholars and their teachings was determined by the degree of their dogmatic literal belief in the Bible (or Rovelli's book or what precisely plays the role of the Bible in this context)? Should we dismiss people as non-experts just because they know evolutionary biology or string theory - or they are sinners in yet another way? Sorry if you don't like to hear it but Nicolai and Peeters are among the leading LQG experts in the current world and your humble correspondent belongs to the group that follows after them.

I confirm that the paper not only covers the most essential material about LQG that we know but it also lists fully legitimate issues with LQG that would have to be resolved before LQG could be claimed to be a promising candidate theory of quantum gravity.

There is a significant asymmetry in the relation between string theory and LQG, of course. It is very likely that no "LQG specialist" would be able to write a meaningful review of string theory that others could cite. It's simply because LQG is, in comparison with string theory, a naive game for children. Quantum gravity is a pretty specialized field and it is just a wrong idea to try to isolate separate branches that should not talk to each other. People who are quantum gravity experts - such as Nicolai and Peeters - are just naturally interested in all these questions that LQG and other approaches try to answer. So am I. And they reach certain conclusions. It is not surprising that some people whose technical errors and oversimplified and flawed assumptions are revealed don't necessarily like these developments. But it is absolutely crucial for science to move on.

If there is any controversy, it is not really between string theory and LQG. It is between narrow-minded physicists who don't want to study the insights found by others and who want to be confined in prisons defined by hundreds of their mostly incorrect assumptions - especially the assumption that the world must be a version of LEGO and everyone who thinks otherwise must be wrong because the world surely can't be more complicated than LEGO; and those who do study physics without prejudices, who are able to change their opinions and ideas by looking at the evidence obtained by others, who are ready to eliminate conjectures that have been falsified, and who actually make some progress.

And that was the memo.

8 comments:

  1. Dear Lubos,

    "We could go on and on and on. String theory reproduces special relativity; LQG does not. String theory is consistent with the existence of other forces and fields (which moreover seem necessary not only according to the experiments but also because of internal consistency of quantum gravity) and it in fact predicts them; LQG does not." - Lubos Motl

    Special relativity not exactly a defence of string theory as general relativity is quite different, and anyway special relativity conflicts with quantum field theory:

    Quantum field theory is moving towards an ether picture of the Feynman path integral, due to problems with renormalisation in the purely abstract mathematical model. See arXiv: hep-th/0510040 p85, the virtual particles in the vacuum contradict special relativity and imply a Dirac sea as: 'it is not possible anymore to define a state which would be recognised as the vacuum by all observers'. There is evidence that 'string theory' is hogwash as it makes no testable predictions, but its major rival is the 'spin foam vacuum' of loop quantum gravity, which again is a Dirac sea.

    General relativity is entirely different to special/restricted relativity:

    EINSTEIN REPUDIATED SPECIAL RELATIVITY AS FOLLOWS:

    ‘The special theory of relativity … does not extend to non-uniform motion … The laws of physics must be of such a nature that they apply to systems of reference in any kind of motion. Along this road we arrive at an extension of the postulate of relativity… The general laws of nature are to be expressed by equations which hold good for all systems of co-ordinates, that is, are co-variant with respect to any substitutions whatever (generally co-variant). …’ – Albert Einstein, ‘The Foundation of the General Theory of Relativity’, Annalen der Physik, v49, 1916.

    You can get the equations of "special relativity" from electrodynamics as Lorentz and others did before Einstein. Yes Einstein was the biggest genius of all time, but he ADMITTED SPECIAL RELATIVITY DOESN'T APPLY TO A REAL WORLD WHERE THERE ARE ACCELERATIONS (SEE QUOTATION ABOVE). Which is why general relativity is different. At best, the EQUATIONS of special relativity are the same as those of the correct theory; at worse they are completely wrong and lead to paradoxes by not including acceleration effects. You cannot rely on the framework of special framework since all real motion involves acceleration and thus force.

    It is really to be expected that the same people who admire Einstein's early errors as if they were better than his major work in general relativity, are the people who also work on string theory. It is not coincidence. You are just brainwashed bigots. You can't see that physics tied to facts, such as FitzGerald and Lorentz's approach to the equations of "special relativity" a decade before Einstein are MORE BEAUTIFUL PHYSICALLY, because they are connected to reality.

    Instead, you ignore the reality of general relativity and the spin foam vacuum which seems to connect the experimental facts of gravity with those of quantum field theory, and you instead try to connect 10/11 dimensional speculation with spin-2 gauge boson (graviton) speculation, by way of unobservable string speculation.

    You think speculation is beautiful because the level of maths, or what you call maths, is so technically involved it will not be understood or checked by outsiders. The whole approach of you big mouthed crackpots is insulting, speculative, trivial, unconnected to any reality, and frankly insane.

    Grow up!

    Nigel

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  2. Dear Nigel,

    I have been amazed twice by statements of some other readers that you seem to know something about physics.

    Saying that quantum field theory contradicts special relativity is an example of a breathtaking ignorance, and be sure that many of us knew that it is stupid already at the high school.

    Quantum field theory IS the framework to reconcile multi-body quantum mechanics with special relativity. It is its very purpose, and it does it.

    I hope that you don't want to read me the rest of your crap.

    Best
    Lubos

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  3. This comment has been removed by a blog administrator.

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  4. Sorry Nigel, could now you try to apply your "knowledge" elsewhere where they appreciate it? Go to Peter Woit's blog, for example.

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  5. Lubos:

    Nigel clearly does NOT understand physics. I keep pointing out to him why his "377 Ohm space" is totally wrong but he could never understand. I do not know who is the one that twice told you that Nigel knows something of physics. I hope you did not mean me because I have never made such a statement.

    You are absolutely correct that QFT is the result of QM plus SR. But you are really stretch it by saying that string theory leads to SR. I keep hearing string theorists claiming that SST leads to GR because it predicts graviton. But I never heard any one claiming that you can derive SR from SST. With due respect, please show me how SST leads to the notion that light speed is invariant, or how SST will allow you to derive E=MC^2.

    The universe is indeed made of legos of sort, because we the observers, as well as all our observation instruments are made of individual legos: our brains contains a finite and definite number of neuron cells, approximately 15 billion of them. So our intelligence would not allow us to comprehend anything that requires memory capacity beyond 150 billion neurons to comprehend. You can observe that colonies of insects like bees or ants demonstrate an amazing collective intelligence, but there is no hope you can teach an ant about super string theory, or use some futurist devices to download the knowledge into an ant's brain. Because their brains simply have too small a capacity. Our brains are larger, but it's only a quantitative difference from insects.

    To us humen who are made of legos, the universe looks like legos and feels like legos, anything that's more complicated than a lego-ed structure, is beyond our observational and comprehensive capability of our lego brains, and thus is irrelevant. For all purposes that is relevant and verifiable, the universe is a lego universe, evidence being that everything we look at more or less behave quantum mechanically, not classical.

    Quantoken

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  6. Quantoken,

    You aren't an electronics engineer, so you may have an excuse for being ignorant about Maxwell's vacuum "displacement current (in the equation for curl.B) having 377 ohm (= permeability times c) impedance.

    Last time you claimed it is dimensionless, which is false. Learn about the vacuum properties before asserting opinions:

    ‘The special theory of relativity … does not extend to non-uniform motion … The laws of physics must be of such a nature that they apply to systems of reference in any kind of motion. Along this road we arrive at an extension of the postulate of relativity… The general laws of nature are to be expressed by equations which hold good for all systems of co-ordinates, that is, are co-variant with respect to any substitutions whatever (generally co-variant). …’ – Albert Einstein, ‘The Foundation of the General Theory of Relativity’, Annalen der Physik, v49, 1916.

    Quantum field theory involks the Dirac sea. ‘… with the new theory of electrodynamics [vacuum filled with virtual particles] we are rather forced to have an aether.’ – Paul A. M. Dirac, ‘Is There an Aether?,’ Nature, v168, 1951, p906.

    Thanks,
    Nigel

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  7. Nigel:
    You are just an electric engineer. No wonder you don't know physics. The Maxwell Equation is taught in kindergarten so every one who has average intelligence knows.
    Your 377 Ohm Vacuum comes from this fact:
    377 = U0 x C
    = 4*PI*10^-7 x 2.99792458^8

    The magnetic constant, 4*PI*10^-7, is certainly an artificial number, choosen by no reason of natures but certain physicist liked the number. So the 377 number is just an artificial number which bears no secret of the nature. Now go back to the drawing board and re-work your crackpot theory.

    Quantoken

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  8. Quantoken,

    I don't use 377 ohms on my site in any argument or calculation, and I'm not an 'electric engineer'. I don't have a theory, I've just put a few facts together. You are just repeating what I said, uc = 377 ohms.

    There are 4 electromagnetic constants based on two measurements, so 50% are always calculated:

    c = 1/(root of product of permeability and permittivity)

    Z = product of permeability and c

    From these relations and algebra yu see 4 numbers.

    I can measure u by measuring magnetic force, as it appears in the magnetic force equation.

    I can measure the impedance 377 ohms using a sampling oscilloscope and a cable.

    I can measure speed of light or electric TEM wave propagated by two conductors in a vacuum or other medium.

    I can measure permittivity as it appears in Coulomb's law.

    The reason why in the rationalised system of units since 1960, u is defined as the value you specify is simply that magnetic field and hence magnetic constant u derive from electric current, and 1 Amp was defined around 1960 as the amount of current producing a magnetic force of 1 Newton between two infinitely long conductors 1 metres apart...

    ‘… the innovator has for enemies all those who have done well under the old conditions, and lukewarm defenders in those who may do well under the new. This coolness arises partly from fear of the opponents, who have the laws on their side, and partly from the incredulity of men, who do not readily believe in new things until they have had a long experience of them. Thus it happens that whenever those who are hostile have the opportunity to attack they do it like partisans, whilst the others defend lukewarmly…’ - http://www.constitution.org/mac/prince06.htm

    ‘(1). The idea is nonsense. (2). Somebody thought of it before you did. (3). We believed it all the time.’ - Professor R.A. Lyttleton's summary of inexcusable censorship (quoted by Sir Fred Hoyle in ‘Home is Where the Wind Blows’ Oxford University Press, 1997, p154).

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