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Bee's top ten

  1. Jul 8, 2006 #1

    marcus

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    there has been a rumbling of lists in the Bloggery and Bee rose to the occasion at Backreaction http://backreaction.blogspot.com/
    by presenting her LIST OF STUPID TITLES of professional articles on arxiv
    http://backreaction.blogspot.com/2006/07/stupid-title-list.html
    Bee also linked to a good discussion of major open problems in physics by John Baez:
    http://math.ucr.edu/home/baez/physics/General/open_questions.html

    Bee had some other lists as well, and some links to other people's lists of physics questions. It included thoughtful speculation by David Mermin
    http://www.physicstoday.org/pt/vol-54/iss-2/p11.html
    (begins with a not-very-informative list published in 2000 by the NY Times but then Mermin tells you his own ideas)
    ==================

    this is just by way of introduction to Bee's TOP TEN PHYSICS QUESTIONS, which is a very handy list. It would be interesting to know how each of us would guess the answers. these are major unsolved problems, stated very concisely. One can't know the answers, but one can have suspicions and ideas about how it will go. I like this list so much I will copy it from here:
    http://backreaction.blogspot.com/2006/07/top-ten.html#my
     
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  3. Jul 8, 2006 #2

    marcus

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    here is Bee's top ten
    http://backreaction.blogspot.com/2006/07/top-ten.html#my

    Unsolved Questions in Theoretical Physics
    1) How can the apparent disagreement between general relativity (GR) and quantum gravity be resolved? Does it require to quantize gravity? If so, how? If not, see 2 and 3.

    2) Do black holes destroy information? If not, what happens to the matter that collapses to a black hole?

    3) Are there really singularities in GR (inside black holes/big bang)? If so, how can we understand what happens there? If not, how are they avoided?

    4) How can we explain the data (supernovae, WMAP) which seems to indicate that the universe is filled with dark energy. Is there really dark energy? If so, what is it? Why does it become important just now (coincidence problem)?

    5) How can we understand the rotation curves of galaxies, and the too large sizes of voids between galaxies. Does dark matter exist? If so, what is it made of?

    6) What happened in the very early stages of the universe? How can we solve the horizon/flatness/homogeneity problem? Did inflation really take place? If so, what is the inflaton? How does electroweak symmetry breaking work? Where does the baryon-antibaryon asymmetry come from?

    7) Why do we experience 3+1 dimensions? Are there extra dimensions? If so, why haven't we yet noticed them?

    8) Are the electroweak and strong interaction unified at high energies? If so, are the currently known particles of the standard model (SM) elementary? Are there more yet unobserved particles? Why are the parameters of the SM what they are and are they in yet unknown ways related to each other (or are they related to 1. or 6.?). Why are the gauge groups of the SM what they are?

    9) Can we understand quantization?

    10) What causes particles to have masses and why are these so much smaller than the Planck mass (and hence the gravitational interaction so weak, alias the hierarchy problem)?

    I want to add that imo solving 3 will solve 2, and 9 will help with 1.

    ====HERE ARE MY REACTIONS=====
    Please, if you feel like it, put down your guesses too. Here are mine:

    1. How can the disagreement be resolved? By implementing spacetime geometry and matter in the same 4D world model. Will this quantize spacetime geometry (i.e. gravity)? Yes---or else it will uncover something deeper from which quantum theory emerges.

    2. No they don't destroy information. The matter comes out the bottom in a re-expansion. Evolution is unitary but time forks.

    3. No there are no singularities. Singularities are just the failure of a theory and do not exist in nature. Gravity is repellent at high density and there is a bounce.

    4.

    5.

    6.

    7. We experience 3+1 dimensions because there are 3+1 dimensions. No, there are not "extra" dimensions.

    8. There are no particles. The "particles" of the SM model are not elementary. Matter is an aspect of spacetime geometry (see recent Freidel, recent Baez, Smolin, and others). Electroweak and strong DO unify because all interaction of matter with matter and of matter with geometry is PART of the same comprehensive spacetime-matter model.

    9. Yes. If you are good you will be allowed to learn the secret. :smile:

    10. This was already asked as part of question 8. so I think Bee put it on just so she would have a full ten.
     
    Last edited: Jul 8, 2006
  4. Jul 8, 2006 #3

    Kea

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    I'll have a go!

    1)Machian principle from Gray categories. Quantum theory more fundamental - GR derived.

    2)No - and stop thinking classically.

    3)Clearly GR does not apply in such regimes.

    4)DE - see Padmanabhan (think cohomology). Coincidence problem - new form of locality kills this paradox.

    5)TeVeS MOND.

    6)Flatness follows from the basic principle. Inflation (effective) - do old BC idea in new state sum context to get eqn of state.

    7)Naive extra dimensions are a REALLY silly idea.

    8)We're working on this one....but even Heisenberg knew that particles were not elementary.

    9)Yes.

    10)The broken parity cube helps 'generate geometry'. Of course masses are just quantum numbers.
     
  5. Jul 8, 2006 #4

    marcus

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    Kea's answers seem interesting, so for easier reading I will put them with the questions
    ======================
    Unsolved Questions in Theoretical Physics

    1) How can the apparent disagreement between general relativity (GR) and quantum gravity be resolved? Does it require to quantize gravity? If so, how? If not, see 2 and 3.
    Machian principle from Gray categories. Quantum theory more fundamental - GR derived.

    2) Do black holes destroy information? If not, what happens to the matter that collapses to a black hole?
    No - and stop thinking classically.

    3) Are there really singularities in GR (inside black holes/big bang)? If so, how can we understand what happens there? If not, how are they avoided?
    Clearly GR does not apply in such regimes.

    4) How can we explain the data (supernovae, WMAP) which seems to indicate that the universe is filled with dark energy. Is there really dark energy? If so, what is it? Why does it become important just now (coincidence problem)?
    DE - see Padmanabhan (think cohomology). Coincidence problem - new form of locality kills this paradox.

    5) How can we understand the rotation curves of galaxies, and the too large sizes of voids between galaxies. Does dark matter exist? If so, what is it made of?
    TeVeS MOND.

    6) What happened in the very early stages of the universe? How can we solve the horizon/flatness/homogeneity problem? Did inflation really take place? If so, what is the inflaton? How does electroweak symmetry breaking work? Where does the baryon-antibaryon asymmetry come from?
    Flatness follows from the basic principle. Inflation (effective) - do old BC idea in new state sum context to get eqn of state.

    7) Why do we experience 3+1 dimensions? Are there extra dimensions? If so, why haven't we yet noticed them?
    Naive extra dimensions are a REALLY silly idea.

    8) Are the electroweak and strong interaction unified at high energies? If so, are the currently known particles of the standard model (SM) elementary? Are there more yet unobserved particles? Why are the parameters of the SM what they are and are they in yet unknown ways related to each other (or are they related to 1. or 6.?). Why are the gauge groups of the SM what they are?
    We're working on this one....but even Heisenberg knew that particles were not elementary.

    9) Can we understand quantization?
    Yes.

    10) What causes particles to have masses and why are these so much smaller than the Planck mass (and hence the gravitational interaction so weak, alias the hierarchy problem)?
    The broken parity cube helps 'generate geometry'. Of course masses are just quantum numbers.
     
    Last edited: Jul 8, 2006
  6. Jul 8, 2006 #5

    Kea

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    Ooooooh, Marcus, this is fun! Can we have more questions? GZK maybe?

    :tongue2:
     
  7. Jul 9, 2006 #6

    marcus

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    Your answers were fun---more than that: provocative, consciousness-raising:smile: ----not sure what the word is.

    but no one else answered yet. maybe top ten questions don't work here at PF right at the moment.

    or no! maybe what we should do is NOT WAIT for other people to put their answers. we should just discuss the replies (yours and mine) that we already have

    the trouble is that several of your answers, when I scan back at what is written in blue, look so SENSIBLE!!!
    I can't imagine how anyone could have any doubt! Isn't that strange. these look like the obvious answers, so who could disagree and how is controversy possible?

    And then what you said for #10, I don't understand at all, or #4, so I can't disagree with those either. Either it is self-evident or incomprehensible----like so many things. :smile:
     
    Last edited: Jul 9, 2006
  8. Jul 9, 2006 #7

    Kea

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    Hi Marcus

    I think 4 needs the most work. 10 is clear, but involves lots of diagrams.
     
  9. Jul 10, 2006 #8

    Kea

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    Well, we could be completely wrong, I suppose. Maybe we're all deluding ourselves into thinking it actually makes sense after too many years of confusing ourselves silly.

    It does seem to me, though, that any other candidate picture has to at least have a decent go at answering all the questions. Maybe we should make up a String theory answer list - but I can't get one to make sense!

    :smile:
     
  10. Jul 11, 2006 #9

    Kea

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    To write down a (2-topological) spin foam model including masses we just need to remember that instead of functors made out of tetrahedra we now have functors using Street's orientals in the next dimension up. The problem with horizontal composition of 2-cells forces one to put in parity cubes (bits of weak cohomology).

    The 4-arrow inside the cube obeys a nice Stasheff polytope cocycle condition...related to a modified tetrahedron equation, solutions to which have naturally been studied by some of the 3D lattice model gurus:

    The modified tetrahedron equation and its solutions
    G. von Gehlen, S. Pakuliak, S.Sergeev
    http://arxiv.org/abs/nlin/0303043

    Zamolodchikov's Tetrahedron Equation and Hidden Structure of Quantum Groups
    Vladimir V. Bazhanov, Sergey M. Sergeev
    http://arxiv.org/abs/hep-th/0509181

    Thus we get our 14j (or whatever) symbols. Piece of cake. Well, I'm off to a conference tomorrow, so have fun.
     
    Last edited: Jul 11, 2006
  11. Jul 11, 2006 #10

    marcus

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    Arrrgh, Kea! Arrgh arrgh arrgh!
     
  12. Jul 11, 2006 #11

    Kea

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    Here is a Crans paper that talks about the ordinary tetrahedron equation:

    Lie 2-Algebras
    Alissa S. Crans
    http://arxiv.org/abs/math/0409602

    Maybe John can tell us more about this.
     
  13. Jul 11, 2006 #12

    Kea

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    It has not escaped our attention that the weak cohomology thus indicates the existence of a mass gap, a cosmological constant and three generations in the standard model.
     
  14. Jul 11, 2006 #13

    f-h

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    I like you Kea. You do Algebra (even when you do geometry) and it's pretty, but I still don't understand a fraction of it.

    Still interferes with my idea of doing physics by digging deep in the known and working theories rather then building high out of pure abstraction, but damn it looks pretty. Building into the sky is always so much clearer then digging deep. ;)

    I'll have a go with some wild (minimalist, conservative) speculation:

    "1) How can the apparent disagreement between general relativity (GR) and quantum gravity be resolved? Does it require to quantize gravity? If so, how? If not, see 2 and 3."

    No quantization of GR required, though it supplies good hints. some modifications to quantum mechanics in the "Problem of time" sense, but Hilbertspace+Operator algebra is sufficient. Both ordinary QM of particles and geometry of spacetime are limits of a unified form of degrees of freedom.

    "2) Do black holes destroy information? If not, what happens to the matter that collapses to a black hole?"

    No, though not all information may be accessible in all timeslices.

    "3) Are there really singularities in GR (inside black holes/big bang)? "

    I think you meant in reality, in GR, yes of course. See the singularity theorems.

    "If so, how can we understand what happens there?"

    Theres no problem with that as long as censorship holds. It's just a bit of infinity brought closer to home. I really would like to emphasize this point, people always say that singularities indicate the breakdown of GR, as far as I have seen there is nothing in the singularities ordinarily encountered in GR that renders the theory inconsistent. These singularities could be a perfectly consistent way for reality to operate.

    "If not, how are they avoided?"

    Our notions of space and time break down. See unified degrees of freedom above.

    "4) How can we explain the data (supernovae, WMAP) which seems to indicate that the universe is filled with dark energy. Is there really dark energy? If so, what is it? Why does it become important just now (coincidence problem)?"

    Cosmological constant. A term in the effective action of GR.

    "5) How can we understand the rotation curves of galaxies, and the too large sizes of voids between galaxies. Does dark matter exist? If so, what is it made of?"

    TeVeS MOND.

    6) What happened in the very early stages of the universe? How can we solve the horizon/flatness/homogeneity problem? Did inflation really take place? If so, what is the inflaton? How does electroweak symmetry breaking work? Where does the baryon-antibaryon asymmetry come from?

    Flatness arises from renormalization.

    7) Why do we experience 3+1 dimensions? Are there extra dimensions? If so, why haven't we yet noticed them?

    Because you can't tie your shoes in more (or less) then 3 spatial dimensions.
    As the concept of geometry breaks down for the fundamental DoF, so does the notion of dimension. (Stringtheorists 10=11)

    "8) Are the electroweak and strong interaction unified at high energies?"

    In some sense, yes. They are both effective forces arising in the low energy limit after all.

    "If so, are the currently known particles of the standard model (SM) elementary?"

    No. The very concept of particle is not fundamental.

    "Are there more yet unobserved particles?"

    Yes.

    "Why are the parameters of the SM what they are and are they in yet unknown ways related to each other (or are they related to 1. or 6.?)."

    Accidental.

    "Why are the gauge groups of the SM what they are?"

    A handfull of generic features of the underlying theory, plus the fact that at low energies all theories look like renormalizable theories via Wilsons argument.

    9) Can we understand quantization?

    Do we have to? I never liked it to begin with.

    Can we understand the opposite process though? How a classical low energy theory arises.

    10) What causes particles to have masses and why are these so much smaller than the Planck mass (and hence the gravitational interaction so weak, alias the hierarchy problem)?

    Mass is generated effectively via the Higgs mechanism. What it effectively arises from? Same as above.
     
  15. Jul 11, 2006 #14

    selfAdjoint

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    If you get too woozy from the stratospheric math, take a hard analysis break with this important string field theory paper by Schnabl. All the old time goodies; Bernouilli numbers, Stirling numbers of the first kind, the whole down-and-dirty works.

    (Thanks to Cosmic Variance for the link)
     
  16. Jul 11, 2006 #15

    marcus

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    Both Kea's answers and FH's seem interesting, so for easier reading I will put them with the questions
    ======================
    Unsolved Questions in Theoretical Physics

    1) How can the apparent disagreement between general relativity (GR) and quantum gravity be resolved? Does it require to quantize gravity? If so, how? If not, see 2 and 3.
    Machian principle from Gray categories. Quantum theory more fundamental - GR derived.
    No quantization of GR required, though it supplies good hints. some modifications to quantum mechanics in the "Problem of time" sense, but Hilbertspace+Operator algebra is sufficient. Both ordinary QM of particles and geometry of spacetime are limits of a unified form of degrees of freedom.

    2) Do black holes destroy information? If not, what happens to the matter that collapses to a black hole?
    No - and stop thinking classically.
    No, though not all information may be accessible in all timeslices.

    3) Are there really singularities in GR (inside black holes/big bang)? If so, how can we understand what happens there? If not, how are they avoided?
    Clearly GR does not apply in such regimes.
    I think you meant in reality, in GR, yes of course. See the singularity theorems.
    "If so, how can we understand what happens there?"
    Theres no problem with that as long as censorship holds. It's just a bit of infinity brought closer to home. I really would like to emphasize this point, people always say that singularities indicate the breakdown of GR, as far as I have seen there is nothing in the singularities ordinarily encountered in GR that renders the theory inconsistent. These singularities could be a perfectly consistent way for reality to operate.
    "If not, how are they avoided?"
    Our notions of space and time break down. See unified degrees of freedom above.


    4) How can we explain the data (supernovae, WMAP) which seems to indicate that the universe is filled with dark energy. Is there really dark energy? If so, what is it? Why does it become important just now (coincidence problem)?
    DE - see Padmanabhan (think cohomology). Coincidence problem - new form of locality kills this paradox.
    Cosmological constant. A term in the effective action of GR.

    5) How can we understand the rotation curves of galaxies, and the too large sizes of voids between galaxies. Does dark matter exist? If so, what is it made of?
    TeVeS MOND.
    TeVeS MOND.

    6) What happened in the very early stages of the universe? How can we solve the horizon/flatness/homogeneity problem? Did inflation really take place? If so, what is the inflaton? How does electroweak symmetry breaking work? Where does the baryon-antibaryon asymmetry come from?
    Flatness follows from the basic principle. Inflation (effective) - do old BC idea in new state sum context to get eqn of state.
    Flatness arises from renormalization.

    7) Why do we experience 3+1 dimensions? Are there extra dimensions? If so, why haven't we yet noticed them?
    Naive extra dimensions are a REALLY silly idea.
    Because you can't tie your shoes in more (or less) then 3 spatial dimensions.
    As the concept of geometry breaks down for the fundamental DoF, so does the notion of dimension. (Stringtheorists 10=11)


    8) Are the electroweak and strong interaction unified at high energies? If so, are the currently known particles of the standard model (SM) elementary? Are there more yet unobserved particles? Why are the parameters of the SM what they are and are they in yet unknown ways related to each other (or are they related to 1. or 6.?). Why are the gauge groups of the SM what they are?
    We're working on this one....but even Heisenberg knew that particles were not elementary.
    In some sense, yes. They are both effective forces arising in the low energy limit after all.
    "If so, are the currently known particles of the standard model (SM) elementary?"
    No. The very concept of particle is not fundamental.
    "Are there more yet unobserved particles?"
    Yes.
    "Why are the parameters of the SM what they are and are they in yet unknown ways related to each other (or are they related to 1. or 6.?)."
    Accidental.
    "Why are the gauge groups of the SM what they are?"
    A handfull of generic features of the underlying theory, plus the fact that at low energies all theories look like renormalizable theories via Wilsons argument.


    9) Can we understand quantization?
    Yes.
    Do we have to? I never liked it to begin with.
    Can we understand the opposite process though? How a classical low energy theory arises.


    10) What causes particles to have masses and why are these so much smaller than the Planck mass (and hence the gravitational interaction so weak, alias the hierarchy problem)?
    The broken parity cube helps 'generate geometry'. Of course masses are just quantum numbers.

    Mass is generated effectively via the Higgs mechanism. What it effectively arises from? Same as above.
     
  17. Jul 11, 2006 #16
    Let me confess a ``secret''. Some six years ago - when I had to give a presentation for a course - I gave a lecture about the de Rahm theorem in which Cech cohomology and more abstract ideas concerning long exact cohomology sequences associated to chain complexes played an important role. I was enchanted by the elegance and simplicity of the ideas involved, studied some part of Saunders Mc Lane's book and looked into some papers about quantum gravity based upon quantum logic. I thought about using these abstract cohomology ideas in connection to the inverse problem in causal sets but in a way that is not based upon sprinklings of course (that was ``cheating'' in some sense). But, there was a big but. Causal sets are generically not like (topological) manifolds at all - manifoldness being a very delicate property - so whatever definition one would come up with, it would generically give ``bad'' results. Likewise, one immediately realised that finding a dynamics which would result in a manifold -like universe on sufficiently large scales is far from trivial - actually almost a miracle (although the Sorkin-Rideout dynamics has partial succes here; note that these observations are not bounded to causal sets).

    Therefore, it occured to me that these ideas - despite how beautiful they are - are not going to give us much chance for deeper insight in nature - at least not in the forseeable future. They are simply too ``untestable'' and general (and unfortunately didn't shed much new light on QM). For example, the preon idea is cute but it occurs to me that this view is again an effective picture ((10^{15})^4 orders of magnitude larger) of a fundamental Planck scale dynamics (a common idea to most QG approaches). Is it realistic to expect a natural reason to exist why this should be so ? Also, I was remembered at the blissfull properties of the continuum which makes it so much easier to write down dynamical laws; and why should the continuum decription not be adequate ? Why should a renormalization transformation (rescaling) reveal (flat ?) manifold like behavior when the foam is crazy as hell ?

    Is the world truely that crazy ? Isn't it better to understand less wild alternatives first, at a deeper level and test them by a realistic experiment (not about how many angels ... ) ? Again, I and many others (I believe) would truely appreciate it if you could give an example which would make this somehow more plausible.

    Careful
     
    Last edited: Jul 11, 2006
  18. Jul 11, 2006 #17

    Kea

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    I like you too, fh.

    Well, that's category theory - geometry and algebra - and logic.

    There is no substitute for digging and digging. Remember that I have the advantage of being a little older than you, and I did my time.

    :smile:
     
  19. Jul 11, 2006 #18

    Kea

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    :smile: :smile: :smile:

    Ideas have their time. For a long while now, people have thought about these things. But only in 1995 did Gordon, Power and Street figure out how to define tricategories, and they are still not very well understood.

    You are right, Careful. A great deal of work remains to be done before we can say we understand 2-topologies. We are still awaiting a new PhD thesis by a mathematician who has been working on these structures.

    As always, yes.
     
  20. Jul 11, 2006 #19

    Hurkyl

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    So just what is a 2-topology? Is it like a topological category, or a Grothendieck topology, or something else?
     
  21. Jul 11, 2006 #20

    Kea

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    A Grothendieck topology is a proper 1-topology (meaning it knows about topos theory) because it is a topology for 1-categories. Like I say, no one really understands the mathematics of 2-topologies. However, for practical physicists it shouldn't be that complicated. There are hints to the right way to do it in Baratin-Freidel.

    Actually, even 2-topology is the wrong word, because we are into 3-categories here. That is, the topos point of view is telling us to think more carefully about categorification (going up one dimension).
     
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