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Open Problems in Theoretical Physics

  1. Aug 25, 2014 #1
    Dear PFer's,
    I just finished reading the first chapter of Lee Smolin's famous "The Trouble with Physics". The first chapter of the book is titled "The Five Great Problems in Theoretical Physics" and is the source of the questions I want to post.
    I searched PF for similar posts, but I found nothing, so I will post here. In case you do not have access to the book, you can find more details and the full quote of the problems here: http://physics.about.com/od/physics101thebasics/a/fiveproblems.htm.
    Here follows the description of the problems that Prof. Smolin gives. I have remarks on Problems 2 and 5 only.
    1. Problem 2: Resolve the problem of the foundations of quantum mechanics, either by making sense or by inventing a new theory that does make sense. Here professor Smolin seems to be concerned with the issue of realism in QM - sorry I cannot post the two pages he devotes to the topic. I do not quite understand why realism is a QM issue rather than a general one. Unless one brings in the so called problem of hidden variables, in which case I believe we'd better talk of locality/causality alternatives. Then, I suspect the issue is solved by the results of the various EPR experiments - possibly in favor of non-locality. Again, pardon me if my opinions are a bit naive or over-simplified.
    2. Problem 5: Explain dark matter and dark energy. Or, if they don't exist, determine how and why gravity is modified on large scales. More generally, explain why the constants of the standard model of cosmology, including the dark energy, have the values they do. I do not see why the problems of dark matter and dark energy would be put together. Unless one is sure they have one and the same solution, which I suspect is not very likely. Also, I am not sure why inflation (and the associated field) and vacuum energy (i.e., the discrepancy between the QED prediction and the measured value) do not deserve a place in the list, au pair with dark matter and dark energy.
    I would appreciate the point of view of someone (much) more knowledgeable than I am.
    I am sure I gave you a lot of material for discussion ;-).
  2. jcsd
  3. Aug 25, 2014 #2


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    I think you are right, and I believe Smolin would agree with you too, maybe he's just writing a bit loosely here.
  4. Aug 25, 2014 #3
    It seems both of the problems in physics that you mention here require an understanding of the foundations of physics, why QM, why GR. And if we need to go that deep, then there is probably a common principle that gives rise to both GR and QM.
  5. Aug 25, 2014 #4


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    Re Problem Two: One can have two of the three, reality, locality or causality to explain the EPR paradox. FWIW, I see a lot of merit in dispensing with causality, or more precisely, allowing causes to propagate backward in time to a point of entanglement within a light cone before propagating forward in time. But, there are lots of good physics that make other assumptions.

    Re Problem Five: This is by far the most profound problem in fundamental physics, because it accounts for the predominance of the circumstances in which bare GR and SM QM do not explain empirical observations without new physics. However, the dark energy issue which can be explained fully in a manner consistent with observation simply by including the cosmological constant in the equations of GR, which is the accepted "orthodoxy" if you will, is by far the less serious issue.

    The reason that the Higgs vev is not the observed level of dark energy is inherent in the dark energy problem and is really part of the more general issue that GR and the SM aren't a perfect fit for each other.

    The issue if inflation might or might not be related to dark energy - many theories try to unify the two and the simplest naive solution to each (other than through the form of the GR equations) is with a scalar field (although if BICEP's findings that there is a primordial tensor mode gravitational wave in the cosmic background radiation, this simple solution is problematic or just a first order approximation; but BICEP's findings are currently "on probation" until confirmed with more data which should happen or not happen within the year). Inflation is certainly a less pressing issue, because it concerns a cause of the "almost initial conditions" of the universe and hence is a "why" issue rather than a question concerning the laws of physics as they apply to phenomena today.

    No particle has been observed, and no modification of gravity has been directly measured that would explain dark matter phenomena, although there are quite meaningful experimental constraints on efforts to explain dark matter phenomena in either fashion.

    This area of inquiry is also complicated because the way that terms like "cold dark matter" and "neutrino flavor" are used in the prevailing six parameter lamdaCDM model of cosmology, and the way that those terms are used in particle physicists and people who look at dark matter phenomena at sub-cosmological scales are not really the same. LamdaCDM includes particles of hundreds of eV mass of greater as "cold dark matter" while other contexts limit that to particles of GeV mass or greater. LamdaCDM defines a neutrino to be something with masses in the 10 eV or less range (arguably even less than that), while particle physicists define it as a lepton with a zero electric charge that may or not may interact via the weak force.

    Something that is called a "sterile neutrino" could count as cold dark matter, rather than as a neutrino for LamdaCDM model purposes.

    LamdaCDM also defines CDM as "almost collisionless" which is good enough for cosmology purposes, but leaves much to be desired at smaller scales where slight cross sections of interaction can have huge phenomenological impacts on galaxy structure, for example.

    Specifically, the trick in the dark matter paradigm is to find the right dark matter particle with the right kind of dark matter self-interactions and interactions with SM particles to produce the correct large scale structure of the universe, the right abundance of DM, and the correct rugby ball shaped dark matter halos in galaxies as those observed in a way that holds uniformly at all scales. One leading paradigm called "warm dark matter" expects that DM is made up of a single kind of thermal relic particles of about 2 keV that interact via Fermi contact forces and gravity but nothing else. Another assumes somewhat heavier particles of DM that interact with each other via massive photon-like bosons in the MeV mass range. The traditional WIMP paradigm inspired by supersymmetry theories assumed that dark matter was a lightest supersymmetric particle in the GeV mass range that interacts only via the weak force - lots of data seems to strongly disfavor this approach but proponents argue that this is because of an analysis that underestimates the impact of gravitational interactions between ordinary baryonic matter and dark matter.

    WIMP searches are predominant even though this is like looking for the keys you think you lost in the dark alley under the streetlight because the problems with the GeV scale CDM WIMP paradigm were not widely known when the current direct detection experiments were devised, and because it is very, very hard to directly detect 2 keV dark matter particle that interact only via gravity and Fermi contact forces. These particle, if they exist, relative to the already almost invisible and collisionless neutrinos, are like mosquitos to neutrinos as African elephants. It is easier to try to directly detect or rule out something that you might be able to see, than to detect something you know for sure that you don't have experimental techniques good enough to directly observe right now.

    In a modification to gravity paradigm the big tricks are to find a correct modification of weak gravitational fields to reproduce galactic dynamics, to produce the right magnitude of dark matter effects in galactic clusters which simpler gravity modification theories fail to do, and to explain the apparent separation of baryonic and dark matter effects in the Bullet Cluster collision, in a mathematically consistent way. Evaluating these theories is hard in part because the GR equations are not easy to apply to complex systems without making simplifying assumptions so it is hard to know what the default theory prediction is because the wrong simplifying assumptions could lead to the wrong result.

    This study is also complicated because we have an incomplete knowledge of the dynamics by which galaxies and other large scale structures are formed and accurate understandings of the distribution of non-luminious ordinary matter like interstellar gas and dust and incorporating both DM and ordinary matter into simulations and equations is necessary to accurately pin down the true nature of DM phenomena as distinct from ordinary GR applied to ordinary matter.
    Last edited: Aug 25, 2014
  6. Aug 26, 2014 #5


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    1. You may find more details on this type of problems in
      especially in Secs. 5 and 6.
  7. Aug 28, 2014 #6

    Thanks a lot to all those who find the time to answer my post. I have more food for thought now.
  8. Aug 28, 2014 #7


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