LHC - the last chance for all theories of everything?

  1. The LHC is probably the last of the large accelerators so should the main contenders for the title of the theory of everything (if such a thing exists) now, at the dawn of the LHC start up, clearly state what findings would a) prove their theory b) support their theory & c) eliminate their theory? Physics, after all, is still an empirical science & at some point even the most elegant mathematical theories need to connect with experimental fact.
     
  2. jcsd
  3. I don't think it will be the last. Why do you think this? The LHC is unlikely to find conclusive evidence for any theory of everything. Of coarse one would like to prove or disprove theories but in reality this isn't likely. Finding support for a theory such as strings is probably the best we can hope for. For conclusive evidence we will have to wait for a larger accelerator or some novel way of testing fundamental physics that has not yet been thought up. Another hope would be more cosmological data that supports theories.

    I think the things that the LHC should be able to prove or disprove, at least to some degree, is the Higgs boson and supersymmetry. And even if in these cases there may well be ways out if they don't find them at the energy levels of the LHC (theorists who love SUSY will just claim its at a higher energy scale). All we can really do is hope that they find something significant so that governments continue to fund fundamental physics experiments otherwise we may enter an age where there is relatively little empirical evidence to guide new theories.
     
  4. marcus

    marcus 24,517
    Science Advisor
    Gold Member
    2014 Award

    A lot of evidence is coming from astrophysics.
    This was part of the message in a recent talk that Edward Witten gave at Cern, called something like "Physics away from the high energy frontier".

    The notes are on line. If you move away from the collider energy frontier, you still have high energy phenomena to observe. Cosmic rays, astrophysical gamma radiation. Some interesting things can be deduced from such data.
     
  5. Yes. Don't forget that perhaps something totally UNEXPECTED may arise....Wouldn't that be most exciting of all possibilities!!...something to set science off in new directions, perhaps towards a different theory of everything from current forms. Something that really alters our perceptions...like Hubbles work, or the discovery of dark energy and dark matter...
     
  6.  
  7. marcus

    marcus 24,517
    Science Advisor
    Gold Member
    2014 Award

    Adrian the future of bigger colliders is in part a political question.
    There are alternative ways of deducing facts about the universe (Witten discussed a bunch, but one sees evidence of this all the time) so who builds the next big machine, and when they build it, is likely to be determined at least as much by politics---somewhat like the race to the moon there is an element of national prestige and proving to the world one's command of the most challenging technologies.

    So I would say that you have to think about the aspirations of the Chinese People's Republic.
    Think what it would mean to the Chinese if they could build a more successful collider than the French and Swiss! It would demonstrate scientific, economic, and engineering strength of major proportions. And also would make China the center of particle physics activity and the host to a great international collaboration of minds.

    Given the political motives, I would imagine that it is almost certain that a post-LHC collider will be built, regardless of the strictly scientific need for it, and that it will very likely be reliable ("with a vengeance" so to speak). That is, will experience a less eventful start-up than LHC.

    Just a guess though.
     
  8. Sounds quite possible. In fact, it would not be surprising to see India and China in a race to build the most powerful collider.
     
  9. marcus

    marcus 24,517
    Science Advisor
    Gold Member
    2014 Award

    National aspirations can be achieved within the context of international collaboration. How about an International Collider which the Chinese manage to get built on their soil. This would be an amazing boost to their universities, technology companies, and scientific establishment. Also their prestige in the EastAsian region. Doubtless there is an initiative to do this and they are already working on it. I wonder if something like that will happen.
     
  10. So if we do not build large colliders anymore, Science will have to wait for powerful tabletop accelerators. We'll not see that in our lifetime, but there is no reason why it should be impossible in principle.
     
  11. marcus

    marcus 24,517
    Science Advisor
    Gold Member
    2014 Award

    In case anyone wants to look at it, here are slides for Witten's talk about the prospects of physics "away from the high energy frontier":
    http://indico.cern.ch/getFile.py/ac...onId=1&resId=0&materialId=slides&confId=51128
    And here is a video of the talk itself:
    http://cdsweb.cern.ch/record/1176909/
    It is in the first session of the workshop, which is the default. And it is minute 4:30 to minute 32:20 (followed by questions from audience until 37:30)
    So you just start the first session and drag the button to 4:30 to skip the chairman's introduction.
    A lot of the talk is about how to get new physics out of astrophysical observation and cosmology, without relying on colliders or high energy machines in general.
     
    Last edited: Aug 14, 2009
  12. RUTA

    RUTA 741
    Science Advisor

    Has anyone knowledge of a unification approach that doesn't require larger accelerators? I saw a popular program last week about superstring theory in which they said we'd need an accelerator as big as the galaxy to find evidence of strings. I'm not aware of a GUT or SUT that doesn't require much larger accelerators. Have you?
     
  13. marcus

    marcus 24,517
    Science Advisor
    Gold Member
    2014 Award

    Ruta, why don't you watch the first few minutes of Witten's talk? He is talking about unification (GUTs) almost every slide, but the focus is on what can be discovered and tested away from the high energy frontier. That is by other means than machines like the LHC.

    I think it is unrealistic to imagine that there will not be some development of colliders beyond LHC. But it is also unrealistic to suppose that the whole progress in fundamental physics depends on making larger and larger colliders. Listen to the talk, which is number one talk in a well-attended CERN workshop. It illustrates that there are already clever people thinking about how physics can advance by other avenues.
     
    Last edited: Aug 14, 2009
  14. MTd2

    MTd2 1,986
    Gold Member

    I would just like to know if there is any progress towards using high temperature superconductors for the magnets in particle accelerators.
     
  15. The Qweak experiment at Jefferson Lab will run around 1 GeV, but measuring parity violation with precision and provide orthogonal (complementary)to very high energy colliders constraints on physics BSM.
    [​IMG]
    High precision parity violation is not new, it just can be done better today, and it's quite sensitive to tiny vacuum fluctuations.
     
    Last edited: Aug 15, 2009
  16. RUTA

    RUTA 741
    Science Advisor

    Thanks much, Marcus. Of course I've been hearing about low energy predictions of GUTs and SUTs, but these, as you know, are not definitive given the extremely high unification energies. I'm not asking for indirect, secondary consequences of high energy unification schemes. I'm wondering if someone has any ideas which don't entail high energies to unify physics in the first place.
     
  17.  
  18. tom.stoer

    tom.stoer 5,489
    Science Advisor

    That's the very core of the discussion. The LHC can find evidence for entities like Higgs, SUSY, etc. iff they are in the LHC's energy range.

    For the SM Higgs it's pretty clear: if the LHC does not prove its existence it automatically disproves the SM. For all other topics it can only push the limits of their "existence" to higher energies.

    So that automatically means the LHC will be absolutely mute about any "ToE". It can support theories with respect to their low energy regime, but nothing else.

    For example string theory (with small extra dimensions): as its low energy limit is a certain SUGRA, the LHC cannot distinguish between the two scenarios "ST is the ToE" and "SUGRA is the ToE".

    My conclusion is that something like a ToE cannot exist in a physical sense:
    a) you can only "prove", support or disprove a theory in a very restricted sense = in certain regimes
    b) a ToE should be able to tell you why it is the ToE; I don't think that ST does only because it is allowed to exist in a 10-D spacetime: it does not tell you why other approaches must fail
    c) in a certain sense the discussions regarding duality show that there may be not one fundamental theory but only certain dual descriptions of something we like to call "reality" - whatever that means;
    look at QFT: what are the fundamental entities of a "quantum ontology"? state vectors in Fock space and field operators - or path integrals and "trajectories in field configuration space";
    look at ST: there are dualities between very different string theories, compactifications etc.
    so in terms of ontology this question is undecidable;
    d) any mathematical approach to a ToE is not able to prove why this approach must be mathematical at all
     
  19. marcus

    marcus 24,517
    Science Advisor
    Gold Member
    2014 Award

    Tom
    How does your perspective compare with the one Weinberg presented towards the end of his 6 July Cern talk?

    I expect you may have watched it but in case not I'll get the link.
    http://cdsweb.cern.ch/record/1188567/

    I mentioned in this 11 July post
    https://www.physicsforums.com/showthread.php?p=2268482#post2268482
    that to save time you can drag the time button to minute 58 (if you want to skip the historical part) and get to the core message
    which is in the last 12 minutes.

    The slides are here:
    http://itpworkshop.unibe.ch/MaKaC/g...mp;resId=0&materialId=slides&confId=2

    There was further discussion here:
    https://www.physicsforums.com/showthread.php?t=324841

    The title of the Cern talk was "The Quantum Theory of Fields--Effective or Fundamental?"
     
    Last edited: Aug 22, 2009
  20. tom.stoer

    tom.stoer 5,489
    Science Advisor

    Marcus, which statement or perspective do you mean?

    The first one regarding the LHC energy range?

    The second one regarding the low energy limit?

    Or my general conclusion regarding possible existence (or non-existence) of a ToE
     
  21. marcus

    marcus 24,517
    Science Advisor
    Gold Member
    2014 Award

    Tom I often find your informed opinion on things very helpful but in this case what you say does not make sense to me and I contrast it with the cautious optimism in Weinberg's talk.

    I think Weinberg was using a very pragmatic idea of a ToE when he referred to "how nature is". Talking about a predictive theory that appears good to arbitrary high energy.
    The commonsense view of science as a process and a community---not as an ultimate.
    A theory is a ToE if it acts like a ToE and most of the community accepts it provisionally as such.
    And he was saying that string may be irrelevant. String may not turn out to be "how nature is". He suggested an alternative line he is currently pursuing based on asymsafe qg and quantum field theory.

    Therefore I think your references to string theory (ST) in your quote may be irrelevant and distracting. Maybe I will take them out and look at the bare bones skeleton of the argument, minus the flesh of that example.

    ==skeleton argument==
    My conclusion is that something like a ToE cannot exist in a physical sense:
    a) you can only "prove", support or disprove a theory in a very restricted sense = in certain regimes

    b) a ToE should be able to tell you why it is the ToE

    c) in a certain sense the discussions regarding duality show that there may be not one fundamental theory but only certain dual descriptions of something we like to call "reality" - whatever that means;
    look at QFT: what are the fundamental entities of a "quantum ontology"? state vectors in Fock space and field operators - or path integrals and "trajectories in field configuration space"

    d) any mathematical approach to a ToE is not able to prove why this approach must be mathematical at all
    ==end of excerpt==

    Let me think a bit about this
    d) is right of course. physical theories are mathematical. the fact that math works to describe regularity in nature is a mystery. but we do not have to address that. A ToE is just a physical theory that works predictively to arbitrary high energy, it does not have to explain all the mysteries.
    Like "why does existence exist?" That is a good question but ToE does not have to address it.

    c) does not have to worry us. There can be alternative equivalent formulations of the regularities in nature. Sometimes we eventually find that one is better. Sometimes we find a more general mathematics that comprehends both. It is OK. There can still be a ToE even if it comes in several equivalent formulations.

    b) pragmatically, I do not see why any physical theory has to explain why it is an adequate theory. If something works and acts like a ToE then it is a ToE.
    It does not have to contain a "theory of theories". Science is a communal process governed in part by tradition---and the tradition says that a theory is accepted provisionally until and if a better one is found. That is as good as it gets, in the tradition. All acceptance by the community is provisional. No acceptance is ultimate. So b) is no problem.

    a) What you say here is absolutely right, except that we do not know in advance the ultimate limit that we can probe empirically.
    Clever ways of testing a model may be invented which surprise us and which go beyond the domain of verifiability that we would have expected.
    Pragmatically, the meaning of infinity is "way beyond what anybody expected".
    If an theory predicts accurately way beyond what anybody ever thought we would be able to test, because of some clever ways to test that people think up, then it will be acting like a ToE. Predictive out to arbitrarily high energies. It's possible that humans will get such a theory. I don't think that on purely logical grounds you can deny us the possibility. :biggrin:

    But I like your argument and think that it is one worth making.
     
    Last edited: Aug 24, 2009
Know someone interested in this topic? Share a link to this question via email, Google+, Twitter, or Facebook

Have something to add?