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I Are there new observables/operators in BSM?

  1. Aug 9, 2017 #1
    In beyond standard models like string theories with multidimensional and holographic objects or other quantum gravity theories or models. Are there additional observable in addition to the standard ones like position, spin, charge, momentum and the like? For example. Are there holographic (or other unusual) observable where you can create operators to act on the quantum state?

    for example

    Kaluza observable?

    Malcadamia observable?

    which we could add to the common Quantum Mechanics observables like position, spin, charge, momentum and the like..

    But in QFT, the QM observables were no longer focused. So what is the right way to ask this in the context of QFT about new observables or operators pertaining to higher multidimensional, holographic or other exotic states.
  2. jcsd
  3. Aug 10, 2017 #2


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    In some ways this is a very deep question, and in some ways, it seems like you are a bit muddled about what an "observable" is.

    Certainly, one could imagine a BSM theory in which particles have new kinds of properties not found in the SM. Indeed, as often as not, they do. But, to have a new observable, you would need a new way to make observations than the ones currently available to us.

    For example, many SUSY theories have a property called "R" which is conserved or not imperfectly conserved. But, R isn't an "observable" in this theory. One infers the "R" number of particles by figuring out what decays of other particles happen and what decays never happen, even though they would but for R conservation.

    Indeed, there are important properties of particles in the SM that are not observables, such as QCD color charge, which is inferred, but can't be directly measured. Even the non-top quark masses are not themselves observables in the SM, due to the confinement of all quarks in hadrons except the top quark which doesn't hadronize, even though they are fundamental parameters of the theory. But, if one had a BSM theory in which the gluon fields of SM QCD could be suppressed somehow allowing for free quarks, the up, down, strange, charm, and bottom quark masses might become observables in the BSM theory.

    This is another ambiguity in your question. Certainly, a BSM theory could have new quantities that can be observed. For example, in SM4 model with four generations of fermions instead of three, there would be a tau prime lepton which would have an observable mass, spin, parity, charge, half life, etc.

    But, it seems to me that this is not what you are really asking. Instead, you are asking if a BSM theory could have a new "kind" of observable, rather than just a new phenomena that can be experimentally observed (which almost by definition would be true of almost every BSM theory except a true "within the Standard Model" BSM theory that merely explains the properties of the SM at a deep level without actually proposing any phenomenological differences at all, and maybe that's a WSM theory for "within the Standard Model" rather than a BSM theory).

    In principle, a BSM theory could include both a new property of particles and an associated way to directly observe that property.

    It could even be something that scientists have seen forever but never previously thought to conceptualize as a separate meaningful fundamental "observable" like "chirality" which we usually think about in everyday life as merely a non-fundamental emergent property of the location of different particles in a larger system at the same time rather than as a property that a point-like property could have.

    Something along the same lines in string theory is the character of a string as an open string or closed string in a particle that is fundamental although not truly point-like. I could imagine that there could be a way to directly observe the character of a particle as open-string-like or closed-string-like thereby creating a new observable that really doesn't exactly correspond to any other existing observables even though, like a point-particle's chirality, it has a classical/macroscopic physics analogy which is emergent rather than fundamental.

    For example, by analogy to your suggested "Kaluza observable" one can imagine that there might be an observable property of space-time such that the apparent speed of light though that area of space-time which appears to be a vacuum is <c because it is actually riddled with unavoidable detours through an extra Kaluza-Klein dimension, unlike the ordinary parts of space-time that we know and live.

    Even there, it is hard to say if the property of space-time we are observing is really an "observable" as we aren't really measuring something different that time discrepancies and distance measurements and speed and momentum - the stuff of Newtonian mechanics as modified conceptually so that it can have rigor in the context of general relativity and special relativity. Deep down, we are really just observing when and where photons are launched and when and where they land, and only indirectly observing the different property that the space-time between the launching spot and the landing spot seems to exhibit.

    Similarly, the concept of a negative probability is mathematically well defined and doesn't generate impossible events so long as it infers with another set of positive probabilities, so that the "observables" (i.e. how often something happens when all the relevant probabilities are added up) is always positive and real. Negative probabilities, imaginary numbers and all sorts of other properties of the world that don't exist in the SM that don't create non-physical true observables are all good and well and lots of BSM theories propose new ones, but they aren't observables themselves.

    One could imagine a BSM theory in which there is "negative" mass, but it only appears in a manner that interferes with "positive" mass terms, resulting in net positive mass.

    Part of the problem in defining "observable" is figuring out how to draw the line between direct and indirect observation in a way that meaningfully distinguishes a non-observable property that only explains the results with unobservable intermediate steps, and a direct observable, when in real life with all of its true complexity, the chain of events from a physical event that happens to the layers of hardware, software and wetware that intervenes between the physical event and our awareness of that physical event as human beings is so multi-layered and complex.
    Last edited: Aug 10, 2017
  4. Aug 10, 2017 #3
    Thanks for such an enlightening post. Have so many gems to ponder for the weekend.

    I noticed that in the quantum observables like position, spin, charge, momentum, the Hamiltonian.. spin is unique in that it requires special relativity to even exist. I wonder if there are other observables that can occur as a result of the marriage of general relativity to Quantum theory?

    About the holographic principle.. I know our current model just uses anti deSitter space which is not our spacetime. But if someday they discovered our spacetime had a surface where all the physics take place.. won't there be some kind of Malcadamia observable for this holographic translation? the observable being the ease by which different particles differ in how they can translate between the holographic domain and our universe (or other description like this that makes more sense)?
  5. Aug 10, 2017 #4


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    What you describe as a Malcadamia observable sure doesn't sound like an observable to me.
  6. Aug 10, 2017 #5
    to be an observable.. the ordinary Schrodinger Equation and even Dirac Equation has to produce all the spectrums? How do you categorize when something is an observable or not? (in the company of position, spin, charge, momentum, the Hamiltonian.. )
  7. Aug 10, 2017 #6


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    Something has to be observed by an observer to be an observable. The Hamiltonian is not an observable and is not even necessarily unique. Position and momentum are clearly observables. And you can also measure charge and spin.
  8. Aug 10, 2017 #7
    makes sense.. an observable is something that can be observed.. of course..

    so let's say in the future particles are known to have Malcadamia holographic address or tag where it can be read or observed by future devices (to know the location of the address bits in the holographic principle surface where it originated).. then for all intent and purposes it can be called an observable.. this is what I meant but I'll ponder on your statements earlier where you describe BSM observables...
  9. Aug 10, 2017 #8
    Here is a question that makes more sense...

    if there are quantum non-local (even non deterministic) hidden variables.. are they called observables?

    What are the official candidates of new BSM observables?
  10. Aug 11, 2017 #9


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    No. Hidden variables are, by definition, not observables.
  11. Aug 11, 2017 #10
    But what if the hidden variables can be observed.. for example.. manipulating the hidden variables can make matter translate or turn into spacetime.. so matter and spacetime is really just two faces of the coin or one thing only.. is this not called an observable?

    btw.. what quantum gravity theories and professional researched sources have this matter and spacetime just the same object and the matter and spacetime just two faces of a coin where they can be converted to each other?
  12. Aug 12, 2017 #11


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    The fact that a property has an inferred impact doesn't mean that it can be observed. If it is an observable you can measure it and not just its inferred impact. Obverable is not a synonym for "things that exist."

    LQG is one class of theories in which matter can be emergent from space-time in some of them. I imagine that kindred theories to LQG would behave similarly.
  13. Aug 12, 2017 #12
    Returning to this. Why did you categorize open string or closed string and if they could be directly observed as emergent rather than fundamental? Chirality is emergent because it is not fundamental because just it is just arrangement of location of different particles. But open string or closed string are not emergent but fundamental. Yet you referred to it as emergent, how come?

    I guess what I'm looking for is an observable that is not a BSM but within the standard model that is missed.. is the following classification accurate?

    Low energy observable are things like position, spin, charge, momentum..

    while High energy observables are BSM phenomena that can be observed like 4th generation spin, half, etc.

    I'm looking for possibility of a hidden low energy observable. If particle has gender and we haven't detected it yet but someday we detect it. then a particle gender is a bonafide observable? Note this is just an example. Are there no physicists looking for a hidden low energy observable? (remember they are trying to look for local Lorentz invariance violation.. and I remember this proton radius problem which if true can make sense if there is a new hidden low energy observable)

    Thanks a lot for your help!

  14. Aug 13, 2017 #13
    If someone sees any articles in arxiv about this.. please share the link...

    If the universe has say 50 dimensions and every particle can only use 11 dimensions, with each particle able to use different dimensions combinations out of the 50.. can't we call it as a dimensional observable? Also in the case of the holographic principle, we don't know if it even applies to our world so it's not impossible there are new low energy observation as pertaining to higher dimensions or holographic nature. I'd just like to acquaint myself and be versatile in this discernment so I can recognize a new within the standard model low energy observable if I saw one (as distinct to high energy BSM dynamics which may not be a genuine observable). If others have different criteria than ohwilleke for distinguishing them, do drop your thoughts.. thanks.
  15. Aug 16, 2017 #14
    What the original poster was calling "observables", I would call "properties", which refers to things like mass, electric charge, and spin. The word "observable" has a precise technical meaning in quantum mechanics where it refers to self-adjoint operators in a Hilbert space.

    Quantum mechanics also involves conjugate variables, the most famous example of which is position and momentum but there are many others. Squeezed light involves conjugate variables, which were discovered more recently, but does not count as discovering "new" observables. There is nothing particularly profound about this, and it does not involve physics beyond the Standard Model.

    In the past, properties that were thought to be fundamental, turned out to be not important. "Isospin" and "strangeness" used to be considered as fundamental as mass or electric charge, but today, we realize that they are not really fundamental.

    You can also group properties of particles in three categories.

    1. Properties that are part of the definition of particles or the theory, that are necessary to define in order to define the particle, but not, even in principle, observable. How could you, in principle, measure the R-parity of SUSY?

    2. Properties that are, in principle, observable, but in practice, can't be observed with current technology.

    3. Properties that can be observed with current technology.
  16. Aug 16, 2017 #15
    If the holographic principle is right and physical objects are just hologram and the real dynamics occurs in some surface "somewhere".. and someday we can create technology to dematerialize any macroscopic object (i.e. make them vanish by just accessing the holographic surface and reprogramming it).. then how do you describe these properties of objects to vanish.. is it an observable or properties?
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