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Thread: Quantum Gravity territory

  1. #1 Quantum Gravity territory 
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    I think that both models-GR and QM are accepted as incomplete.

    Also I think that the aim of research is to integrate both models into one "joined up" version.

    I have a general question about this endeavour that I probably phrased badly before ,so here goes.

    When (or if) this integrated model is put forward is there ,broadly speaking going to be some causative link from those areas of Physics at present mainly covered by QM to those covered by GR?

    (Simplistically and unscientifically ,does the little rule over the large ? Do the micro effects "cause" the macro effects?)


    Or ,when a Quantum Gravity theory is successfully established ,should we perhaps not notice any micro/macro seams whatever ?


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    Interesting questions - a little too early.


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    Moderator Moderator Markus Hanke's Avatar
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    There is no definitive answer to this one as per yet, but I think the final model will probably come in the form of a duality, meaning that certain microscopic and macroscopic constructs will turn out to be just two sides of the same coin. Somewhat like the whole ER=EPR thing.

    We‘ll see.
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    Quote Originally Posted by Markus Hanke View Post
    There is no definitive answer to this one as per yet, but I think the final model will probably come in the form of a duality, meaning that certain microscopic and macroscopic constructs will turn out to be just two sides of the same coin. Somewhat like the whole ER=EPR thing.

    We‘ll see.
    Hopefully I am homing in on a specific area of the possible correlations between quantum and classic effects with this follow up question.

    Particles such as quarks are said to have "spin" and classical objects can rotate.

    To date all I have heard is the warning not to confuse the two phenomena .So is there any relationship between these two phenomena?

    Is there any sense that quantum spin "leads to" classical rotation or at least is a causative component in classical rotation?

    Or might all apparent similarity between the two phenomena be extremely distant ?

    I think the concept of a quantum particle's spin being the sum of its component parts may be part of the standard theory . Would that have any relevance? (I am thinking of the proton and its quarks which is being discussed over at another forum just now)
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    Moderator Moderator Markus Hanke's Avatar
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    So is there any relationship between these two phenomena?
    There is a mathematical relationship between the concept of spin, and rotations. However, the relationship is not one of “a little ball of matter rotating around its own axis”; it’s a lot less intuitive than this, and I have yet to find an analogy that is easily understandable.

    In my opinion it is best to think of spin as that property of a particle, that determines what kind of mathematical object is needed to represent its associated field ( more technically - what the wavefunction needs to look like in order to behave “correctly” under Lorentz transformations ). So for example, a spin-0 particle would be represented by a scalar field. A spin-1 particle would be represented by a vector field. A spin-2 particle would be represented by a rank-2 tensor field. And so on. A spin-˝ particle such as the electron would be represented by a spinor field of a certain type, which is a somewhat more general concept than a tensor. And so on. So it tells you what kind of object the wavefunction is.

    All of these seemingly disparate objects are actually all just different representations of a mathematical object called the Lorentz group, under different circumstances. This group represents the physical symmetries of spacetime in Special Relativity. And you may have heard also that Lorentz transformations are actually nothing else but rotations and boosts in spacetime, which gives your connection to rotations. This whole discipline of studying this is called Lorentz representation theory - a very important, very fundamental mathematical area, but also quite complex.

    So to make a long story short - spin tells you what a field needs to look like ( as in - what type of object it needs to be ) in order to behave “correctly” under rotations in spacetime. It has nothing to do with classical rotations of little balls around an axis. So essentially, spin is actually a relativistic phenomenon, that has to do with the symmetries of spacetime.

    I think the concept of a quantum particle's spin being the sum of its component parts may be part of the standard theory
    Just to say that spins do not simply add up in a linear manner - yes, you can figure out the overall spin of a composite particle from its component spins, but it’s not a very straightforward calculation.
    Last edited by Markus Hanke; December 3rd, 2017 at 08:58 AM.
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