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    A Weinberg's gauge-fixed quantum gravity

    In this 1965 paper by Weinberg,, he describes a quantum field theory of the graviton in a Coulomb-like fixed gauge, where the free graviton has only space-space components and is traceless. This of course makes the field dynamics...
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    I Field Renormalization vs. Interaction Picture

    When introducing renormalization of fields, we define the "free Lagrangian" to be the kinetic and mass terms, using the renormalized fields. The remaining kinetic term is treated as an "interaction" counterterm. If we write down the Hamiltonian, the split between "free" and "interaction" terms...
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    A Does Dim Reg really avoid a photon mass?

    In the loop integral for the one-loop correction to the photon propagator in QED, the dominant term, after Wick rotation and angular averaging, has the form (omitting uninteresting factors) $$(1-2/d) e^2 \eta^{\mu\nu}\int_0^\infty \frac{p^{d+1}}{(p^2+\Delta)^2}dp,$$ where ##p## is the absolute...
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    I Do photons decay?

    QED predicts photon-photon scattering, via a fourth-order Feynman diagram with four external photon legs connected to an electron loop. It seems to me that the same diagram should also predict a process where a single incoming photon decays spontaneously into three photons. This is not forbidden...
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    A Current status of LQG?

    Where does the LQG approach stand right now? What are its biggest successes and failures? Does it have any known difficulties that might be "fatal"? What major questions (on the fundamental level) are still open in the approach, and is there "momentum" toward solving them? In particular, is it...
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    I Commuting observables for Fermion fields?

    In nonrelativistic QM, we usually describe the Hilbert space by choosing a complete set of commuting observables, so that the set of states that are eigenstates of all the observables can be used as a basis. For instance, the "wavefunction" is the state as expressed in terms of "states" with...
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    I Heavy elements from neutron star collisions?

    I have seen it claimed online that the recently announced observation of a neutron-star merger by LIGO provides strong support for the hypothesis that heavy elements - gold and platinum were mentioned in particular - are mostly created in neutron-star collisions rather than in supernovas. Is...
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    I Does global topology lead to a preferred frame in SR?

    According to this author,, a locally Minkowski spacetime with a nontrivial global topology may have a preferred inertial frame, in the sense that hypersurfaces of constant time can only be defined using particular time...
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    Resolution to Gibbs' entropy paradox?

    It seems to me that Gibbs' Paradox (that the entropy of a classical ideal gas, calculated by phase-space volume, is not extensive) can be resolved without assuming that particles are indistinguishable. Suppose instead the opposite: that particles are distinguishable, meaning that each one can...
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    I Conservation of energy in quantum measurement

    A measurement of an observable that does not commute with energy will generally cause a change in the expectation value of the energy. Is there a clear formalism to describe how energy is conserved overall?
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    How can a point charge exert force on itself?

    An accelerated charge emits radiation and so must lose energy. This implies that it feels a "reaction force" in the direction opposite the motion. Since EM interactions conserve energy/momentum, it must be possible to describe the reaction force in terms of EM fields acting on the charge. These...
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    Definition of energy

    This thread was triggered by @Anonymous Vegetable 's question re. nuclear fusion. In GR, energy density (in some coordinate system) is a parameter with physical implications- it is the (0,0) element in the stress energy tensor. This is in contrast with the situation in classical mechanics...
  13. M

    Conservation law for any potential field?

    Consider a free particle moving in a general time-dependent scalar potential. Energy & momentum are not conserved. However, there is a symmetry in the lagrangian: the velocity appears only as its square, so we can rotate it without affecting the value of L. What conservation law results from...
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    Are there currently any proposals for a complete physics?

    Has any model of physics yet been proposed that meets the following criteria? 1. The model is believed to, in principle, yield a full description of behavior at all energy levels. (Nonnegative probabilities summing to 1, no singularities without full mathematical treatment, no infinite values to...
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    Old black holes in LQC

    According to LQC, did the universe before the bounce contain black holes? If so, would they still be around? What I'm getting at is this: I've read that LQC predicts that the high densities around the bounce tend to smooth out inhomogeneities. If I understand correctly, this is quite promising...
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    EM determined by gauge freedom?

    I've heard the claim that the gauge freedom of the general Lagrangian can be used to derive the Lorentz force on a charged particle. I understand that Langrangian gauge freedom allows A⋅v -∇φ to be unaffected by the gauge freedom in defining the EM potentials, but this seems like a convenient...
  17. M

    Up & down quarks

    Is there a simple explanation for why up & down quarks have different charge?
  18. M

    How do the continents float?

    Given that both the continents and the oceans are "floating" on magma, and the continents are much heavier, why doesn't the pressure created by the continents force the ocean floor upward?
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    Mass equivalence solved?

    In formulating GR, Einstein was motivated by the otherwise strange coincidence of the equivalence of gravitational and inertial mass. He removed this by discovering that trajectories determined by gravitation are actually inertial. However, gravity's source is still the tensor T, which is...
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    John Wheeler & QM

    Does anyone know how Wheeler understood QM? Here are some well-known sayings of his (slightly paraphrased): "the wave function does not represent the electron- it represents what we know about the electron; the joint product of all the evidence that is available to those who communicate" "it is...
  21. M

    Objective reduction

    Roger Penrose suggested that wf collapse is an objective phenomenon caused by gravity. Is there any actual model for this? For instance, how would the nonlocal collapse work with relativity?
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    Does the wf entangle everything in the neighborhood?

    According to the GRW approach, the wf collapses objectively whenever large numbers of particles are entangled in it. My question is, how can this ever not be the case? Take a photon passing through a prism. Its path, if reconstructed after detection, will include a refraction obeying Snell's...
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    Non-Euclidean complex plane?

    If the Parallel Axiom is just one of several possible assumptions, why is it that so many mathematical relationships seem to only be expressible in the Euclidean plane? Do planes with positive or negative curvature give analogues to the Agrand plane for complex algebra, or the Cartesian plane...
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    Non-real localism?

    Bell & Aspect proved you can't have local realism. Bohm set up a model that is realistic but not local. Is there a meaningful model that is local but not realistic? How would it explain the way measurement choices affect distant outcomes?
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    Why doesn't light cancel itself?

    In a beam of unpolarised light, there are thousands of light waves of all polarisations. At a given point along the beam, each of these elements represents an electric and/or magnetic field in some direction. Since all directions are more or less equally represented, shouldn't all the fields sum...