Lorentz symmetry: Broken, intact or deformed?



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Abstract

Is aether technically natural?
I will discuss whether higher energy Lorentz violation should be considered a natural expectation in theories of quantum gravity with a preferred frame.
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If spacetime is a causal set then Lorentz symmetry is unbroken
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Quantum superpositions of the speed of light
If the metric is an operator, it can exist in superpositions.  The
simplest case one can look at is a superposition of flat spaces which
differ only in the value of the speed of light. I will lay out how such
superpositions can be incorporated into quantum field theory, and
discuss the fate of Lorentz-invariance in this scenario.
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Relative locality and fate of Lorentz symmetry
In my talk I will briefly introduce the idea of relative locality,
being a particular regime of quantum gravity characterized by
negligible Planck length and finite Planck mass.  Then I will discuss
possible scenarios concerning the fate of Lorentz symmetry in this
regime.
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Observational constraints on scale hierarchy

in Horava-Lifshiftz gravity
Horava-Lifshitz gravity models contain higher order operators suppressed by a characteristic scale, which is required to be parametrically smaller than the Planck scale. We show that recomputed synchrotron radiation constraints from the Crab nebula suffice to exclude the possibility that this scale is of the same order of magnitude as the Lorentz breaking scale in the matter sector. This highlights the need for a mechanism that suppresses the percolation of Lorentz violation in the matter sector and is effective for higher order operators as well.
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Breaking Lorentz invariance: the Universe loves it!

I show how the local Lorentz and / or diffeomorphism invariances may be broken by a varying speed of light, softly or harshly, depending on taste.  Regardless of the fundamental implications of such dramas, these smmetry breakings may be of great practical use in cosmology.  They may solve the horizon and flatness problesm.  A near scale-invariant sprectrum of fluctuation may arise, even without inflation.  Distinct observational imprints may be left.

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Quantum Gravity Phenomenology without

Lorentz
Invariance Violation

Is there hope to see quantum gravity effects if the underlying theory is strictly respecting of Lorentz invariance?  I will discuss a novel class of possibilities, suggested by analogy with some simple solid state physics, including one that has lead to an actual experiment, which has placed the first relevant constraints on these kind of effects