| ESPERIMENTO GS51, RESPONSABILE: Giovanni Amelino Camelia
Fenomenologia della Fisica alla scala di Planck
Da lacuni anni si e' realizzato che il regime della Gravita' Quantistica (che ci si apetta cominci a prevalere a energie/lunghezze confrontabili con quelle di Planck) potrebbe avere conseguenze fenomenologicamente verificabili anche a scale energetiche molto piu' basse ed accessibili ad esperimenti attuali o in
fase di definizione. Un elenco (non esaustivo) di quese possibilita' riguarda per esempio condizioni iniziali di Gravita' Quantistica per la Cosmologia, deviazioni dalla legge di Newton e deviazioni parametrizzate dalla scala di Planck da simmetrie spaziotemporali classiche come CPT e l' invarianza relativistica.
Questa iniziativa specifica e' indirizzata a chiarificare l' ambito teorico di questi aspetti verificabilli della fisica alla sala di Planck, mettendo soprattutto l' accento sulle possibile modifihe delle simmetrie, per esempio attraverso deformazioni non lineari dell' invarianza relativistica o attraverso una sua eventuale rottura; allo stesso tempo si cerca di individuare tutte le possibilita'
di verificare/falsificare questi effetti attraverso esperimenti di Astrofisica o Raggi Cosmici e studi di cosmologia.
| OBIETTIVI DELL'ESPERIMENTO GS51
|PHENOMENOLGY OF PLANCK SCALE PHYSICS
Since a few years it emerged the realization that it is conceivable that effects of Planck scale physics,generally believed to follow from Quantum Gravity, could be phenomenologically testable.
Moreover, although a Quantum Gravity theory is still far
from being formulated, and there are in fact several
approaches to it, there are indications that a non
trivial space-time structure will be relevant nearby the
Planck scale. This structure could lead to phenomena
which will be therefore characteristic of quantum gravity. Some of the proposed ideas are: loss of quantum coherence or
state collapse, QG imprint on initial cosmological
perturbations, extra dimensions and low-scale QG, deviations from Newton's law, black holes produced in colliders,and Planck-scale deviations from classical spacetime symmetries,
such as CPT and Lorentz invariance (LI).
This research initiative aims to clarify the theoretical
framework of these testable effects of Planck scale
physics, focusing primarily on the issues relevant for the fate of classical symmetries at the Planck scale,and at the same time to explore all the opportunities that planned astrophysics observatories and cosmology studies
may provide for testing/falsifying such Planck-scale effects.
From this perspective we identify three main lines of research:
1) Development of the framework
Indications in favour of the possibility of testable Planck-scale effects have recently emerged from several approaches to the quantum-gravity problem, and in particular from loop quantum gravity and non-commutative geometry. The analysis of condensed-matter analogs of gravity also provides consistent results.
However, due to the complexity of the relevant formalisms, in most cases the analyses, as presently formulated, are only applicable to a limited number of experimental contexts.
It would be valuable, from a phenomenological perspective, to achieve a more comprehensive description of the new effects, which would allow to combine limits obtained in different experimental contexts, with an associated improved capability to test/falsify the relevant theories.
2) Analysis of possible phenomenological signatures.
Among the possible effects of Planck scale physics the largest effort in the recent literature has been devoted to the possibility of modified dispersion relations
(E=SQRT(p^2+m^2)+f(p,Eplanck)), with associated departures from standard Lorentz invariance.
This type of modifications appear to be unavoidable when the
fundamental description of spacetime is based on noncommutative geometry and they also emerge naturally in the loop-quantum-gravity approach.
Evidence of a general applicability of modified dispersion
relations is also growing in studies of noncritical string theory, while for the most studied critical superstring theory analogous modifications of the dispersion relation only arise in presence of an external B-tensor background.
It has been shown that phenomenological signatures of such modified dispersion relations can have impact on high energy astrophysics observations,particularly in the study of ultra high energy cosmic rays and the study of radiation from high energy objects (like gamma ray bursters, AGN and supernova remnants).
The absence of such deviations would put constraints on proposed models of quantum gravity, and (independently of the quantum-gravity motivation) would allow an improvement on the present level of verification of Lorentz symmetry.
Analogous remarks apply to the study of the type of departures from CPT symmetry which could be induced by Planck scale physics.
Already laboratory experiments involving neutral mesons have reached good sensitivity to Planck-scale departures from CPT symmetry, and we expect that observations of neutrinos in astrophysics, now that the presence of neutrino masses is being established, should lead to competitive experimental limits.
3) Investigations of the nature of the departures from
From a conceptual perspective the fact that some approaches to spacetime quantization invite one to consider the possibility of departures from Lorentz symmetry of course deserves careful scrutiny.
In particular, in the recent quantum-gravity literature
there has been strong interest in the issue of whether the
departures form Lorentz symmetry are of a type that allows to select a preferred class of inertial observers.
If the mentioned modifications of the dispersion relation are a manifestation of a symmetry-breaking mechanism (possibly a spacetime analog of the mechanism of spontaneous symmetry breaking that is commonly used in field theory)
then inevitably one does find violations of the Relativity
Principle, i.e. the possibility of selecting a preferred
class of inertial observers emerges.
An interesting perspective on this scenario can be provided
by the so called analog models of gravity: condensed matter
systems, such as acoustics in flowing fluids, light in
moving dielectrics, or quasi-particles in a moving
superfluid, can be used to mimic aspects of general
relativity. More precisely in these systems linearized
perturbations propagate on the background as fields in
curved spacetimes. From the point of view of Planck scale phenomenology even more relevant is the fact that these perturbations propagating in these analog systems are characterized by standard dispersion relations (invariant under Lorentz transformations with the speed of light replaced by the speed of sound) at low energies but acquire modified dispersion relation of the kind discussed at point (2) in the high energy regime where the discrete structure of the condensed matter background (the analog spacetime) is probed.
In particular one might try to develop systems that closely mimic a gravitational theory with a preferred frame (or a deformed relativity in some regime) and then investigate which conditions/symmetries these requirements enforce on the underling condensed matter microscopic theory (which would correspond to the quantum gravity regime).
In this sense these models might represent test fields for the basic ideas motivating the research of point (2) and even more interestingly they can provide systems in which these ideas can be put to an experimental test mixing concepts and expertise among three fundamental fields such has particle, gravitational and condensed matter physics.
In alternative to a framework with a preferred frame one can imagine a situation somewhat analogous to the one that a century ago led to replacing Galilei Relativity with Special Relativity: the Galilei dispersion relation was not consistent with the indications of the Maxwell theory and at first it was thought that the existence of a preferred frame (ether) might be responsible, but eventually the new dispersion relation was understood in terms of a "deformation" of the Galilei boost, with the speed scale "c" as deformation scale, which did not require a violation of the Relativity Principle.
It is conceivable that some quantum pictures of spacetime might analogously lead to modified dispersion relations without violating the Relativity Principle, but as a result of an associated "Planck-scale deformation" of the Lorentz boosts.
Recent results establish that the simplest noncommutative geometries (called "canonical") simply break Lorentz symmetry, while there is growing evidence that a more complex type of noncommutative geometry (the so-called "Lie-algebra" geometries) lead to a deformation of Lorentz symmetry.
The studies of modified dispersion relations in loop quantum gravity are still too preliminary to distinguish between broken and deformed Lorentz symmetry.
We intend to contribute to the study of this issue in noncommutative geometry and loop quantum gravity.
And primarily we intend to establish whether the nature of the planned astrophysical observations searching for Planck-scale modifications of the dispersion rleation could in principle be used to distinguish between the broken-LI and the deformed-LI scenarios. This possibility is encouraged by recent results on the analysis of UHE cosmic rays, where a deformation of Lorentz symmetry appears to inevitably lead to smaller effects with respect to the case of broken Lorentz symmetry, even when the same modification of the dispersion relation is adopted, while still giving rise to potentially measurable effects in other sectors.
4) Cosmological tests of departures from standard GR
The possibility that fundamental spacetime symmetries (as local Lorentz
invariance) could be violated due to QG effects leads naturally also to
consider the possibility that GR could just be a low energy effective field
theory limit of some more fundamental theory not necessarily characterized
by the same symmetries. In this sense it is interesting to explore more
general effective actions for gravity (like f(R) or Einstein-Aether, gravity)
which could play a role in explaining the current (puzzling) cosmological
observations. The aim of the research in this sense is to develop a proper
formalism for testing and constraining such alternative theories of
gravitation, possibly with the aid of next coming high precision cosmological
experiments like Planck and SNAP or astrophysical observations of strong
gravity effects (e.g. binary pulsar spin down).
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