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Giovanni Amelino-Camelia

Publications and source records attributed to Giovanni Amelino-Camelia.

At least 145 records · Page 8Linked to original sources
arXiv2002

Quantum Space-Time: Deformed Symmetries Versus Broken Symmetries

Several recent studies have concerned the faith of classical symmetries in quantum space-time. In particular, it appears likely that quantum (discretized, noncommutative,...) versions of Minkowski space-time would not enjoy the classical Lorentz symmetries. I compare two interesting cases: the case in which the classical symmetries are "broken", i.e. at the quantum level some classical symmetries are lost, and the case in which the classical symmetries are "deformed", i.e. the quantum space-time has as many symmetries as its classical counterpart but the nature of these symmetries is affected by the space-time quantization procedure. While some general features, such as the emergence of deformed dispersion relations, characterize both the symmetry-breaking case and the symmetry-deformation case, the two scenarios are also characterized by sharp differences, even concerning the nature of the new effects predicted. I illustrate this point within an illustrative calculation concerning the role of space-time symmetries in the evaluation of particle-decay amplitudes. The results of the analysis here reported also show that the indications obtained by certain dimensional arguments, such as the ones recently considered in hep-ph/0106309 may fail to uncover some key features of quantum space-time symmetries.

gr-qc↗
arXiv2001

Coproduct and star product in field theories on Lie-algebra non-commutative space-times

We propose a new approach to field theory on $κ$-Minkowski non-commutative space-time, a popular example of Lie-algebra space-time. Our proposal is essentially based on the introduction of a star product, a technique which is proving to be very fruitful in analogous studies of canonical non-commutative space-times, such as the ones recently found to play a role in the description of certain string-theory backgrounds. We find to be incorrect the expectation, previously reported in the literature, that the lack of symmetry of the $κ$-Poincare' coproduct should lead to interaction vertices that are not symmetric under exchanges of the momenta of identical particles entering the relevant processes. We show that in $κ$-Minkowski the coproduct and the star product must indeed treat momenta in a non-symmetric way, but the overall structure of interaction vertices is symmetric under exchange of identical particles. We also show that in $κ$-Minkowski field theories it is convenient to introduce the concepts of "planar" and "non-planar" Feynman loop-diagrams, again in close analogy with the corresponding concepts previously introduced in the study of field theories in canonical non-commutative space-times.

hep-th↗
arXiv2001

Space-time quantum solves three experimental paradoxes

I show that a Planck-scale deformation of the relativistic dispersion relation, which has been independently considered in the quantum-gravity literature, can explain the surprising results of three classes of experiments: (1) observations of cosmic rays above the expected GZK limit, (2) observations of multi-TeV photons from the BL Lac object Markarian 501, (3) studies of the longitudinal development of the air showers produced by ultra-high-energy hadronic particles. Experiments now in preparation, such as the ones planned for the GLAST space telescope, will provide an independent test of this solution of the three experimental paradoxes.

gr-qc↗
arXiv2001

Status of Relativity with observer-independent length and velocity scales

I have recently shown that it is possible to formulate the Relativity postulates in a way that does not lead to inconsistencies in the case of space-times whose structure is governed by observer-independent scales of both velocity and length. Here I give an update on the status of this proposal, including a brief review of some very recent developments. I also emphasize the role that one of the kappa-Poincare' Hopf algebras could play in the realization of a particular example of the new type of postulates. I show that the new ideas on Relativity require us to extend the set of tools provided by kappa-Poincare' and to revise our understanding of certain already available tools, such as the energy-momentum coproduct.

gr-qc↗
arXiv2001

Observed threshold anomalies as the first hope of a manifestation of Planck-length physics

The observations of photons from the BL Lac object Mk501 with energies above 10 TeV and of cosmic rays with energies above the GZK threshold appear to be inconsistent with conventional theories. Remarkably, among the recent new-physics proposals of solutions of these threshold paradoxes a prominent role has been played by proposals based on quantum properties of space-time. While the experimental evidence (and theory work attempting to interpret it) is much too preliminary to justify any serious hopes that we might have stumbled upon the first manifestation of a "quantum gravity", the fact that for the first time phenomenological models involving quantum-gravity ideas are competing on level ground with other new-physics proposals clearly marks the beginning of a new stage of quantum-gravity research. I emphasize one important aspect of this new phenomenology: combining the determination of the relevant thresholds with data on the time/energy structure of gamma-ray bursts it is possible to distinguish between alternative quantum-gravity scenarios. This point is illustrated focusing on 3 specific scenarios: dispersion-inducing space-time foam, string-theory-motivated non-commutative space-time, and this author's recent proposal of a relativistic theory in which the Planck length has the role of fundamental observer-independent minimum length.

gr-qc↗
arXiv2001

Phenomenological description of space-time foam

The expectation that it should not be possible to gain experimental insight on the structure of space-time at Planckian distance scales has been recently challenged by several studies. With respect to space-time fluctuations, one of the conjectured features of quantum-gravity foam, the experiments that have the best sensitivity are the ones which were originally devised for searches of the classical-physics phenomenon of gravity waves. In experiments searching for classical gravity waves the presence of space-time fluctuations would introduce a source of noise just like the ordinary (non-gravitational) quantum properties of the photons composing the laser beam used in interferometry introduce a source of noise. Earlier studies of the noise induced by quantum properties of space-time have shown that certain simple pictures of fluctuations of space-time occuring genuinely at the Planck scale would lead to an observably large effect. Experimentalists would benefit from the guidance of detailed description of this noise, but quantum-gravity theories are not yet developed to the point of allowing such detailed analysis of physical processes. I propose a new phenomenological approach to the description of foam-induced noise.

gr-qc↗
arXiv2000

Testable scenario for Relativity with minimum-length

I propose a general class of space-times whose structure is governed by observer-independent scales of both velocity ($c$) and length (Planck length), and I observe that these space-times can naturally host a modification of FitzGerald-Lorentz contraction such that lengths which in their inertial rest frame are bigger than a "minimum length" are also bigger than the minimum length in all other inertial frames. With an analysis in leading order in the minimum length, I show that this is the case in a specific illustrative example of postulates for Relativity with velocity and length observer-independent scales.

hep-th↗
arXiv2000

Relativity in space-times with short-distance structure governed by an observer-independent (Planckian) length scale

I show that it is possible to formulate the Relativity postulates in a way that does not lead to inconsistencies in the case of space-times whose short-distance structure is governed by an observer-independent length scale. The consistency of these postulates proves incorrect the expectation that modifications of the rules of kinematics involving the Planck length would necessarily require the introduction of a preferred class of inertial observers. In particular, it is possible for every inertial observer to agree on physical laws supporting deformed dispersion relations of the type $E^2- c^2 p^2- c^4 m^2 + f(E,p,m;L_p)=0$, at least for certain types of $f$.

gr-qc↗
arXiv2000

Quantum theory's last challenge

Quantum mechanics is now 100 years old and still going strong. Combining general relativity with quantum mechanics is the last hurdle to be overcome in the "quantum revolution".

gr-qc↗
arXiv2000

Planck-scale deformation of Lorentz symmetry as a solution to the UHECR and the TeV-$γ$ paradoxes

One of the most puzzling current experimental physics paradoxes is the arrival on Earth of Ultra High Energy Cosmic Rays with energies above the GZK threshold. The recent observation of 20TeV photons from Mk 501 is another somewhat similar paradox. Several models have been proposed for the UHECR paradox. No solution has yet been proposed for the TeV-$γ$ paradox. Remarkably, the drastic assumption of a violation of ordinary Lorentz invariance would resolve both paradoxes. We present a formalism for the description of the type of Lorentz-invariance deformation (LID) that could be induced by non-trivial short-distance structure of space-time, and we show that this formalism is well suited for comparison of experimental data with LID predictions. We use the UHECR and TeV-$γ$ data, as well as bounds on time-of-flight differences between photons of different energies, to constrain the LID parameter space. A model with only two parameters, an energy scale and a dimensionless parameter characterizing the functional dependence on the energy scale, is shown to be sufficient to solve both the UHECR and the TeV-$γ$ threshold anomalies while satisfying the time-of-flight bounds. The allowed region of the two-parameter space is relatively small, but, remarkably, it fits perfectly the expectations of the quantum-gravity-motivated space-time models known to support such deformations of Lorentz invariance: integer value of the dimensionless parameter and characteristic energy scale constrained to a narrow interval in the neighborhood of the Planck scale.

astro-ph↗
arXiv2000

Planck-Length Phenomenology

This author's recent proposal of interferometric tests of Planck-scale-related properties of space-time is here revisited from a strictly phenomenological viewpoint. The results announced previously are rederived using elementary dimensional considerations. The dimensional analysis is then extended to the other two classes of experiments (observations of neutral kaons at particle accelerators and observations of the gamma rays we detect from distant astrophysical sources) which have been recently considered as opportunities to explore "foamy" properties of space-time. The emerging picture suggests that there is an objective and intuitive way to connect the sensitivities of these three experiments with the Planck length. While in previous studies the emphasis was always on some quantum-gravity scenario and the analysis was always primarily aimed at showing that the chosen scenario would leave a trace in a certain class of doable experiments, the analysis here reported takes as starting point the experiments and, by relating in a direct quantitative way the sensitivities to the Planck length, provides a model-independent description of the status of Planck-length phenomenology.

gr-qc↗
arXiv2000

Cosmic rays and TeV photons as probes of quantum properties of space-time

It has been recently observed that small violations of Lorentz invariance, of a type which may arise in quantum gravity, could explain both the observations of cosmic rays above the GZK cutoff and the observations of 20-TeV gamma rays from Markarian 501. We show here that different pictures of the short-distance structure of space-time would lead to different manifestations of Lorentz-invariance violation. Specifically, the deformation of Lorentz invariance needed to resolve these observational paradoxes can only arise within commutative short-distance pictures of space-time. In noncommutative space-times there is no anomalous effect, at least at leading order. Also exploiting the fact that arrival-time delays between high energy photons with different energies would arise in both the commutative and the noncommutative Lorentz-violation pictures, we describe an experimental programme, based on time-of-arrival analysis of high energy photons and searches of violations of GZK and TeV-photon limits, which could discriminate between alternative scenarios of Lorentz-invariance breakdown and could provide and unexpected window on the (quantum) nature of space-time at very short distances.

hep-ph↗
arXiv2000

A testable description of space-time foam as a fundamental stochastic gravity-wave background

I develop a phenomenological approach to the description of the noise levels that the space-time foam of quantum gravity could induce in modern gravity-wave detectors. Various possibilities are considered, including white noise and random-walk noise. In particular, I find that the sensitivity level expected for the planned LIGO and VIRGO interferometers and for the next upgrade of the NAUTILUS resonant-bar detector corresponds to a white-noise level which can be naturally associated with the Planck length.

gr-qc↗
arXiv2000

On a possible quantum limit for the stabilization of moduli in brane-world scenarios

I consider the implications for brane-world scenarios of the rather robust quantum-gravity expectation that there should be a quantum minimum limit on the uncertainty of all physical length scales. In order to illustrate the possible significance of this issue, I observe that, according to a plausible estimate, the quantum limit on the length scales that characterize the bulk geometry could affect severely the phenomenology of a recently-proposed brane-world scenario.

hep-th↗
arXiv2000

Testing kappa-Poincare' with neutral kaons

In recent work on experimental tests of quantum-gravity-motivated phenomenological models, a significant role has been played by the so-called ``$κ$'' deformations of Poincaré symmetries. Sensitivity to values of the relevant deformation length $λ$ as small as $5 \cdot 10^{-33}m$ has been achieved in recent analyses comparing the structure of $κ$-Poincaré symmetries with data on the gamma rays we detect from distant astrophysical sources. We investigate violations of CPT symmetry which may be associated with $κ$-Poincaré in the physics of the neutral-kaon system. A simple estimate indicates that experiments on the neutral kaons may actually be more $λ$-sensitive than corresponding astrophysical experiments, and may already allow to probe values of $λ$ of order the Planck length.

hep-ph↗
arXiv1999

Are we at the dawn of quantum-gravity phenomenology?

A handful of recent papers has been devoted to proposals of experiments capable of testing some candidate quantum-gravity phenomena. These lecture notes emphasize those aspects that are most relevant to the questions that come to mind when one is exposed for the first time to these research developments: How come theory and experiments are finally meeting in spite of all the gloomy forecasts that pervade traditional reviews? Is this a case of theorists having put forward more and more speculative ideas until a point was reached at which conventional experiments could rule out the proposed phenomena? Or has there been such a remarkable improvement in experimental techniques and ideas that we are now capable of testing plausible candidate quantum-gravity phenomena? These questions are analysed rather carefully for the recent proposals of interferometry-based tests and tests using observations of gamma rays of astrophysical origin. I also briefly discuss other proposed experiments (including tests of quantum-gravity-induced decoherence using the neutral-kaon system and accelerator tests of models with large extra dimensions). The emerging picture suggests that we are finally starting the exploration of a large class of plausible quantum-gravity effects. However, our chances to obtain positive (discovery) experimental results depend crucially on the magnitude of these effects. In most cases the level of sensitivity that the relevant experiments should achieve within a few years corresponds to effects suppressed only linearly by the Planck length.

gr-qc↗
arXiv1999

On the Salecker-Wigner limit and the use of interferometers in space-time-foam studies

The recent paper gr-qc/9909017 criticizes the limit on the measurability of distances that was derived by Salecker and Wigner in the 1950s. If justified, this criticism would have important implications for all the recent studies that have used in various ways the celebrated Salecker-Wigner result, but I show here that the analysis reported in gr-qc/9909017 is incorrect. Whereas Salecker and Wigner sought an operative definition of distances suitable for the Planck regime, the analysis in gr-qc/9909017 relies on several assumptions that appear to be natural in the context of most present-day experiments but are not even meaningful in the Planck regime. Moreover, contrary to the claim made in gr-qc/9909017, a relevant quantum uncertainty which is used in the Salecker-Wigner derivation cannot be truly eliminated; unsurprisingly, it can only be traded for another comparable contribution to the total uncertainty in the measurement. I also comment on the role played by the Salecker-Wigner limit in my recent proposal of interferometry-based tests of quantum properties of space-time, which was incorrectly described in gr-qc/9909017. In particular, I emphasize that, as discussed in detail in gr-qc/9903080, only some of the quantum-gravity ideas that can be probed with modern interferometers are motivated by the Salecker-Wigner limit. The bulk of the insight we can expect from such interferometric studies concerns the properties of "foamy" models of space-time, which are intrinsically interesting independently of the Salecker-Wigner limit.

gr-qc↗