Title: High-Energy gamma-ray Astronomy and String Theory
ArXiv ID: 0903.0318
Date: 2009-03-02
Authors: Nick E. Mavromatos
📝 Abstract
There have been observations, first from the MAGIC Telescope (July 2005) and quite recently (September 2008) from the FERMI Satellite Telescope, on non-simultaneous arrival of high-energy photons from distant celestial sources. In each case, the highest energy photons were delayed, as compared to their lower-energy counterparts. Although the astrophysics at the source of these energetic photons is still not understood, and such non simultaneous arrival might be due to non simultaneous emission as a result of conventional physics effects, nevertheless, rather surprisingly, the observed time delays can also fit excellently some scenarios in quantum gravity, predicting Lorentz violating space-time "foam" backgrounds with a non-trivial subluminal vacuum refractive index suppressed linearly by a quantum gravity scale of the order of the reduced Planck mass. In this pedagogical talk, I discuss the MAGIC and FERMI findings in this context and I argue on a theoretical model of space-time foam in string/brane theory that can accommodate the findings of those experiments in agreement with all other stringent tests of Lorentz invariance. However, I stress the current ambiguities/uncertainties on the source mechanisms, which need to be resolved first before definite conclusions are reached regarding quantum gravity foam scenarios.
💡 Deep Analysis
Deep Dive into High-Energy gamma-ray Astronomy and String Theory.
There have been observations, first from the MAGIC Telescope (July 2005) and quite recently (September 2008) from the FERMI Satellite Telescope, on non-simultaneous arrival of high-energy photons from distant celestial sources. In each case, the highest energy photons were delayed, as compared to their lower-energy counterparts. Although the astrophysics at the source of these energetic photons is still not understood, and such non simultaneous arrival might be due to non simultaneous emission as a result of conventional physics effects, nevertheless, rather surprisingly, the observed time delays can also fit excellently some scenarios in quantum gravity, predicting Lorentz violating space-time “foam” backgrounds with a non-trivial subluminal vacuum refractive index suppressed linearly by a quantum gravity scale of the order of the reduced Planck mass. In this pedagogical talk, I discuss the MAGIC and FERMI findings in this context and I argue on a theoretical model of space-time foam
📄 Full Content
An alternative title for the talk could be MAGIC and String Theory: Usually the terminology M-theory, with M standing for either Magical or Marvelous or Mysterious, is attributed to the underlying (yet not completely understood) unifying theory of all known string theories [1], as a result of the many appealing duality and other symmetries it possesses, which result in the unification of the five known string theories, viewed as a low-energy limit of M-theory. This is a super-unification picture, which may prompt the way for a detailed understanding of the yet elusive theory of the quantum structure of space-time, otherwise termed as "Quantum Gravity" (QG). Our current knowledge/understanding of M-theory is limited. Nevertheless, for an analysis of some of the predictions of string theory that could have some relevance to observable low-energy physics this may not be an obstacle, as we shall attempt to discuss in this work.
In this review the word MAGIC is used for something completely different. It is an acronym (M.A.G.I.C = M ajor Atmospheric Gamma-ray I maging C herenkov telescope), pertaining to the initials describing the full name of a Physics Instrument, specifically a Telescope based on the Canary Islands observatory (c.f. fig. 1), dedicated to the study of Cherenkov radiation emitted by highly energetic cosmic particles as they enter our atmosphere From the study of the emitted Cherenkov radiation, when a highly energetic cosmic particle enters the Earth’s atmosphere, which is characterised by a non-trivial refractive index, one can deduce several important conclusions on the nature of the initial particle and through this to try to understand the mechanisms of production of such energetic cosmic particles.
Figure 1. Major Atmospheric Gamma-ray Imaging Cherenkov Telescope at the Canary Islands (Spain) Observatory (right panel). The telescope observed very high energy gamma rays from the active galactic nucleus Markarian 501 (radio image on left panel, by J.M. Wrobel and J.E. Konway, picture taken from http://www.vlba.nrao.edu/whatis/mark.html
) with energies up to the order of 10 TeV (i.e. 10 12 eV).
On July 9th 2005, the telescope observed [2] (c.f. fig. 1) very high energy gamma rays from the active galactic nucleus Markarian 501 (Mkn 501), which lies at red-shift z = 0.034 (i.e. about half a million light years away) from Earth, with energies up to the order of 10 TeV (1 TeV = 10 3 GeV = 10 12 eV), which were delayed up to four minutes as compared with their lower-energy counterparts (in the 0.6 TeV or lower range) (c.f. fig. 2). It was the first and currently the only observation of such a distinct delay. The effect may be related to the astrophysics of the active galactic nucleus (source effect), which, as we shall discuss below, is not well understood at present and hence there is no consensus among the astrophysicists on the appropriate mechanism for the production of such high-energy photons at the source.
These uncertainties prompted more ambitious, although admittedly looking far-fetched, explanations [3], pertaining to new fundamental physics, affecting the photon propagation due, for instance, to space-time foamy vacuum structures that lead to modified dispersion relations for photons, that is a departure from the Lorentz invariant energy (E)-momentum ( p) relations of Special Relativity, E = | p|c. Indeed, the reader should bear in mind that at small length scales, of the order of the Planck length, ℓ P = hG N c 3 = h M P c ∼ 10 -35 m, which is the characteristic scale that quantum gravity effects are expected to be dominant, the structure of space time may be quite different from what we perceive at our (low-energy) scales, and it might be even discrete and non-commutative, that is the space-time coordinates (as we perceive them at present) might be average values of non commutative quantum operators. Moreover, one may have highly curved non-trivial fluctuations of the space-time metric, giving space time a “foamy” structure. In such complicated Quantum Gravity (QG) vacua, therefore, the very concept of Lorentz symmetry might break down at these short length scales, pointing towards the possibility of spontaneous Lorentz symmetry breaking by the QG vacuum, since if Lorentz symmetry is intact, it strictly [2] regarding very high energy Gamma Rays (with energies in the TeV range) showed that the most energetic photons were delayed up to four minutes as compared with their lower-energy counterparts (in the 0.6 TeV or lower range). The figures show light curves (LC), i.e. the photon flux vs. time of arrival. Observations like this may be used to prompt new fundamental physics on the structure of space-time. The lower panel shows LC at different energy ranges, demonstrating clearly the time delay (of order of 4 ±1 minutes) of the more energetic photons. prohibits such modifications in the dispersion relations. Such departures from the standard Special Relativity form of dispersion rela
