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## Q2C III , 10 july 2008

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**Q2CIII, 10 july 2008**Planck-scale physics in space Giovanni AMELINO-CAMELIA Univ. of Rome “LA SAPIENZA” the “Quantum Gravity problem” and the type of “Quantum Gravity Phenomenology” it can motivate genuine Planck-scale sensitivity is achievable difficult phenomenology but the Planck scale often provides associated “target sensitivities”, and this can in some cases be used to strengthen proposals of fundamental-physics experiments in space (“we shall improve the limits by 2 or 3 orders of magnitude” “we shall improve the limits to the level needed to probe Planck scale”) GAC,”Fundamental physics in space: a quantum-gravity perspective”,GenRelGrav36(2004)539-560 [needs updating]**●Quantum Mechanics and GR are very successful in their**respective domains of typical applicability, but inconsistencies are unavoidable if we combine them naively in the analysis of situations in which they both should be relevant: there should be a “quantum gravity”, i.e. some nontrivial “unification” of QM and GR ● most robust “theoretical evidence” on the quantum-gravity realm: Planck-scale nonlocality; Planck-scale spacetime quantization (discretization, noncommutativity, fuzziness); ………… Planck scale Ep1028eV (Planck length Lp1/Ep10-35m) 2**Effects of spacetime quantization should be small but**striking!!! in most approaches to this “QG problem” spacetime ends up being described by a nonclassical (“quantum”) geometry, with some nonlocality, spacetime fuzziness, spacetime noncommutativity, and this can indeed lead to striking consequences, including •violations of Poincarè/Lorentz symmetry •spacetime fuzziness •some implications for the Equivalence Principle •decoherence •............... The fact that such striking effects would be plausible (though not necessary) at the Planck scale has been acknowledge for decades now, but for a long time it was thought that the smallness of the effects (due to the smallness of the Planck length) would be an unsurmountable difficulty for experimental tests 3**c**we should “walk the plank”: we have robust evidence that something new must be there at the Planck scale…. some authors would argue that quantum-gravity effects could show up already at a lower scale, but nobody will argue against the claim that something new must be there by the time we get to the Planck scale…. so we better sharpen our tools for the Planck scale!! also because there may well be nothing (of quantum-gravity relevance) below the Planck scale: e.g. scenarios with large extra space dimensions ELEP;MW EP EGUT**In which sense we have “proven sensitivity to**effects introduced genuinely at the Planck scale”? imagine space is discrete with lattice scale the Planck length, then naturally you end up with something like m2+ p2 E2 -E4/Ep2 see,e.g.,t’Hooft, CQG(1996) then compute the threshold energy requirement for photopion production p +γCMBR => p+π with this modified dispersion relation and one finds a shift of the threshold, which implies an observably large shift of the “GZK scale” for the cosmic-ray spectrum Kifune, Astr.Journ.Lett.(1999) GAC+Piran, PhysRevD(2001) GAC,Nature(2000) O(1) correction important already when Ep2 (kth,0 ) 4/(mprotm)***following this line of analysis (and data recently gathered**at the Pierre Auger cosmic-ray observatory) we are now close to establishing as a scientific fact that rigid Planck-scale discretizations of spacetime are not allowed *so we do have, at least in some cases, a chance to probe effects introduced genuinely at the Planck scale * and this “threshold-anomaly analyses” provide examples of how wrong the naive “Yang-Mills integrated-out-gauge-bosons intuition” is about the magnitude of Planck-scale-induced effects can be: “must require Planckian energies” requires large boosts (even at relatively “low” energies)!! 6**Another case of wrong intuition**many QG theorists would favour a picture in which the world lines of particles are “fuzzy” with fuzziness at scales of Planck-length magnitude per Planck time interval and a simple-minded but effective way to give a first crude estimate of the size of this effect could rely on a picture in which particles still propagate in a classical geometry but their trajectory is affected by stochasticity at the level of Planck-length-size fluctuations with Planck-time frequency naive intuition: study of this effect requires operating an interferometer at the huge Planck frequency (1044Hz) 7**But if indeed the particles random walk with Planck-length**fluctuations occurring each Planck time then we should expect the characteristic random-walk power spectrum which insteads is mostly low-frequency GAC, Nature 398(1999) Ng+VanDam, FoundPhys(2000) GAC,PhysRevD(2000) GAC,Nature410(2001) Schiller et al, PhysRevD(2004) 8**model too crude….but naivety of “Planck-frequency**expectation‘‘ robustly exposed… should be increasingly important as we gain access to lower and lower frequencies…. GAC, Nature 398(1999)216 Ng+VanDam, FoundPhys(2000) GAC,Nature410(2001) * * Quantum Gravity? also see GAC+Lämmerzahl, ClassicalQuantGrav(2004) [for in-vacuo dispersion]**an example where the numbers work out just perfectly**for a “Planck-scale target sensitivity”: GLAST and Planck-scale-induced in-vacuo dispersion GAC+Ellis+Mavromatos+Nanopoulos+Sarkar, Nature393(1998) Schaefer, PhysRevLett82(1999) Gambini+Pullin, PhysRevD59(1999) from modified dispersion relation dE v= 1- ηE/Ep dp wavelength-dependent speed for photons This would mean that two (nearly-)simultaneously-emitted photons would reach the Earth with a relative time-of-arrival difference of t = T ηE/Ep where T is the overall time travelled since this needs sharp time resolution, long distances travelled and (possibly) particles of high energy, gamma-ray bursts: - travel distances of order 1010 light years - microbursts within a burst can have duration 10-3 seconds - relatively large E (10 MeV... 100 MeV...possibly a few GeV...) GLAST numbers work out to provide sensitivity to ||1 (e.g. BATSE at ||10-3) (N.B.: focusing here on linear effect but quadratic effect within reach applying the same strategy to some UHE neutrino observations) Jacob+Piran,NaturePhysics3(2007) GAC, Nature Physics 3(2007)**Planck-scale physics in space:**already a reality *GLAST collaboration well aware; dedicated analyses planned *most EUSO colleagues are aware *spacetime fuzziness analysis also relevant for LISA, but I am not informed of any dedicated plans *ability to manipulate atoms (often improved in space environment) is likely to be next opportunity…. 11**example: atom recoil studies:**atom absorbs a photon with frequency tuned on resonance, gets in an excited level and recoils, then a photon with a doppler-shifted frequency ’ deexcites the atom, inducing stimulated emission = ? • a calculation for “deformed Lorentz symmetry”: • modify Lorentz symmetry via some • Planck-scale effects, but introduce • mathematics (Hopf algebra) • to preserve equivalence of inertial frames GAC, PhysLettB(2001) Magueijo+Smolin, PhysRevLett(2002) GAC,Nature(2002) 12**Doppler effect not fully understood from a**“deformed-symmetry” perspective , but for some scenarios preliminary result is GAC+Lämmerzahl F. Mercati for m124GeV and 352 THz (Caesium, D2 line)**Summary: “target sensitivity” and other features render**Planck-scale phenomenology well suited for proposals of “fund phys in space” QGphenomenology made significant progress in just a few years “Are we at dawn of Quantum-Gravity Phenomenology” GAC,@Karpacz1999,Lect.NotesPhys.541,1 GAC,Nature408,661-664(2000) BEFORE: even Isham in his reviews only includes 3 lines on “experimental tests” just to argue that it could not possibly be done Isham, “Structural issues in quantum gravity”, Proc. of General Relativity and Gravitation 1995 AFTER: even general (rather formal) QG reviews acknowledge importance of QGphenomenology for the overall development of QG Rovelli, gr-qc0006061 Carlip, Rept.Prog.Phys.64,885 Smolin, hep-th0408048 and it appears likely that, now that finally awareness of the futility of the pursuit of a “theory of everything” is spreading, efforts in the QGphenomenology direction will further increase**Einstein’s utopia of a theory of everything:**“I would like to state a theorem…: there are no arbitrary constants ... that is to say, Nature is so constituted that it is possible logically to lay down such strongly determined laws that within these laws only rationally completely determined constants occur”