Table Of ContentFIAS Interdisciplinary Science Series
Sabine Hossenfelder Editor
Experimental
Search for
Quantum Gravity
FIAS Interdisciplinary Science Series
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Sabine Hossenfelder
Editor
Experimental Search for
Quantum Gravity
123
Editor
SabineHossenfelder
FrankfurtInstitute for AdvancedStudies
(FIAS)
Frankfurtam Main
Germany
FIAS Interdisciplinary ScienceSeries
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Preface
Ifyouhavegoodeyes,thesmallestobjectsyoucanmakeoutareaboutatenthofa
millimeter, roughly the width of a human hair. Add technology, and the smallest
structures we have measured so far are approximately 10−19 m, that is, the wave-
lengthoftheprotonscollidedattheLHC.Ithastakenusabout400yearsfromthe
invention of the microscope to the construction of the LHC—400 years to cross
15 orders of magnitude.
Quantum effects of gravity are estimated to become relevant on distance scales
ofapproximately10−35m,knownasthePlancklength.Thatisanother16ordersof
magnitudetogo.Itmakesyouwonderwhetheritispossibleatallorwhetherallthe
effort to find a quantum theory of gravity is just idle speculation.
Iamoptimistic.Thehistoryofscienceisfullwithpeoplewhothoughtthingsto
beimpossiblethathavemeanwhilebeendone:measuringthelightdeflectiononthe
sun, heavier-than-airflying machines, detecting gravitational waves. Hence, I don’t
think it is impossible to experimentally test quantum gravity. Maybe it will take
some decades,ormaybe itwill takesome centuries—butifonlywekeeppushing,
one day we will measure quantum gravitational effects. Not by directly crossing
these 15 orders of magnitude, I believe, but instead by indirect detections at lower
energies.
From nothing comes nothing though. If we do not think about how quantum
gravitationaleffectscanlooklikeandwheretheymightshowup,wewillcertainly
never find them. But fueling my optimism is the steadily increasing interest in the
phenomenologyofquantumgravity,theresearchareadedicatedtostudyinghowto
best find evidence for quantum gravitational effects.
Since there is not any one agreed-upon theory for quantum gravity, existing
effortstofindobservablephenomenafocusonfindingwaystotestgeneralfeatures
of the theory, properties that have been found in several different approaches to
quantumgravity.Quantumfluctuationsofspace-time,forexample,orthepresence
of a “minimal length” would impose a fundamental resolution limit. Such effects
can be quantified in mathematical models, which can then be used to estimate the
strength of the effects and thus to find out which experiments are most promising.
v
vi Preface
Thisvolumecollectssomerecentdevelopmentsinthefieldofphenomenological
quantum gravity that were subject of the recent conference on “Experimental
SearchforQuantumGravity.”ThismeetingtookplaceattheFrankfurtInstituteof
Advanced Studies (FIAS) in September 2016 as part of the first Giersch
Symposion.
Frankfurt am Main, Germany Sabine Hossenfelder
June 2017
Contents
Astroparticle Physics Connections to the Quantum
Gravity Problem... .... .... ..... .... .... .... .... .... ..... .... 1
Matthias Lorentz
The Search for a Tiny Hint from Quantum Gravity
in the Cosmic Relic Radiation ..... .... .... .... .... .... ..... .... 9
David Brizuela and Manuel Krämer
Superfluid Helium: The Volovik Lessons .... .... .... .... ..... .... 15
Tim Lappe
On the Paradigms of Quantum Gravity 2016. .... .... .... ..... .... 21
Fabian Müller
On the Measurement of the Speed of Light in a Cavity. .... ..... .... 29
Fabienne Schneiter
Neutrino: The Elusive Particle Bringing us Closer
to the World of Quantum Gravity.. .... .... .... .... .... ..... .... 37
Giacomo D’Amico
Gravitational Waves: The “Sound” of the Universe.... .... ..... .... 43
José Manuel Carmona
Essay on Planck Star Phenomenology... .... .... .... .... ..... .... 49
Alexander Maximilian Eller
Essay About Gravitational Measurements at Small Distances..... .... 55
Helena Schmidt
Return on Investment in Quantum-Gravity Research .. .... ..... .... 61
Giovanni Amelino-Camelia
Semiclassical Gravity: A Testable Theory of Quantum Gravity ... .... 69
Sabina Scully
vii
viii Contents
Quantum Gravity Deformations ... .... .... .... .... .... ..... .... 77
Antonia Micol Frassino
General Relativity, Black Holes and Planck Stars . .... .... ..... .... 85
Matteo Trudu
Spacetime Structure: Analogy in Condensed Matter
and Quantum Information... ..... .... .... .... .... .... ..... .... 91
Martin Seltmann
Experimental Search for Quantum Gravity Using Cosmology .... .... 105
Manon Bischoff and Vincent Vennin
The Cosmological Constant and Its Problems: A Review
of Gravitational Aether.. .... ..... .... .... .... .... .... ..... .... 109
Michael Florian Wondrak
Astroparticle Physics Connections
to the Quantum Gravity Problem
MatthiasLorentz
Introduction
The Universe can be used as a formidable laboratory for high-energy physics. In
contrasttoparticlephysicsatcolliderswheretheexperimentalenvironmentiswell
controlled, astroparticle physics deals with elementary particles in “natural” envi-
ronmentswherediverseastrophysicalprocessesareatplay.Whilethiscanintroduce
anadditionallevelofcomplexityinthephysicalinterpretationofdata,theparticle
energiesachievablebycosmicacceleratorsarebeyondthecapabilitiesofman-made
accelerators(seeFig.1)andthesehigh-energyparticlespropagateovercosmological
distances,offeringuniqueopportunitiestoaddressfundamentalphysicsaspects.
Thequestionofquantumgravity(QG)remainsunanswered,asnoconsistentthe-
ory combining quantum field theory and general relativity have been found so far.
TheoriesofQGhavebeendevelopedbasedonmathematicalconsistencyarguments
butthelackofobservationalevidenceduetotheextremelysmallexpectedeffectsren-
dersverydifficulttoproperlytestandguidesuchspeculativetheories.Thissituationis
changingasmoreandmoreeffortsareputintodevelopingaphenomenologyofQG,
searching for viable experimental tests in various fields, and astroparticle physics
offers many attractive possibilities. We briefly present here selected astroparticle
aspectsrelevantintheexperimentalsearchforQGmainlyfocusedonPlanck-scale
tests of Lorentz symmetry, in the perspective of current and future experimental
facilities.
B
M.Lorentz ( )
Irfu,CEASaclay,91191Gif-sur-Yvette,France
e-mail:[email protected]
©SpringerInternationalPublishingAG2018 1
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