Transcript Giovanni_Camelia - UCLA Physics & Astronomy

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 Ep 1028eV (Planck length Lp1/Ep 10-35m)
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,
e.g. scenarios with
but nobody will argue against the claim that
large extra space dimensions
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:
ELEP;MW
EGUT
EP
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
2
2
2
4
2
m + p  E -E /Ep
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)
kth,QG 
2m protm

#
kth3 , 0 kth, 0
E p2 
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)
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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
S( f ) 

f
 L2   df S ( f )  T
2
GAC, Nature 398(1999)
Ng+VanDam, FoundPhys(2000)
GAC,PhysRevD(2000)
GAC,Nature410(2001)
Schiller et al, PhysRevD(2004)
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model too crude….but naivety of “Planck-frequency expectation‘‘ robustly exposed…
should be increasingly important as we
GAC, Nature 398(1999)216
Ng+VanDam, FoundPhys(2000)
gain access to lower and lower frequencies….
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)
dE
Schaefer, PhysRevLett82(1999)
 1- η E/Ep
Gambini+Pullin, PhysRevD59(1999)
dp
wavelength-dependent speed for photons
from modified dispersion relation
v =
This would mean that two (nearly-)simultaneously-emitted photons would
reach the Earth with a relative time-of-arrival difference of
t = T η E/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 Jacob+Piran,NaturePhysics3(2007)
GAC, Nature Physics 3(2007)
some UHE neutrino observations)
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….
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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)
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Doppler effect not fully understood from a
“deformed-symmetry” perspective , but
for some scenarios preliminary result is
GAC+Lämmerzahl
for m124GeV and 352 THz (Caesium, D2 line)
F. Mercati
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”