Perspectives in Fundamental Physics in Space: Orfeu Bertolami
Perspectives in Fundamental Physics in Space: Orfeu Bertolami
Perspectives in Fundamental Physics in Space: Orfeu Bertolami
Orfeu Bertolami
Instituto Superior Técnico (IST), Departamento de Física, 1049-001 Lisbon,
e-mail: orfeu@cosmos.ist.utl.pt
Olivier Minster
ESA-ESTEC, HME-GAP, NL-2201 Noordwijk, e-mail: Olivier.Minster@esa.int
Sergio Volonte
ESA-HQ, SCI-CA, F-75015 Paris, e-mail: Sergio.Volonte@esa.int
Abstract
We discuss the fundamental principles underlying the current physical theories and
the prospects of further improving their knowledge through experiments in space.
Key words:
Gravitational waves, gravitomagnetism, Equivalence Principle, Antimatter,
Pioneer Anomaly, Lorentz invariance.
PACS:
1 Introduction
General Relativity (GR) and Quantum Mechanics (QM) are the most fun-
damental and encompassing physical theories of the XX th century. They are
2
of the gravitational interaction from the quantum mechanical point of view.
1. Physical laws are independent of the position and velocity of the frame of
reference thanks to the invariance of the world-line distance between events
in the spacetime continuum. This is ultimately related with the fact that the
3
speed of propagation of the electromagnetic and gravitational interactions is
constant and independent of the frame of reference. This speed is the speed
of light in the vacuum.
2. The acceleration of a test body falling under the single influence of the
gravitational interaction is independent of its mass. This can be understood
only if inertial and gravitational masses are exactly equal to each other.
The first set of experiments is associated with the invariance of the physical
systems under translations and rotations in spacetime usually referred to as:
The second set of experiments concerns the so-called Weak Equivalence Prin-
ciple (WEP)or Strong Equivalence Principle (SEP) when gravitational self
interaction is important (see e.g. [12]).
In the limit of weak gravitational fields and low velocities compared with the
speed of light, GR yields small corrections to Newtonian gravity through the
addition of terms proportional to GM/rc2 , where G is Newton’s gravitational
constant, M the mass and r the radius of the source of the gravitational
field under consideration. Thus, general relativistic corrections will become
important in the case of compact astronomical objects, such as neutron stars
(GM/rc2 = O(10−1)) and black holes (GM/rc2 = O(1)), and for the Universe
as a whole.
4
have already been indirectly detected in binary pulsar systems [13] via tracking
of the Post-Keplerian Parameters of the system, and comparison with GR.
LISA will lead to a fundamentally new window for observing the Universe
through observation of sources of gravitational waves. Astronomy has so far
mostly observed the sources of electromagnetic radiation in the Universe.
Gravitational astronomy will allow scientists to achieve a deeper understand-
ing of the dynamics of the cosmos since gravitational waves couple very weakly
with matter and therefore suffer little scattering and absorption on the way
from the source to the observer.
GR also predicts, in the weak field limit and at first order beyond Newtonian
gravity, that for certain mass configurations (a current like one), the metric
can be decomposed into two vector fields. The first one, usually referred to
as gravitoelectric field, corresponds to Newton’s gravitational field. The sec-
ond corresponds to a ”new” field, the so-called gravitomagnetic field. These
designations arise from the fact that in this approximation, Einstein’s field
equations can be formulated in a way that resembles Maxwell’s equations for
the electromagnetic field. Clearly, direct experimental detection of gravita-
tional waves and the gravitomagnetic field produced by Earth’s rotation, are
important tests of GR.
ESA Hyper (Hyper precision cold atom interferometry in space) concept [15]
and the NASA Gravity Probe-B mission [16], a Stanford University mission
which has been launched last 20th April 2004, are dedicated to the detection
of Earth’s gravitomagnetic field. The Gravity Probe B satellite circles the
Earth in a polar orbit at an altitude of 650 km. Data taking was concluded in
August 2005 and results are expected in 2007. The mission concept consists
5
in using four spinning gyroscopes and a telescope. The telescope has been
pointed to a guiding star, IM Pegasi, and the gyroscopes were electrically
induced to align parallel to the telescope axis. Over a year of operation about
5000 orbits are expected. The gyroscopes are left undisturbed as the telescope
is kept pointing toward the guiding star through attitude control thrusters of
the spacecraft. According to GR, the drift angle between the gyroscopes and
the telescope is about 6.6′′ , due to the Earth’s geodetic effect, while a smaller
angle of 0.041′′ should open up in the direction of the Earth’s rotation, due to
the Lens-Thirring effect.
Ground based experiments designed to verify the WEP are limited by the
unavoidable micro seismic activity of Earth. Space experiments offer the pos-
sibility of improving the precision of current tests by a factor of 102 to 105 .
6
Principe d’Equivalence) [20] is a collaborative CNES - ESA mission to be
launched in 2009, designed to evaluate the WEP through the monitoring of
the free fall of two pairs of masses orbiting the Earth located in a drag free
environment at room temperature. The measured signal is the force required
to keep the test masses in a pair centered on each other. Microscope will
evaluate the WEP with a precision expected to reach 1 part in 1015 .
Another promising possibility for testing the WEP uses cold atom interferom-
etry. Ground based High-precision gravimetric measurements have been made
using the interferometry of free-falling Cesium atoms, and allowed to reach
a precision of 7 parts in 109 [23]. Ultimate precision of this method can only
be achieved in space. As an example, the resolution provided by the atom
interferometers to be used in ESA’s Hyper concept mission which could be
sufficient to perform a test of the WEP with an improved precision by a fac-
tor of 106 . Hyper would carry two cold-atom Sagnac interferometers (based on
the negative Michelson-Morley experiment for detection of the ether drift). By
comparing the rate of fall of Cesium and Rubidium atoms in two independent
interferometers a precision of the order of 1 part in 1015 could be achieved, and
this would represent an independent confirmation of, or perhaps a disagree-
ment with, the results of MICROSCOPE. As already mentioned, a concrete
mission to fulfil Hyper concept will be the “Fundamental Physics Explorer”
described at ESA’s Cosmic Vision 2015-2025 [17].
Invariance under Lorentz transformations (LLI), which states that the laws of
physics are independent of the frame velocity, is one of the most fundamental
symmetries of physics and a basic ingredient of all known physical theories.
However, recently some evidence has been found, in the context of String/M-
Theory, that this symmetry can be spontaneously broken. Naturally, this poses
the challenge of verifying this possibility experimentally. The most accurate
laboratory tests of LLI are performed via the so-called Hughes-Drever exper-
iment [24] [25]. In this type of experiment, one searches whether there exists
any anisotropy of inertia through the study of resonant absorption of pho-
tons by a Li7 nucleus in a strong magnetic field. The ground state has spin
3/2 and splits into 4 equally spaced energy levels, given that nuclear physics
laws are rotationally invariant. Therefore, if inertia is not isotropic, then the
four states will not remain exactly equally spaced over the 12 hours period
7
of Earth’s rotation in which the magnetic field is carried to two different lo-
cations with respect to the galactic center. This technique allows achieving
impressive limits, the most stringent being [26].
mI c2
δ ≡| P − 1 |< 3 × 10−22 , (2)
A EA
8
On very large scales, the Hot Big-Bang Model describes the Universe through
GR and the assumption that matter and radiation are homogeneously and
isotropically distributed. Compatibility with data suggests that we are living
in an accelerating, low matter-density Universe. The origin of this acceleration
can be due to either to a cosmological constant [42], or a slow-varying vacuum
energy of some scalar field, usually referred to as Quintessence [43], or due to an
exotic new equation of state, the generalized Chaplygin equation of state [44].
This dark energy amounts for a substantial part of the energy density of the
Universe, ΩΛ ≃ 0.73, with the contribution from matter, dark 1 and baryonic,
P
ΩDM ≃ 0.23, ΩBaryons ≃ 0.04, so that i Ωi = 1 but with no contribution
from the spatial curvature [46] [47] [45] [48] as predicted from Inflation (see
for instance, [49]).
Thus, at late times the rate of expansion of the Universe is controlled by the
dark energy component, which has negative pressure. It should be mentioned
that the understanding of the quantum properties of vacuum and how it relates
with the observed value of energy density are amongst the greatest challenges
for XXI st century physics.
On the other hand, experiments such as EUSO [51] and LOBSTER onboard
the ISS for the space observation of cosmic rays with energies greater than
the ones achievable in particle accelerators, will also help to push even further
our understanding of high-energy physics. Notice that the EUSO experiment
has been postponed until ground-based cosmic-ray observatories like AUGER
[52] yield results.
Testing QM in space is also very important for the future use of novel tech-
1 Most likely candidates for dark matter include a linear combination of neutral
supersymmetric particles, the neutralinos (see eg. [53]), axions [54] and a self-
interacting scalar particle [55].
9
nologies that will rely entirely on the unusual features of the quantum world.
Emerging fields like spintronics, nanotechnology, quantum computing and
quantum communication [56] will certainly represent new technological oppor-
tunities to expand the possibilities of spaceflight. Nevertheless, these technolo-
gies that are still under development on ground, will need proper qualification
for possible use in space. Therefore, quantum physics experiments in space
will not only provide deeper insights; through fundamental physics missions
we will also acquire experience needed to fulfill these qualification steps in the
future.
1. Celestial mechanics has been for centuries the main source of discoveries
in gravitational physics, from Kepler’s laws to the subtle anomalies of Mer-
cury’s orbit. Recently discovered anomalous trajectories of the Pioneer 10
and 11, Ulysses and Galileo probes seem to indicate that some anomalous
gravitational-type force with range beyond several 20 AU or so might exist
[57].
2. Analysis of the free fall of physical systems is, as already discussed, a priv-
ileged experimental tool to test GR. It is remarkable in this respect, that the
free fall of electrically charged particles and of antimatter has been so far
poorly investigated. It is extremely relevant that a novel round of free fall
experiments is carried out for charged particles and antimatter.
Given the importance of these two issues we discuss them in more detail next.
10
studied in space.
The testing of the WEP for antiparticles remains still a largely open problem,
despite recent developments in producing an appreciable number of antihydo-
gen atoms by the ATHENA and ATRAP collaborations at CERN [59] [60]. It
is somewhat urgent that free fall experiments for antimatter are conducted so
as to evaluate to which extent gravity complies with CPT symmetry. This is
a fundamental symmetry of quantum field theory and corresponds to invari-
ance of three conjugate operations, where C stands for charge conjugation,
P for parity, and T for time reversal. In case gravity respects this symmetry,
antimatter will fall exactly like matter in a gravitational field. From the ex-
perimental point of view, it should be mentioned that special Penning trap
devices, magnetic containers, were developed for the purpose of storing sub-
stantial amounts of antimatter over a long time. In this respect, experimental
proposals like WEAX (Weak Equivalence Antimatter experiment) [62] to be
conducted at a cryogenic vacuum facility onboard the ISS and which aim to
measure the free fall of antiprotons while orbiting the Earth are particularly
appealing. The main idea behind this type of experiments is that antiprotons
can be confined for a few weeks in a Penning trap with a geometrical configu-
ration in which the effect of gravity would manifest itself as a perturbation on
their motion. The expected precision of the experiment is 1 part in 106 , three
orders of magnitude better than for a ground experiment. Naturally, testing
the gravitational properties of antihydrogen as well as its spectroscopy, will
allow a deeper understanding of this symmetry. It is worth mentioning that
in some String-Field-Theory models, CPT symmetry can be spontaneously
broken, meaning that although it is a symmetry of the theory, it is not shared
by its ground state.
It is important to point out that these experiments have a high scientific value
as they can provide relevant insights on extensions of the Standard Model. In
this context, it is interesting to remark, that free-fall experiments with charged
particles are also particularly relevant given the fact that they are very poorly
tested experimentally. In the case of ground-based experiments, they involve,
at least for the electron and the positron, the Schiff-Barnhill effect [61] (see in
Ref. [72], [64]).
11
mission. An alternative to a dedicated mission, would involve a somewhat
more limited experiment mounted as piggy-bag onboard deep space missions
like, for instance the Pluto-express mission or the NASA Interstellar Probe
mission [65].
A dedicated mission would in its simplest form consist in launching into deep
space a spherical probe whose behaviour (mechanical, thermal, electromag-
netic, etc.) is very well known [72] [63]. Accurate tracking of its orbit would
allow for precise evaluation of the Pioneer anomaly, as any deviation from the
predicted trajectory would be used to evaluate the un-modeled Pioneer ac-
celeration. Alternative mission concepts were discussed in Refs [73], [74]. The
use of laser ranging techniques and the flying formation concept to the test
the Pioneer anomaly were recently discussed [75].
4 Conclusion
For more than half a century, classical physics has provided the knowledge
required to propel and transport manned and unmanned missions throughout
the Solar System. Contemporary physics however, has not so far played a
12
similar role. Advances in quantum and relativistic mechanics were not yet
fully implemented so to lead to propulsion breakthroughs and to allow for a
more efficient exploration and utilization of space.
Suitable space platforms can provide the proper drag free environment for
carrying out research in many critical areas of modern physics. It is an exciting
prospect to think that fundamental physics missions in space may provide
important insights into the nature of the theory still to emerge that would
harmoniously encompass QM and GR. In turn, unification and a synthesis of
QM and GR may lead to technological breakthroughs that will further push
the boundaries of current space systems.
5 Acknowledgement
The authors would like to thank J. Páramos and A. Rathke for useful sugges-
tions on the topics of this paper.
References
[2] http://aether.lbl.gov/www/projects/cobe/
[4] http://www.sci.esa.int/science-e/www/area/index.cfm?fareaid=17
[5] http://cossc.gsfc.nasa.gov/
[6] http://www.sci.esa.int/science-e/www/area/index.cfm?fareaid=21
[7] http://xmm.vilspa.esa.es/
13
[8] http://heasarc.gsfc.nasa.gov/docs/einstein/heao2.html
[9] http://heasarc.gsfc.nasa.gov/docs/rosat/rosgof.html
[10] http://chandra.harvard.edu/
[11] http://www.sci.esa.int/science-e/www/area/index.cfm?fareaid=31
[14] http://sci.esa.int/science-e/www/area/index.cfm?fareaid=27
[16] http://einstein.stanford.edu/
[19] S. Baessler et al., Phys. Rev. Lett. 83 (1999) 3585; G.L. Smith et al., Phys. Rev.
D61 (2000) 022001.
[20] http://smsc.cnes.fr/MICROSCOPE/Fr/
[21] http://einstein.stanford.edu/STEP/index.html
[22] T. Damour, A.M. Polyakov, Nucl. Phys. B423 (1994) 532; Gen. Rel. Grav. 26
(1994) 1171.
[24] V.W. Hughes, H.G. Robinson, V. Beltran-Lopez, Phys. Rev. Lett. 4 (1960) 342.
[26] S.K. Lamoreaux, J.P. Jacobs, B.R. Heckel, F.J. Raab, E.N. Fortson, Phys. Rev.
Lett. 57 (1986) 3125.
[27] C.M. Will, “The Confrontation between General Relativity and Experiment”,
gr-qc/0103036; “Theory and Experiment in Gravitational Physics” (Cambridge
University Press 1993).
[30] K. Greisen, Phys. Rev. Lett. 16 (1966) 748; G.T. Zatsepin, V.A. Kuzmin, JETP
Lett. 41 (1966) 78.
[31] N. Hayashida et al., (AGASA Collab.), Phys. Rev. Lett. 73 (1994) 3491; M.
Takeda et al., (AGASA Collab.), Phys. Rev. Lett. 81 (1998) 1163.
14
[32] D.J. Bird et al., (Fly’s Eye Collab.), Phys. Rev. Lett. 71 (1993) 3401; Astrophys.
J. 424 (1994) 491; 441 (1995) 144.
[33] M.A. Lawrence, R.J.O. Reid, A.A. Watson (Haverah Park Collab.), J. Phys.
G17 (1991) 733.
[35] http://www-akeno.icrr.u-tokyo.ac.jp/AGASA/
[37] S. Coleman, S.L. Glashow, Phys. Lett. B405 (1997) 249; Phys. Rev. D59 (1999)
116008.
[42] M.C. Bento, O. Bertolami, Gen. Rel. Grav. 31 (1999) 1461; M.C. Bento, O.
Bertolami, P.T. Silva, Phys. Lett. B498 (2001) 62 and references therein.
[43] R.R. Caldwell, R. Dave, P. Steinhardt, Phys. Rev. Lett. 80 (1998) 1582; L.
Wang, R.R. Caldwell, J.P. Ostriker, P. Steinhardt, Astrophys. J. 530 (2000) 17.
[44] M.C. Bento, O. Bertolami, A.A. Sen, Phys. Rev. D66 (2002) 043507; D67 (2003)
063003; Phys. Lett. 575 B (2003) 172; Gen. Rel. Grav. 35 (2003) 2063; P.T. Silva,
O. Bertolami, Astrophys. J. 599 (2003) 829.
[45] M.C. Bento, O. Bertolami, A.A. Sen, Phys. Rev. D70 (2004) 083519; O.
Bertolami, P.T. Silva, Mon. Not. Roy. Ast. Soc. 365 (2006) 1149; M.C. Bento,
O. Bertolami, M.J. Reboucas, P.T. Silva, Phys. Rev. D73 (2006) 043504
[47] P.M. Garnavich et al., Astrophys. J. Lett. 493 (1998) L53; Science 279 (1998)
1298.
[51] http://www.euso-mission.org/
[52] http://www.auger.org/
15
[53] A. Bottino, F. Donato, N. Fornengo, S. Scopel, Nuvl. Phys. B (Proc. Suppl.)
113 (2002) 50
[54] S. Asztalos et al., Phys. Rev. D64 (2001) 092003.
[55] M.C. Bento, O. Bertolami, R. Rosenfeld, L. Teodoro, Phys. Rev. D62 (2000)
041302 (R); M.C. Bento, O. Bertolami, R. Rosenfeld, Phys. Lett. B518 (2001)
276.
[56] A., Zeilinger, ”Quantum Communication in Space”, ESA Contract 16358/02,
General Studies Programme
[57] J.D. Anderson, et al., Physics Review Letters 81 (1998) 2858; J.D. Anderson,
et al. Phys. Rev. D65 (2002) 082004
[58] M. Tajmar, C.J. de Matos, Physica C 420 (2005)
[59] http://athena.cern.ch/athena
[60] http://hulse.harvard.edu/atrap/
[61] L.I. Schiff, M.V. Barnhill, Phys. Rev. 151 (1966) 1067.
[62] R.A. Lewis, G.A. Smith, F.M. Huber, E.W. Messerschmid, “Measuring the
gravitational force of anti-protons in space”, ESA SP-385
[63] O. Bertolami, M. Tajmar, ”Hypothetical Gravity Control and Spacecraft
Propulsion”, gr-qc/0207123.
[64] H. Dittus, C. Lämmerzahl, H. Selig, Gen. Rel. Grav. 36 (2004) 571.
[65] http://interstellar.jpl.nasa.gov/outreach.html
[66] O. Bertolami, ”Three Key Tests to Gravity”, hep-ph/0301191
[67] O. Bertolami, J. Páramos, Classical and Quantum Gravity 21 (2004) 3309.
[68] O. Bertolami, J. Páramos, “Pioneer’s Final Riddle”, gr-qc/0411020.
[69] O. Bertolami, J. Páramos, Phys. Rev. D71 (2005) 023521.
[70] J.I. Katz, Phys. Rev. Lett. 83 (1999) 1892; L.K. Scheffer, Phys. Rev. D67 (2003)
084201.
[71] O. Bertolami, P. Vieira, “Pioneer Anomaly and the Kuipper Belt mass
distribution”, astro-ph/0506330; to appear in Classical and quantum Gravity
(2006).
[72] O. Bertolami, M. Tajmar, ”Gravity Control and Possible Influence on Space
Propulsion: A Scientific Study”, ESA-CR(P) 4365
[73] J.D. Anderson, M.M. Nieto, S.G. Turyshev, Int. J. Mod. Phys. D11 (2002)
1545; M.M. Nieto, S.G. Turyshev, ”Finding the Origin of the Pioneer Anomaly”
gr-gc/0308017.
[74] U. Johann, R. Forstner, ”Enigma Mission”, EADS Astrium (2003)
[75] PIONEER Collaboration, “A Mission to Explore the Pioneer Anomaly”, H.
Dittus et al., gr-qc/0506139.
16
A Nomenclature List
17